The Bison Parser Algorithm
-* Look-Ahead:: Parser looks one token ahead when deciding what to do.
+* Lookahead:: 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.
@unnumbered Introduction
@cindex introduction
-@dfn{Bison} is a general-purpose parser generator that converts a grammar
-description for an @acronym{LALR}(1) or @acronym{GLR} context-free grammar
-into a C or C++ program to parse that grammar. Once you are proficient with
+@dfn{Bison} is a general-purpose parser generator that converts an
+annotated context-free grammar into an @acronym{LALR}(1) or
+@acronym{GLR} parser for that grammar. Once you are proficient with
Bison, you can use it to develop a wide range of language parsers, from those
used in simple desk calculators to complex programming languages.
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
+token of lookahead. 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
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
+(called a @dfn{lookahead}) 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.
+lookahead 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
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,
+Such grammars typically require more than one symbol of lookahead,
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}).
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
+With normal @acronym{LALR}(1) one-token lookahead 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
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
+lookahead 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.
@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.
+the lookahead 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.
+lookahead 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}.
+@xref{Lookahead, ,Lookahead Tokens}.
@findex yyclearin
@cindex @acronym{GLR} parsers and @code{yyclearin}
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}.
+@code{last_column}. By conventions, and in accordance with the GNU
+Coding Standards and common practice, the line and column count both
+start at 1.
@node Ltcalc Rules
@subsection Grammar Rules for @code{ltcalc}
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
+The @var{Prologue} section is terminated by the first occurrence
of @samp{%@}} that is outside a comment, a string literal, or a
character constant.
@smallexample
%@{
+ #define _GNU_SOURCE
#include <stdio.h>
#include "ptypes.h"
%@}
@dots{}
@end smallexample
+When in doubt, it is usually safer to put prologue code before all
+Bison declarations, rather than after. For example, any definitions
+of feature test macros like @code{_GNU_SOURCE} or
+@code{_POSIX_C_SOURCE} should appear before all Bison declarations, as
+feature test macros can affect the behavior of Bison-generated
+@code{#include} directives.
+
+@findex %requires
+@findex %code
+If you've instructed Bison to generate a header file (@pxref{Table of Symbols,
+,%defines}), you probably want @code{#include "ptypes.h"} to appear
+in that header file as well.
+In that case, use @code{%requires}, @code{%provides}, and
+@code{%code} instead of @var{Prologue} sections
+(@pxref{Table of Symbols, ,%requires}):
+
+@smallexample
+%requires @{
+ #include "ptypes.h"
+@}
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%code @{
+ #include <stdio.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)
@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
+Bison normally uses the type @code{int} for semantic values if your
+program uses the same data type for all language constructs. To
specify some other type, define @code{YYSTYPE} as a macro, like this:
@example
@end example
@noindent
+@code{YYSTYPE}'s replacement list should be a type name
+that does not contain parentheses or square brackets.
This macro definition must go in the prologue of the grammar file
(@pxref{Grammar Outline, ,Outline of a Bison Grammar}).
@itemize @bullet
@item
-Specify the entire collection of possible data types, with the
+Specify the entire collection of possible data types, either by using the
@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of
-Value Types}).
+Value Types}), or by using a @code{typedef} or a @code{#define} to
+define @code{YYSTYPE} to be a union type whose member names are
+the type tags.
@item
Choose one of those types for each symbol (terminal or nonterminal) for
definition of @code{foo}.
@vindex yylval
-It is also possible to access the semantic value of the look-ahead token, if
+It is also possible to access the semantic value of the lookahead token, if
any, from a semantic action.
This semantic value is stored in @code{yylval}.
@xref{Action Features, ,Special Features for Use in Actions}.
restoring it.
Thus, @code{$<context>5} needs a destructor (@pxref{Destructor Decl, , Freeing
Discarded Symbols}).
-However, Bison currently provides no means to declare a destructor for a
-mid-rule action's semantic value.
+However, Bison currently provides no means to declare a destructor specific to
+a particular mid-rule action's semantic value.
