--- /dev/null
+\input texinfo @c -*-texinfo-*-
+@comment %**start of header
+@setfilename bison.info
+@settitle Bison 1.20
+@setchapternewpage odd
+
+@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 next time, consider using @set for edition number, etc...
+
+@c Set following if you have the new `shorttitlepage' command
+@c @clear shorttitlepage-enabled
+@c @set shorttitlepage-enabled
+
+@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
+
+@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
+
+@ifinfo
+This file documents the Bison parser generator.
+
+Copyright (C) 1988, 1989, 1990, 1991, 1992 Free Software Foundation, Inc.
+
+Permission is granted to make and distribute verbatim copies of
+this manual provided the copyright notice and this permission notice
+are preserved on all copies.
+
+@ignore
+Permission is granted to process this file through Tex and print the
+results, provided the printed document carries copying permission
+notice identical to this one except for the removal of this paragraph
+(this paragraph not being relevant to the printed manual).
+
+@end ignore
+Permission is granted to copy and distribute modified versions of this
+manual under the conditions for verbatim copying, provided also that the
+sections entitled ``GNU General Public License'' and ``Conditions for
+Using Bison'' are included exactly as in the original, and provided that
+the entire resulting derived work is distributed under the terms of a
+permission notice identical to this one.
+
+Permission is granted to copy and distribute translations of this manual
+into another language, under the above conditions for modified versions,
+except that the sections entitled ``GNU General Public License'',
+``Conditions for Using Bison'' and this permission notice may be
+included in translations approved by the Free Software Foundation
+instead of in the original English.
+@end ifinfo
+
+@ifset shorttitlepage-enabled
+@shorttitlepage Bison
+@end ifset
+@titlepage
+@title Bison
+@subtitle The YACC-compatible Parser Generator
+@subtitle December 1992, Bison Version 1.20
+
+@author by Charles Donnelly and Richard Stallman
+
+@page
+@vskip 0pt plus 1filll
+Copyright @copyright{} 1988, 1989, 1990, 1991, 1992 Free Software
+Foundation
+
+@sp 2
+Published by the Free Software Foundation @*
+675 Massachusetts Avenue @*
+Cambridge, MA 02139 USA @*
+Printed copies are available for $15 each.@*
+ISBN-1-882114-30-2
+
+Permission is granted to make and distribute verbatim copies of
+this manual provided the copyright notice and this permission notice
+are preserved on all copies.
+
+@ignore
+Permission is granted to process this file through TeX and print the
+results, provided the printed document carries copying permission
+notice identical to this one except for the removal of this paragraph
+(this paragraph not being relevant to the printed manual).
+
+@end ignore
+Permission is granted to copy and distribute modified versions of this
+manual under the conditions for verbatim copying, provided also that the
+sections entitled ``GNU General Public License'' and ``Conditions for
+Using Bison'' are included exactly as in the original, and provided that
+the entire resulting derived work is distributed under the terms of a
+permission notice identical to this one.
+
+Permission is granted to copy and distribute translations of this manual
+into another language, under the above conditions for modified versions,
+except that the sections entitled ``GNU General Public License'',
+``Conditions for Using Bison'' and this permission notice may be
+included in translations approved by the Free Software Foundation
+instead of in the original English.
+@sp 2
+Cover art by Etienne Suvasa.
+@end titlepage
+@page
+
+@node Top, Introduction, (dir), (dir)
+
+@ifinfo
+This manual documents version 1.20 of Bison.
+@end ifinfo
+
+@menu
+* Introduction::
+* Conditions::
+* Copying:: The 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:: Debugging Bison parsers that parse wrong.
+* Invocation:: How to run Bison (to produce the parser source file).
+* Table of Symbols:: All the keywords of the Bison language are explained.
+* Glossary:: Basic concepts are explained.
+* Index:: Cross-references to the text.
+
+ --- 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.
+* 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.
+
+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.
+* 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. Bison and C 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::
+
+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.
+* 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
+
+* C Declarations:: Syntax and usage of the C declarations section.
+* Bison Declarations:: Syntax and usage of the Bison declarations section.
+* Grammar Rules:: Syntax and usage of the grammar rules section.
+* C Code:: Syntax and usage of the additional C code section.
+
+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.
+
+Bison Declarations
+
+* 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.
+* Expect Decl:: Suppressing warnings about shift/reduce 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.
