During deterministic GLR operation, user actions should be able to

[bison.git] / doc / bison.texinfo

\input texinfo @c -*-texinfo-*-
@comment %**start of header
@setfilename bison.info
@include version.texi
@settitle Bison @value{VERSION}
@setchapternewpage odd

@finalout

@c SMALL BOOK version
@c This edition has been formatted so that you can format and print it in
@c the smallbook format.
@c @smallbook

@c Set following if you have the new `shorttitlepage' command
@c @clear shorttitlepage-enabled
@c @set shorttitlepage-enabled

@c Set following if you want to document %default-prec and %no-default-prec.
@c This feature is experimental and may change in future Bison versions.
@c @set defaultprec

@c ISPELL CHECK: done, 14 Jan 1993 --bob

@c Check COPYRIGHT dates.  should be updated in the titlepage, ifinfo
@c titlepage; should NOT be changed in the GPL.  --mew

@c FIXME: I don't understand this `iftex'.  Obsolete? --akim.
@iftex
@syncodeindex fn cp
@syncodeindex vr cp
@syncodeindex tp cp
@end iftex
@ifinfo
@synindex fn cp
@synindex vr cp
@synindex tp cp
@end ifinfo
@comment %**end of header

@copying

This manual is for @acronym{GNU} Bison (version @value{VERSION},
@value{UPDATED}), the @acronym{GNU} parser generator.

Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the @acronym{GNU} Free Documentation License,
Version 1.2 or any later version published by the Free Software
Foundation; with no Invariant Sections, with the Front-Cover texts
being ``A @acronym{GNU} Manual,'' and with the Back-Cover Texts as in
(a) below.  A copy of the license is included in the section entitled
``@acronym{GNU} Free Documentation License.''

(a) The @acronym{FSF}'s Back-Cover Text is: ``You have freedom to copy
and modify this @acronym{GNU} Manual, like @acronym{GNU} software.
Copies published by the Free Software Foundation raise funds for
@acronym{GNU} development.''
@end quotation
@end copying

@dircategory Software development
@direntry
* bison: (bison).       @acronym{GNU} parser generator (Yacc replacement).
@end direntry

@ifset shorttitlepage-enabled
@shorttitlepage Bison
@end ifset
@titlepage
@title Bison
@subtitle The Yacc-compatible Parser Generator
@subtitle @value{UPDATED}, Bison Version @value{VERSION}

@author by Charles Donnelly and Richard Stallman

@page
@vskip 0pt plus 1filll
@insertcopying
@sp 2
Published by the Free Software Foundation @*
51 Franklin Street, Fifth Floor @*
Boston, MA  02110-1301  USA @*
Printed copies are available from the Free Software Foundation.@*
@acronym{ISBN} 1-882114-44-2
@sp 2
Cover art by Etienne Suvasa.
@end titlepage

@contents

@ifnottex
@node Top
@top Bison
@insertcopying
@end ifnottex

@menu
* Introduction::
* Conditions::
* Copying::           The @acronym{GNU} General Public License says
                        how you can copy and share Bison

Tutorial sections:
* Concepts::          Basic concepts for understanding Bison.
* Examples::          Three simple explained examples of using Bison.

Reference sections:
* Grammar File::      Writing Bison declarations and rules.
* Interface::         C-language interface to the parser function @code{yyparse}.
* Algorithm::         How the Bison parser works at run-time.
* Error Recovery::    Writing rules for error recovery.
* Context Dependency::  What to do if your language syntax is too
                        messy for Bison to handle straightforwardly.
* Debugging::         Understanding or debugging Bison parsers.
* Invocation::        How to run Bison (to produce the parser source file).
* C++ Language Interface::  Creating C++ parser objects.
* FAQ::               Frequently Asked Questions
* Table of Symbols::  All the keywords of the Bison language are explained.
* Glossary::          Basic concepts are explained.
* Copying This Manual::  License for copying this manual.
* Index::             Cross-references to the text.

@detailmenu
 --- The Detailed Node Listing ---

The Concepts of Bison

* Language and Grammar::  Languages and context-free grammars,
                            as mathematical ideas.
* Grammar in Bison::  How we represent grammars for Bison's sake.
* Semantic Values::   Each token or syntactic grouping can have
                        a semantic value (the value of an integer,
                        the name of an identifier, etc.).
* Semantic Actions::  Each rule can have an action containing C code.
* GLR Parsers::       Writing parsers for general context-free languages.
* Locations Overview::    Tracking Locations.
* Bison Parser::      What are Bison's input and output,
                        how is the output used?
* Stages::            Stages in writing and running Bison grammars.
* Grammar Layout::    Overall structure of a Bison grammar file.

Writing @acronym{GLR} Parsers

* Simple GLR Parsers::      Using @acronym{GLR} parsers on unambiguous grammars.
* Merging GLR Parses::      Using @acronym{GLR} parsers to resolve ambiguities.
* GLR Semantic Actions::    Deferred semantic actions have special concerns.
* Compiler Requirements::   @acronym{GLR} parsers require a modern C compiler.

Examples

* RPN Calc::          Reverse polish notation calculator;
                        a first example with no operator precedence.
* Infix Calc::        Infix (algebraic) notation calculator.
                        Operator precedence is introduced.
* Simple Error Recovery::  Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
* Multi-function Calc::  Calculator with memory and trig functions.
                           It uses multiple data-types for semantic values.
* Exercises::         Ideas for improving the multi-function calculator.

Reverse Polish Notation Calculator

* Decls: Rpcalc Decls.  Prologue (declarations) for rpcalc.
* Rules: Rpcalc Rules.  Grammar Rules for rpcalc, with explanation.
* Lexer: Rpcalc Lexer.  The lexical analyzer.
* Main: Rpcalc Main.    The controlling function.
* Error: Rpcalc Error.  The error reporting function.
* Gen: Rpcalc Gen.      Running Bison on the grammar file.
* Comp: Rpcalc Compile. Run the C compiler on the output code.

Grammar Rules for @code{rpcalc}

* Rpcalc Input::
* Rpcalc Line::
* Rpcalc Expr::

Location Tracking Calculator: @code{ltcalc}

* Decls: Ltcalc Decls.  Bison and C declarations for ltcalc.
* Rules: Ltcalc Rules.  Grammar rules for ltcalc, with explanations.
* Lexer: Ltcalc Lexer.  The lexical analyzer.

Multi-Function Calculator: @code{mfcalc}

* Decl: Mfcalc Decl.      Bison declarations for multi-function calculator.
* Rules: Mfcalc Rules.    Grammar rules for the calculator.
* Symtab: Mfcalc Symtab.  Symbol table management subroutines.

Bison Grammar Files

* Grammar Outline::   Overall layout of the grammar file.
* Symbols::           Terminal and nonterminal symbols.
* Rules::             How to write grammar rules.
* Recursion::         Writing recursive rules.
* Semantics::         Semantic values and actions.
* Locations::         Locations and actions.
* Declarations::      All kinds of Bison declarations are described here.
* Multiple Parsers::  Putting more than one Bison parser in one program.

Outline of a Bison Grammar

* Prologue::          Syntax and usage of the prologue.
* Bison Declarations::  Syntax and usage of the Bison declarations section.
* Grammar Rules::     Syntax and usage of the grammar rules section.
* Epilogue::          Syntax and usage of the epilogue.

Defining Language Semantics

* Value Type::        Specifying one data type for all semantic values.
* Multiple Types::    Specifying several alternative data types.
* Actions::           An action is the semantic definition of a grammar rule.
* Action Types::      Specifying data types for actions to operate on.
* Mid-Rule Actions::  Most actions go at the end of a rule.
                      This says when, why and how to use the exceptional
                        action in the middle of a rule.

Tracking Locations

* Location Type::               Specifying a data type for locations.
* Actions and Locations::       Using locations in actions.
* Location Default Action::     Defining a general way to compute locations.

Bison Declarations

* Require Decl::      Requiring a Bison version.
* Token Decl::        Declaring terminal symbols.
* Precedence Decl::   Declaring terminals with precedence and associativity.
* Union Decl::        Declaring the set of all semantic value types.
* Type Decl::         Declaring the choice of type for a nonterminal symbol.
* Initial Action Decl::  Code run before parsing starts.
* Destructor Decl::   Declaring how symbols are freed.
* Expect Decl::       Suppressing warnings about parsing conflicts.
* Start Decl::        Specifying the start symbol.
* Pure Decl::         Requesting a reentrant parser.
* Decl Summary::      Table of all Bison declarations.

Parser C-Language Interface

* Parser Function::   How to call @code{yyparse} and what it returns.
* Lexical::           You must supply a function @code{yylex}
                        which reads tokens.
* Error Reporting::   You must supply a function @code{yyerror}.
* Action Features::   Special features for use in actions.
* Internationalization::  How to let the parser speak in the user's
                        native language.

The Lexical Analyzer Function @code{yylex}

* Calling Convention::  How @code{yyparse} calls @code{yylex}.
* Token Values::      How @code{yylex} must return the semantic value
                        of the token it has read.
* Token Locations::   How @code{yylex} must return the text location
                        (line number, etc.) of the token, if the
                        actions want that.
* Pure Calling::      How the calling convention differs
                        in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).

The Bison Parser Algorithm

* Look-Ahead::        Parser looks one token ahead when deciding what to do.
* Shift/Reduce::      Conflicts: when either shifting or reduction is valid.
* Precedence::        Operator precedence works by resolving conflicts.
* Contextual Precedence::  When an operator's precedence depends on context.
* Parser States::     The parser is a finite-state-machine with stack.
* Reduce/Reduce::     When two rules are applicable in the same situation.
* Mystery Conflicts::  Reduce/reduce conflicts that look unjustified.
* Generalized LR Parsing::  Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted.  How to avoid it.

Operator Precedence

* Why Precedence::    An example showing why precedence is needed.
* Using Precedence::  How to specify precedence in Bison grammars.
* Precedence Examples::  How these features are used in the previous example.
* How Precedence::    How they work.

Handling Context Dependencies

* Semantic Tokens::   Token parsing can depend on the semantic context.
* Lexical Tie-ins::   Token parsing can depend on the syntactic context.
* Tie-in Recovery::   Lexical tie-ins have implications for how
                        error recovery rules must be written.

Debugging Your Parser

* Understanding::     Understanding the structure of your parser.
* Tracing::           Tracing the execution of your parser.

Invoking Bison

* Bison Options::     All the options described in detail,
                        in alphabetical order by short options.
* Option Cross Key::  Alphabetical list of long options.
* Yacc Library::      Yacc-compatible @code{yylex} and @code{main}.

C++ Language Interface

* C++ Parsers::                 The interface to generate C++ parser classes
* A Complete C++ Example::      Demonstrating their use

C++ Parsers

* C++ Bison Interface::         Asking for C++ parser generation
* C++ Semantic Values::         %union vs. C++
* C++ Location Values::         The position and location classes
* C++ Parser Interface::        Instantiating and running the parser
* C++ Scanner Interface::       Exchanges between yylex and parse

A Complete C++ Example

* Calc++ --- C++ Calculator::   The specifications
* Calc++ Parsing Driver::       An active parsing context
* Calc++ Parser::               A parser class
* Calc++ Scanner::              A pure C++ Flex scanner
* Calc++ Top Level::            Conducting the band

Frequently Asked Questions

* Memory Exhausted::           Breaking the Stack Limits
* How Can I Reset the Parser:: @code{yyparse} Keeps some State
* Strings are Destroyed::      @code{yylval} Loses Track of Strings
* Implementing Gotos/Loops::   Control Flow in the Calculator

Copying This Manual

* GNU Free Documentation License::  License for copying this manual.

@end detailmenu
@end menu

@node Introduction
@unnumbered Introduction
@cindex introduction

@dfn{Bison} is a general-purpose parser generator that converts a
grammar description for an @acronym{LALR}(1) context-free grammar into a C
program to parse that grammar.  Once you are proficient with Bison,
you may use it to develop a wide range of language parsers, from those
used in simple desk calculators to complex programming languages.

Bison is upward compatible with Yacc: all properly-written Yacc grammars
ought to work with Bison with no change.  Anyone familiar with Yacc
should be able to use Bison with little trouble.  You need to be fluent in
C programming in order to use Bison or to understand this manual.

We begin with tutorial chapters that explain the basic concepts of using
Bison and show three explained examples, each building on the last.  If you
don't know Bison or Yacc, start by reading these chapters.  Reference
chapters follow which describe specific aspects of Bison in detail.

Bison was written primarily by Robert Corbett; Richard Stallman made it
Yacc-compatible.  Wilfred Hansen of Carnegie Mellon University added
multi-character string literals and other features.

This edition corresponds to version @value{VERSION} of Bison.

@node Conditions
@unnumbered Conditions for Using Bison

As of Bison version 1.24, we have changed the distribution terms for
@code{yyparse} to permit using Bison's output in nonfree programs when
Bison is generating C code for @acronym{LALR}(1) parsers.  Formerly, these
parsers could be used only in programs that were free software.

The other @acronym{GNU} programming tools, such as the @acronym{GNU} C
compiler, have never
had such a requirement.  They could always be used for nonfree
software.  The reason Bison was different was not due to a special
policy decision; it resulted from applying the usual General Public
License to all of the Bison source code.

The output of the Bison utility---the Bison parser file---contains a
verbatim copy of a sizable piece of Bison, which is the code for the
@code{yyparse} function.  (The actions from your grammar are inserted
into this function at one point, but the rest of the function is not
changed.)  When we applied the @acronym{GPL} terms to the code for
@code{yyparse},
the effect was to restrict the use of Bison output to free software.

We didn't change the terms because of sympathy for people who want to
make software proprietary.  @strong{Software should be free.}  But we
concluded that limiting Bison's use to free software was doing little to
encourage people to make other software free.  So we decided to make the
practical conditions for using Bison match the practical conditions for
using the other @acronym{GNU} tools.

This exception applies only when Bison is generating C code for an
@acronym{LALR}(1) parser; otherwise, the @acronym{GPL} terms operate
as usual.  You can
tell whether the exception applies to your @samp{.c} output file by
inspecting it to see whether it says ``As a special exception, when
this file is copied by Bison into a Bison output file, you may use
that output file without restriction.''

@include gpl.texi

@node Concepts
@chapter The Concepts of Bison

This chapter introduces many of the basic concepts without which the
details of Bison will not make sense.  If you do not already know how to
use Bison or Yacc, we suggest you start by reading this chapter carefully.

@menu
* Language and Grammar::  Languages and context-free grammars,
                            as mathematical ideas.
* Grammar in Bison::  How we represent grammars for Bison's sake.
* Semantic Values::   Each token or syntactic grouping can have
                        a semantic value (the value of an integer,
                        the name of an identifier, etc.).
* Semantic Actions::  Each rule can have an action containing C code.
* GLR Parsers::       Writing parsers for general context-free languages.
* Locations Overview::    Tracking Locations.
* Bison Parser::      What are Bison's input and output,
                        how is the output used?
* Stages::            Stages in writing and running Bison grammars.
* Grammar Layout::    Overall structure of a Bison grammar file.
@end menu

@node Language and Grammar
@section Languages and Context-Free Grammars

@cindex context-free grammar
@cindex grammar, context-free
In order for Bison to parse a language, it must be described by a
@dfn{context-free grammar}.  This means that you specify one or more
@dfn{syntactic groupings} and give rules for constructing them from their
parts.  For example, in the C language, one kind of grouping is called an
`expression'.  One rule for making an expression might be, ``An expression
can be made of a minus sign and another expression''.  Another would be,
``An expression can be an integer''.  As you can see, rules are often
recursive, but there must be at least one rule which leads out of the
recursion.

@cindex @acronym{BNF}
@cindex Backus-Naur form
The most common formal system for presenting such rules for humans to read
is @dfn{Backus-Naur Form} or ``@acronym{BNF}'', which was developed in
order to specify the language Algol 60.  Any grammar expressed in
@acronym{BNF} is a context-free grammar.  The input to Bison is
essentially machine-readable @acronym{BNF}.

@cindex @acronym{LALR}(1) grammars
@cindex @acronym{LR}(1) grammars
There are various important subclasses of context-free grammar.  Although it
can handle almost all context-free grammars, Bison is optimized for what
are called @acronym{LALR}(1) grammars.
In brief, in these grammars, it must be possible to
tell how to parse any portion of an input string with just a single
token of look-ahead.  Strictly speaking, that is a description of an
@acronym{LR}(1) grammar, and @acronym{LALR}(1) involves additional
restrictions that are
hard to explain simply; but it is rare in actual practice to find an
@acronym{LR}(1) grammar that fails to be @acronym{LALR}(1).
@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for
more information on this.

@cindex @acronym{GLR} parsing
@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
@cindex ambiguous grammars
@cindex nondeterministic parsing

Parsers for @acronym{LALR}(1) grammars are @dfn{deterministic}, meaning
roughly that the next grammar rule to apply at any point in the input is
uniquely determined by the preceding input and a fixed, finite portion
(called a @dfn{look-ahead}) of the remaining input.  A context-free
grammar can be @dfn{ambiguous}, meaning that there are multiple ways to
apply the grammar rules to get the same inputs.  Even unambiguous
grammars can be @dfn{nondeterministic}, meaning that no fixed
look-ahead always suffices to determine the next grammar rule to apply.
With the proper declarations, Bison is also able to parse these more
general context-free grammars, using a technique known as @acronym{GLR}
parsing (for Generalized @acronym{LR}).  Bison's @acronym{GLR} parsers
are able to handle any context-free grammar for which the number of
possible parses of any given string is finite.

@cindex symbols (abstract)
@cindex token
@cindex syntactic grouping
@cindex grouping, syntactic
In the formal grammatical rules for a language, each kind of syntactic
unit or grouping is named by a @dfn{symbol}.  Those which are built by
grouping smaller constructs according to grammatical rules are called
@dfn{nonterminal symbols}; those which can't be subdivided are called
@dfn{terminal symbols} or @dfn{token types}.  We call a piece of input
corresponding to a single terminal symbol a @dfn{token}, and a piece
corresponding to a single nonterminal symbol a @dfn{grouping}.

We can use the C language as an example of what symbols, terminal and
nonterminal, mean.  The tokens of C are identifiers, constants (numeric
and string), and the various keywords, arithmetic operators and
punctuation marks.  So the terminal symbols of a grammar for C include
`identifier', `number', `string', plus one symbol for each keyword,
operator or punctuation mark: `if', `return', `const', `static', `int',
`char', `plus-sign', `open-brace', `close-brace', `comma' and many more.
(These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)

Here is a simple C function subdivided into tokens:

@ifinfo
@example
int             /* @r{keyword `int'} */
square (int x)  /* @r{identifier, open-paren, keyword `int',}
                   @r{identifier, close-paren} */
@{               /* @r{open-brace} */
  return x * x; /* @r{keyword `return', identifier, asterisk,
                   identifier, semicolon} */
@}               /* @r{close-brace} */
@end example
@end ifinfo
@ifnotinfo
@example
int             /* @r{keyword `int'} */
square (int x)  /* @r{identifier, open-paren, keyword `int', identifier, close-paren} */
@{               /* @r{open-brace} */
  return x * x; /* @r{keyword `return', identifier, asterisk, identifier, semicolon} */
@}               /* @r{close-brace} */
@end example
@end ifnotinfo

The syntactic groupings of C include the expression, the statement, the
declaration, and the function definition.  These are represented in the
grammar of C by nonterminal symbols `expression', `statement',
`declaration' and `function definition'.  The full grammar uses dozens of
additional language constructs, each with its own nonterminal symbol, in
order to express the meanings of these four.  The example above is a
function definition; it contains one declaration, and one statement.  In
the statement, each @samp{x} is an expression and so is @samp{x * x}.

Each nonterminal symbol must have grammatical rules showing how it is made
out of simpler constructs.  For example, one kind of C statement is the
@code{return} statement; this would be described with a grammar rule which
reads informally as follows:

@quotation
A `statement' can be made of a `return' keyword, an `expression' and a
`semicolon'.
@end quotation

@noindent
There would be many other rules for `statement', one for each kind of
statement in C.

@cindex start symbol
One nonterminal symbol must be distinguished as the special one which
defines a complete utterance in the language.  It is called the @dfn{start
symbol}.  In a compiler, this means a complete input program.  In the C
language, the nonterminal symbol `sequence of definitions and declarations'
plays this role.

For example, @samp{1 + 2} is a valid C expression---a valid part of a C
program---but it is not valid as an @emph{entire} C program.  In the
context-free grammar of C, this follows from the fact that `expression' is
not the start symbol.

The Bison parser reads a sequence of tokens as its input, and groups the
tokens using the grammar rules.  If the input is valid, the end result is
that the entire token sequence reduces to a single grouping whose symbol is
the grammar's start symbol.  If we use a grammar for C, the entire input
must be a `sequence of definitions and declarations'.  If not, the parser
reports a syntax error.

@node Grammar in Bison
@section From Formal Rules to Bison Input
@cindex Bison grammar
@cindex grammar, Bison
@cindex formal grammar

A formal grammar is a mathematical construct.  To define the language
for Bison, you must write a file expressing the grammar in Bison syntax:
a @dfn{Bison grammar} file.  @xref{Grammar File, ,Bison Grammar Files}.

A nonterminal symbol in the formal grammar is represented in Bison input
as an identifier, like an identifier in C@.  By convention, it should be
in lower case, such as @code{expr}, @code{stmt} or @code{declaration}.

The Bison representation for a terminal symbol is also called a @dfn{token
type}.  Token types as well can be represented as C-like identifiers.  By
convention, these identifiers should be upper case to distinguish them from
nonterminals: for example, @code{INTEGER}, @code{IDENTIFIER}, @code{IF} or
@code{RETURN}.  A terminal symbol that stands for a particular keyword in
the language should be named after that keyword converted to upper case.
The terminal symbol @code{error} is reserved for error recovery.
@xref{Symbols}.

A terminal symbol can also be represented as a character literal, just like
a C character constant.  You should do this whenever a token is just a
single character (parenthesis, plus-sign, etc.): use that same character in
a literal as the terminal symbol for that token.

A third way to represent a terminal symbol is with a C string constant
containing several characters.  @xref{Symbols}, for more information.

The grammar rules also have an expression in Bison syntax.  For example,
here is the Bison rule for a C @code{return} statement.  The semicolon in
quotes is a literal character token, representing part of the C syntax for
the statement; the naked semicolon, and the colon, are Bison punctuation
used in every rule.

@example
stmt:   RETURN expr ';'
        ;
@end example

@noindent
@xref{Rules, ,Syntax of Grammar Rules}.

@node Semantic Values
@section Semantic Values
@cindex semantic value
@cindex value, semantic

A formal grammar selects tokens only by their classifications: for example,
if a rule mentions the terminal symbol `integer constant', it means that
@emph{any} integer constant is grammatically valid in that position.  The
precise value of the constant is irrelevant to how to parse the input: if
@samp{x+4} is grammatical then @samp{x+1} or @samp{x+3989} is equally
grammatical.

But the precise value is very important for what the input means once it is
parsed.  A compiler is useless if it fails to distinguish between 4, 1 and
3989 as constants in the program!  Therefore, each token in a Bison grammar
has both a token type and a @dfn{semantic value}.  @xref{Semantics,
,Defining Language Semantics},
for details.

The token type is a terminal symbol defined in the grammar, such as
@code{INTEGER}, @code{IDENTIFIER} or @code{','}.  It tells everything
you need to know to decide where the token may validly appear and how to
group it with other tokens.  The grammar rules know nothing about tokens
except their types.

The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
identifier.  (A token such as @code{','} which is just punctuation doesn't
need to have any semantic value.)

For example, an input token might be classified as token type
@code{INTEGER} and have the semantic value 4.  Another input token might
have the same token type @code{INTEGER} but value 3989.  When a grammar
rule says that @code{INTEGER} is allowed, either of these tokens is
acceptable because each is an @code{INTEGER}.  When the parser accepts the
token, it keeps track of the token's semantic value.

Each grouping can also have a semantic value as well as its nonterminal
symbol.  For example, in a calculator, an expression typically has a
semantic value that is a number.  In a compiler for a programming
language, an expression typically has a semantic value that is a tree
structure describing the meaning of the expression.

@node Semantic Actions
@section Semantic Actions
@cindex semantic actions
@cindex actions, semantic

In order to be useful, a program must do more than parse input; it must
also produce some output based on the input.  In a Bison grammar, a grammar
rule can have an @dfn{action} made up of C statements.  Each time the
parser recognizes a match for that rule, the action is executed.
@xref{Actions}.

Most of the time, the purpose of an action is to compute the semantic value
of the whole construct from the semantic values of its parts.  For example,
suppose we have a rule which says an expression can be the sum of two
expressions.  When the parser recognizes such a sum, each of the
subexpressions has a semantic value which describes how it was built up.
The action for this rule should create a similar sort of value for the
newly recognized larger expression.

For example, here is a rule that says an expression can be the sum of
two subexpressions:

@example
expr: expr '+' expr   @{ $$ = $1 + $3; @}
        ;
@end example

@noindent
The action says how to produce the semantic value of the sum expression
from the values of the two subexpressions.

@node GLR Parsers
@section Writing @acronym{GLR} Parsers
@cindex @acronym{GLR} parsing
@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
@findex %glr-parser
@cindex conflicts
@cindex shift/reduce conflicts
@cindex reduce/reduce conflicts

In some grammars, Bison's standard
@acronym{LALR}(1) parsing algorithm cannot decide whether to apply a
certain grammar rule at a given point.  That is, it may not be able to
decide (on the basis of the input read so far) which of two possible
reductions (applications of a grammar rule) applies, or whether to apply
a reduction or read more of the input and apply a reduction later in the
input.  These are known respectively as @dfn{reduce/reduce} conflicts
(@pxref{Reduce/Reduce}), and @dfn{shift/reduce} conflicts
(@pxref{Shift/Reduce}).

To use a grammar that is not easily modified to be @acronym{LALR}(1), a
more general parsing algorithm is sometimes necessary.  If you include
@code{%glr-parser} among the Bison declarations in your file
(@pxref{Grammar Outline}), the result is a Generalized @acronym{LR}
(@acronym{GLR}) parser.  These parsers handle Bison grammars that
contain no unresolved conflicts (i.e., after applying precedence
declarations) identically to @acronym{LALR}(1) parsers.  However, when
faced with unresolved shift/reduce and reduce/reduce conflicts,
@acronym{GLR} parsers use the simple expedient of doing both,
effectively cloning the parser to follow both possibilities.  Each of
the resulting parsers can again split, so that at any given time, there
can be any number of possible parses being explored.  The parsers
proceed in lockstep; that is, all of them consume (shift) a given input
symbol before any of them proceed to the next.  Each of the cloned
parsers eventually meets one of two possible fates: either it runs into
a parsing error, in which case it simply vanishes, or it merges with
another parser, because the two of them have reduced the input to an
identical set of symbols.

During the time that there are multiple parsers, semantic actions are
recorded, but not performed.  When a parser disappears, its recorded
semantic actions disappear as well, and are never performed.  When a
reduction makes two parsers identical, causing them to merge, Bison
records both sets of semantic actions.  Whenever the last two parsers
merge, reverting to the single-parser case, Bison resolves all the
outstanding actions either by precedences given to the grammar rules
involved, or by performing both actions, and then calling a designated
user-defined function on the resulting values to produce an arbitrary
merged result.

@menu
* Simple GLR Parsers::      Using @acronym{GLR} parsers on unambiguous grammars.
* Merging GLR Parses::      Using @acronym{GLR} parsers to resolve ambiguities.
* GLR Semantic Actions::    Deferred semantic actions have special concerns.
* Compiler Requirements::   @acronym{GLR} parsers require a modern C compiler.
@end menu

@node Simple GLR Parsers
@subsection Using @acronym{GLR} on Unambiguous Grammars
@cindex @acronym{GLR} parsing, unambiguous grammars
@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars
@findex %glr-parser
@findex %expect-rr
@cindex conflicts
@cindex reduce/reduce conflicts
@cindex shift/reduce conflicts

In the simplest cases, you can use the @acronym{GLR} algorithm
to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1).
Such grammars typically require more than one symbol of look-ahead,
or (in rare cases) fall into the category of grammars in which the
@acronym{LALR}(1) algorithm throws away too much information (they are in
@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery Conflicts}).

Consider a problem that
arises in the declaration of enumerated and subrange types in the
programming language Pascal.  Here are some examples:

@example
type subrange = lo .. hi;
type enum = (a, b, c);
@end example

@noindent
The original language standard allows only numeric
literals and constant identifiers for the subrange bounds (@samp{lo}
and @samp{hi}), but Extended Pascal (@acronym{ISO}/@acronym{IEC}
10206) and many other
Pascal implementations allow arbitrary expressions there.  This gives
rise to the following situation, containing a superfluous pair of
parentheses:

@example
type subrange = (a) .. b;
@end example

@noindent
Compare this to the following declaration of an enumerated
type with only one value:

@example
type enum = (a);
@end example

@noindent
(These declarations are contrived, but they are syntactically
valid, and more-complicated cases can come up in practical programs.)

These two declarations look identical until the @samp{..} token.
With normal @acronym{LALR}(1) one-token look-ahead it is not
possible to decide between the two forms when the identifier
@samp{a} is parsed.  It is, however, desirable
for a parser to decide this, since in the latter case
@samp{a} must become a new identifier to represent the enumeration
value, while in the former case @samp{a} must be evaluated with its
current meaning, which may be a constant or even a function call.

You could parse @samp{(a)} as an ``unspecified identifier in parentheses'',
to be resolved later, but this typically requires substantial
contortions in both semantic actions and large parts of the
grammar, where the parentheses are nested in the recursive rules for
expressions.

You might think of using the lexer to distinguish between the two
forms by returning different tokens for currently defined and
undefined identifiers.  But if these declarations occur in a local
scope, and @samp{a} is defined in an outer scope, then both forms
are possible---either locally redefining @samp{a}, or using the
value of @samp{a} from the outer scope.  So this approach cannot
work.

A simple solution to this problem is to declare the parser to
use the @acronym{GLR} algorithm.
When the @acronym{GLR} parser reaches the critical state, it
merely splits into two branches and pursues both syntax rules
simultaneously.  Sooner or later, one of them runs into a parsing
error.  If there is a @samp{..} token before the next
@samp{;}, the rule for enumerated types fails since it cannot
accept @samp{..} anywhere; otherwise, the subrange type rule
fails since it requires a @samp{..} token.  So one of the branches
fails silently, and the other one continues normally, performing
all the intermediate actions that were postponed during the split.

If the input is syntactically incorrect, both branches fail and the parser
reports a syntax error as usual.

The effect of all this is that the parser seems to ``guess'' the
correct branch to take, or in other words, it seems to use more
look-ahead than the underlying @acronym{LALR}(1) algorithm actually allows
for.  In this example, @acronym{LALR}(2) would suffice, but also some cases
that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way.

In general, a @acronym{GLR} parser can take quadratic or cubic worst-case time,
and the current Bison parser even takes exponential time and space
for some grammars.  In practice, this rarely happens, and for many
grammars it is possible to prove that it cannot happen.
The present example contains only one conflict between two
rules, and the type-declaration context containing the conflict
cannot be nested.  So the number of
branches that can exist at any time is limited by the constant 2,
and the parsing time is still linear.

Here is a Bison grammar corresponding to the example above.  It
parses a vastly simplified form of Pascal type declarations.

@example
%token TYPE DOTDOT ID

@group
%left '+' '-'
%left '*' '/'
@end group

%%

@group
type_decl : TYPE ID '=' type ';'
     ;
@end group

@group
type : '(' id_list ')'
     | expr DOTDOT expr
     ;
@end group

@group
id_list : ID
     | id_list ',' ID
     ;
@end group

@group
expr : '(' expr ')'
     | expr '+' expr
     | expr '-' expr
     | expr '*' expr
     | expr '/' expr
     | ID
     ;
@end group
@end example

When used as a normal @acronym{LALR}(1) grammar, Bison correctly complains
about one reduce/reduce conflict.  In the conflicting situation the
parser chooses one of the alternatives, arbitrarily the one
declared first.  Therefore the following correct input is not
recognized:

@example
type t = (a) .. b;
@end example

The parser can be turned into a @acronym{GLR} parser, while also telling Bison
to be silent about the one known reduce/reduce conflict, by
adding these two declarations to the Bison input file (before the first
@samp{%%}):

@example
%glr-parser
%expect-rr 1
@end example

@noindent
No change in the grammar itself is required.  Now the
parser recognizes all valid declarations, according to the
limited syntax above, transparently.  In fact, the user does not even
notice when the parser splits.