One solution is to bury the mid-rule action inside a nonterminal symbol and to
declare a destructor for that symbol:
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}.)
+the @dfn{lookahead} token at this time, since the parser is still
+deciding what to do about it. @xref{Lookahead, ,Lookahead Tokens}.)
You might think that you could correct the problem by putting identical
actions into the two rules, like this:
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}.
+You can specify the type of locations by defining a macro called
+@code{YYLTYPE}, just as you can specify the semantic value type by
+defining a @code{YYSTYPE} macro (@pxref{Value Type}).
When @code{YYLTYPE} is not defined, Bison uses a default structure type with
four members:
@} YYLTYPE;
@end example
+At the beginning of the parsing, Bison initializes all these fields to 1
+for @code{yylloc}.
+
@node Actions and Locations
@subsection Actions and Locations
@cindex location actions
@end example
@vindex yylloc
-It is also possible to access the location of the look-ahead token, if any,
+It is also possible to access the location of the lookahead token, if any,
from a semantic action.
This location is stored in @code{yylloc}.
@xref{Action Features, ,Special Features for Use in Actions}.
Note that, unlike making a @code{union} declaration in C, you need not write
a semicolon after the closing brace.
+Instead of @code{%union}, you can define and use your own union type
+@code{YYSTYPE} if your grammar contains at least one
+@samp{<@var{type}>} tag. For example, you can put the following into
+a header file @file{parser.h}:
+
+@example
+@group
+union YYSTYPE @{
+ double val;
+ symrec *tptr;
+@};
+typedef union YYSTYPE YYSTYPE;
+@end group
+@end example
+
+@noindent
+and then your grammar can use the following
+instead of @code{%union}:
+
+@example
+@group
+%@{
+#include "parser.h"
+%@}
+%type <val> expr
+%token <tptr> ID
+@end group
+@end example
+
@node Type Decl
@subsection Nonterminal Symbols
@cindex declaring value types, nonterminals
@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{$$} and
-@code{@@$} --- initial value and location of the look-ahead --- and the
+@code{@@$} --- initial value and location of the lookahead --- and the
@code{%parse-param}.
@end deffn
@subsection Freeing Discarded Symbols
@cindex freeing discarded symbols
@findex %destructor
+@findex %symbol-default
During error recovery (@pxref{Error Recovery}), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded
Invoke the braced @var{code} whenever the parser discards one of the
@var{symbols}.
Within @var{code}, @code{$$} designates the semantic value associated
-with the discarded symbol. The additional parser parameters are also
-available (@pxref{Parser Function, , The Parser Function
-@code{yyparse}}).
+with the discarded symbol, and @code{@@$} designates its location.
+The additional parser parameters are also available (@pxref{Parser Function, ,
+The Parser Function @code{yyparse}}).
+
+When a symbol is listed among @var{symbols}, its @code{%destructor} is called a
+per-symbol @code{%destructor}.
+You may also define a per-type @code{%destructor} by listing a semantic type
+among @var{symbols}.
+In that case, the parser will invoke this @var{code} whenever it discards any
+grammar symbol that has that semantic type unless that symbol has its own
+per-symbol @code{%destructor}.
+
+Finally, you may define a default @code{%destructor} by placing
+@code{%symbol-default} in the @var{symbols} list of exactly one
+@code{%destructor} declaration in your grammar file.
+In that case, the parser will invoke the associated @var{code} whenever it
+discards any user-defined grammar symbol for which there is no per-type or
+per-symbol @code{%destructor}.
@end deffn
+@noindent
For instance:
@smallexample
-%union
-@{
- char *string;
-@}
-%token <string> STRING
-%type <string> string
-%destructor @{ free ($$); @} STRING string
+%union @{ char *string; @}
+%token <string> STRING1
+%token <string> STRING2
+%type <string> string1
+%type <string> string2
+%union @{ char character; @}
+%token <character> CHR
+%type <character> chr
+%destructor @{ free ($$); @} %symbol-default
+%destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
+%destructor @{ @} <character>
@end smallexample
@noindent
-guarantees that when a @code{STRING} or a @code{string} is discarded,
-its associated memory will be freed.