+
+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 Positions:: How @code{yylex} must return the text position
+ (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.
+* Stack Overflow:: What happens when stack gets full. 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.
+
+Invoking Bison
+
+* Bison Options:: All the options described in detail,
+ in alphabetical order by short options.
+* Option Cross Key:: Alphabetical list of long options.
+* VMS Invocation:: Bison command syntax on VMS.
+@end menu
+
+@node Introduction, Conditions, Top, Top
+@unnumbered Introduction
+@cindex introduction
+
+@dfn{Bison} is a general-purpose parser generator that converts a
+grammar description for an 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. This edition corresponds to version 1.20 of Bison.
+
+@node Conditions, Copying, Introduction, Top
+@unnumbered Conditions for Using Bison
+
+Bison grammars can be used only in programs that are free software. This
+is in contrast to what happens with the GNU C compiler and the other
+GNU programming tools.
+
+The reason Bison is special is that 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.)
+
+As a result, the Bison parser file is covered by the same copying
+conditions that cover Bison itself and the rest of the GNU system: any
+program containing it has to be distributed under the standard GNU copying
+conditions.
+
+Occasionally people who would like to use Bison to develop proprietary
+programs complain about this.
+
+We don't particularly sympathize with their complaints. The purpose of the
+GNU project is to promote the right to share software and the practice of
+sharing software; it is a means of changing society. The people who
+complain are planning to be uncooperative toward the rest of the world; why
+should they deserve our help in doing so?
+
+However, it's possible that a change in these conditions might encourage
+computer companies to use and distribute the GNU system. If so, then we
+might decide to change the terms on @code{yyparse} as a matter of the
+strategy of promoting the right to share. Such a change would be
+irrevocable. Since we stand by the copying permissions we have announced,
+we cannot withdraw them once given.
+
+We mustn't make an irrevocable change hastily. We have to wait until there
+is a complete GNU system and there has been time to learn how this issue
+affects its reception.
+
+@node Copying, Concepts, Conditions, Top
+@unnumbered GNU GENERAL PUBLIC LICENSE
+@center Version 2, June 1991
+
+@display
+Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc.
+675 Mass Ave, Cambridge, MA 02139, USA
+
+Everyone is permitted to copy and distribute verbatim copies
+of this license document, but changing it is not allowed.
+@end display
+
+@unnumberedsec Preamble
+
+ The licenses for most software are designed to take away your
+freedom to share and change it. By contrast, the GNU General Public
+License is intended to guarantee your freedom to share and change free
+software---to make sure the software is free for all its users. This
+General Public License applies to most of the Free Software
+Foundation's software and to any other program whose authors commit to
+using it. (Some other Free Software Foundation software is covered by
+the GNU Library General Public License instead.) You can apply it to
+your programs, too.
+
+ When we speak of free software, we are referring to freedom, not
+price. Our General Public Licenses are designed to make sure that you
+have the freedom to distribute copies of free software (and charge for
+this service if you wish), that you receive source code or can get it
+if you want it, that you can change the software or use pieces of it
+in new free programs; and that you know you can do these things.
+
+ To protect your rights, we need to make restrictions that forbid
+anyone to deny you these rights or to ask you to surrender the rights.
+These restrictions translate to certain responsibilities for you if you
+distribute copies of the software, or if you modify it.
+
+ For example, if you distribute copies of such a program, whether
+gratis or for a fee, you must give the recipients all the rights that
+you have. You must make sure that they, too, receive or can get the
+source code. And you must show them these terms so they know their
+rights.
+
+ We protect your rights with two steps: (1) copyright the software, and
+(2) offer you this license which gives you legal permission to copy,
+distribute and/or modify the software.
+
+ Also, for each author's protection and ours, we want to make certain
+that everyone understands that there is no warranty for this free
+software. If the software is modified by someone else and passed on, we
+want its recipients to know that what they have is not the original, so
+that any problems introduced by others will not reflect on the original
+authors' reputations.
+
+ Finally, any free program is threatened constantly by software
+patents. We wish to avoid the danger that redistributors of a free
+program will individually obtain patent licenses, in effect making the
+program proprietary. To prevent this, we have made it clear that any
+patent must be licensed for everyone's free use or not licensed at all.
+
+ The precise terms and conditions for copying, distribution and
+modification follow.