So here we have a case where we can use the benefits of @acronym{GLR},
almost without disadvantages.  Even in simple cases like this, however,
there are at least two potential problems to beware.  First, always
analyze the conflicts reported by Bison to make sure that @acronym{GLR}
splitting is only done where it is intended.  A @acronym{GLR} parser
splitting inadvertently may cause problems less obvious than an
@acronym{LALR} parser statically choosing the wrong alternative in a
conflict.  Second, consider interactions with the lexer (@pxref{Semantic
Tokens}) with great care.  Since a split parser consumes tokens without
performing any actions during the split, the lexer cannot obtain
information via parser actions.  Some cases of lexer interactions can be
eliminated by using @acronym{GLR} to shift the complications from the
lexer to the parser.  You must check the remaining cases for
correctness.

In our example, it would be safe for the lexer to return tokens based on
their current meanings in some symbol table, because no new symbols are
defined in the middle of a type declaration.  Though it is possible for
a parser to define the enumeration constants as they are parsed, before
the type declaration is completed, it actually makes no difference since
they cannot be used within the same enumerated type declaration.

@node Merging GLR Parses
@subsection Using @acronym{GLR} to Resolve Ambiguities
@cindex @acronym{GLR} parsing, ambiguous grammars
@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, ambiguous grammars
@findex %dprec
@findex %merge
@cindex conflicts
@cindex reduce/reduce conflicts

Let's consider an example, vastly simplified from a C++ grammar.

@example
%@{
  #include <stdio.h>
  #define YYSTYPE char const *
  int yylex (void);
  void yyerror (char const *);
%@}

%token TYPENAME ID

%right '='
%left '+'

%glr-parser

%%

prog :
     | prog stmt   @{ printf ("\n"); @}
     ;

stmt : expr ';'  %dprec 1
     | decl      %dprec 2
     ;

expr : ID               @{ printf ("%s ", $$); @}
     | TYPENAME '(' expr ')'
                        @{ printf ("%s <cast> ", $1); @}
     | expr '+' expr    @{ printf ("+ "); @}
     | expr '=' expr    @{ printf ("= "); @}
     ;

decl : TYPENAME declarator ';'
                        @{ printf ("%s <declare> ", $1); @}
     | TYPENAME declarator '=' expr ';'
                        @{ printf ("%s <init-declare> ", $1); @}
     ;

declarator : ID         @{ printf ("\"%s\" ", $1); @}
     | '(' declarator ')'
     ;
@end example

@noindent
This models a problematic part of the C++ grammar---the ambiguity between
certain declarations and statements.  For example,

@example
T (x) = y+z;
@end example

@noindent
parses as either an @code{expr} or a @code{stmt}
(assuming that @samp{T} is recognized as a @code{TYPENAME} and
@samp{x} as an @code{ID}).
Bison detects this as a reduce/reduce conflict between the rules
@code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the
time it encounters @code{x} in the example above.  Since this is a
@acronym{GLR} parser, it therefore splits the problem into two parses, one for
each choice of resolving the reduce/reduce conflict.
Unlike the example from the previous section (@pxref{Simple GLR Parsers}),
however, neither of these parses ``dies,'' because the grammar as it stands is
ambiguous.  One of the parsers eventually reduces @code{stmt : expr ';'} and
the other reduces @code{stmt : decl}, after which both parsers are in an
identical state: they've seen @samp{prog stmt} and have the same unprocessed
input remaining.  We say that these parses have @dfn{merged.}

At this point, the @acronym{GLR} parser requires a specification in the
grammar of how to choose between the competing parses.
In the example above, the two @code{%dprec}
declarations specify that Bison is to give precedence
to the parse that interprets the example as a
@code{decl}, which implies that @code{x} is a declarator.
The parser therefore prints

@example
"x" y z + T <init-declare>
@end example

The @code{%dprec} declarations only come into play when more than one
parse survives.  Consider a different input string for this parser:

@example
T (x) + y;
@end example

@noindent
This is another example of using @acronym{GLR} to parse an unambiguous
construct, as shown in the previous section (@pxref{Simple GLR Parsers}).
Here, there is no ambiguity (this cannot be parsed as a declaration).
However, at the time the Bison parser encounters @code{x}, it does not
have enough information to resolve the reduce/reduce conflict (again,
between @code{x} as an @code{expr} or a @code{declarator}).  In this
case, no precedence declaration is used.  Again, the parser splits
into two, one assuming that @code{x} is an @code{expr}, and the other
assuming @code{x} is a @code{declarator}.  The second of these parsers
then vanishes when it sees @code{+}, and the parser prints

@example
x T <cast> y +
@end example

Suppose that instead of resolving the ambiguity, you wanted to see all
the possibilities.  For this purpose, you must merge the semantic
actions of the two possible parsers, rather than choosing one over the
other.  To do so, you could change the declaration of @code{stmt} as
follows:

@example
stmt : expr ';'  %merge <stmtMerge>
     | decl      %merge <stmtMerge>
     ;
@end example

@noindent
and define the @code{stmtMerge} function as:

@example
static YYSTYPE
stmtMerge (YYSTYPE x0, YYSTYPE x1)
@{
  printf ("<OR> ");
  return "";
@}
@end example

@noindent
with an accompanying forward declaration
in the C declarations at the beginning of the file:

@example
%@{
  #define YYSTYPE char const *
  static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
%@}
@end example

@noindent
With these declarations, the resulting parser parses the first example
as both an @code{expr} and a @code{decl}, and prints

@example
"x" y z + T <init-declare> x T <cast> y z + = <OR>
@end example

Bison requires that all of the
productions that participate in any particular merge have identical
@samp{%merge} clauses.  Otherwise, the ambiguity would be unresolvable,
and the parser will report an error during any parse that results in
the offending merge.

@node GLR Semantic Actions
@subsection GLR Semantic Actions

@cindex deferred semantic actions
By definition, a deferred semantic action is not performed at the same time as
the associated reduction.
This raises caveats for several Bison features you might use in a semantic
action in a @acronym{GLR} parser.

@vindex yychar
@cindex @acronym{GLR} parsers and @code{yychar}
@vindex yylval
@cindex @acronym{GLR} parsers and @code{yylval}
@vindex yylloc
@cindex @acronym{GLR} parsers and @code{yylloc}
In any semantic action, you can examine @code{yychar} to determine the type of
the look-ahead token present at the time of the associated reduction.
After checking that @code{yychar} is not set to @code{YYEMPTY} or @code{YYEOF},
you can then examine @code{yylval} and @code{yylloc} to determine the
look-ahead token's semantic value and location, if any.
In a nondeferred semantic action, you can also modify any of these variables to
influence syntax analysis.
@xref{Look-Ahead, ,Look-Ahead Tokens}.

@findex yyclearin
@cindex @acronym{GLR} parsers and @code{yyclearin}
In a deferred semantic action, it's too late to influence syntax analysis.
In this case, @code{yychar}, @code{yylval}, and @code{yylloc} are set to
shallow copies of the values they had at the time of the associated reduction.
For this reason alone, modifying them is dangerous.
Moreover, the result of modifying them is undefined and subject to change with
future versions of Bison.
For example, if a semantic action might be deferred, you should never write it
to invoke @code{yyclearin} (@pxref{Action Features}) or to attempt to free
memory referenced by @code{yylval}.

@findex YYERROR
@cindex @acronym{GLR} parsers and @code{YYERROR}
Another Bison feature requiring special consideration is @code{YYERROR}
(@pxref{Action Features}), which you can invoke in any semantic action to
initiate error recovery.
During deterministic @acronym{GLR} operation, the effect of @code{YYERROR} is
the same as its effect in an @acronym{LALR}(1) parser.
In a deferred semantic action, its effect is undefined.
@c The effect is probably a syntax error at the split point.

@node Compiler Requirements
@subsection Considerations when Compiling @acronym{GLR} Parsers
@cindex @code{inline}
@cindex @acronym{GLR} parsers and @code{inline}

The @acronym{GLR} parsers require a compiler for @acronym{ISO} C89 or
later.  In addition, they use the @code{inline} keyword, which is not
C89, but is C99 and is a common extension in pre-C99 compilers.  It is
up to the user of these parsers to handle
portability issues.  For instance, if using Autoconf and the Autoconf
macro @code{AC_C_INLINE}, a mere

@example
%@{
  #include <config.h>
%@}
@end example

@noindent
will suffice.  Otherwise, we suggest

@example
%@{
  #if __STDC_VERSION__ < 199901 && ! defined __GNUC__ && ! defined inline
   #define inline
  #endif
%@}
@end example

@node Locations Overview
@section Locations
@cindex location
@cindex textual location
@cindex location, textual

Many applications, like interpreters or compilers, have to produce verbose
and useful error messages.  To achieve this, one must be able to keep track of
the @dfn{textual location}, or @dfn{location}, of each syntactic construct.
Bison provides a mechanism for handling these locations.

Each token has a semantic value.  In a similar fashion, each token has an
associated location, but the type of locations is the same for all tokens and
groupings.  Moreover, the output parser is equipped with a default data
structure for storing locations (@pxref{Locations}, for more details).

Like semantic values, locations can be reached in actions using a dedicated
set of constructs.  In the example above, the location of the whole grouping
is @code{@@$}, while the locations of the subexpressions are @code{@@1} and
@code{@@3}.

When a rule is matched, a default action is used to compute the semantic value
of its left hand side (@pxref{Actions}).  In the same way, another default
action is used for locations.  However, the action for locations is general
enough for most cases, meaning there is usually no need to describe for each
rule how @code{@@$} should be formed.  When building a new location for a given
grouping, the default behavior of the output parser is to take the beginning
of the first symbol, and the end of the last symbol.

@node Bison Parser
@section Bison Output: the Parser File
@cindex Bison parser
@cindex Bison utility
@cindex lexical analyzer, purpose
@cindex parser

When you run Bison, you give it a Bison grammar file as input.  The output
is a C source file that parses the language described by the grammar.
This file is called a @dfn{Bison parser}.  Keep in mind that the Bison
utility and the Bison parser are two distinct programs: the Bison utility
is a program whose output is the Bison parser that becomes part of your
program.

The job of the Bison parser is to group tokens into groupings according to
the grammar rules---for example, to build identifiers and operators into
expressions.  As it does this, it runs the actions for the grammar rules it
uses.

The tokens come from a function called the @dfn{lexical analyzer} that
you must supply in some fashion (such as by writing it in C).  The Bison
parser calls the lexical analyzer each time it wants a new token.  It
doesn't know what is ``inside'' the tokens (though their semantic values
may reflect this).  Typically the lexical analyzer makes the tokens by
parsing characters of text, but Bison does not depend on this.
@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.

The Bison parser file is C code which defines a function named
@code{yyparse} which implements that grammar.  This function does not make
a complete C program: you must supply some additional functions.  One is
the lexical analyzer.  Another is an error-reporting function which the
parser calls to report an error.  In addition, a complete C program must
start with a function called @code{main}; you have to provide this, and
arrange for it to call @code{yyparse} or the parser will never run.
@xref{Interface, ,Parser C-Language Interface}.

Aside from the token type names and the symbols in the actions you
write, all symbols defined in the Bison parser file itself
begin with @samp{yy} or @samp{YY}.  This includes interface functions
such as the lexical analyzer function @code{yylex}, the error reporting
function @code{yyerror} and the parser function @code{yyparse} itself.
This also includes numerous identifiers used for internal purposes.
Therefore, you should avoid using C identifiers starting with @samp{yy}
or @samp{YY} in the Bison grammar file except for the ones defined in
this manual.  Also, you should avoid using the C identifiers
@samp{malloc} and @samp{free} for anything other than their usual
meanings.

In some cases the Bison parser file includes system headers, and in
those cases your code should respect the identifiers reserved by those
headers.  On some non-@acronym{GNU} hosts, @code{<alloca.h>}, @code{<malloc.h>},
@code{<stddef.h>}, and @code{<stdlib.h>} are included as needed to
declare memory allocators and related types.  @code{<libintl.h>} is
included if message translation is in use
(@pxref{Internationalization}).  Other system headers may
be included if you define @code{YYDEBUG} to a nonzero value
(@pxref{Tracing, ,Tracing Your Parser}).

@node Stages
@section Stages in Using Bison
@cindex stages in using Bison
@cindex using Bison

The actual language-design process using Bison, from grammar specification
to a working compiler or interpreter, has these parts:

@enumerate
@item
Formally specify the grammar in a form recognized by Bison
(@pxref{Grammar File, ,Bison Grammar Files}).  For each grammatical rule
in the language, describe the action that is to be taken when an
instance of that rule is recognized.  The action is described by a
sequence of C statements.

@item
Write a lexical analyzer to process input and pass tokens to the parser.
The lexical analyzer may be written by hand in C (@pxref{Lexical, ,The
Lexical Analyzer Function @code{yylex}}).  It could also be produced
using Lex, but the use of Lex is not discussed in this manual.

@item
Write a controlling function that calls the Bison-produced parser.

@item
Write error-reporting routines.
@end enumerate

To turn this source code as written into a runnable program, you
must follow these steps:

@enumerate
@item
Run Bison on the grammar to produce the parser.

@item
Compile the code output by Bison, as well as any other source files.

@item
Link the object files to produce the finished product.
@end enumerate

@node Grammar Layout
@section The Overall Layout of a Bison Grammar
@cindex grammar file
@cindex file format
@cindex format of grammar file
@cindex layout of Bison grammar

The input file for the Bison utility is a @dfn{Bison grammar file}.  The
general form of a Bison grammar file is as follows:

@example
%@{
@var{Prologue}
%@}

@var{Bison declarations}

%%
@var{Grammar rules}
%%
@var{Epilogue}
@end example

@noindent
The @samp{%%}, @samp{%@{} and @samp{%@}} are punctuation that appears
in every Bison grammar file to separate the sections.

The prologue may define types and variables used in the actions.  You can
also use preprocessor commands to define macros used there, and use
@code{#include} to include header files that do any of these things.
You need to declare the lexical analyzer @code{yylex} and the error
printer @code{yyerror} here, along with any other global identifiers
used by the actions in the grammar rules.

The Bison declarations declare the names of the terminal and nonterminal
symbols, and may also describe operator precedence and the data types of
semantic values of various symbols.

The grammar rules define how to construct each nonterminal symbol from its
parts.

The epilogue can contain any code you want to use.  Often the
definitions of functions declared in the prologue go here.  In a
simple program, all the rest of the program can go here.

@node Examples
@chapter Examples
@cindex simple examples
@cindex examples, simple

Now we show and explain three sample programs written using Bison: a
reverse polish notation calculator, an algebraic (infix) notation
calculator, and a multi-function calculator.  All three have been tested
under BSD Unix 4.3; each produces a usable, though limited, interactive
desk-top calculator.

These examples are simple, but Bison grammars for real programming
languages are written the same way.
@ifinfo
You can copy these examples out of the Info file and into a source file
to try them.
@end ifinfo

@menu
* RPN Calc::          Reverse polish notation calculator;
                        a first example with no operator precedence.
* Infix Calc::        Infix (algebraic) notation calculator.
                        Operator precedence is introduced.
* Simple Error Recovery::  Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
* Multi-function Calc::  Calculator with memory and trig functions.
                           It uses multiple data-types for semantic values.
* Exercises::         Ideas for improving the multi-function calculator.
@end menu

@node RPN Calc
@section Reverse Polish Notation Calculator
@cindex reverse polish notation
@cindex polish notation calculator
@cindex @code{rpcalc}
@cindex calculator, simple

The first example is that of a simple double-precision @dfn{reverse polish
notation} calculator (a calculator using postfix operators).  This example
provides a good starting point, since operator precedence is not an issue.
The second example will illustrate how operator precedence is handled.

The source code for this calculator is named @file{rpcalc.y}.  The
@samp{.y} extension is a convention used for Bison input files.

@menu
* Decls: Rpcalc Decls.  Prologue (declarations) for rpcalc.
* Rules: Rpcalc Rules.  Grammar Rules for rpcalc, with explanation.
* Lexer: Rpcalc Lexer.  The lexical analyzer.
* Main: Rpcalc Main.    The controlling function.
* Error: Rpcalc Error.  The error reporting function.
* Gen: Rpcalc Gen.      Running Bison on the grammar file.
* Comp: Rpcalc Compile. Run the C compiler on the output code.
@end menu

@node Rpcalc Decls
@subsection Declarations for @code{rpcalc}

Here are the C and Bison declarations for the reverse polish notation
calculator.  As in C, comments are placed between @samp{/*@dots{}*/}.

@example
/* Reverse polish notation calculator.  */

%@{
  #define YYSTYPE double
  #include <math.h>
  int yylex (void);
  void yyerror (char const *);
%@}

%token NUM

%% /* Grammar rules and actions follow.  */
@end example

The declarations section (@pxref{Prologue, , The prologue}) contains two
preprocessor directives and two forward declarations.

The @code{#define} directive defines the macro @code{YYSTYPE}, thus
specifying the C data type for semantic values of both tokens and
groupings (@pxref{Value Type, ,Data Types of Semantic Values}).  The
Bison parser will use whatever type @code{YYSTYPE} is defined as; if you
don't define it, @code{int} is the default.  Because we specify
@code{double}, each token and each expression has an associated value,
which is a floating point number.

The @code{#include} directive is used to declare the exponentiation
function @code{pow}.

The forward declarations for @code{yylex} and @code{yyerror} are
needed because the C language requires that functions be declared
before they are used.  These functions will be defined in the
epilogue, but the parser calls them so they must be declared in the
prologue.

The second section, Bison declarations, provides information to Bison
about the token types (@pxref{Bison Declarations, ,The Bison
Declarations Section}).  Each terminal symbol that is not a
single-character literal must be declared here.  (Single-character
literals normally don't need to be declared.)  In this example, all the
arithmetic operators are designated by single-character literals, so the
only terminal symbol that needs to be declared is @code{NUM}, the token
type for numeric constants.

@node Rpcalc Rules
@subsection Grammar Rules for @code{rpcalc}

Here are the grammar rules for the reverse polish notation calculator.

@example
input:    /* empty */
        | input line
;

line:     '\n'
        | exp '\n'      @{ printf ("\t%.10g\n", $1); @}
;

exp:      NUM           @{ $$ = $1;           @}
        | exp exp '+'   @{ $$ = $1 + $2;      @}
        | exp exp '-'   @{ $$ = $1 - $2;      @}
        | exp exp '*'   @{ $$ = $1 * $2;      @}
        | exp exp '/'   @{ $$ = $1 / $2;      @}
         /* Exponentiation */
        | exp exp '^'   @{ $$ = pow ($1, $2); @}
         /* Unary minus    */
        | exp 'n'       @{ $$ = -$1;          @}
;
%%
@end example

The groupings of the rpcalc ``language'' defined here are the expression
(given the name @code{exp}), the line of input (@code{line}), and the
complete input transcript (@code{input}).  Each of these nonterminal
symbols has several alternate rules, joined by the @samp{|} punctuator
which is read as ``or''.  The following sections explain what these rules
mean.

The semantics of the language is determined by the actions taken when a
grouping is recognized.  The actions are the C code that appears inside
braces.  @xref{Actions}.

You must specify these actions in C, but Bison provides the means for
passing semantic values between the rules.  In each action, the
pseudo-variable @code{$$} stands for the semantic value for the grouping
that the rule is going to construct.  Assigning a value to @code{$$} is the
main job of most actions.  The semantic values of the components of the
rule are referred to as @code{$1}, @code{$2}, and so on.

@menu
* Rpcalc Input::
* Rpcalc Line::
* Rpcalc Expr::
@end menu

@node Rpcalc Input
@subsubsection Explanation of @code{input}

Consider the definition of @code{input}:

@example
input:    /* empty */
        | input line
;
@end example

This definition reads as follows: ``A complete input is either an empty
string, or a complete input followed by an input line''.  Notice that
``complete input'' is defined in terms of itself.  This definition is said
to be @dfn{left recursive} since @code{input} appears always as the
leftmost symbol in the sequence.  @xref{Recursion, ,Recursive Rules}.

The first alternative is empty because there are no symbols between the
colon and the first @samp{|}; this means that @code{input} can match an
empty string of input (no tokens).  We write the rules this way because it
is legitimate to type @kbd{Ctrl-d} right after you start the calculator.
It's conventional to put an empty alternative first and write the comment
@samp{/* empty */} in it.

The second alternate rule (@code{input line}) handles all nontrivial input.
It means, ``After reading any number of lines, read one more line if
possible.''  The left recursion makes this rule into a loop.  Since the
first alternative matches empty input, the loop can be executed zero or
more times.

The parser function @code{yyparse} continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
input tokens; we will arrange for the latter to happen at end-of-input.

@node Rpcalc Line
@subsubsection Explanation of @code{line}

Now consider the definition of @code{line}:

@example
line:     '\n'
        | exp '\n'  @{ printf ("\t%.10g\n", $1); @}
;
@end example

The first alternative is a token which is a newline character; this means
that rpcalc accepts a blank line (and ignores it, since there is no
action).  The second alternative is an expression followed by a newline.
This is the alternative that makes rpcalc useful.  The semantic value of
the @code{exp} grouping is the value of @code{$1} because the @code{exp} in
question is the first symbol in the alternative.  The action prints this
value, which is the result of the computation the user asked for.

This action is unusual because it does not assign a value to @code{$$}.  As
a consequence, the semantic value associated with the @code{line} is
uninitialized (its value will be unpredictable).  This would be a bug if
that value were ever used, but we don't use it: once rpcalc has printed the
value of the user's input line, that value is no longer needed.

@node Rpcalc Expr
@subsubsection Explanation of @code{expr}

The @code{exp} grouping has several rules, one for each kind of expression.
The first rule handles the simplest expressions: those that are just numbers.
The second handles an addition-expression, which looks like two expressions
followed by a plus-sign.  The third handles subtraction, and so on.

@example
exp:      NUM
        | exp exp '+'     @{ $$ = $1 + $2;    @}
        | exp exp '-'     @{ $$ = $1 - $2;    @}
        @dots{}
        ;
@end example

We have used @samp{|} to join all the rules for @code{exp}, but we could
equally well have written them separately:

@example
exp:      NUM ;
exp:      exp exp '+'     @{ $$ = $1 + $2;    @} ;
exp:      exp exp '-'     @{ $$ = $1 - $2;    @} ;
        @dots{}
@end example

Most of the rules have actions that compute the value of the expression in
terms of the value of its parts.  For example, in the rule for addition,
@code{$1} refers to the first component @code{exp} and @code{$2} refers to
the second one.  The third component, @code{'+'}, has no meaningful
associated semantic value, but if it had one you could refer to it as
@code{$3}.  When @code{yyparse} recognizes a sum expression using this
rule, the sum of the two subexpressions' values is produced as the value of
the entire expression.  @xref{Actions}.

You don't have to give an action for every rule.  When a rule has no
action, Bison by default copies the value of @code{$1} into @code{$$}.
This is what happens in the first rule (the one that uses @code{NUM}).

The formatting shown here is the recommended convention, but Bison does
not require it.  You can add or change white space as much as you wish.
For example, this:

@example
exp   : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ;
@end example

@noindent
means the same thing as this:

@example
exp:      NUM
        | exp exp '+'    @{ $$ = $1 + $2; @}
        | @dots{}
;
@end example

@noindent
The latter, however, is much more readable.

@node Rpcalc Lexer
@subsection The @code{rpcalc} Lexical Analyzer
@cindex writing a lexical analyzer
@cindex lexical analyzer, writing

The lexical analyzer's job is low-level parsing: converting characters
or sequences of characters into tokens.  The Bison parser gets its
tokens by calling the lexical analyzer.  @xref{Lexical, ,The Lexical
Analyzer Function @code{yylex}}.

Only a simple lexical analyzer is needed for the @acronym{RPN}
calculator.  This
lexical analyzer skips blanks and tabs, then reads in numbers as
@code{double} and returns them as @code{NUM} tokens.  Any other character
that isn't part of a number is a separate token.  Note that the token-code
for such a single-character token is the character itself.

The return value of the lexical analyzer function is a numeric code which
represents a token type.  The same text used in Bison rules to stand for
this token type is also a C expression for the numeric code for the type.
This works in two ways.  If the token type is a character literal, then its
numeric code is that of the character; you can use the same
character literal in the lexical analyzer to express the number.  If the
token type is an identifier, that identifier is defined by Bison as a C
macro whose definition is the appropriate number.  In this example,
therefore, @code{NUM} becomes a macro for @code{yylex} to use.

The semantic value of the token (if it has one) is stored into the
global variable @code{yylval}, which is where the Bison parser will look
for it.  (The C data type of @code{yylval} is @code{YYSTYPE}, which was
defined at the beginning of the grammar; @pxref{Rpcalc Decls,
,Declarations for @code{rpcalc}}.)

A token type code of zero is returned if the end-of-input is encountered.
(Bison recognizes any nonpositive value as indicating end-of-input.)

Here is the code for the lexical analyzer:

@example
@group
/* The lexical analyzer returns a double floating point
   number on the stack and the token NUM, or the numeric code
   of the character read if not a number.  It skips all blanks
   and tabs, and returns 0 for end-of-input.  */

#include <ctype.h>
@end group

@group
int
yylex (void)
@{
  int c;

  /* Skip white space.  */
  while ((c = getchar ()) == ' ' || c == '\t')
    ;
@end group
@group
  /* Process numbers.  */
  if (c == '.' || isdigit (c))
    @{
      ungetc (c, stdin);
      scanf ("%lf", &yylval);
      return NUM;
    @}
@end group
@group
  /* Return end-of-input.  */
  if (c == EOF)
    return 0;
  /* Return a single char.  */
  return c;
@}
@end group
@end example

@node Rpcalc Main
@subsection The Controlling Function
@cindex controlling function
@cindex main function in simple example

In keeping with the spirit of this example, the controlling function is
kept to the bare minimum.  The only requirement is that it call
@code{yyparse} to start the process of parsing.

@example
@group
int
main (void)
@{
  return yyparse ();
@}
@end group
@end example

@node Rpcalc Error
@subsection The Error Reporting Routine
@cindex error reporting routine

When @code{yyparse} detects a syntax error, it calls the error reporting
function @code{yyerror} to print an error message (usually but not
always @code{"syntax error"}).  It is up to the programmer to supply
@code{yyerror} (@pxref{Interface, ,Parser C-Language Interface}), so
here is the definition we will use:

@example
@group
#include <stdio.h>

/* Called by yyparse on error.  */
void
yyerror (char const *s)
@{
  fprintf (stderr, "%s\n", s);
@}
@end group
@end example

After @code{yyerror} returns, the Bison parser may recover from the error
and continue parsing if the grammar contains a suitable error rule
(@pxref{Error Recovery}).  Otherwise, @code{yyparse} returns nonzero.  We
have not written any error rules in this example, so any invalid input will
cause the calculator program to exit.  This is not clean behavior for a
real calculator, but it is adequate for the first example.

@node Rpcalc Gen
@subsection Running Bison to Make the Parser
@cindex running Bison (introduction)

Before running Bison to produce a parser, we need to decide how to
arrange all the source code in one or more source files.  For such a
simple example, the easiest thing is to put everything in one file.  The
definitions of @code{yylex}, @code{yyerror} and @code{main} go at the
end, in the epilogue of the file
(@pxref{Grammar Layout, ,The Overall Layout of a Bison Grammar}).

For a large project, you would probably have several source files, and use
@code{make} to arrange to recompile them.

With all the source in a single file, you use the following command to
convert it into a parser file:

@example
bison @var{file}.y
@end example

@noindent
In this example the file was called @file{rpcalc.y} (for ``Reverse Polish
@sc{calc}ulator'').  Bison produces a file named @file{@var{file}.tab.c},
removing the @samp{.y} from the original file name.  The file output by
Bison contains the source code for @code{yyparse}.  The additional
functions in the input file (@code{yylex}, @code{yyerror} and @code{main})
are copied verbatim to the output.

@node Rpcalc Compile
@subsection Compiling the Parser File
@cindex compiling the parser

Here is how to compile and run the parser file:

@example
@group
# @r{List files in current directory.}
$ @kbd{ls}
rpcalc.tab.c  rpcalc.y
@end group

@group
# @r{Compile the Bison parser.}
# @r{@samp{-lm} tells compiler to search math library for @code{pow}.}
$ @kbd{cc -lm -o rpcalc rpcalc.tab.c}
@end group

@group
# @r{List files again.}
$ @kbd{ls}
rpcalc  rpcalc.tab.c  rpcalc.y
@end group
@end example

The file @file{rpcalc} now contains the executable code.  Here is an
example session using @code{rpcalc}.

@example
$ @kbd{rpcalc}
@kbd{4 9 +}
13
@kbd{3 7 + 3 4 5 *+-}
-13
@kbd{3 7 + 3 4 5 * + - n}              @r{Note the unary minus, @samp{n}}
13
@kbd{5 6 / 4 n +}
-3.166666667
@kbd{3 4 ^}                            @r{Exponentiation}
81
@kbd{^D}                               @r{End-of-file indicator}
$
@end example

@node Infix Calc
@section Infix Notation Calculator: @code{calc}
@cindex infix notation calculator
@cindex @code{calc}
@cindex calculator, infix notation

We now modify rpcalc to handle infix operators instead of postfix.  Infix
notation involves the concept of operator precedence and the need for
parentheses nested to arbitrary depth.  Here is the Bison code for
@file{calc.y}, an infix desk-top calculator.

@example
/* Infix notation calculator.  */

%@{
  #define YYSTYPE double
  #include <math.h>
  #include <stdio.h>
  int yylex (void);
  void yyerror (char const *);
%@}

/* Bison declarations.  */
%token NUM
%left '-' '+'
%left '*' '/'
%left NEG     /* negation--unary minus */
%right '^'    /* exponentiation */

%% /* The grammar follows.  */
input:    /* empty */
        | input line
;

line:     '\n'
        | exp '\n'  @{ printf ("\t%.10g\n", $1); @}
;

exp:      NUM                @{ $$ = $1;         @}
        | exp '+' exp        @{ $$ = $1 + $3;    @}
        | exp '-' exp        @{ $$ = $1 - $3;    @}
        | exp '*' exp        @{ $$ = $1 * $3;    @}
        | exp '/' exp        @{ $$ = $1 / $3;    @}
        | '-' exp  %prec NEG @{ $$ = -$2;        @}
        | exp '^' exp        @{ $$ = pow ($1, $3); @}
        | '(' exp ')'        @{ $$ = $2;         @}
;
%%
@end example

@noindent
The functions @code{yylex}, @code{yyerror} and @code{main} can be the
same as before.

There are two important new features shown in this code.

In the second section (Bison declarations), @code{%left} declares token
types and says they are left-associative operators.  The declarations
@code{%left} and @code{%right} (right associativity) take the place of
@code{%token} which is used to declare a token type name without
associativity.  (These tokens are single-character literals, which
ordinarily don't need to be declared.  We declare them here to specify
the associativity.)

Operator precedence is determined by the line ordering of the
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence.  Hence, exponentiation
has the highest precedence, unary minus (@code{NEG}) is next, followed
by @samp{*} and @samp{/}, and so on.  @xref{Precedence, ,Operator
Precedence}.

The other important new feature is the @code{%prec} in the grammar
section for the unary minus operator.  The @code{%prec} simply instructs
Bison that the rule @samp{| '-' exp} has the same precedence as
@code{NEG}---in this case the next-to-highest.  @xref{Contextual
Precedence, ,Context-Dependent Precedence}.

Here is a sample run of @file{calc.y}:

@need 500
@example
$ @kbd{calc}
@kbd{4 + 4.5 - (34/(8*3+-3))}
6.880952381
@kbd{-56 + 2}
-54
@kbd{3 ^ 2}
9
@end example

@node Simple Error Recovery
@section Simple Error Recovery
@cindex error recovery, simple

Up to this point, this manual has not addressed the issue of @dfn{error
recovery}---how to continue parsing after the parser detects a syntax
error.  All we have handled is error reporting with @code{yyerror}.
Recall that by default @code{yyparse} returns after calling
@code{yyerror}.  This means that an erroneous input line causes the
calculator program to exit.  Now we show how to rectify this deficiency.

The Bison language itself includes the reserved word @code{error}, which
may be included in the grammar rules.  In the example below it has
been added to one of the alternatives for @code{line}:

@example
@group
line:     '\n'
        | exp '\n'   @{ printf ("\t%.10g\n", $1); @}
        | error '\n' @{ yyerrok;                  @}
;
@end group
@end example

This addition to the grammar allows for simple error recovery in the
event of a syntax error.  If an expression that cannot be evaluated is
read, the error will be recognized by the third rule for @code{line},
and parsing will continue.  (The @code{yyerror} function is still called
upon to print its message as well.)  The action executes the statement
@code{yyerrok}, a macro defined automatically by Bison; its meaning is
that error recovery is complete (@pxref{Error Recovery}).  Note the
difference between @code{yyerrok} and @code{yyerror}; neither one is a
misprint.

This form of error recovery deals with syntax errors.  There are other
kinds of errors; for example, division by zero, which raises an exception
signal that is normally fatal.  A real calculator program must handle this
signal and use @code{longjmp} to return to @code{main} and resume parsing
input lines; it would also have to discard the rest of the current line of
input.  We won't discuss this issue further because it is not specific to
Bison programs.