+guarantees that, when the parser discards any user-defined symbol that has a
+semantic type tag other than @code{<character>}, it passes its semantic value
+to @code{free}.
+However, when the parser discards a @code{STRING1} or a @code{string1}, it also
+prints its line number to @code{stdout}.
+It performs only the second @code{%destructor} in this case, so it invokes
+@code{free} only once.
+
+Notice that a Bison-generated parser invokes the default @code{%destructor}
+only for user-defined as opposed to Bison-defined symbols.
+For example, the parser will not invoke it for the special Bison-defined
+symbols @code{$accept}, @code{$undefined}, or @code{$end} (@pxref{Table of
+Symbols, ,Bison Symbols}), none of which you can reference in your grammar.
+It also will not invoke it for the @code{error} token (@pxref{Table of Symbols,
+,error}), which is always defined by Bison regardless of whether you reference
+it in your grammar.
+However, it will invoke it for the end token (token 0) if you redefine it from
+@code{$end} to, for example, @code{END}:
+
+@smallexample
+%token END 0
+@end smallexample
+
+@ignore
+@noindent
+In the future, it may be possible to redefine the @code{error} token as a
+nonterminal that captures the discarded symbols.
+In that case, the parser will invoke the default destructor for it as well.
+@end ignore
@sp 1
@item
incoming terminals during the second phase of error recovery,
@item
-the current look-ahead and the entire stack (except the current
+the current lookahead 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.
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}
+For C parsers, the output header declares @code{YYSTYPE} unless
+@code{YYSTYPE} is already defined as a macro or you have used a
+@code{<@var{type}>} tag without using @code{%union}.
+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
+or type definition
(@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
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
+the @code{YYSTYPE} macro and @code{yylval}. @xref{Locations, ,Tracking
Locations}.
This output file is normally essential if you wish to put the definition
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}.
+
+@findex %requires
+@findex %provides
+If you have declared @code{%requires} or @code{%provides}, the output
+header also contains their code.
+@xref{Table of Symbols, ,%requires}.
@end deffn
@deffn {Directive} %destructor
@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
+parser states and what is done for each type of lookahead token in
that state. @xref{Understanding, , Understanding Your Parser}, for more
information.
@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.
+a single value, and only when there is no lookahead token.
It is also disallowed in @acronym{GLR} parsers.
-It installs a look-ahead token with token type @var{token} and
+It installs a lookahead 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 lookahead token already, then it reports a syntax error with
a message @samp{cannot back up} and performs ordinary error
recovery.
@deffn {Macro} YYEMPTY
@vindex YYEMPTY
-Value stored in @code{yychar} when there is no look-ahead token.
+Value stored in @code{yychar} when there is no lookahead token.
@end deffn
@deffn {Macro} YYEOF
@vindex YYEOF
-Value stored in @code{yychar} when the look-ahead is the end of the input
+Value stored in @code{yychar} when the lookahead is the end of the input
stream.
@end deffn
@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.
+@findex YYRECOVERING
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
@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
+Variable containing either the lookahead token, or @code{YYEOF} when the
+lookahead is the end of the input stream, or @code{YYEMPTY} when no lookahead
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}.
+@xref{Lookahead, ,Lookahead Tokens}.
@end deffn
@deffn {Macro} yyclearin;
-Discard the current look-ahead token. This is useful primarily in
+Discard the current lookahead token. This is useful primarily in
error rules.
Do not invoke @code{yyclearin} in a deferred semantic action (@pxref{GLR
Semantic Actions}).
@end deffn
@deffn {Variable} yylloc
-Variable containing the look-ahead token location when @code{yychar} is not set
+Variable containing the lookahead 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}).
@end deffn
@deffn {Variable} yylval
-Variable containing the look-ahead token semantic value when @code{yychar} is
+Variable containing the lookahead 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}).
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.
+* Lookahead:: 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.