+
+@iftex
+@unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+@end iftex
+@ifinfo
+@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+@end ifinfo
+
+@enumerate
+@item
+This License applies to any program or other work which contains
+a notice placed by the copyright holder saying it may be distributed
+under the terms of this General Public License. The ``Program'', below,
+refers to any such program or work, and a ``work based on the Program''
+means either the Program or any derivative work under copyright law:
+that is to say, a work containing the Program or a portion of it,
+either verbatim or with modifications and/or translated into another
+language. (Hereinafter, translation is included without limitation in
+the term ``modification''.) Each licensee is addressed as ``you''.
+
+Activities other than copying, distribution and modification are not
+covered by this License; they are outside its scope. The act of
+running the Program is not restricted, and the output from the Program
+is covered only if its contents constitute a work based on the
+Program (independent of having been made by running the Program).
+Whether that is true depends on what the Program does.
+
+@item
+You may copy and distribute verbatim copies of the Program's
+source code as you receive it, in any medium, provided that you
+conspicuously and appropriately publish on each copy an appropriate
+copyright notice and disclaimer of warranty; keep intact all the
+notices that refer to this License and to the absence of any warranty;
+and give any other recipients of the Program a copy of this License
+along with the Program.
+
+You may charge a fee for the physical act of transferring a copy, and
+you may at your option offer warranty protection in exchange for a fee.
+
+@item
+You may modify your copy or copies of the Program or any portion
+of it, thus forming a work based on the Program, and copy and
+distribute such modifications or work under the terms of Section 1
+above, provided that you also meet all of these conditions:
+
+@enumerate a
+@item
+You must cause the modified files to carry prominent notices
+stating that you changed the files and the date of any change.
+
+@item
+You must cause any work that you distribute or publish, that in
+whole or in part contains or is derived from the Program or any
+part thereof, to be licensed as a whole at no charge to all third
+parties under the terms of this License.
+
+@item
+If the modified program normally reads commands interactively
+when run, you must cause it, when started running for such
+interactive use in the most ordinary way, to print or display an
+announcement including an appropriate copyright notice and a
+notice that there is no warranty (or else, saying that you provide
+a warranty) and that users may redistribute the program under
+these conditions, and telling the user how to view a copy of this
+License. (Exception: if the Program itself is interactive but
+does not normally print such an announcement, your work based on
+the Program is not required to print an announcement.)
+@end enumerate
+
+These requirements apply to the modified work as a whole. If
+identifiable sections of that work are not derived from the Program,
+and can be reasonably considered independent and separate works in
+themselves, then this License, and its terms, do not apply to those
+sections when you distribute them as separate works. But when you
+distribute the same sections as part of a whole which is a work based
+on the Program, the distribution of the whole must be on the terms of
+this License, whose permissions for other licensees extend to the
+entire whole, and thus to each and every part regardless of who wrote it.
+
+Thus, it is not the intent of this section to claim rights or contest
+your rights to work written entirely by you; rather, the intent is to
+exercise the right to control the distribution of derivative or
+collective works based on the Program.
+
+In addition, mere aggregation of another work not based on the Program
+with the Program (or with a work based on the Program) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+@item
+You may copy and distribute the Program (or a work based on it,
+under Section 2) in object code or executable form under the terms of
+Sections 1 and 2 above provided that you also do one of the following:
+
+@enumerate a
+@item
+Accompany it with the complete corresponding machine-readable
+source code, which must be distributed under the terms of Sections
+1 and 2 above on a medium customarily used for software interchange; or,
+
+@item
+Accompany it with a written offer, valid for at least three
+years, to give any third party, for a charge no more than your
+cost of physically performing source distribution, a complete
+machine-readable copy of the corresponding source code, to be
+distributed under the terms of Sections 1 and 2 above on a medium
+customarily used for software interchange; or,
+
+@item
+Accompany it with the information you received as to the offer
+to distribute corresponding source code. (This alternative is
+allowed only for noncommercial distribution and only if you
+received the program in object code or executable form with such
+an offer, in accord with Subsection b above.)
+@end enumerate
+
+The source code for a work means the preferred form of the work for
+making modifications to it. For an executable work, complete source
+code means all the source code for all modules it contains, plus any
+associated interface definition files, plus the scripts used to
+control compilation and installation of the executable. However, as a
+special exception, the source code distributed need not include
+anything that is normally distributed (in either source or binary
+form) with the major components (compiler, kernel, and so on) of the
+operating system on which the executable runs, unless that component
+itself accompanies the executable.