@node Location Tracking Calc
@section Location Tracking Calculator: @code{ltcalc}
@cindex location tracking calculator
@cindex @code{ltcalc}
@cindex calculator, location tracking

This example extends the infix notation calculator with location
tracking.  This feature will be used to improve the error messages.  For
the sake of clarity, this example is a simple integer calculator, since
most of the work needed to use locations will be done in the lexical
analyzer.

@menu
* Decls: Ltcalc Decls.  Bison and C declarations for ltcalc.
* Rules: Ltcalc Rules.  Grammar rules for ltcalc, with explanations.
* Lexer: Ltcalc Lexer.  The lexical analyzer.
@end menu

@node Ltcalc Decls
@subsection Declarations for @code{ltcalc}

The C and Bison declarations for the location tracking calculator are
the same as the declarations for the infix notation calculator.

@example
/* Location tracking calculator.  */

%@{
  #define YYSTYPE int
  #include <math.h>
  int yylex (void);
  void yyerror (char const *);
%@}

/* Bison declarations.  */
%token NUM

%left '-' '+'
%left '*' '/'
%left NEG
%right '^'

%% /* The grammar follows.  */
@end example

@noindent
Note there are no declarations specific to locations.  Defining a data
type for storing locations is not needed: we will use the type provided
by default (@pxref{Location Type, ,Data Types of Locations}), which is a
four member structure with the following integer fields:
@code{first_line}, @code{first_column}, @code{last_line} and
@code{last_column}.

@node Ltcalc Rules
@subsection Grammar Rules for @code{ltcalc}

Whether handling locations or not has no effect on the syntax of your
language.  Therefore, grammar rules for this example will be very close
to those of the previous example: we will only modify them to benefit
from the new information.

Here, we will use locations to report divisions by zero, and locate the
wrong expressions or subexpressions.

@example
@group
input   : /* empty */
        | input line
;
@end group

@group
line    : '\n'
        | exp '\n' @{ printf ("%d\n", $1); @}
;
@end group

@group
exp     : NUM           @{ $$ = $1; @}
        | exp '+' exp   @{ $$ = $1 + $3; @}
        | exp '-' exp   @{ $$ = $1 - $3; @}
        | exp '*' exp   @{ $$ = $1 * $3; @}
@end group
@group
        | exp '/' exp
            @{
              if ($3)
                $$ = $1 / $3;
              else
                @{
                  $$ = 1;
                  fprintf (stderr, "%d.%d-%d.%d: division by zero",
                           @@3.first_line, @@3.first_column,
                           @@3.last_line, @@3.last_column);
                @}
            @}
@end group
@group
        | '-' exp %preg NEG     @{ $$ = -$2; @}
        | exp '^' exp           @{ $$ = pow ($1, $3); @}
        | '(' exp ')'           @{ $$ = $2; @}
@end group
@end example

This code shows how to reach locations inside of semantic actions, by
using the pseudo-variables @code{@@@var{n}} for rule components, and the
pseudo-variable @code{@@$} for groupings.

We don't need to assign a value to @code{@@$}: the output parser does it
automatically.  By default, before executing the C code of each action,
@code{@@$} is set to range from the beginning of @code{@@1} to the end
of @code{@@@var{n}}, for a rule with @var{n} components.  This behavior
can be redefined (@pxref{Location Default Action, , Default Action for
Locations}), and for very specific rules, @code{@@$} can be computed by
hand.

@node Ltcalc Lexer
@subsection The @code{ltcalc} Lexical Analyzer.

Until now, we relied on Bison's defaults to enable location
tracking.  The next step is to rewrite the lexical analyzer, and make it
able to feed the parser with the token locations, as it already does for
semantic values.

To this end, we must take into account every single character of the
input text, to avoid the computed locations of being fuzzy or wrong:

@example
@group
int
yylex (void)
@{
  int c;
@end group

@group
  /* Skip white space.  */
  while ((c = getchar ()) == ' ' || c == '\t')
    ++yylloc.last_column;
@end group

@group
  /* Step.  */
  yylloc.first_line = yylloc.last_line;
  yylloc.first_column = yylloc.last_column;
@end group

@group
  /* Process numbers.  */
  if (isdigit (c))
    @{
      yylval = c - '0';
      ++yylloc.last_column;
      while (isdigit (c = getchar ()))
        @{
          ++yylloc.last_column;
          yylval = yylval * 10 + c - '0';
        @}
      ungetc (c, stdin);
      return NUM;
    @}
@end group

  /* Return end-of-input.  */
  if (c == EOF)
    return 0;

  /* Return a single char, and update location.  */
  if (c == '\n')
    @{
      ++yylloc.last_line;
      yylloc.last_column = 0;
    @}
  else
    ++yylloc.last_column;
  return c;
@}
@end example

Basically, the lexical analyzer performs the same processing as before:
it skips blanks and tabs, and reads numbers or single-character tokens.
In addition, it updates @code{yylloc}, the global variable (of type
@code{YYLTYPE}) containing the token's location.

Now, each time this function returns a token, the parser has its number
as well as its semantic value, and its location in the text.  The last
needed change is to initialize @code{yylloc}, for example in the
controlling function:

@example
@group
int
main (void)
@{
  yylloc.first_line = yylloc.last_line = 1;
  yylloc.first_column = yylloc.last_column = 0;
  return yyparse ();
@}
@end group
@end example

Remember that computing locations is not a matter of syntax.  Every
character must be associated to a location update, whether it is in
valid input, in comments, in literal strings, and so on.

@node Multi-function Calc
@section Multi-Function Calculator: @code{mfcalc}
@cindex multi-function calculator
@cindex @code{mfcalc}
@cindex calculator, multi-function

Now that the basics of Bison have been discussed, it is time to move on to
a more advanced problem.  The above calculators provided only five
functions, @samp{+}, @samp{-}, @samp{*}, @samp{/} and @samp{^}.  It would
be nice to have a calculator that provides other mathematical functions such
as @code{sin}, @code{cos}, etc.

It is easy to add new operators to the infix calculator as long as they are
only single-character literals.  The lexical analyzer @code{yylex} passes
back all nonnumeric characters as tokens, so new grammar rules suffice for
adding a new operator.  But we want something more flexible: built-in
functions whose syntax has this form:

@example
@var{function_name} (@var{argument})
@end example

@noindent
At the same time, we will add memory to the calculator, by allowing you
to create named variables, store values in them, and use them later.
Here is a sample session with the multi-function calculator:

@example
$ @kbd{mfcalc}
@kbd{pi = 3.141592653589}
3.1415926536
@kbd{sin(pi)}
0.0000000000
@kbd{alpha = beta1 = 2.3}
2.3000000000
@kbd{alpha}
2.3000000000
@kbd{ln(alpha)}
0.8329091229
@kbd{exp(ln(beta1))}
2.3000000000
$
@end example

Note that multiple assignment and nested function calls are permitted.

@menu
* Decl: Mfcalc Decl.      Bison declarations for multi-function calculator.
* Rules: Mfcalc Rules.    Grammar rules for the calculator.
* Symtab: Mfcalc Symtab.  Symbol table management subroutines.
@end menu

@node Mfcalc Decl
@subsection Declarations for @code{mfcalc}

Here are the C and Bison declarations for the multi-function calculator.

@smallexample
@group
%@{
  #include <math.h>  /* For math functions, cos(), sin(), etc.  */
  #include "calc.h"  /* Contains definition of `symrec'.  */
  int yylex (void);
  void yyerror (char const *);
%@}
@end group
@group
%union @{
  double    val;   /* For returning numbers.  */
  symrec  *tptr;   /* For returning symbol-table pointers.  */
@}
@end group
%token <val>  NUM        /* Simple double precision number.  */
%token <tptr> VAR FNCT   /* Variable and Function.  */
%type  <val>  exp

@group
%right '='
%left '-' '+'
%left '*' '/'
%left NEG     /* negation--unary minus */
%right '^'    /* exponentiation */
@end group
%% /* The grammar follows.  */
@end smallexample

The above grammar introduces only two new features of the Bison language.
These features allow semantic values to have various data types
(@pxref{Multiple Types, ,More Than One Value Type}).

The @code{%union} declaration specifies the entire list of possible types;
this is instead of defining @code{YYSTYPE}.  The allowable types are now
double-floats (for @code{exp} and @code{NUM}) and pointers to entries in
the symbol table.  @xref{Union Decl, ,The Collection of Value Types}.

Since values can now have various types, it is necessary to associate a
type with each grammar symbol whose semantic value is used.  These symbols
are @code{NUM}, @code{VAR}, @code{FNCT}, and @code{exp}.  Their
declarations are augmented with information about their data type (placed
between angle brackets).

The Bison construct @code{%type} is used for declaring nonterminal
symbols, just as @code{%token} is used for declaring token types.  We
have not used @code{%type} before because nonterminal symbols are
normally declared implicitly by the rules that define them.  But
@code{exp} must be declared explicitly so we can specify its value type.
@xref{Type Decl, ,Nonterminal Symbols}.

@node Mfcalc Rules
@subsection Grammar Rules for @code{mfcalc}

Here are the grammar rules for the multi-function calculator.
Most of them are copied directly from @code{calc}; three rules,
those which mention @code{VAR} or @code{FNCT}, are new.

@smallexample
@group
input:   /* empty */
        | input line
;
@end group

@group
line:
          '\n'
        | exp '\n'   @{ printf ("\t%.10g\n", $1); @}
        | error '\n' @{ yyerrok;                  @}
;
@end group

@group
exp:      NUM                @{ $$ = $1;                         @}
        | VAR                @{ $$ = $1->value.var;              @}
        | VAR '=' exp        @{ $$ = $3; $1->value.var = $3;     @}
        | FNCT '(' exp ')'   @{ $$ = (*($1->value.fnctptr))($3); @}
        | exp '+' exp        @{ $$ = $1 + $3;                    @}
        | exp '-' exp        @{ $$ = $1 - $3;                    @}
        | exp '*' exp        @{ $$ = $1 * $3;                    @}
        | exp '/' exp        @{ $$ = $1 / $3;                    @}
        | '-' exp  %prec NEG @{ $$ = -$2;                        @}
        | exp '^' exp        @{ $$ = pow ($1, $3);               @}
        | '(' exp ')'        @{ $$ = $2;                         @}
;
@end group
/* End of grammar.  */
%%
@end smallexample

@node Mfcalc Symtab
@subsection The @code{mfcalc} Symbol Table
@cindex symbol table example

The multi-function calculator requires a symbol table to keep track of the
names and meanings of variables and functions.  This doesn't affect the
grammar rules (except for the actions) or the Bison declarations, but it
requires some additional C functions for support.

The symbol table itself consists of a linked list of records.  Its
definition, which is kept in the header @file{calc.h}, is as follows.  It
provides for either functions or variables to be placed in the table.

@smallexample
@group
/* Function type.  */
typedef double (*func_t) (double);
@end group

@group
/* Data type for links in the chain of symbols.  */
struct symrec
@{
  char *name;  /* name of symbol */
  int type;    /* type of symbol: either VAR or FNCT */
  union
  @{
    double var;      /* value of a VAR */
    func_t fnctptr;  /* value of a FNCT */
  @} value;
  struct symrec *next;  /* link field */
@};
@end group

@group
typedef struct symrec symrec;

/* The symbol table: a chain of `struct symrec'.  */
extern symrec *sym_table;

symrec *putsym (char const *, int);
symrec *getsym (char const *);
@end group
@end smallexample

The new version of @code{main} includes a call to @code{init_table}, a
function that initializes the symbol table.  Here it is, and
@code{init_table} as well:

@smallexample
#include <stdio.h>

@group
/* Called by yyparse on error.  */
void
yyerror (char const *s)
@{
  printf ("%s\n", s);
@}
@end group

@group
struct init
@{
  char const *fname;
  double (*fnct) (double);
@};
@end group

@group
struct init const arith_fncts[] =
@{
  "sin",  sin,
  "cos",  cos,
  "atan", atan,
  "ln",   log,
  "exp",  exp,
  "sqrt", sqrt,
  0, 0
@};
@end group

@group
/* The symbol table: a chain of `struct symrec'.  */
symrec *sym_table;
@end group

@group
/* Put arithmetic functions in table.  */
void
init_table (void)
@{
  int i;
  symrec *ptr;
  for (i = 0; arith_fncts[i].fname != 0; i++)
    @{
      ptr = putsym (arith_fncts[i].fname, FNCT);
      ptr->value.fnctptr = arith_fncts[i].fnct;
    @}
@}
@end group

@group
int
main (void)
@{
  init_table ();
  return yyparse ();
@}
@end group
@end smallexample

By simply editing the initialization list and adding the necessary include
files, you can add additional functions to the calculator.

Two important functions allow look-up and installation of symbols in the
symbol table.  The function @code{putsym} is passed a name and the type
(@code{VAR} or @code{FNCT}) of the object to be installed.  The object is
linked to the front of the list, and a pointer to the object is returned.
The function @code{getsym} is passed the name of the symbol to look up.  If
found, a pointer to that symbol is returned; otherwise zero is returned.

@smallexample
symrec *
putsym (char const *sym_name, int sym_type)
@{
  symrec *ptr;
  ptr = (symrec *) malloc (sizeof (symrec));
  ptr->name = (char *) malloc (strlen (sym_name) + 1);
  strcpy (ptr->name,sym_name);
  ptr->type = sym_type;
  ptr->value.var = 0; /* Set value to 0 even if fctn.  */
  ptr->next = (struct symrec *)sym_table;
  sym_table = ptr;
  return ptr;
@}

symrec *
getsym (char const *sym_name)
@{
  symrec *ptr;
  for (ptr = sym_table; ptr != (symrec *) 0;
       ptr = (symrec *)ptr->next)
    if (strcmp (ptr->name,sym_name) == 0)
      return ptr;
  return 0;
@}
@end smallexample

The function @code{yylex} must now recognize variables, numeric values, and
the single-character arithmetic operators.  Strings of alphanumeric
characters with a leading letter are recognized as either variables or
functions depending on what the symbol table says about them.

The string is passed to @code{getsym} for look up in the symbol table.  If
the name appears in the table, a pointer to its location and its type
(@code{VAR} or @code{FNCT}) is returned to @code{yyparse}.  If it is not
already in the table, then it is installed as a @code{VAR} using
@code{putsym}.  Again, a pointer and its type (which must be @code{VAR}) is
returned to @code{yyparse}.

No change is needed in the handling of numeric values and arithmetic
operators in @code{yylex}.

@smallexample
@group
#include <ctype.h>
@end group

@group
int
yylex (void)
@{
  int c;

  /* Ignore white space, get first nonwhite character.  */
  while ((c = getchar ()) == ' ' || c == '\t');

  if (c == EOF)
    return 0;
@end group

@group
  /* Char starts a number => parse the number.         */
  if (c == '.' || isdigit (c))
    @{
      ungetc (c, stdin);
      scanf ("%lf", &yylval.val);
      return NUM;
    @}
@end group

@group
  /* Char starts an identifier => read the name.       */
  if (isalpha (c))
    @{
      symrec *s;
      static char *symbuf = 0;
      static int length = 0;
      int i;
@end group

@group
      /* Initially make the buffer long enough
         for a 40-character symbol name.  */
      if (length == 0)
        length = 40, symbuf = (char *)malloc (length + 1);

      i = 0;
      do
@end group
@group
        @{
          /* If buffer is full, make it bigger.        */
          if (i == length)
            @{
              length *= 2;
              symbuf = (char *) realloc (symbuf, length + 1);
            @}
          /* Add this character to the buffer.         */
          symbuf[i++] = c;
          /* Get another character.                    */
          c = getchar ();
        @}
@end group
@group
      while (isalnum (c));

      ungetc (c, stdin);
      symbuf[i] = '\0';
@end group

@group
      s = getsym (symbuf);
      if (s == 0)
        s = putsym (symbuf, VAR);
      yylval.tptr = s;
      return s->type;
    @}

  /* Any other character is a token by itself.        */
  return c;
@}
@end group
@end smallexample

This program is both powerful and flexible.  You may easily add new
functions, and it is a simple job to modify this code to install
predefined variables such as @code{pi} or @code{e} as well.

@node Exercises
@section Exercises
@cindex exercises

@enumerate
@item
Add some new functions from @file{math.h} to the initialization list.

@item
Add another array that contains constants and their values.  Then
modify @code{init_table} to add these constants to the symbol table.
It will be easiest to give the constants type @code{VAR}.

@item
Make the program report an error if the user refers to an
uninitialized variable in any way except to store a value in it.
@end enumerate

@node Grammar File
@chapter Bison Grammar Files

Bison takes as input a context-free grammar specification and produces a
C-language function that recognizes correct instances of the grammar.

The Bison grammar input file conventionally has a name ending in @samp{.y}.
@xref{Invocation, ,Invoking Bison}.

@menu
* Grammar Outline::   Overall layout of the grammar file.
* Symbols::           Terminal and nonterminal symbols.
* Rules::             How to write grammar rules.
* Recursion::         Writing recursive rules.
* Semantics::         Semantic values and actions.
* Locations::         Locations and actions.
* Declarations::      All kinds of Bison declarations are described here.
* Multiple Parsers::  Putting more than one Bison parser in one program.
@end menu

@node Grammar Outline
@section Outline of a Bison Grammar

A Bison grammar file has four main sections, shown here with the
appropriate delimiters:

@example
%@{
  @var{Prologue}
%@}

@var{Bison declarations}

%%
@var{Grammar rules}
%%

@var{Epilogue}
@end example

Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections.
As a @acronym{GNU} extension, @samp{//} introduces a comment that
continues until end of line.

@menu
* Prologue::          Syntax and usage of the prologue.
* Bison Declarations::  Syntax and usage of the Bison declarations section.
* Grammar Rules::     Syntax and usage of the grammar rules section.
* Epilogue::          Syntax and usage of the epilogue.
@end menu

@node Prologue
@subsection The prologue
@cindex declarations section
@cindex Prologue
@cindex declarations

The @var{Prologue} section contains macro definitions and declarations
of functions and variables that are used in the actions in the grammar
rules.  These are copied to the beginning of the parser file so that
they precede the definition of @code{yyparse}.  You can use
@samp{#include} to get the declarations from a header file.  If you
don't need any C declarations, you may omit the @samp{%@{} and
@samp{%@}} delimiters that bracket this section.

The @var{Prologue} section is terminated by the the first occurrence
of @samp{%@}} that is outside a comment, a string literal, or a
character constant.

You may have more than one @var{Prologue} section, intermixed with the
@var{Bison declarations}.  This allows you to have C and Bison
declarations that refer to each other.  For example, the @code{%union}
declaration may use types defined in a header file, and you may wish to
prototype functions that take arguments of type @code{YYSTYPE}.  This
can be done with two @var{Prologue} blocks, one before and one after the
@code{%union} declaration.

@smallexample
%@{
  #include <stdio.h>
  #include "ptypes.h"
%@}

%union @{
  long int n;
  tree t;  /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}

%@{
  static void print_token_value (FILE *, int, YYSTYPE);
  #define YYPRINT(F, N, L) print_token_value (F, N, L)
%@}

@dots{}
@end smallexample

@node Bison Declarations
@subsection The Bison Declarations Section
@cindex Bison declarations (introduction)
@cindex declarations, Bison (introduction)

The @var{Bison declarations} section contains declarations that define
terminal and nonterminal symbols, specify precedence, and so on.
In some simple grammars you may not need any declarations.
@xref{Declarations, ,Bison Declarations}.

@node Grammar Rules
@subsection The Grammar Rules Section
@cindex grammar rules section
@cindex rules section for grammar

The @dfn{grammar rules} section contains one or more Bison grammar
rules, and nothing else.  @xref{Rules, ,Syntax of Grammar Rules}.

There must always be at least one grammar rule, and the first
@samp{%%} (which precedes the grammar rules) may never be omitted even
if it is the first thing in the file.

@node Epilogue
@subsection The epilogue
@cindex additional C code section
@cindex epilogue
@cindex C code, section for additional

The @var{Epilogue} is copied verbatim to the end of the parser file, just as
the @var{Prologue} is copied to the beginning.  This is the most convenient
place to put anything that you want to have in the parser file but which need
not come before the definition of @code{yyparse}.  For example, the
definitions of @code{yylex} and @code{yyerror} often go here.  Because
C requires functions to be declared before being used, you often need
to declare functions like @code{yylex} and @code{yyerror} in the Prologue,
even if you define them in the Epilogue.
@xref{Interface, ,Parser C-Language Interface}.

If the last section is empty, you may omit the @samp{%%} that separates it
from the grammar rules.

The Bison parser itself contains many macros and identifiers whose names
start with @samp{yy} or @samp{YY}, so it is a good idea to avoid using
any such names (except those documented in this manual) in the epilogue
of the grammar file.

@node Symbols
@section Symbols, Terminal and Nonterminal
@cindex nonterminal symbol
@cindex terminal symbol
@cindex token type
@cindex symbol

@dfn{Symbols} in Bison grammars represent the grammatical classifications
of the language.

A @dfn{terminal symbol} (also known as a @dfn{token type}) represents a
class of syntactically equivalent tokens.  You use the symbol in grammar
rules to mean that a token in that class is allowed.  The symbol is
represented in the Bison parser by a numeric code, and the @code{yylex}
function returns a token type code to indicate what kind of token has
been read.  You don't need to know what the code value is; you can use
the symbol to stand for it.

A @dfn{nonterminal symbol} stands for a class of syntactically
equivalent groupings.  The symbol name is used in writing grammar rules.
By convention, it should be all lower case.

Symbol names can contain letters, digits (not at the beginning),
underscores and periods.  Periods make sense only in nonterminals.

There are three ways of writing terminal symbols in the grammar:

@itemize @bullet
@item
A @dfn{named token type} is written with an identifier, like an
identifier in C@.  By convention, it should be all upper case.  Each
such name must be defined with a Bison declaration such as
@code{%token}.  @xref{Token Decl, ,Token Type Names}.

@item
@cindex character token
@cindex literal token
@cindex single-character literal
A @dfn{character token type} (or @dfn{literal character token}) is
written in the grammar using the same syntax used in C for character
constants; for example, @code{'+'} is a character token type.  A
character token type doesn't need to be declared unless you need to
specify its semantic value data type (@pxref{Value Type, ,Data Types of
Semantic Values}), associativity, or precedence (@pxref{Precedence,
,Operator Precedence}).

By convention, a character token type is used only to represent a
token that consists of that particular character.  Thus, the token
type @code{'+'} is used to represent the character @samp{+} as a
token.  Nothing enforces this convention, but if you depart from it,
your program will confuse other readers.

All the usual escape sequences used in character literals in C can be
used in Bison as well, but you must not use the null character as a
character literal because its numeric code, zero, signifies
end-of-input (@pxref{Calling Convention, ,Calling Convention
for @code{yylex}}).  Also, unlike standard C, trigraphs have no
special meaning in Bison character literals, nor is backslash-newline
allowed.

@item
@cindex string token
@cindex literal string token
@cindex multicharacter literal
A @dfn{literal string token} is written like a C string constant; for
example, @code{"<="} is a literal string token.  A literal string token
doesn't need to be declared unless you need to specify its semantic
value data type (@pxref{Value Type}), associativity, or precedence
(@pxref{Precedence}).

You can associate the literal string token with a symbolic name as an
alias, using the @code{%token} declaration (@pxref{Token Decl, ,Token
Declarations}).  If you don't do that, the lexical analyzer has to
retrieve the token number for the literal string token from the
@code{yytname} table (@pxref{Calling Convention}).

@strong{Warning}: literal string tokens do not work in Yacc.

By convention, a literal string token is used only to represent a token
that consists of that particular string.  Thus, you should use the token
type @code{"<="} to represent the string @samp{<=} as a token.  Bison
does not enforce this convention, but if you depart from it, people who
read your program will be confused.

All the escape sequences used in string literals in C can be used in
Bison as well, except that you must not use a null character within a
string literal.  Also, unlike Standard C, trigraphs have no special
meaning in Bison string literals, nor is backslash-newline allowed.  A
literal string token must contain two or more characters; for a token
containing just one character, use a character token (see above).
@end itemize

How you choose to write a terminal symbol has no effect on its
grammatical meaning.  That depends only on where it appears in rules and
on when the parser function returns that symbol.

The value returned by @code{yylex} is always one of the terminal
symbols, except that a zero or negative value signifies end-of-input.
Whichever way you write the token type in the grammar rules, you write
it the same way in the definition of @code{yylex}.  The numeric code
for a character token type is simply the positive numeric code of the
character, so @code{yylex} can use the identical value to generate the
requisite code, though you may need to convert it to @code{unsigned
char} to avoid sign-extension on hosts where @code{char} is signed.
Each named token type becomes a C macro in
the parser file, so @code{yylex} can use the name to stand for the code.
(This is why periods don't make sense in terminal symbols.)
@xref{Calling Convention, ,Calling Convention for @code{yylex}}.

If @code{yylex} is defined in a separate file, you need to arrange for the
token-type macro definitions to be available there.  Use the @samp{-d}
option when you run Bison, so that it will write these macro definitions
into a separate header file @file{@var{name}.tab.h} which you can include
in the other source files that need it.  @xref{Invocation, ,Invoking Bison}.

If you want to write a grammar that is portable to any Standard C
host, you must use only nonnull character tokens taken from the basic
execution character set of Standard C@.  This set consists of the ten
digits, the 52 lower- and upper-case English letters, and the
characters in the following C-language string:

@example
"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_@{|@}~"
@end example

The @code{yylex} function and Bison must use a consistent character set
and encoding for character tokens.  For example, if you run Bison in an
@acronym{ASCII} environment, but then compile and run the resulting
program in an environment that uses an incompatible character set like
@acronym{EBCDIC}, the resulting program may not work because the tables
generated by Bison will assume @acronym{ASCII} numeric values for
character tokens.  It is standard practice for software distributions to
contain C source files that were generated by Bison in an
@acronym{ASCII} environment, so installers on platforms that are
incompatible with @acronym{ASCII} must rebuild those files before
compiling them.

The symbol @code{error} is a terminal symbol reserved for error recovery
(@pxref{Error Recovery}); you shouldn't use it for any other purpose.
In particular, @code{yylex} should never return this value.  The default
value of the error token is 256, unless you explicitly assigned 256 to
one of your tokens with a @code{%token} declaration.

@node Rules
@section Syntax of Grammar Rules
@cindex rule syntax
@cindex grammar rule syntax
@cindex syntax of grammar rules

A Bison grammar rule has the following general form:

@example
@group
@var{result}: @var{components}@dots{}
        ;
@end group
@end example

@noindent
where @var{result} is the nonterminal symbol that this rule describes,
and @var{components} are various terminal and nonterminal symbols that
are put together by this rule (@pxref{Symbols}).

For example,

@example
@group
exp:      exp '+' exp
        ;
@end group
@end example

@noindent
says that two groupings of type @code{exp}, with a @samp{+} token in between,
can be combined into a larger grouping of type @code{exp}.

White space in rules is significant only to separate symbols.  You can add
extra white space as you wish.

Scattered among the components can be @var{actions} that determine
the semantics of the rule.  An action looks like this:

@example
@{@var{C statements}@}
@end example

@noindent
@cindex braced code
This is an example of @dfn{braced code}, that is, C code surrounded by
braces, much like a compound statement in C@.  Braced code can contain
any sequence of C tokens, so long as its braces are balanced.  Bison
does not check the braced code for correctness directly; it merely
copies the code to the output file, where the C compiler can check it.

Within braced code, the balanced-brace count is not affected by braces
within comments, string literals, or character constants, but it is
affected by the C digraphs @samp{<%} and @samp{%>} that represent
braces.  At the top level braced code must be terminated by @samp{@}}
and not by a digraph.  Bison does not look for trigraphs, so if braced
code uses trigraphs you should ensure that they do not affect the
nesting of braces or the boundaries of comments, string literals, or
character constants.

Usually there is only one action and it follows the components.
@xref{Actions}.

@findex |
Multiple rules for the same @var{result} can be written separately or can
be joined with the vertical-bar character @samp{|} as follows:

@ifinfo
@example
@var{result}:   @var{rule1-components}@dots{}
        | @var{rule2-components}@dots{}
        @dots{}
        ;
@end example
@end ifinfo
@iftex
@example
@group
@var{result}:    @var{rule1-components}@dots{}
        | @var{rule2-components}@dots{}
        @dots{}
        ;
@end group
@end example
@end iftex

@noindent
They are still considered distinct rules even when joined in this way.

If @var{components} in a rule is empty, it means that @var{result} can
match the empty string.  For example, here is how to define a
comma-separated sequence of zero or more @code{exp} groupings:

@example
@group
expseq:   /* empty */
        | expseq1
        ;
@end group

@group
expseq1:  exp
        | expseq1 ',' exp
        ;
@end group
@end example

@noindent
It is customary to write a comment @samp{/* empty */} in each rule
with no components.

@node Recursion
@section Recursive Rules
@cindex recursive rule

A rule is called @dfn{recursive} when its @var{result} nonterminal
appears also on its right hand side.  Nearly all Bison grammars need to
use recursion, because that is the only way to define a sequence of any
number of a particular thing.  Consider this recursive definition of a
comma-separated sequence of one or more expressions:

@example
@group
expseq1:  exp
        | expseq1 ',' exp
        ;
@end group
@end example

@cindex left recursion
@cindex right recursion
@noindent
Since the recursive use of @code{expseq1} is the leftmost symbol in the
right hand side, we call this @dfn{left recursion}.  By contrast, here
the same construct is defined using @dfn{right recursion}:

@example
@group
expseq1:  exp
        | exp ',' expseq1
        ;
@end group
@end example

@noindent
Any kind of sequence can be defined using either left recursion or right
recursion, but you should always use left recursion, because it can
parse a sequence of any number of elements with bounded stack space.
Right recursion uses up space on the Bison stack in proportion to the
number of elements in the sequence, because all the elements must be
shifted onto the stack before the rule can be applied even once.
@xref{Algorithm, ,The Bison Parser Algorithm}, for further explanation
of this.

@cindex mutual recursion
@dfn{Indirect} or @dfn{mutual} recursion occurs when the result of the
rule does not appear directly on its right hand side, but does appear
in rules for other nonterminals which do appear on its right hand
side.

For example:

@example
@group
expr:     primary
        | primary '+' primary
        ;
@end group

@group
primary:  constant
        | '(' expr ')'
        ;
@end group
@end example

@noindent
defines two mutually-recursive nonterminals, since each refers to the
other.

@node Semantics
@section Defining Language Semantics
@cindex defining language semantics
@cindex language semantics, defining

The grammar rules for a language determine only the syntax.  The semantics
are determined by the semantic values associated with various tokens and
groupings, and by the actions taken when various groupings are recognized.

For example, the calculator calculates properly because the value
associated with each expression is the proper number; it adds properly
because the action for the grouping @w{@samp{@var{x} + @var{y}}} is to add
the numbers associated with @var{x} and @var{y}.

@menu
* Value Type::        Specifying one data type for all semantic values.
* Multiple Types::    Specifying several alternative data types.
* Actions::           An action is the semantic definition of a grammar rule.
* Action Types::      Specifying data types for actions to operate on.
* Mid-Rule Actions::  Most actions go at the end of a rule.
                      This says when, why and how to use the exceptional
                        action in the middle of a rule.
@end menu

@node Value Type
@subsection Data Types of Semantic Values
@cindex semantic value type
@cindex value type, semantic
@cindex data types of semantic values
@cindex default data type

In a simple program it may be sufficient to use the same data type for
the semantic values of all language constructs.  This was true in the
@acronym{RPN} and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish
Notation Calculator}).

Bison's default is to use type @code{int} for all semantic values.  To
specify some other type, define @code{YYSTYPE} as a macro, like this:

@example
#define YYSTYPE double
@end example

@noindent
This macro definition must go in the prologue of the grammar file
(@pxref{Grammar Outline, ,Outline of a Bison Grammar}).

@node Multiple Types
@subsection More Than One Value Type

In most programs, you will need different data types for different kinds
of tokens and groupings.  For example, a numeric constant may need type
@code{int} or @code{long int}, while a string constant needs type
@code{char *}, and an identifier might need a pointer to an entry in the
symbol table.

To use more than one data type for semantic values in one parser, Bison
requires you to do two things:

@itemize @bullet
@item
Specify the entire collection of possible data types, with the
@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of
Value Types}).

@item
Choose one of those types for each symbol (terminal or nonterminal) for
which semantic values are used.  This is done for tokens with the
@code{%token} Bison declaration (@pxref{Token Decl, ,Token Type Names})
and for groupings with the @code{%type} Bison declaration (@pxref{Type
Decl, ,Nonterminal Symbols}).
@end itemize

@node Actions
@subsection Actions
@cindex action
@vindex $$
@vindex $@var{n}

An action accompanies a syntactic rule and contains C code to be executed
each time an instance of that rule is recognized.  The task of most actions
is to compute a semantic value for the grouping built by the rule from the
semantic values associated with tokens or smaller groupings.