* 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
+@node Lookahead
+@section Lookahead Tokens
+@cindex lookahead 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
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
+@dfn{lookahead 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
+the lookahead token remains off to the side. When no more reductions
+should take place, the lookahead 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
+token type of the lookahead token, some rules may choose to delay their
application.
-Here is a simple case where look-ahead is needed. These three rules define
+Here is a simple case where lookahead is needed. These three rules define
expressions which contain binary addition operators and postfix unary
factorial operators (@samp{!}), and allow parentheses for grouping.
@vindex yychar
@vindex yylval
@vindex yylloc
-The look-ahead token is stored in the variable @code{yychar}.
+The lookahead 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}.
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
+When the @code{ELSE} token is read and becomes the lookahead 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
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
+contains @w{@samp{1 - 2}} and the lookahead token is @samp{-}: shifting
makes right-associativity.
@node Using Precedence
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
+of the rule being considered with that of the lookahead 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
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.
+the lookahead token has no precedence, then the default is to shift.
@node Contextual Precedence
@section Context-Dependent Precedence
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
+Each time a lookahead token is read, the current parser state together
+with the type of lookahead token are looked up in a table. This table
+entry can say, ``Shift the lookahead 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
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
+There is one other alternative: the table can say that the lookahead token
is erroneous in the current state. This causes error processing to begin
(@pxref{Error Recovery}).
@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
+of lookahead: 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).
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
+that the rules would require different lookahead 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).
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.
+based on a summary of the preceding input and on one extra token of lookahead.
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
+lookahead, 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
@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
+lookahead 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
@samp{yyerrok;} is a valid C statement.
@findex yyclearin
-The previous look-ahead token is reanalyzed immediately after an error. If
+The previous lookahead 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.
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
+probably correct. The previous lookahead 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.
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
+Syntax error diagnostics are suppressed while recovering from a syntax
+error.
@node Context Dependency
@chapter Handling Context Dependencies
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).
+representation of it, either textually or graphically (as a DOT file).
The textual file is generated when the options @option{--report} or
@option{--verbose} are specified, see @xref{Invocation, , Invoking
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
+symbol, and the lookahead 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.''
+lookahead 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
+report lists @code{NUM} as a lookahead 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
@end example
@noindent
-the rule 5, @samp{exp: NUM;}, is completed. Whatever the look-ahead token
+the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead 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}).
@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
+because of the item @samp{exp -> exp . '+' exp}, if the lookahead 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
$default reduce using rule 1 (exp)
@end example
-Indeed, there are two actions associated to the look-ahead @samp{/}:
+Indeed, there are two actions associated to the lookahead @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
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
+possible, the lookahead is required to select the action. State 8 is
+one such state: if the lookahead 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
+lookahead 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:
+with some set of possible lookahead tokens. When run with
+@option{--report=lookahead}, Bison specifies these lookahead tokens:
@example
state 8
@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}.
+and @code{YYFPRINTF} 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}.
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}:
+@file{y.tab.h}.
+Also, if generating an @acronym{LALR}(1) parser in C, generate @code{#define}
+statements in addition to an @code{enum} to associate token numbers with token
+names.
+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
@item -b @var{file-prefix}
@itemx --file-prefix=@var{prefix}
-Pretend that @code{%file-prefix} was specified, i.e, specify prefix to use
+Pretend that @code{%file-prefix} was specified, i.e., specify prefix to use
for all Bison output file names. @xref{Decl Summary}.
@item -r @var{things}
Description of the grammar, conflicts (resolved and unresolved), and
@acronym{LALR} automaton.
-@item look-ahead
+@item lookahead
Implies @code{state} and augments the description of the automaton with
-each rule's look-ahead set.
+each rule's lookahead set.
@item itemset
Implies @code{state} and augments the description of the automaton with
@item -v
@itemx --verbose
-Pretend that @code{%verbose} was specified, i.e, write an extra output
+Pretend that @code{%verbose} was specified, i.e., write an extra output
file containing verbose descriptions of the grammar and
parser. @xref{Decl Summary}.
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}.