+
+If distribution of executable or object code is made by offering
+access to copy from a designated place, then offering equivalent
+access to copy the source code from the same place counts as
+distribution of the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+@item
+You may not copy, modify, sublicense, or distribute the Program
+except as expressly provided under this License. Any attempt
+otherwise to copy, modify, sublicense or distribute the Program is
+void, and will automatically terminate your rights under this License.
+However, parties who have received copies, or rights, from you under
+this License will not have their licenses terminated so long as such
+parties remain in full compliance.
+
+@item
+You are not required to accept this License, since you have not
+signed it. However, nothing else grants you permission to modify or
+distribute the Program or its derivative works. These actions are
+prohibited by law if you do not accept this License. Therefore, by
+modifying or distributing the Program (or any work based on the
+Program), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Program or works based on it.
+
+@item
+Each time you redistribute the Program (or any work based on the
+Program), the recipient automatically receives a license from the
+original licensor to copy, distribute or modify the Program subject to
+these terms and conditions. You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties to
+this License.
+
+@item
+If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
+conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License. If you cannot
+distribute so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you
+may not distribute the Program at all. For example, if a patent
+license would not permit royalty-free redistribution of the Program by
+all those who receive copies directly or indirectly through you, then
+the only way you could satisfy both it and this License would be to
+refrain entirely from distribution of the Program.
+
+If any portion of this section is held invalid or unenforceable under
+any particular circumstance, the balance of the section is intended to
+apply and the section as a whole is intended to apply in other
+circumstances.
+
+It is not the purpose of this section to induce you to infringe any
+patents or other property right claims or to contest validity of any
+such claims; this section has the sole purpose of protecting the
+integrity of the free software distribution system, which is
+implemented by public license practices. Many people have made
+generous contributions to the wide range of software distributed
+through that system in reliance on consistent application of that
+system; it is up to the author/donor to decide if he or she is willing
+to distribute software through any other system and a licensee cannot
+impose that choice.
+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+@item
+If the distribution and/or use of the Program is restricted in
+certain countries either by patents or by copyrighted interfaces, the
+original copyright holder who places the Program under this License
+may add an explicit geographical distribution limitation excluding
+those countries, so that distribution is permitted only in or among
+countries not thus excluded. In such case, this License incorporates
+the limitation as if written in the body of this License.
+
+@item
+The Free Software Foundation may publish revised and/or new versions
+of the General Public License from time to time. Such new versions will
+be similar in spirit to the present version, but may differ in detail to
+address new problems or concerns.
+
+Each version is given a distinguishing version number. If the Program
+specifies a version number of this License which applies to it and ``any
+later version'', you have the option of following the terms and conditions
+either of that version or of any later version published by the Free
+Software Foundation. If the Program does not specify a version number of
+this License, you may choose any version ever published by the Free Software
+Foundation.
+
+@item
+If you wish to incorporate parts of the Program into other free
+programs whose distribution conditions are different, write to the author
+to ask for permission. For software which is copyrighted by the Free
+Software Foundation, write to the Free Software Foundation; we sometimes
+make exceptions for this. Our decision will be guided by the two goals
+of preserving the free status of all derivatives of our free software and
+of promoting the sharing and reuse of software generally.
+
+@iftex
+@heading NO WARRANTY
+@end iftex
+@ifinfo
+@center NO WARRANTY
+@end ifinfo
+
+@item
+BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
+FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN
+OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
+PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
+OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS
+TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE
+PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
+REPAIR OR CORRECTION.
+
+@item
+IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
+REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
+INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
+OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
+TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
+YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
+PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGES.
+@end enumerate
+
+@iftex
+@heading END OF TERMS AND CONDITIONS
+@end iftex
+@ifinfo
+@center END OF TERMS AND CONDITIONS
+@end ifinfo
+
+@page
+@unnumberedsec How to Apply These Terms to Your New Programs
+
+ If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+ To do so, attach the following notices to the program. It is safest
+to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least
+the ``copyright'' line and a pointer to where the full notice is found.
+
+@smallexample
+@var{one line to give the program's name and a brief idea of what it does.}
+Copyright (C) 19@var{yy} @var{name of author}
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
+@end smallexample
+
+Also add information on how to contact you by electronic and paper mail.