An action consists of braced code containing C statements, and can be
placed at any position in the rule;
it is executed at that position.  Most rules have just one action at the
end of the rule, following all the components.  Actions in the middle of
a rule are tricky and used only for special purposes (@pxref{Mid-Rule
Actions, ,Actions in Mid-Rule}).

The C code in an action can refer to the semantic values of the components
matched by the rule with the construct @code{$@var{n}}, which stands for
the value of the @var{n}th component.  The semantic value for the grouping
being constructed is @code{$$}.  Bison translates both of these
constructs into expressions of the appropriate type when it copies the
actions into the parser file.  @code{$$} is translated to a modifiable
lvalue, so it can be assigned to.

Here is a typical example:

@example
@group
exp:    @dots{}
        | exp '+' exp
            @{ $$ = $1 + $3; @}
@end group
@end example

@noindent
This rule constructs an @code{exp} from two smaller @code{exp} groupings
connected by a plus-sign token.  In the action, @code{$1} and @code{$3}
refer to the semantic values of the two component @code{exp} groupings,
which are the first and third symbols on the right hand side of the rule.
The sum is stored into @code{$$} so that it becomes the semantic value of
the addition-expression just recognized by the rule.  If there were a
useful semantic value associated with the @samp{+} token, it could be
referred to as @code{$2}.

Note that the vertical-bar character @samp{|} is really a rule
separator, and actions are attached to a single rule.  This is a
difference with tools like Flex, for which @samp{|} stands for either
``or'', or ``the same action as that of the next rule''.  In the
following example, the action is triggered only when @samp{b} is found:

@example
@group
a-or-b: 'a'|'b'   @{ a_or_b_found = 1; @};
@end group
@end example

@cindex default action
If you don't specify an action for a rule, Bison supplies a default:
@w{@code{$$ = $1}.}  Thus, the value of the first symbol in the rule
becomes the value of the whole rule.  Of course, the default action is
valid only if the two data types match.  There is no meaningful default
action for an empty rule; every empty rule must have an explicit action
unless the rule's value does not matter.

@code{$@var{n}} with @var{n} zero or negative is allowed for reference
to tokens and groupings on the stack @emph{before} those that match the
current rule.  This is a very risky practice, and to use it reliably
you must be certain of the context in which the rule is applied.  Here
is a case in which you can use this reliably:

@example
@group
foo:      expr bar '+' expr  @{ @dots{} @}
        | expr bar '-' expr  @{ @dots{} @}
        ;
@end group

@group
bar:      /* empty */
        @{ previous_expr = $0; @}
        ;
@end group
@end example

As long as @code{bar} is used only in the fashion shown here, @code{$0}
always refers to the @code{expr} which precedes @code{bar} in the
definition of @code{foo}.

@vindex yylval
It is also possible to access the semantic value of the look-ahead token, if
any, from a semantic action.
This semantic value is stored in @code{yylval}.
@xref{Action Features, ,Special Features for Use in Actions}.

@node Action Types
@subsection Data Types of Values in Actions
@cindex action data types
@cindex data types in actions

If you have chosen a single data type for semantic values, the @code{$$}
and @code{$@var{n}} constructs always have that data type.

If you have used @code{%union} to specify a variety of data types, then you
must declare a choice among these types for each terminal or nonterminal
symbol that can have a semantic value.  Then each time you use @code{$$} or
@code{$@var{n}}, its data type is determined by which symbol it refers to
in the rule.  In this example,

@example
@group
exp:    @dots{}
        | exp '+' exp
            @{ $$ = $1 + $3; @}
@end group
@end example

@noindent
@code{$1} and @code{$3} refer to instances of @code{exp}, so they all
have the data type declared for the nonterminal symbol @code{exp}.  If
@code{$2} were used, it would have the data type declared for the
terminal symbol @code{'+'}, whatever that might be.

Alternatively, you can specify the data type when you refer to the value,
by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the
reference.  For example, if you have defined types as shown here:

@example
@group
%union @{
  int itype;
  double dtype;
@}
@end group
@end example

@noindent
then you can write @code{$<itype>1} to refer to the first subunit of the
rule as an integer, or @code{$<dtype>1} to refer to it as a double.

@node Mid-Rule Actions
@subsection Actions in Mid-Rule
@cindex actions in mid-rule
@cindex mid-rule actions

Occasionally it is useful to put an action in the middle of a rule.
These actions are written just like usual end-of-rule actions, but they
are executed before the parser even recognizes the following components.

A mid-rule action may refer to the components preceding it using
@code{$@var{n}}, but it may not refer to subsequent components because
it is run before they are parsed.

The mid-rule action itself counts as one of the components of the rule.
This makes a difference when there is another action later in the same rule
(and usually there is another at the end): you have to count the actions
along with the symbols when working out which number @var{n} to use in
@code{$@var{n}}.

The mid-rule action can also have a semantic value.  The action can set
its value with an assignment to @code{$$}, and actions later in the rule
can refer to the value using @code{$@var{n}}.  Since there is no symbol
to name the action, there is no way to declare a data type for the value
in advance, so you must use the @samp{$<@dots{}>@var{n}} construct to
specify a data type each time you refer to this value.

There is no way to set the value of the entire rule with a mid-rule
action, because assignments to @code{$$} do not have that effect.  The
only way to set the value for the entire rule is with an ordinary action
at the end of the rule.

Here is an example from a hypothetical compiler, handling a @code{let}
statement that looks like @samp{let (@var{variable}) @var{statement}} and
serves to create a variable named @var{variable} temporarily for the
duration of @var{statement}.  To parse this construct, we must put
@var{variable} into the symbol table while @var{statement} is parsed, then
remove it afterward.  Here is how it is done:

@example
@group
stmt:   LET '(' var ')'
                @{ $<context>$ = push_context ();
                  declare_variable ($3); @}
        stmt    @{ $$ = $6;
                  pop_context ($<context>5); @}
@end group
@end example

@noindent
As soon as @samp{let (@var{variable})} has been recognized, the first
action is run.  It saves a copy of the current semantic context (the
list of accessible variables) as its semantic value, using alternative
@code{context} in the data-type union.  Then it calls
@code{declare_variable} to add the new variable to that list.  Once the
first action is finished, the embedded statement @code{stmt} can be
parsed.  Note that the mid-rule action is component number 5, so the
@samp{stmt} is component number 6.

After the embedded statement is parsed, its semantic value becomes the
value of the entire @code{let}-statement.  Then the semantic value from the
earlier action is used to restore the prior list of variables.  This
removes the temporary @code{let}-variable from the list so that it won't
appear to exist while the rest of the program is parsed.

Taking action before a rule is completely recognized often leads to
conflicts since the parser must commit to a parse in order to execute the
action.  For example, the following two rules, without mid-rule actions,
can coexist in a working parser because the parser can shift the open-brace
token and look at what follows before deciding whether there is a
declaration or not:

@example
@group
compound: '@{' declarations statements '@}'
        | '@{' statements '@}'
        ;
@end group
@end example

@noindent
But when we add a mid-rule action as follows, the rules become nonfunctional:

@example
@group
compound: @{ prepare_for_local_variables (); @}
          '@{' declarations statements '@}'
@end group
@group
        | '@{' statements '@}'
        ;
@end group
@end example

@noindent
Now the parser is forced to decide whether to run the mid-rule action
when it has read no farther than the open-brace.  In other words, it
must commit to using one rule or the other, without sufficient
information to do it correctly.  (The open-brace token is what is called
the @dfn{look-ahead} token at this time, since the parser is still
deciding what to do about it.  @xref{Look-Ahead, ,Look-Ahead Tokens}.)

You might think that you could correct the problem by putting identical
actions into the two rules, like this:

@example
@group
compound: @{ prepare_for_local_variables (); @}
          '@{' declarations statements '@}'
        | @{ prepare_for_local_variables (); @}
          '@{' statements '@}'
        ;
@end group
@end example

@noindent
But this does not help, because Bison does not realize that the two actions
are identical.  (Bison never tries to understand the C code in an action.)

If the grammar is such that a declaration can be distinguished from a
statement by the first token (which is true in C), then one solution which
does work is to put the action after the open-brace, like this:

@example
@group
compound: '@{' @{ prepare_for_local_variables (); @}
          declarations statements '@}'
        | '@{' statements '@}'
        ;
@end group
@end example

@noindent
Now the first token of the following declaration or statement,
which would in any case tell Bison which rule to use, can still do so.

Another solution is to bury the action inside a nonterminal symbol which
serves as a subroutine:

@example
@group
subroutine: /* empty */
          @{ prepare_for_local_variables (); @}
        ;

@end group

@group
compound: subroutine
          '@{' declarations statements '@}'
        | subroutine
          '@{' statements '@}'
        ;
@end group
@end example

@noindent
Now Bison can execute the action in the rule for @code{subroutine} without
deciding which rule for @code{compound} it will eventually use.  Note that
the action is now at the end of its rule.  Any mid-rule action can be
converted to an end-of-rule action in this way, and this is what Bison
actually does to implement mid-rule actions.

@node Locations
@section Tracking Locations
@cindex location
@cindex textual location
@cindex location, textual

Though grammar rules and semantic actions are enough to write a fully
functional parser, it can be useful to process some additional information,
especially symbol locations.

The way locations are handled is defined by providing a data type, and
actions to take when rules are matched.

@menu
* Location Type::               Specifying a data type for locations.
* Actions and Locations::       Using locations in actions.
* Location Default Action::     Defining a general way to compute locations.
@end menu

@node Location Type
@subsection Data Type of Locations
@cindex data type of locations
@cindex default location type

Defining a data type for locations is much simpler than for semantic values,
since all tokens and groupings always use the same type.

The type of locations is specified by defining a macro called @code{YYLTYPE}.
When @code{YYLTYPE} is not defined, Bison uses a default structure type with
four members:

@example
typedef struct YYLTYPE
@{
  int first_line;
  int first_column;
  int last_line;
  int last_column;
@} YYLTYPE;
@end example

@node Actions and Locations
@subsection Actions and Locations
@cindex location actions
@cindex actions, location
@vindex @@$
@vindex @@@var{n}

Actions are not only useful for defining language semantics, but also for
describing the behavior of the output parser with locations.

The most obvious way for building locations of syntactic groupings is very
similar to the way semantic values are computed.  In a given rule, several
constructs can be used to access the locations of the elements being matched.
The location of the @var{n}th component of the right hand side is
@code{@@@var{n}}, while the location of the left hand side grouping is
@code{@@$}.

Here is a basic example using the default data type for locations:

@example
@group
exp:    @dots{}
        | exp '/' exp
            @{
              @@$.first_column = @@1.first_column;
              @@$.first_line = @@1.first_line;
              @@$.last_column = @@3.last_column;
              @@$.last_line = @@3.last_line;
              if ($3)
                $$ = $1 / $3;
              else
                @{
                  $$ = 1;
                  fprintf (stderr,
                           "Division by zero, l%d,c%d-l%d,c%d",
                           @@3.first_line, @@3.first_column,
                           @@3.last_line, @@3.last_column);
                @}
            @}
@end group
@end example

As for semantic values, there is a default action for locations that is
run each time a rule is matched.  It sets the beginning of @code{@@$} to the
beginning of the first symbol, and the end of @code{@@$} to the end of the
last symbol.

With this default action, the location tracking can be fully automatic.  The
example above simply rewrites this way:

@example
@group
exp:    @dots{}
        | exp '/' exp
            @{
              if ($3)
                $$ = $1 / $3;
              else
                @{
                  $$ = 1;
                  fprintf (stderr,
                           "Division by zero, l%d,c%d-l%d,c%d",
                           @@3.first_line, @@3.first_column,
                           @@3.last_line, @@3.last_column);
                @}
            @}
@end group
@end example

@vindex yylloc
It is also possible to access the location of the look-ahead token, if any,
from a semantic action.
This location is stored in @code{yylloc}.
@xref{Action Features, ,Special Features for Use in Actions}.

@node Location Default Action
@subsection Default Action for Locations
@vindex YYLLOC_DEFAULT

Actually, actions are not the best place to compute locations.  Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each
rule.  The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is
matched, before the associated action is run.  It is also invoked
while processing a syntax error, to compute the error's location.

Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.

The @code{YYLLOC_DEFAULT} macro takes three parameters.  The first one is
the location of the grouping (the result of the computation).  When a
rule is matched, the second parameter identifies locations of
all right hand side elements of the rule being matched, and the third
parameter is the size of the rule's right hand side.  When processing
a syntax error, the second parameter identifies locations of
the symbols that were discarded during error processing, and the third
parameter is the number of discarded symbols.

By default, @code{YYLLOC_DEFAULT} is defined this way:

@smallexample
@group
# define YYLLOC_DEFAULT(Current, Rhs, N)                                \
    do                                                                  \
      if (N)                                                            \
        @{                                                               \
          (Current).first_line   = YYRHSLOC(Rhs, 1).first_line;         \
          (Current).first_column = YYRHSLOC(Rhs, 1).first_column;       \
          (Current).last_line    = YYRHSLOC(Rhs, N).last_line;          \
          (Current).last_column  = YYRHSLOC(Rhs, N).last_column;        \
        @}                                                               \
      else                                                              \
        @{                                                               \
          (Current).first_line   = (Current).last_line   =              \
            YYRHSLOC(Rhs, 0).last_line;                                 \
          (Current).first_column = (Current).last_column =              \
            YYRHSLOC(Rhs, 0).last_column;                               \
        @}                                                               \
    while (0)
@end group
@end smallexample

where @code{YYRHSLOC (rhs, k)} is the location of the @var{k}th symbol
in @var{rhs} when @var{k} is positive, and the location of the symbol
just before the reduction when @var{k} and @var{n} are both zero.

When defining @code{YYLLOC_DEFAULT}, you should consider that:

@itemize @bullet
@item
All arguments are free of side-effects.  However, only the first one (the
result) should be modified by @code{YYLLOC_DEFAULT}.

@item
For consistency with semantic actions, valid indexes within the
right hand side range from 1 to @var{n}.  When @var{n} is zero, only 0 is a
valid index, and it refers to the symbol just before the reduction.
During error processing @var{n} is always positive.

@item
Your macro should parenthesize its arguments, if need be, since the
actual arguments may not be surrounded by parentheses.  Also, your
macro should expand to something that can be used as a single
statement when it is followed by a semicolon.
@end itemize

@node Declarations
@section Bison Declarations
@cindex declarations, Bison
@cindex Bison declarations

The @dfn{Bison declarations} section of a Bison grammar defines the symbols
used in formulating the grammar and the data types of semantic values.
@xref{Symbols}.

All token type names (but not single-character literal tokens such as
@code{'+'} and @code{'*'}) must be declared.  Nonterminal symbols must be
declared if you need to specify which data type to use for the semantic
value (@pxref{Multiple Types, ,More Than One Value Type}).

The first rule in the file also specifies the start symbol, by default.
If you want some other symbol to be the start symbol, you must declare
it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free
Grammars}).

@menu
* Require Decl::      Requiring a Bison version.
* Token Decl::        Declaring terminal symbols.
* Precedence Decl::   Declaring terminals with precedence and associativity.
* Union Decl::        Declaring the set of all semantic value types.
* Type Decl::         Declaring the choice of type for a nonterminal symbol.
* Initial Action Decl::  Code run before parsing starts.
* Destructor Decl::   Declaring how symbols are freed.
* Expect Decl::       Suppressing warnings about parsing conflicts.
* Start Decl::        Specifying the start symbol.
* Pure Decl::         Requesting a reentrant parser.
* Decl Summary::      Table of all Bison declarations.
@end menu

@node Require Decl
@subsection Require a Version of Bison
@cindex version requirement
@cindex requiring a version of Bison
@findex %require

You may require the minimum version of Bison to process the grammar.  If
the requirement is not met, @command{bison} exits with an error (exit
status 63).

@example
%require "@var{version}"
@end example

@node Token Decl
@subsection Token Type Names
@cindex declaring token type names
@cindex token type names, declaring
@cindex declaring literal string tokens
@findex %token

The basic way to declare a token type name (terminal symbol) is as follows:

@example
%token @var{name}
@end example

Bison will convert this into a @code{#define} directive in
the parser, so that the function @code{yylex} (if it is in this file)
can use the name @var{name} to stand for this token type's code.

Alternatively, you can use @code{%left}, @code{%right}, or
@code{%nonassoc} instead of @code{%token}, if you wish to specify
associativity and precedence.  @xref{Precedence Decl, ,Operator
Precedence}.

You can explicitly specify the numeric code for a token type by appending
a decimal or hexadecimal integer value in the field immediately
following the token name:

@example
%token NUM 300
%token XNUM 0x12d // a GNU extension
@end example

@noindent
It is generally best, however, to let Bison choose the numeric codes for
all token types.  Bison will automatically select codes that don't conflict
with each other or with normal characters.

In the event that the stack type is a union, you must augment the
@code{%token} or other token declaration to include the data type
alternative delimited by angle-brackets (@pxref{Multiple Types, ,More
Than One Value Type}).

For example:

@example
@group
%union @{              /* define stack type */
  double val;
  symrec *tptr;
@}
%token <val> NUM      /* define token NUM and its type */
@end group
@end example

You can associate a literal string token with a token type name by
writing the literal string at the end of a @code{%token}
declaration which declares the name.  For example:

@example
%token arrow "=>"
@end example

@noindent
For example, a grammar for the C language might specify these names with
equivalent literal string tokens:

@example
%token  <operator>  OR      "||"
%token  <operator>  LE 134  "<="
%left  OR  "<="
@end example

@noindent
Once you equate the literal string and the token name, you can use them
interchangeably in further declarations or the grammar rules.  The
@code{yylex} function can use the token name or the literal string to
obtain the token type code number (@pxref{Calling Convention}).

@node Precedence Decl
@subsection Operator Precedence
@cindex precedence declarations
@cindex declaring operator precedence
@cindex operator precedence, declaring

Use the @code{%left}, @code{%right} or @code{%nonassoc} declaration to
declare a token and specify its precedence and associativity, all at
once.  These are called @dfn{precedence declarations}.
@xref{Precedence, ,Operator Precedence}, for general information on
operator precedence.

The syntax of a precedence declaration is the same as that of
@code{%token}: either

@example
%left @var{symbols}@dots{}
@end example

@noindent
or

@example
%left <@var{type}> @var{symbols}@dots{}
@end example

And indeed any of these declarations serves the purposes of @code{%token}.
But in addition, they specify the associativity and relative precedence for
all the @var{symbols}:

@itemize @bullet
@item
The associativity of an operator @var{op} determines how repeated uses
of the operator nest: whether @samp{@var{x} @var{op} @var{y} @var{op}
@var{z}} is parsed by grouping @var{x} with @var{y} first or by
grouping @var{y} with @var{z} first.  @code{%left} specifies
left-associativity (grouping @var{x} with @var{y} first) and
@code{%right} specifies right-associativity (grouping @var{y} with
@var{z} first).  @code{%nonassoc} specifies no associativity, which
means that @samp{@var{x} @var{op} @var{y} @var{op} @var{z}} is
considered a syntax error.

@item
The precedence of an operator determines how it nests with other operators.
All the tokens declared in a single precedence declaration have equal
precedence and nest together according to their associativity.
When two tokens declared in different precedence declarations associate,
the one declared later has the higher precedence and is grouped first.
@end itemize

@node Union Decl
@subsection The Collection of Value Types
@cindex declaring value types
@cindex value types, declaring
@findex %union

The @code{%union} declaration specifies the entire collection of
possible data types for semantic values.  The keyword @code{%union} is
followed by braced code containing the same thing that goes inside a
@code{union} in C@.

For example:

@example
@group
%union @{
  double val;
  symrec *tptr;
@}
@end group
@end example

@noindent
This says that the two alternative types are @code{double} and @code{symrec
*}.  They are given names @code{val} and @code{tptr}; these names are used
in the @code{%token} and @code{%type} declarations to pick one of the types
for a terminal or nonterminal symbol (@pxref{Type Decl, ,Nonterminal Symbols}).

As an extension to @acronym{POSIX}, a tag is allowed after the
@code{union}.  For example:

@example
@group
%union value @{
  double val;
  symrec *tptr;
@}
@end group
@end example

@noindent
specifies the union tag @code{value}, so the corresponding C type is
@code{union value}.  If you do not specify a tag, it defaults to
@code{YYSTYPE}.

As another extension to @acronym{POSIX}, you may specify multiple
@code{%union} declarations; their contents are concatenated.  However,
only the first @code{%union} declaration can specify a tag.

Note that, unlike making a @code{union} declaration in C, you need not write
a semicolon after the closing brace.

@node Type Decl
@subsection Nonterminal Symbols
@cindex declaring value types, nonterminals
@cindex value types, nonterminals, declaring
@findex %type

@noindent
When you use @code{%union} to specify multiple value types, you must
declare the value type of each nonterminal symbol for which values are
used.  This is done with a @code{%type} declaration, like this:

@example
%type <@var{type}> @var{nonterminal}@dots{}
@end example

@noindent
Here @var{nonterminal} is the name of a nonterminal symbol, and
@var{type} is the name given in the @code{%union} to the alternative
that you want (@pxref{Union Decl, ,The Collection of Value Types}).  You
can give any number of nonterminal symbols in the same @code{%type}
declaration, if they have the same value type.  Use spaces to separate
the symbol names.

You can also declare the value type of a terminal symbol.  To do this,
use the same @code{<@var{type}>} construction in a declaration for the
terminal symbol.  All kinds of token declarations allow
@code{<@var{type}>}.

@node Initial Action Decl
@subsection Performing Actions before Parsing
@findex %initial-action

Sometimes your parser needs to perform some initializations before
parsing.  The @code{%initial-action} directive allows for such arbitrary
code.

@deffn {Directive} %initial-action @{ @var{code} @}
@findex %initial-action
Declare that the braced @var{code} must be invoked before parsing each time
@code{yyparse} is called.  The @var{code} may use @code{$$} and
@code{@@$} --- initial value and location of the look-ahead --- and the
@code{%parse-param}.
@end deffn

For instance, if your locations use a file name, you may use

@example
%parse-param @{ char const *file_name @};
%initial-action
@{
  @@$.initialize (file_name);
@};
@end example


@node Destructor Decl
@subsection Freeing Discarded Symbols
@cindex freeing discarded symbols
@findex %destructor

During error recovery (@pxref{Error Recovery}), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded
until the parser falls on its feet.  If the parser runs out of memory,
or if it returns via @code{YYABORT} or @code{YYACCEPT}, all the
symbols on the stack must be discarded.  Even if the parser succeeds, it
must discard the start symbol.

When discarded symbols convey heap based information, this memory is
lost.  While this behavior can be tolerable for batch parsers, such as
in traditional compilers, it is unacceptable for programs like shells or
protocol implementations that may parse and execute indefinitely.

The @code{%destructor} directive defines code that is called when a
symbol is automatically discarded.

@deffn {Directive} %destructor @{ @var{code} @} @var{symbols}
@findex %destructor
Invoke the braced @var{code} whenever the parser discards one of the
@var{symbols}.
Within @var{code}, @code{$$} designates the semantic value associated
with the discarded symbol.  The additional parser parameters are also
available (@pxref{Parser Function, , The Parser Function
@code{yyparse}}).
@end deffn

For instance:

@smallexample
%union
@{
  char *string;
@}
%token <string> STRING
%type  <string> string
%destructor @{ free ($$); @} STRING string
@end smallexample

@noindent
guarantees that when a @code{STRING} or a @code{string} is discarded,
its associated memory will be freed.

@sp 1

@cindex discarded symbols
@dfn{Discarded symbols} are the following:

@itemize
@item
stacked symbols popped during the first phase of error recovery,
@item
incoming terminals during the second phase of error recovery,
@item
the current look-ahead and the entire stack (except the current
right-hand side symbols) when the parser returns immediately, and
@item
the start symbol, when the parser succeeds.
@end itemize

The parser can @dfn{return immediately} because of an explicit call to
@code{YYABORT} or @code{YYACCEPT}, or failed error recovery, or memory
exhaustion.

Right-hand size symbols of a rule that explicitly triggers a syntax
error via @code{YYERROR} are not discarded automatically.  As a rule
of thumb, destructors are invoked only when user actions cannot manage
the memory.

@node Expect Decl
@subsection Suppressing Conflict Warnings
@cindex suppressing conflict warnings
@cindex preventing warnings about conflicts
@cindex warnings, preventing
@cindex conflicts, suppressing warnings of
@findex %expect
@findex %expect-rr

Bison normally warns if there are any conflicts in the grammar
(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars
have harmless shift/reduce conflicts which are resolved in a predictable
way and would be difficult to eliminate.  It is desirable to suppress
the warning about these conflicts unless the number of conflicts
changes.  You can do this with the @code{%expect} declaration.

The declaration looks like this:

@example
%expect @var{n}
@end example

Here @var{n} is a decimal integer.  The declaration says there should
be @var{n} shift/reduce conflicts and no reduce/reduce conflicts.
Bison reports an error if the number of shift/reduce conflicts differs
from @var{n}, or if there are any reduce/reduce conflicts.

For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more
serious, and should be eliminated entirely.  Bison will always report
reduce/reduce conflicts for these parsers.  With @acronym{GLR}
parsers, however, both kinds of conflicts are routine; otherwise,
there would be no need to use @acronym{GLR} parsing.  Therefore, it is
also possible to specify an expected number of reduce/reduce conflicts
in @acronym{GLR} parsers, using the declaration:

@example
%expect-rr @var{n}
@end example

In general, using @code{%expect} involves these steps:

@itemize @bullet
@item
Compile your grammar without @code{%expect}.  Use the @samp{-v} option
to get a verbose list of where the conflicts occur.  Bison will also
print the number of conflicts.

@item
Check each of the conflicts to make sure that Bison's default
resolution is what you really want.  If not, rewrite the grammar and
go back to the beginning.

@item
Add an @code{%expect} declaration, copying the number @var{n} from the
number which Bison printed.  With @acronym{GLR} parsers, add an
@code{%expect-rr} declaration as well.
@end itemize

Now Bison will warn you if you introduce an unexpected conflict, but
will keep silent otherwise.

@node Start Decl
@subsection The Start-Symbol
@cindex declaring the start symbol
@cindex start symbol, declaring
@cindex default start symbol
@findex %start

Bison assumes by default that the start symbol for the grammar is the first
nonterminal specified in the grammar specification section.  The programmer
may override this restriction with the @code{%start} declaration as follows:

@example
%start @var{symbol}
@end example

@node Pure Decl
@subsection A Pure (Reentrant) Parser
@cindex reentrant parser
@cindex pure parser
@findex %pure-parser

A @dfn{reentrant} program is one which does not alter in the course of
execution; in other words, it consists entirely of @dfn{pure} (read-only)
code.  Reentrancy is important whenever asynchronous execution is possible;
for example, a nonreentrant program may not be safe to call from a signal
handler.  In systems with multiple threads of control, a nonreentrant
program must be called only within interlocks.

Normally, Bison generates a parser which is not reentrant.  This is
suitable for most uses, and it permits compatibility with Yacc.  (The
standard Yacc interfaces are inherently nonreentrant, because they use
statically allocated variables for communication with @code{yylex},
including @code{yylval} and @code{yylloc}.)

Alternatively, you can generate a pure, reentrant parser.  The Bison
declaration @code{%pure-parser} says that you want the parser to be
reentrant.  It looks like this:

@example
%pure-parser
@end example

The result is that the communication variables @code{yylval} and
@code{yylloc} become local variables in @code{yyparse}, and a different
calling convention is used for the lexical analyzer function
@code{yylex}.  @xref{Pure Calling, ,Calling Conventions for Pure
Parsers}, for the details of this.  The variable @code{yynerrs} also
becomes local in @code{yyparse} (@pxref{Error Reporting, ,The Error
Reporting Function @code{yyerror}}).  The convention for calling
@code{yyparse} itself is unchanged.

Whether the parser is pure has nothing to do with the grammar rules.
You can generate either a pure parser or a nonreentrant parser from any
valid grammar.

@node Decl Summary
@subsection Bison Declaration Summary
@cindex Bison declaration summary
@cindex declaration summary
@cindex summary, Bison declaration

Here is a summary of the declarations used to define a grammar:

@deffn {Directive} %union
Declare the collection of data types that semantic values may have
(@pxref{Union Decl, ,The Collection of Value Types}).
@end deffn

@deffn {Directive} %token
Declare a terminal symbol (token type name) with no precedence
or associativity specified (@pxref{Token Decl, ,Token Type Names}).
@end deffn

@deffn {Directive} %right
Declare a terminal symbol (token type name) that is right-associative
(@pxref{Precedence Decl, ,Operator Precedence}).
@end deffn

@deffn {Directive} %left
Declare a terminal symbol (token type name) that is left-associative
(@pxref{Precedence Decl, ,Operator Precedence}).
@end deffn

@deffn {Directive} %nonassoc
Declare a terminal symbol (token type name) that is nonassociative
(@pxref{Precedence Decl, ,Operator Precedence}).
Using it in a way that would be associative is a syntax error.
@end deffn

@ifset defaultprec
@deffn {Directive} %default-prec
Assign a precedence to rules lacking an explicit @code{%prec} modifier
(@pxref{Contextual Precedence, ,Context-Dependent Precedence}).
@end deffn
@end ifset

@deffn {Directive} %type
Declare the type of semantic values for a nonterminal symbol
(@pxref{Type Decl, ,Nonterminal Symbols}).
@end deffn

@deffn {Directive} %start
Specify the grammar's start symbol (@pxref{Start Decl, ,The
Start-Symbol}).
@end deffn

@deffn {Directive} %expect
Declare the expected number of shift-reduce conflicts
(@pxref{Expect Decl, ,Suppressing Conflict Warnings}).
@end deffn


@sp 1
@noindent
In order to change the behavior of @command{bison}, use the following
directives:

@deffn {Directive} %debug
In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
already defined, so that the debugging facilities are compiled.
@end deffn
@xref{Tracing, ,Tracing Your Parser}.

@deffn {Directive} %defines
Write a header file containing macro definitions for the token type
names defined in the grammar as well as a few other declarations.
If the parser output file is named @file{@var{name}.c} then this file
is named @file{@var{name}.h}.

Unless @code{YYSTYPE} is already defined as a macro, the output header
declares @code{YYSTYPE}.  Therefore, if you are using a @code{%union}
(@pxref{Multiple Types, ,More Than One Value Type}) with components that
require other definitions, or if you have defined a @code{YYSTYPE} macro
(@pxref{Value Type, ,Data Types of Semantic Values}), you need to
arrange for these definitions to be propagated to all modules, e.g., by
putting them in a prerequisite header that is included both by your
parser and by any other module that needs @code{YYSTYPE}.

Unless your parser is pure, the output header declares @code{yylval}
as an external variable.  @xref{Pure Decl, ,A Pure (Reentrant)
Parser}.

If you have also used locations, the output header declares
@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of
@code{YYSTYPE} and @code{yylval}.  @xref{Locations, ,Tracking
Locations}.

This output file is normally essential if you wish to put the definition
of @code{yylex} in a separate source file, because @code{yylex}
typically needs to be able to refer to the above-mentioned declarations
and to the token type codes.  @xref{Token Values, ,Semantic Values of
Tokens}.
@end deffn

@deffn {Directive} %destructor
Specify how the parser should reclaim the memory associated to
discarded symbols.  @xref{Destructor Decl, , Freeing Discarded Symbols}.
@end deffn

@deffn {Directive} %file-prefix="@var{prefix}"
Specify a prefix to use for all Bison output file names.  The names are
chosen as if the input file were named @file{@var{prefix}.y}.
@end deffn

@deffn {Directive} %locations
Generate the code processing the locations (@pxref{Action Features,
,Special Features for Use in Actions}).  This mode is enabled as soon as
the grammar uses the special @samp{@@@var{n}} tokens, but if your
grammar does not use it, using @samp{%locations} allows for more
accurate syntax error messages.
@end deffn

@deffn {Directive} %name-prefix="@var{prefix}"
Rename the external symbols used in the parser so that they start with
@var{prefix} instead of @samp{yy}.  The precise list of symbols renamed
is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
@code{yylval}, @code{yylloc}, @code{yychar}, @code{yydebug}, and
possible @code{yylloc}.  For example, if you use
@samp{%name-prefix="c_"}, the names become @code{c_parse}, @code{c_lex},
and so on.  @xref{Multiple Parsers, ,Multiple Parsers in the Same
Program}.
@end deffn

@ifset defaultprec
@deffn {Directive} %no-default-prec
Do not assign a precedence to rules lacking an explicit @code{%prec}
modifier (@pxref{Contextual Precedence, ,Context-Dependent
Precedence}).
@end deffn
@end ifset

@deffn {Directive} %no-parser
Do not include any C code in the parser file; generate tables only.  The
parser file contains just @code{#define} directives and static variable
declarations.