+Output a graphical representation of the @acronym{LALR}(1) grammar
+automaton computed by Bison, in @uref{http://www.graphviz.org/, Graphviz}
+@uref{http://www.graphviz.org/doc/info/lang.html, @acronym{DOT}} format.
+If the grammar file is @file{foo.y}, the output file will
+be @file{foo.dot}.
@item --graph=@var{graph-file}
The behavior of @var{--graph} is the same than @samp{-g}. The only
@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.
+(Assuming the extension of the input file was @samp{.yy}.) The
+declaration and implementation of the C++ parser class. The basename
+and extension of these two files follow the same rules as with regular C
+parsers (@pxref{Invocation}).
-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
+The header is @emph{mandatory}; you must either pass
+@option{-d}/@option{--defines} to @command{bison}, or use the
@samp{%defines} directive.
@end table
@comment file: calc++-driver.hh
@example
-// Announce to Flex the prototype we want for lexing function, ...
-# define YY_DECL \
+// Tell Flex the lexer's prototype ...
+# define YY_DECL \
yy::calcxx_parser::token_type \
yylex (yy::calcxx_parser::semantic_type* yylval, \
yy::calcxx_parser::location_type* yylloc, \
@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
@end example
@noindent
+@findex %requires
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.
+@xref{Table of Symbols, ,%requires}.
@comment file: calc++-parser.yy
@example
-%@{
+%requires @{
# include <string>
class calcxx_driver;
-%@}
+@}
@end example
@noindent
@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.
+@findex %code
+The code between @samp{%code @{} and @samp{@}} is output in the
+@file{*.cc} file; it needs detailed knowledge about the driver.
@comment file: calc++-parser.yy
@example
-%@{
+%code @{
# include "calc++-driver.hh"
-%@}
+@}
@end example
assignments: assignments assignment @{@}
| /* Nothing. */ @{@};
-assignment: "identifier" ":=" exp @{ driver.variables[*$1] = $3; @};
+assignment:
+ "identifier" ":=" exp
+ @{ driver.variables[*$1] = $3; delete $1; @};
%left '+' '-';
%left '*' '/';
| exp '-' exp @{ $$ = $1 - $3; @}
| exp '*' exp @{ $$ = $1 * $3; @}
| exp '/' exp @{ $$ = $1 / $3; @}
- | "identifier" @{ $$ = driver.variables[*$1]; @}
+ | "identifier" @{ $$ = driver.variables[*$1]; delete $1; @}
| "number" @{ $$ = $1; @};
%%
@end example
# include <string>
# include "calc++-driver.hh"
# include "calc++-parser.hh"
-/* Work around a bug in flex 2.5.31. See Debian bug 333231
- <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>. */
+
+/* Work around an incompatibility in flex (at least versions
+ 2.5.31 through 2.5.33): it generates code that does
+ not conform to C89. See Debian bug 333231
+ <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>. */
# undef yywrap
# define yywrap() 1
+
/* By default yylex returns int, we use token_type.
Unfortunately yyterminate by default returns 0, which is
not of token_type. */
driver.trace_scanning = true;
else
@{
- driver.parse (*argv);
- std::cout << driver.result << std::endl;
+ driver.parse (*argv);
+ std::cout << driver.result << std::endl;
@}
@}
@end example
Start-Symbol}. It cannot be used in the grammar.
@end deffn
+@deffn {Directive} %code @{@var{code}@}
+Specifies code to be inserted into the code file after the contents of the
+header file.
+@xref{Table of Symbols, ,%requires}.
+@end deffn
+
+@deffn {Directive} %provides @{@var{code}@}
+Specifies code to be inserted both into the header file (if generated;
+@pxref{Table of Symbols, ,%defines}) and into the code file after any
+Bison-generated definitions.
+@xref{Table of Symbols, ,%requires}.
+@end deffn
+
+@deffn {Directive} %requires @{@var{code}@}
+Specifies code to be inserted both into the header file (if generated;
+@pxref{Table of Symbols, ,%defines}) and into the code file before any
+Bison-generated definitions.