+
+If the program is interactive, make it output a short notice like this
+when it starts in an interactive mode:
+
+@smallexample
+Gnomovision version 69, Copyright (C) 19@var{yy} @var{name of author}
+Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
+type `show w'.
+This is free software, and you are welcome to redistribute it
+under certain conditions; type `show c' for details.
+@end smallexample
+
+The hypothetical commands @samp{show w} and @samp{show c} should show
+the appropriate parts of the General Public License. Of course, the
+commands you use may be called something other than @samp{show w} and
+@samp{show c}; they could even be mouse-clicks or menu items---whatever
+suits your program.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a ``copyright disclaimer'' for the program, if
+necessary. Here is a sample; alter the names:
+
+@smallexample
+Yoyodyne, Inc., hereby disclaims all copyright interest in the program
+`Gnomovision' (which makes passes at compilers) written by James Hacker.
+
+@var{signature of Ty Coon}, 1 April 1989
+Ty Coon, President of Vice
+@end smallexample
+
+This General Public License does not permit incorporating your program into
+proprietary programs. If your program is a subroutine library, you may
+consider it more useful to permit linking proprietary applications with the
+library. If this is what you want to do, use the GNU Library General
+Public License instead of this License.
+
+@node Concepts, Examples, Copying, Top
+@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.
+* 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, Grammar in Bison, , Concepts
+@section Languages and Context-Free Grammars
+
+@c !!! ``An expression can be an integer'' is not a valid Bison
+@c expression---Bison cannot read English! --rjc 6 Feb 1992
+@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 BNF
+@cindex Backus-Naur form
+The most common formal system for presenting such rules for humans to read
+is @dfn{Backus-Naur Form} or ``BNF'', which was developed in order to
+specify the language Algol 60. Any grammar expressed in BNF is a
+context-free grammar. The input to Bison is essentially machine-readable
+BNF.
+
+Not all context-free languages can be handled by Bison, only those
+that are LALR(1). In brief, this means that 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
+LR(1) grammar, and LALR(1) involves additional restrictions that are
+hard to explain simply; but it is rare in actual practice to find an
+LR(1) grammar that fails to be LALR(1). @xref{Mystery Conflicts, ,
+Mysterious Reduce/Reduce Conflicts}, for more information on this.
+
+@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}.@refill
+
+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:
+
+@example
+int /* @r{keyword `int'} */
+square (x) /* @r{identifier, open-paren,} */
+ /* @r{identifier, close-paren} */
+ int x; /* @r{keyword `int', identifier, semicolon} */
+@{ /* @r{open-brace} */
+ return x * x; /* @r{keyword `return', identifier,} */
+ /* @r{asterisk, identifier, semicolon} */
+@} /* @r{close-brace} */
+@end example
+
+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, Semantic Values, Language and Grammar, Concepts
+@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}.@refill
+
+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.
+
+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, Semantic Actions, Grammar in Bison, Concepts
+@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.@refill
+
+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.@refill
+
+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, Bison Parser, Semantic Values, Concepts
+@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 Bison Parser, Stages, Semantic Actions, Concepts
+@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 variable and function names used in the Bison parser file
+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.
+
+@node Stages, Grammar Layout, Bison Parser, Concepts
+@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, , Stages, Concepts
+@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{C declarations}
+%@}
+
+@var{Bison declarations}
+
+%%
+@var{Grammar rules}
+%%
+@var{Additional C code}
+@end example
+
+@noindent
+The @samp{%%}, @samp{%@{} and @samp{%@}} are punctuation that appears
+in every Bison grammar file to separate the sections.
+
+The C declarations 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.
+
+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 additional C code can contain any C code you want to use. Often the
+definition of the lexical analyzer @code{yylex} goes here, plus subroutines
+called by the actions in the grammar rules. In a simple program, all the
+rest of the program can go here.
+
+@node Examples, Grammar File, Concepts, Top
+@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.
+* 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, Infix Calc, , Examples
+@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. Bison and C 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, Rpcalc Rules, , RPN Calc
+@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>
+%@}
+
+%token NUM
+
+%% /* Grammar rules and actions follow */
+@end example
+
+The C declarations section (@pxref{C Declarations, ,The C Declarations Section}) contains two
+preprocessor directives.