This option also tells Bison to write the C code for the grammar actions
into a file named @file{@var{file}.act}, in the form of a
brace-surrounded body fit for a @code{switch} statement.
@end deffn

@deffn {Directive} %no-lines
Don't generate any @code{#line} preprocessor commands in the parser
file.  Ordinarily Bison writes these commands in the parser file so that
the C compiler and debuggers will associate errors and object code with
your source file (the grammar file).  This directive causes them to
associate errors with the parser file, treating it an independent source
file in its own right.
@end deffn

@deffn {Directive} %output="@var{file}"
Specify @var{file} for the parser file.
@end deffn

@deffn {Directive} %pure-parser
Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure
(Reentrant) Parser}).
@end deffn

@deffn {Directive} %require "@var{version}"
Require version @var{version} or higher of Bison.  @xref{Require Decl, ,
Require a Version of Bison}.
@end deffn

@deffn {Directive} %token-table
Generate an array of token names in the parser file.  The name of the
array is @code{yytname}; @code{yytname[@var{i}]} is the name of the
token whose internal Bison token code number is @var{i}.  The first
three elements of @code{yytname} correspond to the predefined tokens
@code{"$end"},
@code{"error"}, and @code{"$undefined"}; after these come the symbols
defined in the grammar file.

The name in the table includes all the characters needed to represent
the token in Bison.  For single-character literals and literal
strings, this includes the surrounding quoting characters and any
escape sequences.  For example, the Bison single-character literal
@code{'+'} corresponds to a three-character name, represented in C as
@code{"'+'"}; and the Bison two-character literal string @code{"\\/"}
corresponds to a five-character name, represented in C as
@code{"\"\\\\/\""}.

When you specify @code{%token-table}, Bison also generates macro
definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
@code{YYNRULES}, and @code{YYNSTATES}:

@table @code
@item YYNTOKENS
The highest token number, plus one.
@item YYNNTS
The number of nonterminal symbols.
@item YYNRULES
The number of grammar rules,
@item YYNSTATES
The number of parser states (@pxref{Parser States}).
@end table
@end deffn

@deffn {Directive} %verbose
Write an extra output file containing verbose descriptions of the
parser states and what is done for each type of look-ahead token in
that state.  @xref{Understanding, , Understanding Your Parser}, for more
information.
@end deffn

@deffn {Directive} %yacc
Pretend the option @option{--yacc} was given, i.e., imitate Yacc,
including its naming conventions.  @xref{Bison Options}, for more.
@end deffn


@node Multiple Parsers
@section Multiple Parsers in the Same Program

Most programs that use Bison parse only one language and therefore contain
only one Bison parser.  But what if you want to parse more than one
language with the same program?  Then you need to avoid a name conflict
between different definitions of @code{yyparse}, @code{yylval}, and so on.

The easy way to do this is to use the option @samp{-p @var{prefix}}
(@pxref{Invocation, ,Invoking Bison}).  This renames the interface
functions and variables of the Bison parser to start with @var{prefix}
instead of @samp{yy}.  You can use this to give each parser distinct
names that do not conflict.

The precise list of symbols renamed is @code{yyparse}, @code{yylex},
@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yylloc},
@code{yychar} and @code{yydebug}.  For example, if you use @samp{-p c},
the names become @code{cparse}, @code{clex}, and so on.

@strong{All the other variables and macros associated with Bison are not
renamed.} These others are not global; there is no conflict if the same
name is used in different parsers.  For example, @code{YYSTYPE} is not
renamed, but defining this in different ways in different parsers causes
no trouble (@pxref{Value Type, ,Data Types of Semantic Values}).

The @samp{-p} option works by adding macro definitions to the beginning
of the parser source file, defining @code{yyparse} as
@code{@var{prefix}parse}, and so on.  This effectively substitutes one
name for the other in the entire parser file.

@node Interface
@chapter Parser C-Language Interface
@cindex C-language interface
@cindex interface

The Bison parser is actually a C function named @code{yyparse}.  Here we
describe the interface conventions of @code{yyparse} and the other
functions that it needs to use.

Keep in mind that the parser uses many C identifiers starting with
@samp{yy} and @samp{YY} for internal purposes.  If you use such an
identifier (aside from those in this manual) in an action or in epilogue
in the grammar file, you are likely to run into trouble.

@menu
* Parser Function::   How to call @code{yyparse} and what it returns.
* Lexical::           You must supply a function @code{yylex}
                        which reads tokens.
* Error Reporting::   You must supply a function @code{yyerror}.
* Action Features::   Special features for use in actions.
* Internationalization::  How to let the parser speak in the user's
                        native language.
@end menu

@node Parser Function
@section The Parser Function @code{yyparse}
@findex yyparse

You call the function @code{yyparse} to cause parsing to occur.  This
function reads tokens, executes actions, and ultimately returns when it
encounters end-of-input or an unrecoverable syntax error.  You can also
write an action which directs @code{yyparse} to return immediately
without reading further.


@deftypefun int yyparse (void)
The value returned by @code{yyparse} is 0 if parsing was successful (return
is due to end-of-input).

The value is 1 if parsing failed because of invalid input, i.e., input
that contains a syntax error or that causes @code{YYABORT} to be
invoked.

The value is 2 if parsing failed due to memory exhaustion.
@end deftypefun

In an action, you can cause immediate return from @code{yyparse} by using
these macros:

@defmac YYACCEPT
@findex YYACCEPT
Return immediately with value 0 (to report success).
@end defmac

@defmac YYABORT
@findex YYABORT
Return immediately with value 1 (to report failure).
@end defmac

If you use a reentrant parser, you can optionally pass additional
parameter information to it in a reentrant way.  To do so, use the
declaration @code{%parse-param}:

@deffn {Directive} %parse-param @{@var{argument-declaration}@}
@findex %parse-param
Declare that an argument declared by the braced-code
@var{argument-declaration} is an additional @code{yyparse} argument.
The @var{argument-declaration} is used when declaring
functions or prototypes.  The last identifier in
@var{argument-declaration} must be the argument name.
@end deffn

Here's an example.  Write this in the parser:

@example
%parse-param @{int *nastiness@}
%parse-param @{int *randomness@}
@end example

@noindent
Then call the parser like this:

@example
@{
  int nastiness, randomness;
  @dots{}  /* @r{Store proper data in @code{nastiness} and @code{randomness}.}  */
  value = yyparse (&nastiness, &randomness);
  @dots{}
@}
@end example

@noindent
In the grammar actions, use expressions like this to refer to the data:

@example
exp: @dots{}    @{ @dots{}; *randomness += 1; @dots{} @}
@end example


@node Lexical
@section The Lexical Analyzer Function @code{yylex}
@findex yylex
@cindex lexical analyzer

The @dfn{lexical analyzer} function, @code{yylex}, recognizes tokens from
the input stream and returns them to the parser.  Bison does not create
this function automatically; you must write it so that @code{yyparse} can
call it.  The function is sometimes referred to as a lexical scanner.

In simple programs, @code{yylex} is often defined at the end of the Bison
grammar file.  If @code{yylex} is defined in a separate source file, you
need to arrange for the token-type macro definitions to be available there.
To do this, use the @samp{-d} option when you run Bison, so that it will
write these macro definitions into a separate header file
@file{@var{name}.tab.h} which you can include in the other source files
that need it.  @xref{Invocation, ,Invoking Bison}.

@menu
* Calling Convention::  How @code{yyparse} calls @code{yylex}.
* Token Values::      How @code{yylex} must return the semantic value
                        of the token it has read.
* Token Locations::   How @code{yylex} must return the text location
                        (line number, etc.) of the token, if the
                        actions want that.
* Pure Calling::      How the calling convention differs
                        in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
@end menu

@node Calling Convention
@subsection Calling Convention for @code{yylex}

The value that @code{yylex} returns must be the positive numeric code
for the type of token it has just found; a zero or negative value
signifies end-of-input.

When a token is referred to in the grammar rules by a name, that name
in the parser file becomes a C macro whose definition is the proper
numeric code for that token type.  So @code{yylex} can use the name
to indicate that type.  @xref{Symbols}.

When a token is referred to in the grammar rules by a character literal,
the numeric code for that character is also the code for the token type.
So @code{yylex} can simply return that character code, possibly converted
to @code{unsigned char} to avoid sign-extension.  The null character
must not be used this way, because its code is zero and that
signifies end-of-input.

Here is an example showing these things:

@example
int
yylex (void)
@{
  @dots{}
  if (c == EOF)    /* Detect end-of-input.  */
    return 0;
  @dots{}
  if (c == '+' || c == '-')
    return c;      /* Assume token type for `+' is '+'.  */
  @dots{}
  return INT;      /* Return the type of the token.  */
  @dots{}
@}
@end example

@noindent
This interface has been designed so that the output from the @code{lex}
utility can be used without change as the definition of @code{yylex}.

If the grammar uses literal string tokens, there are two ways that
@code{yylex} can determine the token type codes for them:

@itemize @bullet
@item
If the grammar defines symbolic token names as aliases for the
literal string tokens, @code{yylex} can use these symbolic names like
all others.  In this case, the use of the literal string tokens in
the grammar file has no effect on @code{yylex}.

@item
@code{yylex} can find the multicharacter token in the @code{yytname}
table.  The index of the token in the table is the token type's code.
The name of a multicharacter token is recorded in @code{yytname} with a
double-quote, the token's characters, and another double-quote.  The
token's characters are escaped as necessary to be suitable as input
to Bison.

Here's code for looking up a multicharacter token in @code{yytname},
assuming that the characters of the token are stored in
@code{token_buffer}, and assuming that the token does not contain any
characters like @samp{"} that require escaping.

@smallexample
for (i = 0; i < YYNTOKENS; i++)
  @{
    if (yytname[i] != 0
        && yytname[i][0] == '"'
        && ! strncmp (yytname[i] + 1, token_buffer,
                      strlen (token_buffer))
        && yytname[i][strlen (token_buffer) + 1] == '"'
        && yytname[i][strlen (token_buffer) + 2] == 0)
      break;
  @}
@end smallexample

The @code{yytname} table is generated only if you use the
@code{%token-table} declaration.  @xref{Decl Summary}.
@end itemize

@node Token Values
@subsection Semantic Values of Tokens

@vindex yylval
In an ordinary (nonreentrant) parser, the semantic value of the token must
be stored into the global variable @code{yylval}.  When you are using
just one data type for semantic values, @code{yylval} has that type.
Thus, if the type is @code{int} (the default), you might write this in
@code{yylex}:

@example
@group
  @dots{}
  yylval = value;  /* Put value onto Bison stack.  */
  return INT;      /* Return the type of the token.  */
  @dots{}
@end group
@end example

When you are using multiple data types, @code{yylval}'s type is a union
made from the @code{%union} declaration (@pxref{Union Decl, ,The
Collection of Value Types}).  So when you store a token's value, you
must use the proper member of the union.  If the @code{%union}
declaration looks like this:

@example
@group
%union @{
  int intval;
  double val;
  symrec *tptr;
@}
@end group
@end example

@noindent
then the code in @code{yylex} might look like this:

@example
@group
  @dots{}
  yylval.intval = value; /* Put value onto Bison stack.  */
  return INT;            /* Return the type of the token.  */
  @dots{}
@end group
@end example

@node Token Locations
@subsection Textual Locations of Tokens

@vindex yylloc
If you are using the @samp{@@@var{n}}-feature (@pxref{Locations, ,
Tracking Locations}) in actions to keep track of the textual locations
of tokens and groupings, then you must provide this information in
@code{yylex}.  The function @code{yyparse} expects to find the textual
location of a token just parsed in the global variable @code{yylloc}.
So @code{yylex} must store the proper data in that variable.

By default, the value of @code{yylloc} is a structure and you need only
initialize the members that are going to be used by the actions.  The
four members are called @code{first_line}, @code{first_column},
@code{last_line} and @code{last_column}.  Note that the use of this
feature makes the parser noticeably slower.

@tindex YYLTYPE
The data type of @code{yylloc} has the name @code{YYLTYPE}.

@node Pure Calling
@subsection Calling Conventions for Pure Parsers

When you use the Bison declaration @code{%pure-parser} to request a
pure, reentrant parser, the global communication variables @code{yylval}
and @code{yylloc} cannot be used.  (@xref{Pure Decl, ,A Pure (Reentrant)
Parser}.)  In such parsers the two global variables are replaced by
pointers passed as arguments to @code{yylex}.  You must declare them as
shown here, and pass the information back by storing it through those
pointers.

@example
int
yylex (YYSTYPE *lvalp, YYLTYPE *llocp)
@{
  @dots{}
  *lvalp = value;  /* Put value onto Bison stack.  */
  return INT;      /* Return the type of the token.  */
  @dots{}
@}
@end example

If the grammar file does not use the @samp{@@} constructs to refer to
textual locations, then the type @code{YYLTYPE} will not be defined.  In
this case, omit the second argument; @code{yylex} will be called with
only one argument.


If you wish to pass the additional parameter data to @code{yylex}, use
@code{%lex-param} just like @code{%parse-param} (@pxref{Parser
Function}).

@deffn {Directive} lex-param @{@var{argument-declaration}@}
@findex %lex-param
Declare that the braced-code @var{argument-declaration} is an
additional @code{yylex} argument declaration.
@end deffn

For instance:

@example
%parse-param @{int *nastiness@}
%lex-param   @{int *nastiness@}
%parse-param @{int *randomness@}
@end example

@noindent
results in the following signature:

@example
int yylex   (int *nastiness);
int yyparse (int *nastiness, int *randomness);
@end example

If @code{%pure-parser} is added:

@example
int yylex   (YYSTYPE *lvalp, int *nastiness);
int yyparse (int *nastiness, int *randomness);
@end example

@noindent
and finally, if both @code{%pure-parser} and @code{%locations} are used:

@example
int yylex   (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
int yyparse (int *nastiness, int *randomness);
@end example

@node Error Reporting
@section The Error Reporting Function @code{yyerror}
@cindex error reporting function
@findex yyerror
@cindex parse error
@cindex syntax error

The Bison parser detects a @dfn{syntax error} or @dfn{parse error}
whenever it reads a token which cannot satisfy any syntax rule.  An
action in the grammar can also explicitly proclaim an error, using the
macro @code{YYERROR} (@pxref{Action Features, ,Special Features for Use
in Actions}).

The Bison parser expects to report the error by calling an error
reporting function named @code{yyerror}, which you must supply.  It is
called by @code{yyparse} whenever a syntax error is found, and it
receives one argument.  For a syntax error, the string is normally
@w{@code{"syntax error"}}.

@findex %error-verbose
If you invoke the directive @code{%error-verbose} in the Bison
declarations section (@pxref{Bison Declarations, ,The Bison Declarations
Section}), then Bison provides a more verbose and specific error message
string instead of just plain @w{@code{"syntax error"}}.

The parser can detect one other kind of error: memory exhaustion.  This
can happen when the input contains constructions that are very deeply
nested.  It isn't likely you will encounter this, since the Bison
parser normally extends its stack automatically up to a very large limit.  But
if memory is exhausted, @code{yyparse} calls @code{yyerror} in the usual
fashion, except that the argument string is @w{@code{"memory exhausted"}}.

In some cases diagnostics like @w{@code{"syntax error"}} are
translated automatically from English to some other language before
they are passed to @code{yyerror}.  @xref{Internationalization}.

The following definition suffices in simple programs:

@example
@group
void
yyerror (char const *s)
@{
@end group
@group
  fprintf (stderr, "%s\n", s);
@}
@end group
@end example

After @code{yyerror} returns to @code{yyparse}, the latter will attempt
error recovery if you have written suitable error recovery grammar rules
(@pxref{Error Recovery}).  If recovery is impossible, @code{yyparse} will
immediately return 1.

Obviously, in location tracking pure parsers, @code{yyerror} should have
an access to the current location.
This is indeed the case for the @acronym{GLR}
parsers, but not for the Yacc parser, for historical reasons.  I.e., if
@samp{%locations %pure-parser} is passed then the prototypes for
@code{yyerror} are:

@example
void yyerror (char const *msg);                 /* Yacc parsers.  */
void yyerror (YYLTYPE *locp, char const *msg);  /* GLR parsers.   */
@end example

If @samp{%parse-param @{int *nastiness@}} is used, then:

@example
void yyerror (int *nastiness, char const *msg);  /* Yacc parsers.  */
void yyerror (int *nastiness, char const *msg);  /* GLR parsers.   */
@end example

Finally, @acronym{GLR} and Yacc parsers share the same @code{yyerror} calling
convention for absolutely pure parsers, i.e., when the calling
convention of @code{yylex} @emph{and} the calling convention of
@code{%pure-parser} are pure.  I.e.:

@example
/* Location tracking.  */
%locations
/* Pure yylex.  */
%pure-parser
%lex-param   @{int *nastiness@}
/* Pure yyparse.  */
%parse-param @{int *nastiness@}
%parse-param @{int *randomness@}
@end example

@noindent
results in the following signatures for all the parser kinds:

@example
int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness);
int yyparse (int *nastiness, int *randomness);
void yyerror (YYLTYPE *locp,
              int *nastiness, int *randomness,
              char const *msg);
@end example

@noindent
The prototypes are only indications of how the code produced by Bison
uses @code{yyerror}.  Bison-generated code always ignores the returned
value, so @code{yyerror} can return any type, including @code{void}.
Also, @code{yyerror} can be a variadic function; that is why the
message is always passed last.

Traditionally @code{yyerror} returns an @code{int} that is always
ignored, but this is purely for historical reasons, and @code{void} is
preferable since it more accurately describes the return type for
@code{yyerror}.

@vindex yynerrs
The variable @code{yynerrs} contains the number of syntax errors
reported so far.  Normally this variable is global; but if you
request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser})
then it is a local variable which only the actions can access.

@node Action Features
@section Special Features for Use in Actions
@cindex summary, action features
@cindex action features summary

Here is a table of Bison constructs, variables and macros that
are useful in actions.

@deffn {Variable} $$
Acts like a variable that contains the semantic value for the
grouping made by the current rule.  @xref{Actions}.
@end deffn

@deffn {Variable} $@var{n}
Acts like a variable that contains the semantic value for the
@var{n}th component of the current rule.  @xref{Actions}.
@end deffn

@deffn {Variable} $<@var{typealt}>$
Like @code{$$} but specifies alternative @var{typealt} in the union
specified by the @code{%union} declaration.  @xref{Action Types, ,Data
Types of Values in Actions}.
@end deffn

@deffn {Variable} $<@var{typealt}>@var{n}
Like @code{$@var{n}} but specifies alternative @var{typealt} in the
union specified by the @code{%union} declaration.
@xref{Action Types, ,Data Types of Values in Actions}.
@end deffn

@deffn {Macro} YYABORT;
Return immediately from @code{yyparse}, indicating failure.
@xref{Parser Function, ,The Parser Function @code{yyparse}}.
@end deffn

@deffn {Macro} YYACCEPT;
Return immediately from @code{yyparse}, indicating success.
@xref{Parser Function, ,The Parser Function @code{yyparse}}.
@end deffn

@deffn {Macro} YYBACKUP (@var{token}, @var{value});
@findex YYBACKUP
Unshift a token.  This macro is allowed only for rules that reduce
a single value, and only when there is no look-ahead token.
It is also disallowed in @acronym{GLR} parsers.
It installs a look-ahead token with token type @var{token} and
semantic value @var{value}; then it discards the value that was
going to be reduced by this rule.

If the macro is used when it is not valid, such as when there is
a look-ahead token already, then it reports a syntax error with
a message @samp{cannot back up} and performs ordinary error
recovery.

In either case, the rest of the action is not executed.
@end deffn

@deffn {Macro} YYEMPTY
@vindex YYEMPTY
Value stored in @code{yychar} when there is no look-ahead token.
@end deffn

@deffn {Macro} YYEOF
@vindex YYEOF
Value stored in @code{yychar} when the look-ahead is the end of the input
stream.
@end deffn

@deffn {Macro} YYERROR;
@findex YYERROR
Cause an immediate syntax error.  This statement initiates error
recovery just as if the parser itself had detected an error; however, it
does not call @code{yyerror}, and does not print any message.  If you
want to print an error message, call @code{yyerror} explicitly before
the @samp{YYERROR;} statement.  @xref{Error Recovery}.
@end deffn

@deffn {Macro} YYRECOVERING
This macro stands for an expression that has the value 1 when the parser
is recovering from a syntax error, and 0 the rest of the time.
@xref{Error Recovery}.
@end deffn

@deffn {Variable} yychar
Variable containing either the look-ahead token, or @code{YYEOF} when the
look-ahead is the end of the input stream, or @code{YYEMPTY} when no look-ahead
has been performed so the next token is not yet known.
Do not modify @code{yychar} in a deferred semantic action (@pxref{GLR Semantic
Actions}).
@xref{Look-Ahead, ,Look-Ahead Tokens}.
@end deffn

@deffn {Macro} yyclearin;
Discard the current look-ahead token.  This is useful primarily in
error rules.
Do not invoke @code{yyclearin} in a deferred semantic action (@pxref{GLR
Semantic Actions}).
@xref{Error Recovery}.
@end deffn

@deffn {Macro} yyerrok;
Resume generating error messages immediately for subsequent syntax
errors.  This is useful primarily in error rules.
@xref{Error Recovery}.
@end deffn

@deffn {Variable} yylloc
Variable containing the look-ahead token location when @code{yychar} is not set
to @code{YYEMPTY} or @code{YYEOF}.
Do not modify @code{yylloc} in a deferred semantic action (@pxref{GLR Semantic
Actions}).
@xref{Actions and Locations, ,Actions and Locations}.
@end deffn

@deffn {Variable} yylval
Variable containing the look-ahead token semantic value when @code{yychar} is
not set to @code{YYEMPTY} or @code{YYEOF}.
Do not modify @code{yylval} in a deferred semantic action (@pxref{GLR Semantic
Actions}).
@xref{Actions, ,Actions}.
@end deffn

@deffn {Value} @@$
@findex @@$
Acts like a structure variable containing information on the textual location
of the grouping made by the current rule.  @xref{Locations, ,
Tracking Locations}.

@c Check if those paragraphs are still useful or not.

@c @example
@c struct @{
@c   int first_line, last_line;
@c   int first_column, last_column;
@c @};
@c @end example

@c Thus, to get the starting line number of the third component, you would
@c use @samp{@@3.first_line}.

@c In order for the members of this structure to contain valid information,
@c you must make @code{yylex} supply this information about each token.
@c If you need only certain members, then @code{yylex} need only fill in
@c those members.

@c The use of this feature makes the parser noticeably slower.
@end deffn

@deffn {Value} @@@var{n}
@findex @@@var{n}
Acts like a structure variable containing information on the textual location
of the @var{n}th component of the current rule.  @xref{Locations, ,
Tracking Locations}.
@end deffn

@node Internationalization
@section Parser Internationalization
@cindex internationalization
@cindex i18n
@cindex NLS
@cindex gettext
@cindex bison-po

A Bison-generated parser can print diagnostics, including error and
tracing messages.  By default, they appear in English.  However, Bison
also supports outputting diagnostics in the user's native language.  To
make this work, the user should set the usual environment variables.
@xref{Users, , The User's View, gettext, GNU @code{gettext} utilities}.
For example, the shell command @samp{export LC_ALL=fr_CA.UTF-8} might
set the user's locale to French Canadian using the @acronym{UTF}-8
encoding.  The exact set of available locales depends on the user's
installation.

The maintainer of a package that uses a Bison-generated parser enables
the internationalization of the parser's output through the following
steps.  Here we assume a package that uses @acronym{GNU} Autoconf and
@acronym{GNU} Automake.

@enumerate
@item
@cindex bison-i18n.m4
Into the directory containing the @acronym{GNU} Autoconf macros used
by the package---often called @file{m4}---copy the
@file{bison-i18n.m4} file installed by Bison under
@samp{share/aclocal/bison-i18n.m4} in Bison's installation directory.
For example:

@example
cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4
@end example

@item
@findex BISON_I18N
@vindex BISON_LOCALEDIR
@vindex YYENABLE_NLS
In the top-level @file{configure.ac}, after the @code{AM_GNU_GETTEXT}
invocation, add an invocation of @code{BISON_I18N}.  This macro is
defined in the file @file{bison-i18n.m4} that you copied earlier.  It
causes @samp{configure} to find the value of the
@code{BISON_LOCALEDIR} variable, and it defines the source-language
symbol @code{YYENABLE_NLS} to enable translations in the
Bison-generated parser.

@item
In the @code{main} function of your program, designate the directory
containing Bison's runtime message catalog, through a call to
@samp{bindtextdomain} with domain name @samp{bison-runtime}.
For example:

@example
bindtextdomain ("bison-runtime", BISON_LOCALEDIR);
@end example

Typically this appears after any other call @code{bindtextdomain
(PACKAGE, LOCALEDIR)} that your package already has.  Here we rely on
@samp{BISON_LOCALEDIR} to be defined as a string through the
@file{Makefile}.

@item
In the @file{Makefile.am} that controls the compilation of the @code{main}
function, make @samp{BISON_LOCALEDIR} available as a C preprocessor macro,
either in @samp{DEFS} or in @samp{AM_CPPFLAGS}.  For example:

@example
DEFS = @@DEFS@@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
@end example

or:

@example
AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
@end example

@item
Finally, invoke the command @command{autoreconf} to generate the build
infrastructure.
@end enumerate


@node Algorithm
@chapter The Bison Parser Algorithm
@cindex Bison parser algorithm
@cindex algorithm of parser
@cindex shifting
@cindex reduction
@cindex parser stack
@cindex stack, parser

As Bison reads tokens, it pushes them onto a stack along with their
semantic values.  The stack is called the @dfn{parser stack}.  Pushing a
token is traditionally called @dfn{shifting}.

For example, suppose the infix calculator has read @samp{1 + 5 *}, with a
@samp{3} to come.  The stack will have four elements, one for each token
that was shifted.

But the stack does not always have an element for each token read.  When
the last @var{n} tokens and groupings shifted match the components of a
grammar rule, they can be combined according to that rule.  This is called
@dfn{reduction}.  Those tokens and groupings are replaced on the stack by a
single grouping whose symbol is the result (left hand side) of that rule.
Running the rule's action is part of the process of reduction, because this
is what computes the semantic value of the resulting grouping.

For example, if the infix calculator's parser stack contains this:

@example
1 + 5 * 3
@end example

@noindent
and the next input token is a newline character, then the last three
elements can be reduced to 15 via the rule:

@example
expr: expr '*' expr;
@end example

@noindent
Then the stack contains just these three elements:

@example
1 + 15
@end example

@noindent
At this point, another reduction can be made, resulting in the single value
16.  Then the newline token can be shifted.

The parser tries, by shifts and reductions, to reduce the entire input down
to a single grouping whose symbol is the grammar's start-symbol
(@pxref{Language and Grammar, ,Languages and Context-Free Grammars}).

This kind of parser is known in the literature as a bottom-up parser.

@menu
* Look-Ahead::        Parser looks one token ahead when deciding what to do.
* Shift/Reduce::      Conflicts: when either shifting or reduction is valid.
* Precedence::        Operator precedence works by resolving conflicts.
* Contextual Precedence::  When an operator's precedence depends on context.
* Parser States::     The parser is a finite-state-machine with stack.
* Reduce/Reduce::     When two rules are applicable in the same situation.
* Mystery Conflicts::  Reduce/reduce conflicts that look unjustified.
* Generalized LR Parsing::  Parsing arbitrary context-free grammars.
* Memory Management:: What happens when memory is exhausted.  How to avoid it.
@end menu

@node Look-Ahead
@section Look-Ahead Tokens
@cindex look-ahead token

The Bison parser does @emph{not} always reduce immediately as soon as the
last @var{n} tokens and groupings match a rule.  This is because such a
simple strategy is inadequate to handle most languages.  Instead, when a
reduction is possible, the parser sometimes ``looks ahead'' at the next
token in order to decide what to do.

When a token is read, it is not immediately shifted; first it becomes the
@dfn{look-ahead token}, which is not on the stack.  Now the parser can
perform one or more reductions of tokens and groupings on the stack, while
the look-ahead token remains off to the side.  When no more reductions
should take place, the look-ahead token is shifted onto the stack.  This
does not mean that all possible reductions have been done; depending on the
token type of the look-ahead token, some rules may choose to delay their
application.

Here is a simple case where look-ahead is needed.  These three rules define
expressions which contain binary addition operators and postfix unary
factorial operators (@samp{!}), and allow parentheses for grouping.

@example
@group
expr:     term '+' expr
        | term
        ;
@end group

@group
term:     '(' expr ')'
        | term '!'
        | NUMBER
        ;
@end group
@end example

Suppose that the tokens @w{@samp{1 + 2}} have been read and shifted; what
should be done?  If the following token is @samp{)}, then the first three
tokens must be reduced to form an @code{expr}.  This is the only valid
course, because shifting the @samp{)} would produce a sequence of symbols
@w{@code{term ')'}}, and no rule allows this.

If the following token is @samp{!}, then it must be shifted immediately so
that @w{@samp{2 !}} can be reduced to make a @code{term}.  If instead the
parser were to reduce before shifting, @w{@samp{1 + 2}} would become an
@code{expr}.  It would then be impossible to shift the @samp{!} because
doing so would produce on the stack the sequence of symbols @code{expr
'!'}.  No rule allows that sequence.

@vindex yychar
@vindex yylval
@vindex yylloc
The look-ahead token is stored in the variable @code{yychar}.
Its semantic value and location, if any, are stored in the variables
@code{yylval} and @code{yylloc}.
@xref{Action Features, ,Special Features for Use in Actions}.

@node Shift/Reduce
@section Shift/Reduce Conflicts
@cindex conflicts
@cindex shift/reduce conflicts
@cindex dangling @code{else}
@cindex @code{else}, dangling

Suppose we are parsing a language which has if-then and if-then-else
statements, with a pair of rules like this:

@example
@group
if_stmt:
          IF expr THEN stmt
        | IF expr THEN stmt ELSE stmt
        ;
@end group
@end example

@noindent
Here we assume that @code{IF}, @code{THEN} and @code{ELSE} are
terminal symbols for specific keyword tokens.

When the @code{ELSE} token is read and becomes the look-ahead token, the
contents of the stack (assuming the input is valid) are just right for
reduction by the first rule.  But it is also legitimate to shift the
@code{ELSE}, because that would lead to eventual reduction by the second
rule.

This situation, where either a shift or a reduction would be valid, is
called a @dfn{shift/reduce conflict}.  Bison is designed to resolve
these conflicts by choosing to shift, unless otherwise directed by
operator precedence declarations.  To see the reason for this, let's
contrast it with the other alternative.

Since the parser prefers to shift the @code{ELSE}, the result is to attach
the else-clause to the innermost if-statement, making these two inputs
equivalent:

@example
if x then if y then win (); else lose;

if x then do; if y then win (); else lose; end;
@end example

But if the parser chose to reduce when possible rather than shift, the
result would be to attach the else-clause to the outermost if-statement,
making these two inputs equivalent:

@example
if x then if y then win (); else lose;

if x then do; if y then win (); end; else lose;
@end example

The conflict exists because the grammar as written is ambiguous: either
parsing of the simple nested if-statement is legitimate.  The established
convention is that these ambiguities are resolved by attaching the
else-clause to the innermost if-statement; this is what Bison accomplishes
by choosing to shift rather than reduce.  (It would ideally be cleaner to
write an unambiguous grammar, but that is very hard to do in this case.)
This particular ambiguity was first encountered in the specifications of
Algol 60 and is called the ``dangling @code{else}'' ambiguity.

To avoid warnings from Bison about predictable, legitimate shift/reduce
conflicts, use the @code{%expect @var{n}} declaration.  There will be no
warning as long as the number of shift/reduce conflicts is exactly @var{n}.
@xref{Expect Decl, ,Suppressing Conflict Warnings}.

The definition of @code{if_stmt} above is solely to blame for the
conflict, but the conflict does not actually appear without additional
rules.  Here is a complete Bison input file that actually manifests the
conflict:

@example
@group
%token IF THEN ELSE variable
%%
@end group
@group
stmt:     expr
        | if_stmt
        ;
@end group

@group
if_stmt:
          IF expr THEN stmt
        | IF expr THEN stmt ELSE stmt
        ;
@end group

expr:     variable
        ;
@end example

@node Precedence
@section Operator Precedence
@cindex operator precedence
@cindex precedence of operators

Another situation where shift/reduce conflicts appear is in arithmetic
expressions.  Here shifting is not always the preferred resolution; the
Bison declarations for operator precedence allow you to specify when to
shift and when to reduce.