+
+@cindex Prologue
+@findex %union
+@findex %provides
+@findex %code
+For example, the following declaration order in the grammar file reflects the
+order in which Bison will output these code blocks. However, you are free to
+declare these code blocks in your grammar file in whatever order is most
+convenient for you:
+
+@smallexample
+%requires @{
+ /* Bison inserts this block into both the header file and the code
+ * file. In both files, the point of insertion is before any
+ * Bison-generated token, semantic type, location type, and class
+ * definitions. This is a good place to define %union
+ * dependencies, for example. */
+@}
+%union @{
+ /* Unlike the traditional Yacc prologue blocks, the output order
+ * for %requires, %provides or %code blocks is not affected by their
+ * declaration position relative to any %union in the grammar file. */
+@}
+%provides @{
+ /* Bison inserts this block into both the header file and the code
+ * file. In both files, the point of insertion is after the
+ * Bison-generated definitions. This is a good place to declare or
+ * define public functions or data structures that depend on the
+ * Bison-generated definitions. */
+@}
+%code @{
+ /* Bison treats this block like a post-prologue block: it inserts
+ * it into the code file after the contents of the header file. It
+ * does *not* insert it into the header file. This is a good place
+ * to declare or define internal functions or data structures that
+ * depend on the Bison-generated definitions. */
+@}
+@end smallexample
+
+If you have multiple occurrences of any one of the above declarations, Bison
+will concatenate the contents in declaration order.
+
+@xref{Prologue, ,The Prologue}.
+@end deffn
+
+@deffn {Directive} %debug
+Equip the parser for debugging. @xref{Decl Summary}.
+@end deffn
+
@deffn {Directive} %debug
Equip the parser for debugging. @xref{Decl Summary}.
@end deffn
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 @code{error} becomes the current lookahead token. Actions
+corresponding to @code{error} are then executed, and the lookahead
token is reset to the token that originally caused the violation.
@xref{Error Recovery}.
@end deffn
Start-Symbol}.
@end deffn
+@deffn {Directive} %symbol-default
+Used to declare a default @code{%destructor} or default @code{%printer}.
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
@deffn {Directive} %token
Bison declaration to declare token(s) without specifying precedence.
@xref{Token Decl, ,Token Type Names}.
@end deffn
@deffn {Macro} YYBACKUP
-Macro to discard a value from the parser stack and fake a look-ahead
+Macro to discard a value from the parser stack and fake a lookahead
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
+lookahead 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}.
+lookahead token. @xref{Error Recovery}.
@end deffn
@deffn {Macro} YYDEBUG
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.
+In semantic actions, it stores the location of the lookahead token.
@xref{Actions and Locations, ,Actions and Locations}.
@end deffn
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.
+In semantic actions, it stores the semantic value of the lookahead token.
@xref{Actions, ,Actions}.
@end deffn
@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}.
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
+@xref{Action Features, ,Special Features for Use in Actions}.
@end deffn
@deffn {Macro} YYSTACK_USE_ALLOCA
@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
+@item Lookahead token
+A token already read but not yet shifted. @xref{Lookahead, ,Lookahead
Tokens}.
@item @acronym{LALR}(1)
@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.
+lookahead is needed to disambiguate the parsing of any piece of input.
@item Nonterminal symbol
A grammar symbol standing for a grammatical construct that can
@c LocalWords: strncmp intval tindex lvalp locp llocp typealt YYBACKUP
@c LocalWords: YYEMPTY YYEOF YYRECOVERING yyclearin GE def UMINUS maybeword
@c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH
-@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm VCG notype
+@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm notype
@c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args
-@c LocalWords: YYPRINTF infile ypp yxx outfile itemx vcg tex leaderfill
+@c LocalWords: infile ypp yxx outfile itemx tex leaderfill
@c LocalWords: hbox hss hfill tt ly yyin fopen fclose ofirst gcc ll
@c LocalWords: yyrestart nbar yytext fst snd osplit ntwo strdup AST
@c LocalWords: YYSTACK DVI fdl printindex
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