+
+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 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, Rpcalc Lexer, Rpcalc Decls, RPN Calc
+@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, Rpcalc Line, , Rpcalc Rules
+@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 file.
+
+@node Rpcalc Line, Rpcalc Expr, Rpcalc Input, Rpcalc Rules
+@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{$$}. As
+a consequence, the semantic value associated with the @code{line} is
+uninitialized (its value will be unpredictable). This would be a bug if
+that value were ever used, but we don't use it: once rpcalc has printed the
+value of the user's input line, that value is no longer needed.
+
+@node Rpcalc Expr, , Rpcalc Line, Rpcalc Rules
+@subsubsection Explanation of @code{expr}
+
+The @code{exp} grouping has several rules, one for each kind of expression.
+The first rule handles the simplest expressions: those that are just numbers.
+The second handles an addition-expression, which looks like two expressions
+followed by a plus-sign. The third handles subtraction, and so on.
+
+@example
+exp: NUM
+ | exp exp '+' @{ $$ = $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 whitespace 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, Rpcalc Main, Rpcalc Rules, RPN Calc
+@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 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 the ASCII code for that 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-file is encountered.
+(Bison recognizes any nonpositive value as indicating the end of the
+input.)
+
+Here is the code for the lexical analyzer:
+
+@example
+@group
+/* Lexical analyzer returns a double floating point
+ number on the stack and the token NUM, or the ASCII
+ character read if not a number. Skips all blanks
+ and tabs, returns 0 for EOF. */
+
+#include <ctype.h>
+@end group
+
+@group
+yylex ()
+@{
+ 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-file */
+ if (c == EOF)
+ return 0;
+ /* return single chars */
+ return c;
+@}
+@end group
+@end example
+
+@node Rpcalc Main, Rpcalc Error, Rpcalc Lexer, RPN Calc
+@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
+main ()
+@{
+ yyparse ();
+@}
+@end group
+@end example
+
+@node Rpcalc Error, Rpcalc Gen, Rpcalc Main, RPN Calc
+@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{"parse 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>
+
+yyerror (s) /* Called by yyparse on error */
+ char *s;
+@{
+ printf ("%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 in the first example.
+
+@node Rpcalc Gen, Rpcalc Compile, Rpcalc Error, RPN Calc
+@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
+``additional C code'' section 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_name}.y
+@end example
+
+@noindent
+In this example the file was called @file{rpcalc.y} (for ``Reverse Polish
+CALCulator''). Bison produces a file named @file{@var{file_name}.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, , Rpcalc Gen, RPN Calc
+@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.}
+% 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}.}
+% cc rpcalc.tab.c -lm -o rpcalc
+@end group
+
+@group
+# @r{List files again.}
+% 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
+% rpcalc
+4 9 +
+13
+3 7 + 3 4 5 *+-
+-13
+3 7 + 3 4 5 * + - n @r{Note the unary minus, @samp{n}}
+13
+5 6 / 4 n +
+-3.166666667
+3 4 ^ @r{Exponentiation}
+81
+^D @r{End-of-file indicator}
+%
+@end example
+
+@node Infix Calc, Simple Error Recovery, RPN Calc, Examples
+@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--calc */
+
+%@{
+#define YYSTYPE double
+#include <math.h>
+%@}
+
+/* BISON Declarations */
+%token NUM
+%left '-' '+'
+%left '*' '/'
+%left NEG /* negation--unary minus */
+%right '^' /* exponentiation */
+
+/* Grammar follows */
+%%
+input: /* empty string */
+ | 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
+% calc
+4 + 4.5 - (34/(8*3+-3))
+6.880952381
+-56 + 2
+-54
+3 ^ 2
+9
+@end example
+
+@node Simple Error Recovery, Multi-function Calc, Infix Calc, Examples
+@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 parse 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.@refill
+
+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 Multi-function Calc, Exercises, Simple Error Recovery, Examples
+@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 non-number 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
+% acalc
+pi = 3.141592653589
+3.1415926536
+sin(pi)
+0.0000000000
+alpha = beta1 = 2.3
+2.3000000000
+alpha
+2.3000000000
+ln(alpha)
+0.8329091229
+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, Mfcalc Rules, , Multi-function Calc
+@subsection Declarations for @code{mfcalc}
+
+Here are the C and Bison declarations for the multi-function calculator.