@menu
* Why Precedence::    An example showing why precedence is needed.
* Using Precedence::  How to specify precedence in Bison grammars.
* Precedence Examples::  How these features are used in the previous example.
* How Precedence::    How they work.
@end menu

@node Why Precedence
@subsection When Precedence is Needed

Consider the following ambiguous grammar fragment (ambiguous because the
input @w{@samp{1 - 2 * 3}} can be parsed in two different ways):

@example
@group
expr:     expr '-' expr
        | expr '*' expr
        | expr '<' expr
        | '(' expr ')'
        @dots{}
        ;
@end group
@end example

@noindent
Suppose the parser has seen the tokens @samp{1}, @samp{-} and @samp{2};
should it reduce them via the rule for the subtraction operator?  It
depends on the next token.  Of course, if the next token is @samp{)}, we
must reduce; shifting is invalid because no single rule can reduce the
token sequence @w{@samp{- 2 )}} or anything starting with that.  But if
the next token is @samp{*} or @samp{<}, we have a choice: either
shifting or reduction would allow the parse to complete, but with
different results.

To decide which one Bison should do, we must consider the results.  If
the next operator token @var{op} is shifted, then it must be reduced
first in order to permit another opportunity to reduce the difference.
The result is (in effect) @w{@samp{1 - (2 @var{op} 3)}}.  On the other
hand, if the subtraction is reduced before shifting @var{op}, the result
is @w{@samp{(1 - 2) @var{op} 3}}.  Clearly, then, the choice of shift or
reduce should depend on the relative precedence of the operators
@samp{-} and @var{op}: @samp{*} should be shifted first, but not
@samp{<}.

@cindex associativity
What about input such as @w{@samp{1 - 2 - 5}}; should this be
@w{@samp{(1 - 2) - 5}} or should it be @w{@samp{1 - (2 - 5)}}?  For most
operators we prefer the former, which is called @dfn{left association}.
The latter alternative, @dfn{right association}, is desirable for
assignment operators.  The choice of left or right association is a
matter of whether the parser chooses to shift or reduce when the stack
contains @w{@samp{1 - 2}} and the look-ahead token is @samp{-}: shifting
makes right-associativity.

@node Using Precedence
@subsection Specifying Operator Precedence
@findex %left
@findex %right
@findex %nonassoc

Bison allows you to specify these choices with the operator precedence
declarations @code{%left} and @code{%right}.  Each such declaration
contains a list of tokens, which are operators whose precedence and
associativity is being declared.  The @code{%left} declaration makes all
those operators left-associative and the @code{%right} declaration makes
them right-associative.  A third alternative is @code{%nonassoc}, which
declares that it is a syntax error to find the same operator twice ``in a
row''.

The relative precedence of different operators is controlled by the
order in which they are declared.  The first @code{%left} or
@code{%right} declaration in the file declares the operators whose
precedence is lowest, the next such declaration declares the operators
whose precedence is a little higher, and so on.

@node Precedence Examples
@subsection Precedence Examples

In our example, we would want the following declarations:

@example
%left '<'
%left '-'
%left '*'
@end example

In a more complete example, which supports other operators as well, we
would declare them in groups of equal precedence.  For example, @code{'+'} is
declared with @code{'-'}:

@example
%left '<' '>' '=' NE LE GE
%left '+' '-'
%left '*' '/'
@end example

@noindent
(Here @code{NE} and so on stand for the operators for ``not equal''
and so on.  We assume that these tokens are more than one character long
and therefore are represented by names, not character literals.)

@node How Precedence
@subsection How Precedence Works

The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared.  The second effect is to assign
precedence levels to certain rules: each rule gets its precedence from
the last terminal symbol mentioned in the components.  (You can also
specify explicitly the precedence of a rule.  @xref{Contextual
Precedence, ,Context-Dependent Precedence}.)

Finally, the resolution of conflicts works by comparing the precedence
of the rule being considered with that of the look-ahead token.  If the
token's precedence is higher, the choice is to shift.  If the rule's
precedence is higher, the choice is to reduce.  If they have equal
precedence, the choice is made based on the associativity of that
precedence level.  The verbose output file made by @samp{-v}
(@pxref{Invocation, ,Invoking Bison}) says how each conflict was
resolved.

Not all rules and not all tokens have precedence.  If either the rule or
the look-ahead token has no precedence, then the default is to shift.

@node Contextual Precedence
@section Context-Dependent Precedence
@cindex context-dependent precedence
@cindex unary operator precedence
@cindex precedence, context-dependent
@cindex precedence, unary operator
@findex %prec

Often the precedence of an operator depends on the context.  This sounds
outlandish at first, but it is really very common.  For example, a minus
sign typically has a very high precedence as a unary operator, and a
somewhat lower precedence (lower than multiplication) as a binary operator.

The Bison precedence declarations, @code{%left}, @code{%right} and
@code{%nonassoc}, can only be used once for a given token; so a token has
only one precedence declared in this way.  For context-dependent
precedence, you need to use an additional mechanism: the @code{%prec}
modifier for rules.

The @code{%prec} modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that rule.
It's not necessary for that symbol to appear otherwise in the rule.  The
modifier's syntax is:

@example
%prec @var{terminal-symbol}
@end example

@noindent
and it is written after the components of the rule.  Its effect is to
assign the rule the precedence of @var{terminal-symbol}, overriding
the precedence that would be deduced for it in the ordinary way.  The
altered rule precedence then affects how conflicts involving that rule
are resolved (@pxref{Precedence, ,Operator Precedence}).

Here is how @code{%prec} solves the problem of unary minus.  First, declare
a precedence for a fictitious terminal symbol named @code{UMINUS}.  There
are no tokens of this type, but the symbol serves to stand for its
precedence:

@example
@dots{}
%left '+' '-'
%left '*'
%left UMINUS
@end example

Now the precedence of @code{UMINUS} can be used in specific rules:

@example
@group
exp:    @dots{}
        | exp '-' exp
        @dots{}
        | '-' exp %prec UMINUS
@end group
@end example

@ifset defaultprec
If you forget to append @code{%prec UMINUS} to the rule for unary
minus, Bison silently assumes that minus has its usual precedence.
This kind of problem can be tricky to debug, since one typically
discovers the mistake only by testing the code.

The @code{%no-default-prec;} declaration makes it easier to discover
this kind of problem systematically.  It causes rules that lack a
@code{%prec} modifier to have no precedence, even if the last terminal
symbol mentioned in their components has a declared precedence.

If @code{%no-default-prec;} is in effect, you must specify @code{%prec}
for all rules that participate in precedence conflict resolution.
Then you will see any shift/reduce conflict until you tell Bison how
to resolve it, either by changing your grammar or by adding an
explicit precedence.  This will probably add declarations to the
grammar, but it helps to protect against incorrect rule precedences.

The effect of @code{%no-default-prec;} can be reversed by giving
@code{%default-prec;}, which is the default.
@end ifset

@node Parser States
@section Parser States
@cindex finite-state machine
@cindex parser state
@cindex state (of parser)

The function @code{yyparse} is implemented using a finite-state machine.
The values pushed on the parser stack are not simply token type codes; they
represent the entire sequence of terminal and nonterminal symbols at or
near the top of the stack.  The current state collects all the information
about previous input which is relevant to deciding what to do next.

Each time a look-ahead token is read, the current parser state together
with the type of look-ahead token are looked up in a table.  This table
entry can say, ``Shift the look-ahead token.''  In this case, it also
specifies the new parser state, which is pushed onto the top of the
parser stack.  Or it can say, ``Reduce using rule number @var{n}.''
This means that a certain number of tokens or groupings are taken off
the top of the stack, and replaced by one grouping.  In other words,
that number of states are popped from the stack, and one new state is
pushed.

There is one other alternative: the table can say that the look-ahead token
is erroneous in the current state.  This causes error processing to begin
(@pxref{Error Recovery}).

@node Reduce/Reduce
@section Reduce/Reduce Conflicts
@cindex reduce/reduce conflict
@cindex conflicts, reduce/reduce

A reduce/reduce conflict occurs if there are two or more rules that apply
to the same sequence of input.  This usually indicates a serious error
in the grammar.

For example, here is an erroneous attempt to define a sequence
of zero or more @code{word} groupings.

@example
sequence: /* empty */
                @{ printf ("empty sequence\n"); @}
        | maybeword
        | sequence word
                @{ printf ("added word %s\n", $2); @}
        ;

maybeword: /* empty */
                @{ printf ("empty maybeword\n"); @}
        | word
                @{ printf ("single word %s\n", $1); @}
        ;
@end example

@noindent
The error is an ambiguity: there is more than one way to parse a single
@code{word} into a @code{sequence}.  It could be reduced to a
@code{maybeword} and then into a @code{sequence} via the second rule.
Alternatively, nothing-at-all could be reduced into a @code{sequence}
via the first rule, and this could be combined with the @code{word}
using the third rule for @code{sequence}.

There is also more than one way to reduce nothing-at-all into a
@code{sequence}.  This can be done directly via the first rule,
or indirectly via @code{maybeword} and then the second rule.

You might think that this is a distinction without a difference, because it
does not change whether any particular input is valid or not.  But it does
affect which actions are run.  One parsing order runs the second rule's
action; the other runs the first rule's action and the third rule's action.
In this example, the output of the program changes.

Bison resolves a reduce/reduce conflict by choosing to use the rule that
appears first in the grammar, but it is very risky to rely on this.  Every
reduce/reduce conflict must be studied and usually eliminated.  Here is the
proper way to define @code{sequence}:

@example
sequence: /* empty */
                @{ printf ("empty sequence\n"); @}
        | sequence word
                @{ printf ("added word %s\n", $2); @}
        ;
@end example

Here is another common error that yields a reduce/reduce conflict:

@example
sequence: /* empty */
        | sequence words
        | sequence redirects
        ;

words:    /* empty */
        | words word
        ;

redirects:/* empty */
        | redirects redirect
        ;
@end example

@noindent
The intention here is to define a sequence which can contain either
@code{word} or @code{redirect} groupings.  The individual definitions of
@code{sequence}, @code{words} and @code{redirects} are error-free, but the
three together make a subtle ambiguity: even an empty input can be parsed
in infinitely many ways!

Consider: nothing-at-all could be a @code{words}.  Or it could be two
@code{words} in a row, or three, or any number.  It could equally well be a
@code{redirects}, or two, or any number.  Or it could be a @code{words}
followed by three @code{redirects} and another @code{words}.  And so on.

Here are two ways to correct these rules.  First, to make it a single level
of sequence:

@example
sequence: /* empty */
        | sequence word
        | sequence redirect
        ;
@end example

Second, to prevent either a @code{words} or a @code{redirects}
from being empty:

@example
sequence: /* empty */
        | sequence words
        | sequence redirects
        ;

words:    word
        | words word
        ;

redirects:redirect
        | redirects redirect
        ;
@end example

@node Mystery Conflicts
@section Mysterious Reduce/Reduce Conflicts

Sometimes reduce/reduce conflicts can occur that don't look warranted.
Here is an example:

@example
@group
%token ID

%%
def:    param_spec return_spec ','
        ;
param_spec:
             type
        |    name_list ':' type
        ;
@end group
@group
return_spec:
             type
        |    name ':' type
        ;
@end group
@group
type:        ID
        ;
@end group
@group
name:        ID
        ;
name_list:
             name
        |    name ',' name_list
        ;
@end group
@end example

It would seem that this grammar can be parsed with only a single token
of look-ahead: when a @code{param_spec} is being read, an @code{ID} is
a @code{name} if a comma or colon follows, or a @code{type} if another
@code{ID} follows.  In other words, this grammar is @acronym{LR}(1).

@cindex @acronym{LR}(1)
@cindex @acronym{LALR}(1)
However, Bison, like most parser generators, cannot actually handle all
@acronym{LR}(1) grammars.  In this grammar, two contexts, that after
an @code{ID}
at the beginning of a @code{param_spec} and likewise at the beginning of
a @code{return_spec}, are similar enough that Bison assumes they are the
same.  They appear similar because the same set of rules would be
active---the rule for reducing to a @code{name} and that for reducing to
a @code{type}.  Bison is unable to determine at that stage of processing
that the rules would require different look-ahead tokens in the two
contexts, so it makes a single parser state for them both.  Combining
the two contexts causes a conflict later.  In parser terminology, this
occurrence means that the grammar is not @acronym{LALR}(1).

In general, it is better to fix deficiencies than to document them.  But
this particular deficiency is intrinsically hard to fix; parser
generators that can handle @acronym{LR}(1) grammars are hard to write
and tend to
produce parsers that are very large.  In practice, Bison is more useful
as it is now.

When the problem arises, you can often fix it by identifying the two
parser states that are being confused, and adding something to make them
look distinct.  In the above example, adding one rule to
@code{return_spec} as follows makes the problem go away:

@example
@group
%token BOGUS
@dots{}
%%
@dots{}
return_spec:
             type
        |    name ':' type
        /* This rule is never used.  */
        |    ID BOGUS
        ;
@end group
@end example

This corrects the problem because it introduces the possibility of an
additional active rule in the context after the @code{ID} at the beginning of
@code{return_spec}.  This rule is not active in the corresponding context
in a @code{param_spec}, so the two contexts receive distinct parser states.
As long as the token @code{BOGUS} is never generated by @code{yylex},
the added rule cannot alter the way actual input is parsed.

In this particular example, there is another way to solve the problem:
rewrite the rule for @code{return_spec} to use @code{ID} directly
instead of via @code{name}.  This also causes the two confusing
contexts to have different sets of active rules, because the one for
@code{return_spec} activates the altered rule for @code{return_spec}
rather than the one for @code{name}.

@example
param_spec:
             type
        |    name_list ':' type
        ;
return_spec:
             type
        |    ID ':' type
        ;
@end example

For a more detailed exposition of @acronym{LALR}(1) parsers and parser
generators, please see:
Frank DeRemer and Thomas Pennello, Efficient Computation of
@acronym{LALR}(1) Look-Ahead Sets, @cite{@acronym{ACM} Transactions on
Programming Languages and Systems}, Vol.@: 4, No.@: 4 (October 1982),
pp.@: 615--649 @uref{http://doi.acm.org/10.1145/69622.357187}.

@node Generalized LR Parsing
@section Generalized @acronym{LR} (@acronym{GLR}) Parsing
@cindex @acronym{GLR} parsing
@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
@cindex ambiguous grammars
@cindex nondeterministic parsing

Bison produces @emph{deterministic} parsers that choose uniquely
when to reduce and which reduction to apply
based on a summary of the preceding input and on one extra token of look-ahead.
As a result, normal Bison handles a proper subset of the family of
context-free languages.
Ambiguous grammars, since they have strings with more than one possible
sequence of reductions cannot have deterministic parsers in this sense.
The same is true of languages that require more than one symbol of
look-ahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
Finally, as previously mentioned (@pxref{Mystery Conflicts}),
there are languages where Bison's particular choice of how to
summarize the input seen so far loses necessary information.

When you use the @samp{%glr-parser} declaration in your grammar file,
Bison generates a parser that uses a different algorithm, called
Generalized @acronym{LR} (or @acronym{GLR}).  A Bison @acronym{GLR}
parser uses the same basic
algorithm for parsing as an ordinary Bison parser, but behaves
differently in cases where there is a shift-reduce conflict that has not
been resolved by precedence rules (@pxref{Precedence}) or a
reduce-reduce conflict.  When a @acronym{GLR} parser encounters such a
situation, it
effectively @emph{splits} into a several parsers, one for each possible
shift or reduction.  These parsers then proceed as usual, consuming
tokens in lock-step.  Some of the stacks may encounter other conflicts
and split further, with the result that instead of a sequence of states,
a Bison @acronym{GLR} parsing stack is what is in effect a tree of states.

In effect, each stack represents a guess as to what the proper parse
is.  Additional input may indicate that a guess was wrong, in which case
the appropriate stack silently disappears.  Otherwise, the semantics
actions generated in each stack are saved, rather than being executed
immediately.  When a stack disappears, its saved semantic actions never
get executed.  When a reduction causes two stacks to become equivalent,
their sets of semantic actions are both saved with the state that
results from the reduction.  We say that two stacks are equivalent
when they both represent the same sequence of states,
and each pair of corresponding states represents a
grammar symbol that produces the same segment of the input token
stream.

Whenever the parser makes a transition from having multiple
states to having one, it reverts to the normal @acronym{LALR}(1) parsing
algorithm, after resolving and executing the saved-up actions.
At this transition, some of the states on the stack will have semantic
values that are sets (actually multisets) of possible actions.  The
parser tries to pick one of the actions by first finding one whose rule
has the highest dynamic precedence, as set by the @samp{%dprec}
declaration.  Otherwise, if the alternative actions are not ordered by
precedence, but there the same merging function is declared for both
rules by the @samp{%merge} declaration,
Bison resolves and evaluates both and then calls the merge function on
the result.  Otherwise, it reports an ambiguity.

It is possible to use a data structure for the @acronym{GLR} parsing tree that
permits the processing of any @acronym{LALR}(1) grammar in linear time (in the
size of the input), any unambiguous (not necessarily
@acronym{LALR}(1)) grammar in
quadratic worst-case time, and any general (possibly ambiguous)
context-free grammar in cubic worst-case time.  However, Bison currently
uses a simpler data structure that requires time proportional to the
length of the input times the maximum number of stacks required for any
prefix of the input.  Thus, really ambiguous or nondeterministic
grammars can require exponential time and space to process.  Such badly
behaving examples, however, are not generally of practical interest.
Usually, nondeterminism in a grammar is local---the parser is ``in
doubt'' only for a few tokens at a time.  Therefore, the current data
structure should generally be adequate.  On @acronym{LALR}(1) portions of a
grammar, in particular, it is only slightly slower than with the default
Bison parser.

For a more detailed exposition of @acronym{GLR} parsers, please see: Elizabeth
Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style
Generalised @acronym{LR} Parsers, Royal Holloway, University of
London, Department of Computer Science, TR-00-12,
@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps},
(2000-12-24).

@node Memory Management
@section Memory Management, and How to Avoid Memory Exhaustion
@cindex memory exhaustion
@cindex memory management
@cindex stack overflow
@cindex parser stack overflow
@cindex overflow of parser stack

The Bison parser stack can run out of memory if too many tokens are shifted and
not reduced.  When this happens, the parser function @code{yyparse}
calls @code{yyerror} and then returns 2.

Because Bison parsers have growing stacks, hitting the upper limit
usually results from using a right recursion instead of a left
recursion, @xref{Recursion, ,Recursive Rules}.

@vindex YYMAXDEPTH
By defining the macro @code{YYMAXDEPTH}, you can control how deep the
parser stack can become before memory is exhausted.  Define the
macro with a value that is an integer.  This value is the maximum number
of tokens that can be shifted (and not reduced) before overflow.

The stack space allowed is not necessarily allocated.  If you specify a
large value for @code{YYMAXDEPTH}, the parser normally allocates a small
stack at first, and then makes it bigger by stages as needed.  This
increasing allocation happens automatically and silently.  Therefore,
you do not need to make @code{YYMAXDEPTH} painfully small merely to save
space for ordinary inputs that do not need much stack.

However, do not allow @code{YYMAXDEPTH} to be a value so large that
arithmetic overflow could occur when calculating the size of the stack
space.  Also, do not allow @code{YYMAXDEPTH} to be less than
@code{YYINITDEPTH}.

@cindex default stack limit
The default value of @code{YYMAXDEPTH}, if you do not define it, is
10000.

@vindex YYINITDEPTH
You can control how much stack is allocated initially by defining the
macro @code{YYINITDEPTH} to a positive integer.  For the C
@acronym{LALR}(1) parser, this value must be a compile-time constant
unless you are assuming C99 or some other target language or compiler
that allows variable-length arrays.  The default is 200.

Do not allow @code{YYINITDEPTH} to be greater than @code{YYMAXDEPTH}.

@c FIXME: C++ output.
Because of semantical differences between C and C++, the
@acronym{LALR}(1) parsers in C produced by Bison cannot grow when compiled
by C++ compilers.  In this precise case (compiling a C parser as C++) you are
suggested to grow @code{YYINITDEPTH}.  The Bison maintainers hope to fix
this deficiency in a future release.

@node Error Recovery
@chapter Error Recovery
@cindex error recovery
@cindex recovery from errors

It is not usually acceptable to have a program terminate on a syntax
error.  For example, a compiler should recover sufficiently to parse the
rest of the input file and check it for errors; a calculator should accept
another expression.

In a simple interactive command parser where each input is one line, it may
be sufficient to allow @code{yyparse} to return 1 on error and have the
caller ignore the rest of the input line when that happens (and then call
@code{yyparse} again).  But this is inadequate for a compiler, because it
forgets all the syntactic context leading up to the error.  A syntax error
deep within a function in the compiler input should not cause the compiler
to treat the following line like the beginning of a source file.

@findex error
You can define how to recover from a syntax error by writing rules to
recognize the special token @code{error}.  This is a terminal symbol that
is always defined (you need not declare it) and reserved for error
handling.  The Bison parser generates an @code{error} token whenever a
syntax error happens; if you have provided a rule to recognize this token
in the current context, the parse can continue.

For example:

@example
stmnts:  /* empty string */
        | stmnts '\n'
        | stmnts exp '\n'
        | stmnts error '\n'
@end example

The fourth rule in this example says that an error followed by a newline
makes a valid addition to any @code{stmnts}.

What happens if a syntax error occurs in the middle of an @code{exp}?  The
error recovery rule, interpreted strictly, applies to the precise sequence
of a @code{stmnts}, an @code{error} and a newline.  If an error occurs in
the middle of an @code{exp}, there will probably be some additional tokens
and subexpressions on the stack after the last @code{stmnts}, and there
will be tokens to read before the next newline.  So the rule is not
applicable in the ordinary way.

But Bison can force the situation to fit the rule, by discarding part of
the semantic context and part of the input.  First it discards states
and objects from the stack until it gets back to a state in which the
@code{error} token is acceptable.  (This means that the subexpressions
already parsed are discarded, back to the last complete @code{stmnts}.)
At this point the @code{error} token can be shifted.  Then, if the old
look-ahead token is not acceptable to be shifted next, the parser reads
tokens and discards them until it finds a token which is acceptable.  In
this example, Bison reads and discards input until the next newline so
that the fourth rule can apply.  Note that discarded symbols are
possible sources of memory leaks, see @ref{Destructor Decl, , Freeing
Discarded Symbols}, for a means to reclaim this memory.

The choice of error rules in the grammar is a choice of strategies for
error recovery.  A simple and useful strategy is simply to skip the rest of
the current input line or current statement if an error is detected:

@example
stmnt: error ';'  /* On error, skip until ';' is read.  */
@end example

It is also useful to recover to the matching close-delimiter of an
opening-delimiter that has already been parsed.  Otherwise the
close-delimiter will probably appear to be unmatched, and generate another,
spurious error message:

@example
primary:  '(' expr ')'
        | '(' error ')'
        @dots{}
        ;
@end example

Error recovery strategies are necessarily guesses.  When they guess wrong,
one syntax error often leads to another.  In the above example, the error
recovery rule guesses that an error is due to bad input within one
@code{stmnt}.  Suppose that instead a spurious semicolon is inserted in the
middle of a valid @code{stmnt}.  After the error recovery rule recovers
from the first error, another syntax error will be found straightaway,
since the text following the spurious semicolon is also an invalid
@code{stmnt}.

To prevent an outpouring of error messages, the parser will output no error
message for another syntax error that happens shortly after the first; only
after three consecutive input tokens have been successfully shifted will
error messages resume.

Note that rules which accept the @code{error} token may have actions, just
as any other rules can.

@findex yyerrok
You can make error messages resume immediately by using the macro
@code{yyerrok} in an action.  If you do this in the error rule's action, no
error messages will be suppressed.  This macro requires no arguments;
@samp{yyerrok;} is a valid C statement.

@findex yyclearin
The previous look-ahead token is reanalyzed immediately after an error.  If
this is unacceptable, then the macro @code{yyclearin} may be used to clear
this token.  Write the statement @samp{yyclearin;} in the error rule's
action.
@xref{Action Features, ,Special Features for Use in Actions}.

For example, suppose that on a syntax error, an error handling routine is
called that advances the input stream to some point where parsing should
once again commence.  The next symbol returned by the lexical scanner is
probably correct.  The previous look-ahead token ought to be discarded
with @samp{yyclearin;}.

@vindex YYRECOVERING
The macro @code{YYRECOVERING} stands for an expression that has the
value 1 when the parser is recovering from a syntax error, and 0 the
rest of the time.  A value of 1 indicates that error messages are
currently suppressed for new syntax errors.

@node Context Dependency
@chapter Handling Context Dependencies

The Bison paradigm is to parse tokens first, then group them into larger
syntactic units.  In many languages, the meaning of a token is affected by
its context.  Although this violates the Bison paradigm, certain techniques
(known as @dfn{kludges}) may enable you to write Bison parsers for such
languages.

@menu
* Semantic Tokens::   Token parsing can depend on the semantic context.
* Lexical Tie-ins::   Token parsing can depend on the syntactic context.
* Tie-in Recovery::   Lexical tie-ins have implications for how
                        error recovery rules must be written.
@end menu

(Actually, ``kludge'' means any technique that gets its job done but is
neither clean nor robust.)

@node Semantic Tokens
@section Semantic Info in Token Types

The C language has a context dependency: the way an identifier is used
depends on what its current meaning is.  For example, consider this:

@example
foo (x);
@end example

This looks like a function call statement, but if @code{foo} is a typedef
name, then this is actually a declaration of @code{x}.  How can a Bison
parser for C decide how to parse this input?

The method used in @acronym{GNU} C is to have two different token types,
@code{IDENTIFIER} and @code{TYPENAME}.  When @code{yylex} finds an
identifier, it looks up the current declaration of the identifier in order
to decide which token type to return: @code{TYPENAME} if the identifier is
declared as a typedef, @code{IDENTIFIER} otherwise.

The grammar rules can then express the context dependency by the choice of
token type to recognize.  @code{IDENTIFIER} is accepted as an expression,
but @code{TYPENAME} is not.  @code{TYPENAME} can start a declaration, but
@code{IDENTIFIER} cannot.  In contexts where the meaning of the identifier
is @emph{not} significant, such as in declarations that can shadow a
typedef name, either @code{TYPENAME} or @code{IDENTIFIER} is
accepted---there is one rule for each of the two token types.

This technique is simple to use if the decision of which kinds of
identifiers to allow is made at a place close to where the identifier is
parsed.  But in C this is not always so: C allows a declaration to
redeclare a typedef name provided an explicit type has been specified
earlier:

@example
typedef int foo, bar;
int baz (void)
@{
  static bar (bar);      /* @r{redeclare @code{bar} as static variable} */
  extern foo foo (foo);  /* @r{redeclare @code{foo} as function} */
  return foo (bar);
@}
@end example

Unfortunately, the name being declared is separated from the declaration
construct itself by a complicated syntactic structure---the ``declarator''.

As a result, part of the Bison parser for C needs to be duplicated, with
all the nonterminal names changed: once for parsing a declaration in
which a typedef name can be redefined, and once for parsing a
declaration in which that can't be done.  Here is a part of the
duplication, with actions omitted for brevity:

@example
initdcl:
          declarator maybeasm '='
          init
        | declarator maybeasm
        ;

notype_initdcl:
          notype_declarator maybeasm '='
          init
        | notype_declarator maybeasm
        ;
@end example

@noindent
Here @code{initdcl} can redeclare a typedef name, but @code{notype_initdcl}
cannot.  The distinction between @code{declarator} and
@code{notype_declarator} is the same sort of thing.

There is some similarity between this technique and a lexical tie-in
(described next), in that information which alters the lexical analysis is
changed during parsing by other parts of the program.  The difference is
here the information is global, and is used for other purposes in the
program.  A true lexical tie-in has a special-purpose flag controlled by
the syntactic context.

@node Lexical Tie-ins
@section Lexical Tie-ins
@cindex lexical tie-in

One way to handle context-dependency is the @dfn{lexical tie-in}: a flag
which is set by Bison actions, whose purpose is to alter the way tokens are
parsed.

For example, suppose we have a language vaguely like C, but with a special
construct @samp{hex (@var{hex-expr})}.  After the keyword @code{hex} comes
an expression in parentheses in which all integers are hexadecimal.  In
particular, the token @samp{a1b} must be treated as an integer rather than
as an identifier if it appears in that context.  Here is how you can do it:

@example
@group
%@{
  int hexflag;
  int yylex (void);
  void yyerror (char const *);
%@}
%%
@dots{}
@end group
@group
expr:   IDENTIFIER
        | constant
        | HEX '('
                @{ hexflag = 1; @}
          expr ')'
                @{ hexflag = 0;
                   $$ = $4; @}
        | expr '+' expr
                @{ $$ = make_sum ($1, $3); @}
        @dots{}
        ;
@end group

@group
constant:
          INTEGER
        | STRING
        ;
@end group
@end example

@noindent
Here we assume that @code{yylex} looks at the value of @code{hexflag}; when
it is nonzero, all integers are parsed in hexadecimal, and tokens starting
with letters are parsed as integers if possible.

The declaration of @code{hexflag} shown in the prologue of the parser file
is needed to make it accessible to the actions (@pxref{Prologue, ,The Prologue}).
You must also write the code in @code{yylex} to obey the flag.

@node Tie-in Recovery
@section Lexical Tie-ins and Error Recovery

Lexical tie-ins make strict demands on any error recovery rules you have.
@xref{Error Recovery}.

The reason for this is that the purpose of an error recovery rule is to
abort the parsing of one construct and resume in some larger construct.
For example, in C-like languages, a typical error recovery rule is to skip
tokens until the next semicolon, and then start a new statement, like this:

@example
stmt:   expr ';'
        | IF '(' expr ')' stmt @{ @dots{} @}
        @dots{}
        error ';'
                @{ hexflag = 0; @}
        ;
@end example

If there is a syntax error in the middle of a @samp{hex (@var{expr})}
construct, this error rule will apply, and then the action for the
completed @samp{hex (@var{expr})} will never run.  So @code{hexflag} would
remain set for the entire rest of the input, or until the next @code{hex}
keyword, causing identifiers to be misinterpreted as integers.

To avoid this problem the error recovery rule itself clears @code{hexflag}.

There may also be an error recovery rule that works within expressions.
For example, there could be a rule which applies within parentheses
and skips to the close-parenthesis:

@example
@group
expr:   @dots{}
        | '(' expr ')'
                @{ $$ = $2; @}
        | '(' error ')'
        @dots{}
@end group
@end example

If this rule acts within the @code{hex} construct, it is not going to abort
that construct (since it applies to an inner level of parentheses within
the construct).  Therefore, it should not clear the flag: the rest of
the @code{hex} construct should be parsed with the flag still in effect.

What if there is an error recovery rule which might abort out of the
@code{hex} construct or might not, depending on circumstances?  There is no
way you can write the action to determine whether a @code{hex} construct is
being aborted or not.  So if you are using a lexical tie-in, you had better
make sure your error recovery rules are not of this kind.  Each rule must
be such that you can be sure that it always will, or always won't, have to
clear the flag.

@c ================================================== Debugging Your Parser

@node Debugging
@chapter Debugging Your Parser

Developing a parser can be a challenge, especially if you don't
understand the algorithm (@pxref{Algorithm, ,The Bison Parser
Algorithm}).  Even so, sometimes a detailed description of the automaton
can help (@pxref{Understanding, , Understanding Your Parser}), or
tracing the execution of the parser can give some insight on why it
behaves improperly (@pxref{Tracing, , Tracing Your Parser}).

@menu
* Understanding::     Understanding the structure of your parser.
* Tracing::           Tracing the execution of your parser.
@end menu

@node Understanding
@section Understanding Your Parser

As documented elsewhere (@pxref{Algorithm, ,The Bison Parser Algorithm})
Bison parsers are @dfn{shift/reduce automata}.  In some cases (much more
frequent than one would hope), looking at this automaton is required to
tune or simply fix a parser.  Bison provides two different
representation of it, either textually or graphically (as a @acronym{VCG}
file).

The textual file is generated when the options @option{--report} or
@option{--verbose} are specified, see @xref{Invocation, , Invoking
Bison}.  Its name is made by removing @samp{.tab.c} or @samp{.c} from
the parser output file name, and adding @samp{.output} instead.
Therefore, if the input file is @file{foo.y}, then the parser file is
called @file{foo.tab.c} by default.  As a consequence, the verbose
output file is called @file{foo.output}.

The following grammar file, @file{calc.y}, will be used in the sequel:

@example
%token NUM STR
%left '+' '-'
%left '*'
%%
exp: exp '+' exp
   | exp '-' exp
   | exp '*' exp
   | exp '/' exp
   | NUM
   ;
useless: STR;
%%
@end example

@command{bison} reports:

@example
calc.y: warning: 1 useless nonterminal and 1 useless rule
calc.y:11.1-7: warning: useless nonterminal: useless
calc.y:11.10-12: warning: useless rule: useless: STR
calc.y: conflicts: 7 shift/reduce
@end example

When given @option{--report=state}, in addition to @file{calc.tab.c}, it
creates a file @file{calc.output} with contents detailed below.  The
order of the output and the exact presentation might vary, but the
interpretation is the same.

The first section includes details on conflicts that were solved thanks
to precedence and/or associativity:

@example
Conflict in state 8 between rule 2 and token '+' resolved as reduce.
Conflict in state 8 between rule 2 and token '-' resolved as reduce.
Conflict in state 8 between rule 2 and token '*' resolved as shift.
@exdent @dots{}
@end example

@noindent
The next section lists states that still have conflicts.

@example
State 8 conflicts: 1 shift/reduce
State 9 conflicts: 1 shift/reduce
State 10 conflicts: 1 shift/reduce
State 11 conflicts: 4 shift/reduce
@end example

@noindent
@cindex token, useless
@cindex useless token
@cindex nonterminal, useless
@cindex useless nonterminal
@cindex rule, useless
@cindex useless rule
The next section reports useless tokens, nonterminal and rules.  Useless
nonterminals and rules are removed in order to produce a smaller parser,
but useless tokens are preserved, since they might be used by the
scanner (note the difference between ``useless'' and ``not used''
below):

@example
Useless nonterminals:
   useless

Terminals which are not used:
   STR

Useless rules:
#6     useless: STR;
@end example

@noindent
The next section reproduces the exact grammar that Bison used:

@example
Grammar

  Number, Line, Rule
    0   5 $accept -> exp $end
    1   5 exp -> exp '+' exp
    2   6 exp -> exp '-' exp
    3   7 exp -> exp '*' exp
    4   8 exp -> exp '/' exp
    5   9 exp -> NUM
@end example

@noindent
and reports the uses of the symbols:

@example
Terminals, with rules where they appear

$end (0) 0
'*' (42) 3
'+' (43) 1
'-' (45) 2
'/' (47) 4
error (256)
NUM (258) 5

Nonterminals, with rules where they appear

$accept (8)
    on left: 0
exp (9)
    on left: 1 2 3 4 5, on right: 0 1 2 3 4
@end example

@noindent
@cindex item
@cindex pointed rule
@cindex rule, pointed
Bison then proceeds onto the automaton itself, describing each state
with it set of @dfn{items}, also known as @dfn{pointed rules}.  Each
item is a production rule together with a point (marked by @samp{.})
that the input cursor.

@example
state 0

    $accept  ->  . exp $   (rule 0)

    NUM         shift, and go to state 1

    exp         go to state 2
@end example

This reads as follows: ``state 0 corresponds to being at the very
beginning of the parsing, in the initial rule, right before the start
symbol (here, @code{exp}).  When the parser returns to this state right
after having reduced a rule that produced an @code{exp}, the control
flow jumps to state 2.  If there is no such transition on a nonterminal
symbol, and the look-ahead is a @code{NUM}, then this token is shifted on
the parse stack, and the control flow jumps to state 1.  Any other
look-ahead triggers a syntax error.''

@cindex core, item set
@cindex item set core
@cindex kernel, item set
@cindex item set core
Even though the only active rule in state 0 seems to be rule 0, the
report lists @code{NUM} as a look-ahead token because @code{NUM} can be
at the beginning of any rule deriving an @code{exp}.  By default Bison
reports the so-called @dfn{core} or @dfn{kernel} of the item set, but if
you want to see more detail you can invoke @command{bison} with
@option{--report=itemset} to list all the items, include those that can
be derived:

@example
state 0

    $accept  ->  . exp $   (rule 0)
    exp  ->  . exp '+' exp   (rule 1)
    exp  ->  . exp '-' exp   (rule 2)
    exp  ->  . exp '*' exp   (rule 3)
    exp  ->  . exp '/' exp   (rule 4)
    exp  ->  . NUM   (rule 5)

    NUM         shift, and go to state 1

    exp         go to state 2
@end example

@noindent
In the state 1...

@example
state 1

    exp  ->  NUM .   (rule 5)

    $default    reduce using rule 5 (exp)
@end example

@noindent
the rule 5, @samp{exp: NUM;}, is completed.  Whatever the look-ahead token
(@samp{$default}), the parser will reduce it.  If it was coming from
state 0, then, after this reduction it will return to state 0, and will
jump to state 2 (@samp{exp: go to state 2}).

@example
state 2

    $accept  ->  exp . $   (rule 0)
    exp  ->  exp . '+' exp   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)

    $           shift, and go to state 3
    '+'         shift, and go to state 4
    '-'         shift, and go to state 5
    '*'         shift, and go to state 6
    '/'         shift, and go to state 7
@end example

@noindent
In state 2, the automaton can only shift a symbol.  For instance,
because of the item @samp{exp -> exp . '+' exp}, if the look-ahead if
@samp{+}, it will be shifted on the parse stack, and the automaton
control will jump to state 4, corresponding to the item @samp{exp -> exp
'+' . exp}.  Since there is no default action, any other token than
those listed above will trigger a syntax error.

The state 3 is named the @dfn{final state}, or the @dfn{accepting
state}:

@example
state 3

    $accept  ->  exp $ .   (rule 0)

    $default    accept
@end example

@noindent
the initial rule is completed (the start symbol and the end
of input were read), the parsing exits successfully.

The interpretation of states 4 to 7 is straightforward, and is left to
the reader.

@example
state 4

    exp  ->  exp '+' . exp   (rule 1)

    NUM         shift, and go to state 1

    exp         go to state 8

state 5

    exp  ->  exp '-' . exp   (rule 2)

    NUM         shift, and go to state 1

    exp         go to state 9

state 6

    exp  ->  exp '*' . exp   (rule 3)

    NUM         shift, and go to state 1

    exp         go to state 10

state 7

    exp  ->  exp '/' . exp   (rule 4)

    NUM         shift, and go to state 1

    exp         go to state 11
@end example

As was announced in beginning of the report, @samp{State 8 conflicts:
1 shift/reduce}:

@example
state 8

    exp  ->  exp . '+' exp   (rule 1)
    exp  ->  exp '+' exp .   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)

    '*'         shift, and go to state 6
    '/'         shift, and go to state 7

    '/'         [reduce using rule 1 (exp)]
    $default    reduce using rule 1 (exp)
@end example

Indeed, there are two actions associated to the look-ahead @samp{/}:
either shifting (and going to state 7), or reducing rule 1.  The
conflict means that either the grammar is ambiguous, or the parser lacks
information to make the right decision.  Indeed the grammar is
ambiguous, as, since we did not specify the precedence of @samp{/}, the
sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM /
NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) /
NUM}, which corresponds to reducing rule 1.

Because in @acronym{LALR}(1) parsing a single decision can be made, Bison
arbitrarily chose to disable the reduction, see @ref{Shift/Reduce, ,
Shift/Reduce Conflicts}.  Discarded actions are reported in between
square brackets.

Note that all the previous states had a single possible action: either
shifting the next token and going to the corresponding state, or
reducing a single rule.  In the other cases, i.e., when shifting
@emph{and} reducing is possible or when @emph{several} reductions are
possible, the look-ahead is required to select the action.  State 8 is
one such state: if the look-ahead is @samp{*} or @samp{/} then the action
is shifting, otherwise the action is reducing rule 1.  In other words,
the first two items, corresponding to rule 1, are not eligible when the
look-ahead token is @samp{*}, since we specified that @samp{*} has higher
precedence than @samp{+}.  More generally, some items are eligible only
with some set of possible look-ahead tokens.  When run with
@option{--report=look-ahead}, Bison specifies these look-ahead tokens:

@example
state 8

    exp  ->  exp . '+' exp  [$, '+', '-', '/']   (rule 1)
    exp  ->  exp '+' exp .  [$, '+', '-', '/']   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)

    '*'         shift, and go to state 6
    '/'         shift, and go to state 7

    '/'         [reduce using rule 1 (exp)]
    $default    reduce using rule 1 (exp)
@end example

The remaining states are similar:

@example
state 9

    exp  ->  exp . '+' exp   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp '-' exp .   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)

    '*'         shift, and go to state 6
    '/'         shift, and go to state 7

    '/'         [reduce using rule 2 (exp)]
    $default    reduce using rule 2 (exp)

state 10

    exp  ->  exp . '+' exp   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp '*' exp .   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)

    '/'         shift, and go to state 7

    '/'         [reduce using rule 3 (exp)]
    $default    reduce using rule 3 (exp)

state 11

    exp  ->  exp . '+' exp   (rule 1)
    exp  ->  exp . '-' exp   (rule 2)
    exp  ->  exp . '*' exp   (rule 3)
    exp  ->  exp . '/' exp   (rule 4)
    exp  ->  exp '/' exp .   (rule 4)

    '+'         shift, and go to state 4
    '-'         shift, and go to state 5
    '*'         shift, and go to state 6
    '/'         shift, and go to state 7

    '+'         [reduce using rule 4 (exp)]
    '-'         [reduce using rule 4 (exp)]
    '*'         [reduce using rule 4 (exp)]
    '/'         [reduce using rule 4 (exp)]
    $default    reduce using rule 4 (exp)
@end example

@noindent
Observe that state 11 contains conflicts not only due to the lack of
precedence of @samp{/} with respect to @samp{+}, @samp{-}, and
@samp{*}, but also because the
associativity of @samp{/} is not specified.


@node Tracing
@section Tracing Your Parser
@findex yydebug
@cindex debugging
@cindex tracing the parser

If a Bison grammar compiles properly but doesn't do what you want when it
runs, the @code{yydebug} parser-trace feature can help you figure out why.

There are several means to enable compilation of trace facilities:

@table @asis
@item the macro @code{YYDEBUG}
@findex YYDEBUG
Define the macro @code{YYDEBUG} to a nonzero value when you compile the
parser.  This is compliant with @acronym{POSIX} Yacc.  You could use
@samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define
YYDEBUG 1} in the prologue of the grammar file (@pxref{Prologue, , The
Prologue}).

@item the option @option{-t}, @option{--debug}
Use the @samp{-t} option when you run Bison (@pxref{Invocation,
,Invoking Bison}).  This is @acronym{POSIX} compliant too.

@item the directive @samp{%debug}
@findex %debug
Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison
Declaration Summary}).  This is a Bison extension, which will prove
useful when Bison will output parsers for languages that don't use a
preprocessor.  Unless @acronym{POSIX} and Yacc portability matter to
you, this is
the preferred solution.
@end table

We suggest that you always enable the debug option so that debugging is
always possible.

The trace facility outputs messages with macro calls of the form
@code{YYFPRINTF (stderr, @var{format}, @var{args})} where
@var{format} and @var{args} are the usual @code{printf} format and
arguments.  If you define @code{YYDEBUG} to a nonzero value but do not
define @code{YYFPRINTF}, @code{<stdio.h>} is automatically included
and @code{YYPRINTF} is defined to @code{fprintf}.

Once you have compiled the program with trace facilities, the way to
request a trace is to store a nonzero value in the variable @code{yydebug}.
You can do this by making the C code do it (in @code{main}, perhaps), or
you can alter the value with a C debugger.

Each step taken by the parser when @code{yydebug} is nonzero produces a
line or two of trace information, written on @code{stderr}.  The trace
messages tell you these things:

@itemize @bullet
@item
Each time the parser calls @code{yylex}, what kind of token was read.

@item
Each time a token is shifted, the depth and complete contents of the
state stack (@pxref{Parser States}).

@item
Each time a rule is reduced, which rule it is, and the complete contents
of the state stack afterward.
@end itemize

To make sense of this information, it helps to refer to the listing file
produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking
Bison}).  This file shows the meaning of each state in terms of
positions in various rules, and also what each state will do with each
possible input token.  As you read the successive trace messages, you
can see that the parser is functioning according to its specification in
the listing file.  Eventually you will arrive at the place where
something undesirable happens, and you will see which parts of the
grammar are to blame.

The parser file is a C program and you can use C debuggers on it, but it's
not easy to interpret what it is doing.  The parser function is a
finite-state machine interpreter, and aside from the actions it executes
the same code over and over.  Only the values of variables show where in
the grammar it is working.

@findex YYPRINT
The debugging information normally gives the token type of each token
read, but not its semantic value.  You can optionally define a macro
named @code{YYPRINT} to provide a way to print the value.  If you define
@code{YYPRINT}, it should take three arguments.  The parser will pass a
standard I/O stream, the numeric code for the token type, and the token
value (from @code{yylval}).

Here is an example of @code{YYPRINT} suitable for the multi-function
calculator (@pxref{Mfcalc Decl, ,Declarations for @code{mfcalc}}):

@smallexample
%@{
  static void print_token_value (FILE *, int, YYSTYPE);
  #define YYPRINT(file, type, value) print_token_value (file, type, value)
%@}

@dots{} %% @dots{} %% @dots{}

static void
print_token_value (FILE *file, int type, YYSTYPE value)
@{
  if (type == VAR)
    fprintf (file, "%s", value.tptr->name);
  else if (type == NUM)
    fprintf (file, "%d", value.val);
@}
@end smallexample

@c ================================================= Invoking Bison

@node Invocation
@chapter Invoking Bison
@cindex invoking Bison
@cindex Bison invocation
@cindex options for invoking Bison

The usual way to invoke Bison is as follows:

@example
bison @var{infile}
@end example

Here @var{infile} is the grammar file name, which usually ends in
@samp{.y}.  The parser file's name is made by replacing the @samp{.y}
with @samp{.tab.c} and removing any leading directory.  Thus, the
@samp{bison foo.y} file name yields
@file{foo.tab.c}, and the @samp{bison hack/foo.y} file name yields
@file{foo.tab.c}.  It's also possible, in case you are writing
C++ code instead of C in your grammar file, to name it @file{foo.ypp}
or @file{foo.y++}.  Then, the output files will take an extension like
the given one as input (respectively @file{foo.tab.cpp} and
@file{foo.tab.c++}).
This feature takes effect with all options that manipulate file names like
@samp{-o} or @samp{-d}.

For example :

@example
bison -d @var{infile.yxx}
@end example
@noindent
will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}, and

@example
bison -d -o @var{output.c++} @var{infile.y}
@end example
@noindent
will produce @file{output.c++} and @file{outfile.h++}.

For compatibility with @acronym{POSIX}, the standard Bison
distribution also contains a shell script called @command{yacc} that
invokes Bison with the @option{-y} option.

@menu
* Bison Options::     All the options described in detail,
                        in alphabetical order by short options.
* Option Cross Key::  Alphabetical list of long options.
* Yacc Library::      Yacc-compatible @code{yylex} and @code{main}.
@end menu

@node Bison Options
@section Bison Options

Bison supports both traditional single-letter options and mnemonic long
option names.  Long option names are indicated with @samp{--} instead of
@samp{-}.  Abbreviations for option names are allowed as long as they
are unique.  When a long option takes an argument, like
@samp{--file-prefix}, connect the option name and the argument with
@samp{=}.

Here is a list of options that can be used with Bison, alphabetized by
short option.  It is followed by a cross key alphabetized by long
option.

@c Please, keep this ordered as in `bison --help'.
@noindent
Operations modes:
@table @option
@item -h
@itemx --help
Print a summary of the command-line options to Bison and exit.

@item -V
@itemx --version
Print the version number of Bison and exit.

@item --print-localedir
Print the name of the directory containing locale-dependent data.

@item -y
@itemx --yacc
Act more like the traditional Yacc command.  This can cause
different diagnostics to be generated, and may change behavior in
other minor ways.  Most importantly, imitate Yacc's output
file name conventions, so that the parser output file is called
@file{y.tab.c}, and the other outputs are called @file{y.output} and
@file{y.tab.h}.  Thus, the following shell script can substitute
for Yacc, and the Bison distribution contains such a script for
compatibility with @acronym{POSIX}:

@example
#! /bin/sh
bison -y "$@@"
@end example

The @option{-y}/@option{--yacc} option is intended for use with
traditional Yacc grammars.  If your grammar uses a Bison extension
like @samp{%glr-parser}, Bison might not be Yacc-compatible even if
this option is specified.

@end table

@noindent
Tuning the parser:

@table @option
@item -S @var{file}
@itemx --skeleton=@var{file}
Specify the skeleton to use.  You probably don't need this option unless
you are developing Bison.

@item -t
@itemx --debug
In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
already defined, so that the debugging facilities are compiled.
@xref{Tracing, ,Tracing Your Parser}.

@item --locations
Pretend that @code{%locations} was specified.  @xref{Decl Summary}.

@item -p @var{prefix}
@itemx --name-prefix=@var{prefix}
Pretend that @code{%name-prefix="@var{prefix}"} was specified.
@xref{Decl Summary}.

@item -l
@itemx --no-lines
Don't put any @code{#line} preprocessor commands in the parser file.
Ordinarily Bison puts them in the parser file so that the C compiler
and debuggers will associate errors with your source file, the
grammar file.  This option causes them to associate errors with the
parser file, treating it as an independent source file in its own right.

@item -n
@itemx --no-parser
Pretend that @code{%no-parser} was specified.  @xref{Decl Summary}.

@item -k
@itemx --token-table
Pretend that @code{%token-table} was specified.  @xref{Decl Summary}.
@end table

@noindent
Adjust the output:

@table @option
@item -d
@itemx --defines
Pretend that @code{%defines} was specified, i.e., write an extra output
file containing macro definitions for the token type names defined in
the grammar, as well as a few other declarations.  @xref{Decl Summary}.

@item --defines=@var{defines-file}
Same as above, but save in the file @var{defines-file}.

@item -b @var{file-prefix}
@itemx --file-prefix=@var{prefix}
Pretend that @code{%verbose} was specified, i.e, specify prefix to use
for all Bison output file names.  @xref{Decl Summary}.

@item -r @var{things}
@itemx --report=@var{things}
Write an extra output file containing verbose description of the comma
separated list of @var{things} among:

@table @code
@item state
Description of the grammar, conflicts (resolved and unresolved), and
@acronym{LALR} automaton.

@item look-ahead
Implies @code{state} and augments the description of the automaton with
each rule's look-ahead set.

@item itemset
Implies @code{state} and augments the description of the automaton with
the full set of items for each state, instead of its core only.
@end table

@item -v
@itemx --verbose
Pretend that @code{%verbose} was specified, i.e, write an extra output
file containing verbose descriptions of the grammar and
parser.  @xref{Decl Summary}.

@item -o @var{file}
@itemx --output=@var{file}
Specify the @var{file} for the parser file.

The other output files' names are constructed from @var{file} as
described under the @samp{-v} and @samp{-d} options.

@item -g
Output a @acronym{VCG} definition of the @acronym{LALR}(1) grammar
automaton computed by Bison.  If the grammar file is @file{foo.y}, the
@acronym{VCG} output file will
be @file{foo.vcg}.

@item --graph=@var{graph-file}
The behavior of @var{--graph} is the same than @samp{-g}.  The only
difference is that it has an optional argument which is the name of
the output graph file.
@end table

@node Option Cross Key
@section Option Cross Key

Here is a list of options, alphabetized by long option, to help you find
the corresponding short option.

@tex
\def\leaderfill{\leaders\hbox to 1em{\hss.\hss}\hfill}

{\tt
\line{ --debug \leaderfill -t}
\line{ --defines \leaderfill -d}
\line{ --file-prefix \leaderfill -b}
\line{ --graph \leaderfill -g}
\line{ --help \leaderfill -h}
\line{ --name-prefix \leaderfill -p}
\line{ --no-lines \leaderfill -l}
\line{ --no-parser \leaderfill -n}
\line{ --output \leaderfill -o}
\line{ --print-localedir}
\line{ --token-table \leaderfill -k}
\line{ --verbose \leaderfill -v}
\line{ --version \leaderfill -V}
\line{ --yacc \leaderfill -y}
}
@end tex

@ifinfo
@example
--debug                               -t
--defines=@var{defines-file}          -d
--file-prefix=@var{prefix}                  -b @var{file-prefix}
--graph=@var{graph-file}              -d
--help                                -h
--name-prefix=@var{prefix}                  -p @var{name-prefix}
--no-lines                            -l
--no-parser                           -n
--output=@var{outfile}                      -o @var{outfile}
--print-localedir
--token-table                         -k
--verbose                             -v
--version                             -V
--yacc                                -y
@end example
@end ifinfo

@node Yacc Library
@section Yacc Library

The Yacc library contains default implementations of the
@code{yyerror} and @code{main} functions.  These default
implementations are normally not useful, but @acronym{POSIX} requires
them.  To use the Yacc library, link your program with the
@option{-ly} option.  Note that Bison's implementation of the Yacc
library is distributed under the terms of the @acronym{GNU} General
Public License (@pxref{Copying}).

If you use the Yacc library's @code{yyerror} function, you should
declare @code{yyerror} as follows:

@example
int yyerror (char const *);
@end example

Bison ignores the @code{int} value returned by this @code{yyerror}.
If you use the Yacc library's @code{main} function, your
@code{yyparse} function should have the following type signature:

@example
int yyparse (void);
@end example

@c ================================================= C++ Bison

@node C++ Language Interface
@chapter C++ Language Interface

@menu
* C++ Parsers::                 The interface to generate C++ parser classes
* A Complete C++ Example::      Demonstrating their use
@end menu

@node C++ Parsers
@section C++ Parsers

@menu
* C++ Bison Interface::         Asking for C++ parser generation
* C++ Semantic Values::         %union vs. C++
* C++ Location Values::         The position and location classes
* C++ Parser Interface::        Instantiating and running the parser
* C++ Scanner Interface::       Exchanges between yylex and parse
@end menu

@node C++ Bison Interface
@subsection C++ Bison Interface
@c - %skeleton "lalr1.cc"
@c - Always pure
@c - initial action

The C++ parser @acronym{LALR}(1) skeleton is named @file{lalr1.cc}.  To select
it, you may either pass the option @option{--skeleton=lalr1.cc} to
Bison, or include the directive @samp{%skeleton "lalr1.cc"} in the
grammar preamble.  When run, @command{bison} will create several
files:
@table @file
@item position.hh
@itemx location.hh
The definition of the classes @code{position} and @code{location},
used for location tracking.  @xref{C++ Location Values}.

@item stack.hh
An auxiliary class @code{stack} used by the parser.

@item @var{file}.hh
@itemx @var{file}.cc
The declaration and implementation of the C++ parser class.
@var{file} is the name of the output file.  It follows the same
rules as with regular C parsers.

Note that @file{@var{file}.hh} is @emph{mandatory}, the C++ cannot
work without the parser class declaration.  Therefore, you must either
pass @option{-d}/@option{--defines} to @command{bison}, or use the
@samp{%defines} directive.
@end table

All these files are documented using Doxygen; run @command{doxygen}
for a complete and accurate documentation.

@node C++ Semantic Values
@subsection C++ Semantic Values
@c - No objects in unions
@c - YSTYPE
@c - Printer and destructor

The @code{%union} directive works as for C, see @ref{Union Decl, ,The
Collection of Value Types}.  In particular it produces a genuine
@code{union}@footnote{In the future techniques to allow complex types
within pseudo-unions (similar to Boost variants) might be implemented to
alleviate these issues.}, which have a few specific features in C++.
@itemize @minus
@item
The type @code{YYSTYPE} is defined but its use is discouraged: rather
you should refer to the parser's encapsulated type
@code{yy::parser::semantic_type}.
@item
Non POD (Plain Old Data) types cannot be used.  C++ forbids any
instance of classes with constructors in unions: only @emph{pointers}
to such objects are allowed.
@end itemize

Because objects have to be stored via pointers, memory is not
reclaimed automatically: using the @code{%destructor} directive is the
only means to avoid leaks.  @xref{Destructor Decl, , Freeing Discarded
Symbols}.


@node C++ Location Values
@subsection C++ Location Values
@c - %locations
@c - class Position
@c - class Location
@c - %define "filename_type" "const symbol::Symbol"

When the directive @code{%locations} is used, the C++ parser supports
location tracking, see @ref{Locations, , Locations Overview}.  Two
auxiliary classes define a @code{position}, a single point in a file,
and a @code{location}, a range composed of a pair of
@code{position}s (possibly spanning several files).

@deftypemethod {position} {std::string*} file
The name of the file.  It will always be handled as a pointer, the
parser will never duplicate nor deallocate it.  As an experimental
feature you may change it to @samp{@var{type}*} using @samp{%define
"filename_type" "@var{type}"}.
@end deftypemethod

@deftypemethod {position} {unsigned int} line
The line, starting at 1.
@end deftypemethod

@deftypemethod {position} {unsigned int} lines (int @var{height} = 1)
Advance by @var{height} lines, resetting the column number.
@end deftypemethod

@deftypemethod {position} {unsigned int} column
The column, starting at 0.
@end deftypemethod

@deftypemethod {position} {unsigned int} columns (int @var{width} = 1)
Advance by @var{width} columns, without changing the line number.
@end deftypemethod

@deftypemethod {position} {position&} operator+= (position& @var{pos}, int @var{width})
@deftypemethodx {position} {position} operator+ (const position& @var{pos}, int @var{width})
@deftypemethodx {position} {position&} operator-= (const position& @var{pos}, int @var{width})
@deftypemethodx {position} {position} operator- (position& @var{pos}, int @var{width})
Various forms of syntactic sugar for @code{columns}.
@end deftypemethod

@deftypemethod {position} {position} operator<< (std::ostream @var{o}, const position& @var{p})
Report @var{p} on @var{o} like this:
@samp{@var{file}:@var{line}.@var{column}}, or
@samp{@var{line}.@var{column}} if @var{file} is null.
@end deftypemethod

@deftypemethod {location} {position} begin
@deftypemethodx {location} {position} end
The first, inclusive, position of the range, and the first beyond.
@end deftypemethod

@deftypemethod {location} {unsigned int} columns (int @var{width} = 1)
@deftypemethodx {location} {unsigned int} lines (int @var{height} = 1)
Advance the @code{end} position.
@end deftypemethod

@deftypemethod {location} {location} operator+ (const location& @var{begin}, const location& @var{end})
@deftypemethodx {location} {location} operator+ (const location& @var{begin}, int @var{width})
@deftypemethodx {location} {location} operator+= (const location& @var{loc}, int @var{width})
Various forms of syntactic sugar.
@end deftypemethod

@deftypemethod {location} {void} step ()
Move @code{begin} onto @code{end}.
@end deftypemethod


@node C++ Parser Interface
@subsection C++ Parser Interface
@c - define parser_class_name
@c - Ctor
@c - parse, error, set_debug_level, debug_level, set_debug_stream,
@c   debug_stream.
@c - Reporting errors

The output files @file{@var{output}.hh} and @file{@var{output}.cc}
declare and define the parser class in the namespace @code{yy}.  The
class name defaults to @code{parser}, but may be changed using
@samp{%define "parser_class_name" "@var{name}"}.  The interface of
this class is detailed below.  It can be extended using the
@code{%parse-param} feature: its semantics is slightly changed since
it describes an additional member of the parser class, and an
additional argument for its constructor.

@defcv {Type} {parser} {semantic_value_type}
@defcvx {Type} {parser} {location_value_type}
The types for semantics value and locations.
@end defcv

@deftypemethod {parser} {} parser (@var{type1} @var{arg1}, ...)
Build a new parser object.  There are no arguments by default, unless
@samp{%parse-param @{@var{type1} @var{arg1}@}} was used.
@end deftypemethod

@deftypemethod {parser} {int} parse ()
Run the syntactic analysis, and return 0 on success, 1 otherwise.
@end deftypemethod

@deftypemethod {parser} {std::ostream&} debug_stream ()
@deftypemethodx {parser} {void} set_debug_stream (std::ostream& @var{o})
Get or set the stream used for tracing the parsing.  It defaults to
@code{std::cerr}.
@end deftypemethod

@deftypemethod {parser} {debug_level_type} debug_level ()
@deftypemethodx {parser} {void} set_debug_level (debug_level @var{l})
Get or set the tracing level.  Currently its value is either 0, no trace,
or nonzero, full tracing.
@end deftypemethod

@deftypemethod {parser} {void} error (const location_type& @var{l}, const std::string& @var{m})
The definition for this member function must be supplied by the user:
the parser uses it to report a parser error occurring at @var{l},
described by @var{m}.
@end deftypemethod


@node C++ Scanner Interface
@subsection C++ Scanner Interface
@c - prefix for yylex.
@c - Pure interface to yylex
@c - %lex-param

The parser invokes the scanner by calling @code{yylex}.  Contrary to C
parsers, C++ parsers are always pure: there is no point in using the
@code{%pure-parser} directive.  Therefore the interface is as follows.

@deftypemethod {parser} {int} yylex (semantic_value_type& @var{yylval}, location_type& @var{yylloc}, @var{type1} @var{arg1}, ...)
Return the next token.  Its type is the return value, its semantic
value and location being @var{yylval} and @var{yylloc}.  Invocations of
@samp{%lex-param @{@var{type1} @var{arg1}@}} yield additional arguments.
@end deftypemethod


@node A Complete C++ Example
@section A Complete C++ Example

This section demonstrates the use of a C++ parser with a simple but
complete example.  This example should be available on your system,
ready to compile, in the directory @dfn{../bison/examples/calc++}.  It
focuses on the use of Bison, therefore the design of the various C++
classes is very naive: no accessors, no encapsulation of members etc.
We will use a Lex scanner, and more precisely, a Flex scanner, to
demonstrate the various interaction.  A hand written scanner is
actually easier to interface with.

@menu
* Calc++ --- C++ Calculator::   The specifications
* Calc++ Parsing Driver::       An active parsing context
* Calc++ Parser::               A parser class
* Calc++ Scanner::              A pure C++ Flex scanner
* Calc++ Top Level::            Conducting the band
@end menu

@node Calc++ --- C++ Calculator
@subsection Calc++ --- C++ Calculator

Of course the grammar is dedicated to arithmetics, a single
expression, possibly preceded by variable assignments.  An
environment containing possibly predefined variables such as
@code{one} and @code{two}, is exchanged with the parser.  An example
of valid input follows.

@example
three := 3
seven := one + two * three
seven * seven
@end example

@node Calc++ Parsing Driver
@subsection Calc++ Parsing Driver
@c - An env
@c - A place to store error messages
@c - A place for the result

To support a pure interface with the parser (and the scanner) the
technique of the ``parsing context'' is convenient: a structure
containing all the data to exchange.  Since, in addition to simply
launch the parsing, there are several auxiliary tasks to execute (open
the file for parsing, instantiate the parser etc.), we recommend
transforming the simple parsing context structure into a fully blown
@dfn{parsing driver} class.

The declaration of this driver class, @file{calc++-driver.hh}, is as
follows.  The first part includes the CPP guard and imports the
required standard library components, and the declaration of the parser
class.

@comment file: calc++-driver.hh
@example
#ifndef CALCXX_DRIVER_HH
# define CALCXX_DRIVER_HH
# include <string>
# include <map>
# include "calc++-parser.hh"
@end example


@noindent
Then comes the declaration of the scanning function.  Flex expects
the signature of @code{yylex} to be defined in the macro
@code{YY_DECL}, and the C++ parser expects it to be declared.  We can
factor both as follows.

@comment file: calc++-driver.hh
@example
// Announce to Flex the prototype we want for lexing function, ...
# define YY_DECL                                                 \
  int yylex (yy::calcxx_parser::semantic_type* yylval,           \
             yy::calcxx_parser::location_type* yylloc,           \
             calcxx_driver& driver)
// ... and declare it for the parser's sake.
YY_DECL;
@end example

@noindent
The @code{calcxx_driver} class is then declared with its most obvious
members.

@comment file: calc++-driver.hh
@example
// Conducting the whole scanning and parsing of Calc++.
class calcxx_driver
@{
public:
  calcxx_driver ();
  virtual ~calcxx_driver ();

  std::map<std::string, int> variables;

  int result;
@end example

@noindent
To encapsulate the coordination with the Flex scanner, it is useful to
have two members function to open and close the scanning phase.
members.

@comment file: calc++-driver.hh
@example
  // Handling the scanner.
  void scan_begin ();
  void scan_end ();
  bool trace_scanning;
@end example

@noindent
Similarly for the parser itself.

@comment file: calc++-driver.hh
@example
  // Handling the parser.
  void parse (const std::string& f);
  std::string file;
  bool trace_parsing;
@end example

@noindent
To demonstrate pure handling of parse errors, instead of simply
dumping them on the standard error output, we will pass them to the
compiler driver using the following two member functions.  Finally, we
close the class declaration and CPP guard.

@comment file: calc++-driver.hh
@example
  // Error handling.
  void error (const yy::location& l, const std::string& m);
  void error (const std::string& m);
@};
#endif // ! CALCXX_DRIVER_HH
@end example

The implementation of the driver is straightforward.  The @code{parse}
member function deserves some attention.  The @code{error} functions
are simple stubs, they should actually register the located error
messages and set error state.

@comment file: calc++-driver.cc
@example
#include "calc++-driver.hh"
#include "calc++-parser.hh"

calcxx_driver::calcxx_driver ()
  : trace_scanning (false), trace_parsing (false)
@{
  variables["one"] = 1;
  variables["two"] = 2;
@}

calcxx_driver::~calcxx_driver ()
@{
@}

void
calcxx_driver::parse (const std::string &f)
@{
  file = f;
  scan_begin ();
  yy::calcxx_parser parser (*this);
  parser.set_debug_level (trace_parsing);
  parser.parse ();
  scan_end ();
@}

void
calcxx_driver::error (const yy::location& l, const std::string& m)
@{
  std::cerr << l << ": " << m << std::endl;
@}

void
calcxx_driver::error (const std::string& m)
@{
  std::cerr << m << std::endl;
@}
@end example

@node Calc++ Parser
@subsection Calc++ Parser

The parser definition file @file{calc++-parser.yy} starts by asking for
the C++ LALR(1) skeleton, the creation of the parser header file, and
specifies the name of the parser class.  Because the C++ skeleton
changed several times, it is safer to require the version you designed
the grammar for.

@comment file: calc++-parser.yy
@example
%skeleton "lalr1.cc"                          /*  -*- C++ -*- */
%require "2.1a"
%defines
%define "parser_class_name" "calcxx_parser"
@end example

@noindent
Then come the declarations/inclusions needed to define the
@code{%union}.  Because the parser uses the parsing driver and
reciprocally, both cannot include the header of the other.  Because the
driver's header needs detailed knowledge about the parser class (in
particular its inner types), it is the parser's header which will simply
use a forward declaration of the driver.

@comment file: calc++-parser.yy
@example
%@{
# include <string>
class calcxx_driver;
%@}
@end example

@noindent
The driver is passed by reference to the parser and to the scanner.
This provides a simple but effective pure interface, not relying on
global variables.

@comment file: calc++-parser.yy
@example
// The parsing context.
%parse-param @{ calcxx_driver& driver @}
%lex-param   @{ calcxx_driver& driver @}
@end example

@noindent
Then we request the location tracking feature, and initialize the
first location's file name.  Afterwards new locations are computed
relatively to the previous locations: the file name will be
automatically propagated.

@comment file: calc++-parser.yy
@example
%locations
%initial-action
@{
  // Initialize the initial location.
  @@$.begin.filename = @@$.end.filename = &driver.file;
@};
@end example

@noindent
Use the two following directives to enable parser tracing and verbose
error messages.

@comment file: calc++-parser.yy
@example
%debug
%error-verbose
@end example

@noindent
Semantic values cannot use ``real'' objects, but only pointers to
them.

@comment file: calc++-parser.yy
@example
// Symbols.
%union
@{
  int          ival;
  std::string *sval;
@};
@end example

@noindent
The code between @samp{%@{} and @samp{%@}} after the introduction of the
@samp{%union} is output in the @file{*.cc} file; it needs detailed
knowledge about the driver.

@comment file: calc++-parser.yy
@example
%@{
# include "calc++-driver.hh"
%@}
@end example


@noindent
The token numbered as 0 corresponds to end of file; the following line
allows for nicer error messages referring to ``end of file'' instead
of ``$end''.  Similarly user friendly named are provided for each
symbol.  Note that the tokens names are prefixed by @code{TOKEN_} to
avoid name clashes.

@comment file: calc++-parser.yy
@example
%token        END      0 "end of file"
%token        ASSIGN     ":="
%token <sval> IDENTIFIER "identifier"
%token <ival> NUMBER     "number"
%type  <ival> exp        "expression"
@end example

@noindent
To enable memory deallocation during error recovery, use
@code{%destructor}.

@c FIXME: Document %printer, and mention that it takes a braced-code operand.
@comment file: calc++-parser.yy
@example
%printer    @{ debug_stream () << *$$; @} "identifier"
%destructor @{ delete $$; @} "identifier"

%printer    @{ debug_stream () << $$; @} "number" "expression"
@end example

@noindent
The grammar itself is straightforward.

@comment file: calc++-parser.yy
@example
%%
%start unit;
unit: assignments exp  @{ driver.result = $2; @};

assignments: assignments assignment @{@}
           | /* Nothing.  */        @{@};

assignment: "identifier" ":=" exp @{ driver.variables[*$1] = $3; @};

%left '+' '-';
%left '*' '/';
exp: exp '+' exp   @{ $$ = $1 + $3; @}
   | exp '-' exp   @{ $$ = $1 - $3; @}
   | exp '*' exp   @{ $$ = $1 * $3; @}
   | exp '/' exp   @{ $$ = $1 / $3; @}
   | "identifier"  @{ $$ = driver.variables[*$1]; @}
   | "number"      @{ $$ = $1; @};
%%
@end example

@noindent
Finally the @code{error} member function registers the errors to the
driver.

@comment file: calc++-parser.yy
@example
void
yy::calcxx_parser::error (const yy::calcxx_parser::location_type& l,
                          const std::string& m)
@{
  driver.error (l, m);
@}
@end example

@node Calc++ Scanner
@subsection Calc++ Scanner

The Flex scanner first includes the driver declaration, then the
parser's to get the set of defined tokens.

@comment file: calc++-scanner.ll
@example
%@{                                            /* -*- C++ -*- */
# include <cstdlib>
# include <errno.h>
# include <limits.h>
# include <string>
# include "calc++-driver.hh"
# include "calc++-parser.hh"
%@}
@end example

@noindent
Because there is no @code{#include}-like feature we don't need
@code{yywrap}, we don't need @code{unput} either, and we parse an
actual file, this is not an interactive session with the user.
Finally we enable the scanner tracing features.

@comment file: calc++-scanner.ll
@example
%option noyywrap nounput batch debug
@end example

@noindent
Abbreviations allow for more readable rules.

@comment file: calc++-scanner.ll
@example
id    [a-zA-Z][a-zA-Z_0-9]*
int   [0-9]+
blank [ \t]
@end example

@noindent
The following paragraph suffices to track locations accurately.  Each
time @code{yylex} is invoked, the begin position is moved onto the end
position.  Then when a pattern is matched, the end position is
advanced of its width.  In case it matched ends of lines, the end
cursor is adjusted, and each time blanks are matched, the begin cursor
is moved onto the end cursor to effectively ignore the blanks
preceding tokens.  Comments would be treated equally.

@comment file: calc++-scanner.ll
@example
%@{
# define YY_USER_ACTION  yylloc->columns (yyleng);
%@}
%%
%@{
  yylloc->step ();
%@}
@{blank@}+   yylloc->step ();
[\n]+      yylloc->lines (yyleng); yylloc->step ();
@end example

@noindent
The rules are simple, just note the use of the driver to report errors.
It is convenient to use a typedef to shorten
@code{yy::calcxx_parser::token::identifier} into
@code{token::identifier} for instance.

@comment file: calc++-scanner.ll
@example
%@{
  typedef yy::calcxx_parser::token token;
%@}

[-+*/]     return yytext[0];
":="       return token::ASSIGN;
@{int@}      @{
  errno = 0;
  long n = strtol (yytext, NULL, 10);
  if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
    driver.error (*yylloc, "integer is out of range");
  yylval->ival = n;
  return token::NUMBER;
@}
@{id@}       yylval->sval = new std::string (yytext); return token::IDENTIFIER;
.          driver.error (*yylloc, "invalid character");
%%
@end example

@noindent
Finally, because the scanner related driver's member function depend
on the scanner's data, it is simpler to implement them in this file.

@comment file: calc++-scanner.ll
@example
void
calcxx_driver::scan_begin ()
@{
  yy_flex_debug = trace_scanning;
  if (!(yyin = fopen (file.c_str (), "r")))
    error (std::string ("cannot open ") + file);
@}

void
calcxx_driver::scan_end ()
@{
  fclose (yyin);
@}
@end example

@node Calc++ Top Level
@subsection Calc++ Top Level

The top level file, @file{calc++.cc}, poses no problem.

@comment file: calc++.cc
@example
#include <iostream>
#include "calc++-driver.hh"

int
main (int argc, char *argv[])
@{
  calcxx_driver driver;
  for (++argv; argv[0]; ++argv)
    if (*argv == std::string ("-p"))
      driver.trace_parsing = true;
    else if (*argv == std::string ("-s"))
      driver.trace_scanning = true;
    else
      @{
        driver.parse (*argv);
        std::cout << driver.result << std::endl;
      @}
@}
@end example

@c ================================================= FAQ

@node FAQ
@chapter Frequently Asked Questions
@cindex frequently asked questions
@cindex questions

Several questions about Bison come up occasionally.  Here some of them
are addressed.

@menu
* Memory Exhausted::           Breaking the Stack Limits
* How Can I Reset the Parser:: @code{yyparse} Keeps some State
* Strings are Destroyed::      @code{yylval} Loses Track of Strings
* Implementing Gotos/Loops::   Control Flow in the Calculator
@end menu

@node Memory Exhausted
@section Memory Exhausted

@display
My parser returns with error with a @samp{memory exhausted}
message.  What can I do?
@end display

This question is already addressed elsewhere, @xref{Recursion,
,Recursive Rules}.

@node How Can I Reset the Parser
@section How Can I Reset the Parser

The following phenomenon has several symptoms, resulting in the
following typical questions:

@display
I invoke @code{yyparse} several times, and on correct input it works
properly; but when a parse error is found, all the other calls fail
too.  How can I reset the error flag of @code{yyparse}?
@end display

@noindent
or

@display
My parser includes support for an @samp{#include}-like feature, in
which case I run @code{yyparse} from @code{yyparse}.  This fails
although I did specify I needed a @code{%pure-parser}.
@end display

These problems typically come not from Bison itself, but from
Lex-generated scanners.  Because these scanners use large buffers for
speed, they might not notice a change of input file.  As a
demonstration, consider the following source file,
@file{first-line.l}:

@verbatim
%{
#include <stdio.h>
#include <stdlib.h>
%}
%%
.*\n    ECHO; return 1;
%%
int
yyparse (char const *file)
{
  yyin = fopen (file, "r");
  if (!yyin)
    exit (2);
  /* One token only.  */
  yylex ();
  if (fclose (yyin) != 0)
    exit (3);
  return 0;
}

int
main (void)
{
  yyparse ("input");
  yyparse ("input");
  return 0;
}
@end verbatim

@noindent
If the file @file{input} contains

@verbatim
input:1: Hello,
input:2: World!
@end verbatim

@noindent
then instead of getting the first line twice, you get:

@example
$ @kbd{flex -ofirst-line.c first-line.l}
$ @kbd{gcc  -ofirst-line   first-line.c -ll}
$ @kbd{./first-line}
input:1: Hello,
input:2: World!
@end example

Therefore, whenever you change @code{yyin}, you must tell the
Lex-generated scanner to discard its current buffer and switch to the
new one.  This depends upon your implementation of Lex; see its
documentation for more.  For Flex, it suffices to call
@samp{YY_FLUSH_BUFFER} after each change to @code{yyin}.  If your
Flex-generated scanner needs to read from several input streams to
handle features like include files, you might consider using Flex
functions like @samp{yy_switch_to_buffer} that manipulate multiple
input buffers.

If your Flex-generated scanner uses start conditions (@pxref{Start
conditions, , Start conditions, flex, The Flex Manual}), you might
also want to reset the scanner's state, i.e., go back to the initial
start condition, through a call to @samp{BEGIN (0)}.

@node Strings are Destroyed
@section Strings are Destroyed

@display
My parser seems to destroy old strings, or maybe it loses track of
them.  Instead of reporting @samp{"foo", "bar"}, it reports
@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}.
@end display

This error is probably the single most frequent ``bug report'' sent to
Bison lists, but is only concerned with a misunderstanding of the role
of scanner.  Consider the following Lex code:

@verbatim
%{
#include <stdio.h>
char *yylval = NULL;
%}
%%
.*    yylval = yytext; return 1;
\n    /* IGNORE */
%%
int
main ()
{
  /* Similar to using $1, $2 in a Bison action.  */
  char *fst = (yylex (), yylval);
  char *snd = (yylex (), yylval);
  printf ("\"%s\", \"%s\"\n", fst, snd);
  return 0;
}
@end verbatim

If you compile and run this code, you get:

@example
$ @kbd{flex -osplit-lines.c split-lines.l}
$ @kbd{gcc  -osplit-lines   split-lines.c -ll}
$ @kbd{printf 'one\ntwo\n' | ./split-lines}
"one
two", "two"
@end example

@noindent
this is because @code{yytext} is a buffer provided for @emph{reading}
in the action, but if you want to keep it, you have to duplicate it
(e.g., using @code{strdup}).  Note that the output may depend on how
your implementation of Lex handles @code{yytext}.  For instance, when
given the Lex compatibility option @option{-l} (which triggers the
option @samp{%array}) Flex generates a different behavior:

@example
$ @kbd{flex -l -osplit-lines.c split-lines.l}
$ @kbd{gcc     -osplit-lines   split-lines.c -ll}
$ @kbd{printf 'one\ntwo\n' | ./split-lines}
"two", "two"
@end example


@node Implementing Gotos/Loops
@section Implementing Gotos/Loops

@display
My simple calculator supports variables, assignments, and functions,
but how can I implement gotos, or loops?
@end display

Although very pedagogical, the examples included in the document blur
the distinction to make between the parser---whose job is to recover
the structure of a text and to transmit it to subsequent modules of
the program---and the processing (such as the execution) of this
structure.  This works well with so called straight line programs,
i.e., precisely those that have a straightforward execution model:
execute simple instructions one after the others.

@cindex abstract syntax tree
@cindex @acronym{AST}
If you want a richer model, you will probably need to use the parser
to construct a tree that does represent the structure it has
recovered; this tree is usually called the @dfn{abstract syntax tree},
or @dfn{@acronym{AST}} for short.  Then, walking through this tree,
traversing it in various ways, will enable treatments such as its
execution or its translation, which will result in an interpreter or a
compiler.

This topic is way beyond the scope of this manual, and the reader is
invited to consult the dedicated literature.



@c ================================================= Table of Symbols

@node Table of Symbols
@appendix Bison Symbols
@cindex Bison symbols, table of
@cindex symbols in Bison, table of

@deffn {Variable} @@$
In an action, the location of the left-hand side of the rule.
@xref{Locations, , Locations Overview}.
@end deffn

@deffn {Variable} @@@var{n}
In an action, the location of the @var{n}-th symbol of the right-hand
side of the rule.  @xref{Locations, , Locations Overview}.
@end deffn

@deffn {Variable} $$
In an action, the semantic value of the left-hand side of the rule.
@xref{Actions}.
@end deffn

@deffn {Variable} $@var{n}
In an action, the semantic value of the @var{n}-th symbol of the
right-hand side of the rule.  @xref{Actions}.
@end deffn

@deffn {Delimiter} %%
Delimiter used to separate the grammar rule section from the
Bison declarations section or the epilogue.
@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}.
@end deffn

@c Don't insert spaces, or check the DVI output.
@deffn {Delimiter} %@{@var{code}%@}
All code listed between @samp{%@{} and @samp{%@}} is copied directly to
the output file uninterpreted.  Such code forms the prologue of the input
file.  @xref{Grammar Outline, ,Outline of a Bison
Grammar}.
@end deffn

@deffn {Construct} /*@dots{}*/
Comment delimiters, as in C.
@end deffn

@deffn {Delimiter} :
Separates a rule's result from its components.  @xref{Rules, ,Syntax of
Grammar Rules}.
@end deffn

@deffn {Delimiter} ;
Terminates a rule.  @xref{Rules, ,Syntax of Grammar Rules}.
@end deffn

@deffn {Delimiter} |
Separates alternate rules for the same result nonterminal.
@xref{Rules, ,Syntax of Grammar Rules}.
@end deffn

@deffn {Symbol} $accept
The predefined nonterminal whose only rule is @samp{$accept: @var{start}
$end}, where @var{start} is the start symbol.  @xref{Start Decl, , The
Start-Symbol}.  It cannot be used in the grammar.
@end deffn

@deffn {Directive} %debug
Equip the parser for debugging.  @xref{Decl Summary}.
@end deffn

@ifset defaultprec
@deffn {Directive} %default-prec
Assign a precedence to rules that lack an explicit @samp{%prec}
modifier.  @xref{Contextual Precedence, ,Context-Dependent
Precedence}.
@end deffn
@end ifset

@deffn {Directive} %defines
Bison declaration to create a header file meant for the scanner.
@xref{Decl Summary}.
@end deffn

@deffn {Directive} %destructor
Specify how the parser should reclaim the memory associated to
discarded symbols.  @xref{Destructor Decl, , Freeing Discarded Symbols}.
@end deffn

@deffn {Directive} %dprec
Bison declaration to assign a precedence to a rule that is used at parse
time to resolve reduce/reduce conflicts.  @xref{GLR Parsers, ,Writing
@acronym{GLR} Parsers}.
@end deffn

@deffn {Symbol} $end
The predefined token marking the end of the token stream.  It cannot be
used in the grammar.
@end deffn

@deffn {Symbol} error
A token name reserved for error recovery.  This token may be used in
grammar rules so as to allow the Bison parser to recognize an error in
the grammar without halting the process.  In effect, a sentence
containing an error may be recognized as valid.  On a syntax error, the
token @code{error} becomes the current look-ahead token.  Actions
corresponding to @code{error} are then executed, and the look-ahead
token is reset to the token that originally caused the violation.
@xref{Error Recovery}.
@end deffn

@deffn {Directive} %error-verbose
Bison declaration to request verbose, specific error message strings
when @code{yyerror} is called.
@end deffn

@deffn {Directive} %file-prefix="@var{prefix}"
Bison declaration to set the prefix of the output files.  @xref{Decl
Summary}.
@end deffn

@deffn {Directive} %glr-parser
Bison declaration to produce a @acronym{GLR} parser.  @xref{GLR
Parsers, ,Writing @acronym{GLR} Parsers}.
@end deffn

@deffn {Directive} %initial-action
Run user code before parsing.  @xref{Initial Action Decl, , Performing Actions before Parsing}.
@end deffn

@deffn {Directive} %left
Bison declaration to assign left associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
@end deffn

@deffn {Directive} %lex-param @{@var{argument-declaration}@}
Bison declaration to specifying an additional parameter that
@code{yylex} should accept.  @xref{Pure Calling,, Calling Conventions
for Pure Parsers}.
@end deffn

@deffn {Directive} %merge
Bison declaration to assign a merging function to a rule.  If there is a
reduce/reduce conflict with a rule having the same merging function, the
function is applied to the two semantic values to get a single result.
@xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}.
@end deffn

@deffn {Directive} %name-prefix="@var{prefix}"
Bison declaration to rename the external symbols.  @xref{Decl Summary}.
@end deffn

@ifset defaultprec
@deffn {Directive} %no-default-prec
Do not assign a precedence to rules that lack an explicit @samp{%prec}
modifier.  @xref{Contextual Precedence, ,Context-Dependent
Precedence}.
@end deffn
@end ifset

@deffn {Directive} %no-lines
Bison declaration to avoid generating @code{#line} directives in the
parser file.  @xref{Decl Summary}.
@end deffn

@deffn {Directive} %nonassoc
Bison declaration to assign nonassociativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
@end deffn

@deffn {Directive} %output="@var{file}"
Bison declaration to set the name of the parser file.  @xref{Decl
Summary}.
@end deffn

@deffn {Directive} %parse-param @{@var{argument-declaration}@}
Bison declaration to specifying an additional parameter that
@code{yyparse} should accept.  @xref{Parser Function,, The Parser
Function @code{yyparse}}.
@end deffn

@deffn {Directive} %prec
Bison declaration to assign a precedence to a specific rule.
@xref{Contextual Precedence, ,Context-Dependent Precedence}.
@end deffn

@deffn {Directive} %pure-parser
Bison declaration to request a pure (reentrant) parser.
@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
@end deffn

@deffn {Directive} %require "@var{version}"
Require version @var{version} or higher of Bison.  @xref{Require Decl, ,
Require a Version of Bison}.
@end deffn

@deffn {Directive} %right
Bison declaration to assign right associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
@end deffn

@deffn {Directive} %start
Bison declaration to specify the start symbol.  @xref{Start Decl, ,The
Start-Symbol}.
@end deffn

@deffn {Directive} %token
Bison declaration to declare token(s) without specifying precedence.
@xref{Token Decl, ,Token Type Names}.
@end deffn

@deffn {Directive} %token-table
Bison declaration to include a token name table in the parser file.
@xref{Decl Summary}.
@end deffn

@deffn {Directive} %type
Bison declaration to declare nonterminals.  @xref{Type Decl,
,Nonterminal Symbols}.
@end deffn

@deffn {Symbol} $undefined
The predefined token onto which all undefined values returned by
@code{yylex} are mapped.  It cannot be used in the grammar, rather, use
@code{error}.
@end deffn

@deffn {Directive} %union
Bison declaration to specify several possible data types for semantic
values.  @xref{Union Decl, ,The Collection of Value Types}.
@end deffn

@deffn {Macro} YYABORT
Macro to pretend that an unrecoverable syntax error has occurred, by
making @code{yyparse} return 1 immediately.  The error reporting
function @code{yyerror} is not called.  @xref{Parser Function, ,The
Parser Function @code{yyparse}}.
@end deffn

@deffn {Macro} YYACCEPT
Macro to pretend that a complete utterance of the language has been
read, by making @code{yyparse} return 0 immediately.
@xref{Parser Function, ,The Parser Function @code{yyparse}}.
@end deffn

@deffn {Macro} YYBACKUP
Macro to discard a value from the parser stack and fake a look-ahead
token.  @xref{Action Features, ,Special Features for Use in Actions}.
@end deffn

@deffn {Variable} yychar
External integer variable that contains the integer value of the
look-ahead token.  (In a pure parser, it is a local variable within
@code{yyparse}.)  Error-recovery rule actions may examine this variable.
@xref{Action Features, ,Special Features for Use in Actions}.
@end deffn

@deffn {Variable} yyclearin
Macro used in error-recovery rule actions.  It clears the previous
look-ahead token.  @xref{Error Recovery}.
@end deffn

@deffn {Macro} YYDEBUG
Macro to define to equip the parser with tracing code.  @xref{Tracing,
,Tracing Your Parser}.
@end deffn

@deffn {Variable} yydebug
External integer variable set to zero by default.  If @code{yydebug}
is given a nonzero value, the parser will output information on input
symbols and parser action.  @xref{Tracing, ,Tracing Your Parser}.
@end deffn

@deffn {Macro} yyerrok
Macro to cause parser to recover immediately to its normal mode
after a syntax error.  @xref{Error Recovery}.
@end deffn

@deffn {Macro} YYERROR
Macro to pretend that a syntax error has just been detected: call
@code{yyerror} and then perform normal error recovery if possible
(@pxref{Error Recovery}), or (if recovery is impossible) make
@code{yyparse} return 1.  @xref{Error Recovery}.
@end deffn

@deffn {Function} yyerror
User-supplied function to be called by @code{yyparse} on error.
@xref{Error Reporting, ,The Error
Reporting Function @code{yyerror}}.
@end deffn

@deffn {Macro} YYERROR_VERBOSE
An obsolete macro that you define with @code{#define} in the prologue
to request verbose, specific error message strings
when @code{yyerror} is called.  It doesn't matter what definition you
use for @code{YYERROR_VERBOSE}, just whether you define it.  Using
@code{%error-verbose} is preferred.
@end deffn

@deffn {Macro} YYINITDEPTH
Macro for specifying the initial size of the parser stack.
@xref{Memory Management}.
@end deffn

@deffn {Function} yylex
User-supplied lexical analyzer function, called with no arguments to get
the next token.  @xref{Lexical, ,The Lexical Analyzer Function
@code{yylex}}.
@end deffn

@deffn {Macro} YYLEX_PARAM
An obsolete macro for specifying an extra argument (or list of extra
arguments) for @code{yyparse} to pass to @code{yylex}.  The use of this
macro is deprecated, and is supported only for Yacc like parsers.
@xref{Pure Calling,, Calling Conventions for Pure Parsers}.
@end deffn

@deffn {Variable} yylloc
External variable in which @code{yylex} should place the line and column
numbers associated with a token.  (In a pure parser, it is a local
variable within @code{yyparse}, and its address is passed to
@code{yylex}.)
You can ignore this variable if you don't use the @samp{@@} feature in the
grammar actions.
@xref{Token Locations, ,Textual Locations of Tokens}.
In semantic actions, it stores the location of the look-ahead token.
@xref{Actions and Locations, ,Actions and Locations}.
@end deffn

@deffn {Type} YYLTYPE
Data type of @code{yylloc}; by default, a structure with four
members.  @xref{Location Type, , Data Types of Locations}.
@end deffn

@deffn {Variable} yylval
External variable in which @code{yylex} should place the semantic
value associated with a token.  (In a pure parser, it is a local
variable within @code{yyparse}, and its address is passed to
@code{yylex}.)
@xref{Token Values, ,Semantic Values of Tokens}.
In semantic actions, it stores the semantic value of the look-ahead token.
@xref{Actions, ,Actions}.
@end deffn

@deffn {Macro} YYMAXDEPTH
Macro for specifying the maximum size of the parser stack.  @xref{Memory
Management}.
@end deffn

@deffn {Variable} yynerrs
Global variable which Bison increments each time it reports a syntax error.
(In a pure parser, it is a local variable within @code{yyparse}.)
@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
@end deffn

@deffn {Function} yyparse
The parser function produced by Bison; call this function to start
parsing.  @xref{Parser Function, ,The Parser Function @code{yyparse}}.
@end deffn

@deffn {Macro} YYPARSE_PARAM
An obsolete macro for specifying the name of a parameter that
@code{yyparse} should accept.  The use of this macro is deprecated, and
is supported only for Yacc like parsers.  @xref{Pure Calling,, Calling
Conventions for Pure Parsers}.
@end deffn

@deffn {Macro} YYRECOVERING
Macro whose value indicates whether the parser is recovering from a
syntax error.  @xref{Action Features, ,Special Features for Use in Actions}.
@end deffn

@deffn {Macro} YYSTACK_USE_ALLOCA
Macro used to control the use of @code{alloca} when the C
@acronym{LALR}(1) parser needs to extend its stacks.  If defined to 0,
the parser will use @code{malloc} to extend its stacks.  If defined to
1, the parser will use @code{alloca}.  Values other than 0 and 1 are
reserved for future Bison extensions.  If not defined,
@code{YYSTACK_USE_ALLOCA} defaults to 0.

In the all-too-common case where your code may run on a host with a
limited stack and with unreliable stack-overflow checking, you should
set @code{YYMAXDEPTH} to a value that cannot possibly result in
unchecked stack overflow on any of your target hosts when
@code{alloca} is called.  You can inspect the code that Bison
generates in order to determine the proper numeric values.  This will
require some expertise in low-level implementation details.
@end deffn

@deffn {Type} YYSTYPE
Data type of semantic values; @code{int} by default.
@xref{Value Type, ,Data Types of Semantic Values}.
@end deffn

@node Glossary
@appendix Glossary
@cindex glossary

@table @asis
@item Backus-Naur Form (@acronym{BNF}; also called ``Backus Normal Form'')
Formal method of specifying context-free grammars originally proposed
by John Backus, and slightly improved by Peter Naur in his 1960-01-02
committee document contributing to what became the Algol 60 report.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.

@item Context-free grammars
Grammars specified as rules that can be applied regardless of context.
Thus, if there is a rule which says that an integer can be used as an
expression, integers are allowed @emph{anywhere} an expression is
permitted.  @xref{Language and Grammar, ,Languages and Context-Free
Grammars}.

@item Dynamic allocation
Allocation of memory that occurs during execution, rather than at
compile time or on entry to a function.

@item Empty string
Analogous to the empty set in set theory, the empty string is a
character string of length zero.

@item Finite-state stack machine
A ``machine'' that has discrete states in which it is said to exist at
each instant in time.  As input to the machine is processed, the
machine moves from state to state as specified by the logic of the
machine.  In the case of the parser, the input is the language being
parsed, and the states correspond to various stages in the grammar
rules.  @xref{Algorithm, ,The Bison Parser Algorithm}.

@item Generalized @acronym{LR} (@acronym{GLR})
A parsing algorithm that can handle all context-free grammars, including those
that are not @acronym{LALR}(1).  It resolves situations that Bison's
usual @acronym{LALR}(1)
algorithm cannot by effectively splitting off multiple parsers, trying all
possible parsers, and discarding those that fail in the light of additional
right context.  @xref{Generalized LR Parsing, ,Generalized
@acronym{LR} Parsing}.

@item Grouping
A language construct that is (in general) grammatically divisible;
for example, `expression' or `declaration' in C@.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.

@item Infix operator
An arithmetic operator that is placed between the operands on which it
performs some operation.

@item Input stream
A continuous flow of data between devices or programs.

@item Language construct
One of the typical usage schemas of the language.  For example, one of
the constructs of the C language is the @code{if} statement.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.

@item Left associativity
Operators having left associativity are analyzed from left to right:
@samp{a+b+c} first computes @samp{a+b} and then combines with
@samp{c}.  @xref{Precedence, ,Operator Precedence}.

@item Left recursion
A rule whose result symbol is also its first component symbol; for
example, @samp{expseq1 : expseq1 ',' exp;}.  @xref{Recursion, ,Recursive
Rules}.

@item Left-to-right parsing
Parsing a sentence of a language by analyzing it token by token from
left to right.  @xref{Algorithm, ,The Bison Parser Algorithm}.

@item Lexical analyzer (scanner)
A function that reads an input stream and returns tokens one by one.
@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.

@item Lexical tie-in
A flag, set by actions in the grammar rules, which alters the way
tokens are parsed.  @xref{Lexical Tie-ins}.

@item Literal string token
A token which consists of two or more fixed characters.  @xref{Symbols}.

@item Look-ahead token
A token already read but not yet shifted.  @xref{Look-Ahead, ,Look-Ahead
Tokens}.

@item @acronym{LALR}(1)
The class of context-free grammars that Bison (like most other parser
generators) can handle; a subset of @acronym{LR}(1).  @xref{Mystery
Conflicts, ,Mysterious Reduce/Reduce Conflicts}.

@item @acronym{LR}(1)
The class of context-free grammars in which at most one token of
look-ahead is needed to disambiguate the parsing of any piece of input.

@item Nonterminal symbol
A grammar symbol standing for a grammatical construct that can
be expressed through rules in terms of smaller constructs; in other
words, a construct that is not a token.  @xref{Symbols}.

@item Parser
A function that recognizes valid sentences of a language by analyzing
the syntax structure of a set of tokens passed to it from a lexical
analyzer.

@item Postfix operator
An arithmetic operator that is placed after the operands upon which it
performs some operation.

@item Reduction
Replacing a string of nonterminals and/or terminals with a single
nonterminal, according to a grammar rule.  @xref{Algorithm, ,The Bison
Parser Algorithm}.

@item Reentrant
A reentrant subprogram is a subprogram which can be in invoked any
number of times in parallel, without interference between the various
invocations.  @xref{Pure Decl, ,A Pure (Reentrant) Parser}.

@item Reverse polish notation
A language in which all operators are postfix operators.

@item Right recursion
A rule whose result symbol is also its last component symbol; for
example, @samp{expseq1: exp ',' expseq1;}.  @xref{Recursion, ,Recursive
Rules}.

@item Semantics
In computer languages, the semantics are specified by the actions
taken for each instance of the language, i.e., the meaning of
each statement.  @xref{Semantics, ,Defining Language Semantics}.

@item Shift
A parser is said to shift when it makes the choice of analyzing
further input from the stream rather than reducing immediately some
already-recognized rule.  @xref{Algorithm, ,The Bison Parser Algorithm}.

@item Single-character literal
A single character that is recognized and interpreted as is.
@xref{Grammar in Bison, ,From Formal Rules to Bison Input}.

@item Start symbol
The nonterminal symbol that stands for a complete valid utterance in
the language being parsed.  The start symbol is usually listed as the
first nonterminal symbol in a language specification.
@xref{Start Decl, ,The Start-Symbol}.

@item Symbol table
A data structure where symbol names and associated data are stored
during parsing to allow for recognition and use of existing
information in repeated uses of a symbol.  @xref{Multi-function Calc}.

@item Syntax error
An error encountered during parsing of an input stream due to invalid
syntax.  @xref{Error Recovery}.

@item Token
A basic, grammatically indivisible unit of a language.  The symbol
that describes a token in the grammar is a terminal symbol.
The input of the Bison parser is a stream of tokens which comes from
the lexical analyzer.  @xref{Symbols}.

@item Terminal symbol
A grammar symbol that has no rules in the grammar and therefore is
grammatically indivisible.  The piece of text it represents is a token.
@xref{Language and Grammar, ,Languages and Context-Free Grammars}.
@end table

@node Copying This Manual
@appendix Copying This Manual

@menu
* GNU Free Documentation License::  License for copying this manual.
@end menu

@include fdl.texi

@node Index
@unnumbered Index

@printindex cp

@bye

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@c LocalWords: ifinfo smallbook shorttitlepage titlepage GPL FIXME iftex
@c LocalWords: akim fn cp syncodeindex vr tp synindex dircategory direntry
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