+
+@smallexample
+%@{
+#include <math.h> /* For math functions, cos(), sin(), etc. */
+#include "calc.h" /* Contains definition of `symrec' */
+%@}
+%union @{
+double val; /* For returning numbers. */
+symrec *tptr; /* For returning symbol-table pointers */
+@}
+
+%token <val> NUM /* Simple double precision number */
+%token <tptr> VAR FNCT /* Variable and Function */
+%type <val> exp
+
+%right '='
+%left '-' '+'
+%left '*' '/'
+%left NEG /* Negation--unary minus */
+%right '^' /* Exponentiation */
+
+/* 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, Mfcalc Symtab, Mfcalc Decl, Multi-function Calc
+@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
+input: /* empty */
+ | input line
+;
+
+line:
+ '\n'
+ | exp '\n' @{ printf ("\t%.10g\n", $1); @}
+ | error '\n' @{ yyerrok; @}
+;
+
+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 of grammar */
+%%
+@end smallexample
+
+@node Mfcalc Symtab, , Mfcalc Rules, Multi-function Calc
+@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
+/* 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 */
+ double (*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 ();
+symrec *getsym ();
+@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
+@group
+#include <stdio.h>
+
+main ()
+@{
+ init_table ();
+ yyparse ();
+@}
+@end group
+
+@group
+yyerror (s) /* Called by yyparse on error */
+ char *s;
+@{
+ printf ("%s\n", s);
+@}
+
+struct init
+@{
+ char *fname;
+ double (*fnct)();
+@};
+@end group
+
+@group
+struct init arith_fncts[]
+ = @{
+ "sin", sin,
+ "cos", cos,
+ "atan", atan,
+ "ln", log,
+ "exp", exp,
+ "sqrt", sqrt,
+ 0, 0
+ @};
+
+/* The symbol table: a chain of `struct symrec'. */
+symrec *sym_table = (symrec *)0;
+@end group
+
+@group
+init_table () /* puts arithmetic functions in table. */
+@{
+ 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
+@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 (sym_name,sym_type)
+ char *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 (sym_name)
+ char *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 nondigit 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}.@refill
+
+No change is needed in the handling of numeric values and arithmetic
+operators in @code{yylex}.
+
+@smallexample
+@group
+#include <ctype.h>
+yylex ()
+@{
+ int c;
+
+ /* Ignore whitespace, 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 (c != EOF && 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, , Multi-function Calc, Examples
+@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, Interface, Examples, Top
+@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}.
+
+@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.
+* 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, Symbols, , Grammar File
+@section Outline of a Bison Grammar
+
+A Bison grammar file has four main sections, shown here with the
+appropriate delimiters:
+
+@example
+%@{
+@var{C declarations}
+%@}
+
+@var{Bison declarations}
+
+%%
+@var{Grammar rules}
+%%
+
+@var{Additional C code}
+@end example
+
+Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections.
+
+@menu
+* C Declarations:: Syntax and usage of the C declarations section.
+* Bison Declarations:: Syntax and usage of the Bison declarations section.
+* Grammar Rules:: Syntax and usage of the grammar rules section.
+* C Code:: Syntax and usage of the additional C code section.
+@end menu
+
+@node C Declarations, Bison Declarations, , Grammar Outline
+@subsection The C Declarations Section
+@cindex C declarations section
+@cindex declarations, C
+
+The @var{C declarations} 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.
+
+@node Bison Declarations, Grammar Rules, C Declarations, Grammar Outline
+@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, C Code, Bison Declarations, Grammar Outline
+@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 C Code, , Grammar Rules, Grammar Outline
+@subsection The Additional C Code Section
+@cindex additional C code section
+@cindex C code, section for additional
+
+The @var{additional C code} section is copied verbatim to the end of
+the parser file, just as the @var{C declarations} section 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. @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 static variables whose names start
+with @samp{yy} and many macros whose names start with @samp{YY}. It is a
+good idea to avoid using any such names (except those documented in this
+manual) in the additional C code section of the grammar file.
+
+@node Symbols, Rules, Grammar Outline, Grammar File
+@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 two 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 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 ASCII code, zero, is the code
+@code{yylex} returns for end-of-input (@pxref{Calling Convention, ,Calling Convention for @code{yylex}}).
+@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
+(or 0 for 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 ASCII code for
+the character, so @code{yylex} can use the identical character constant to
+generate the requisite code. 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}.
+
+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.
+
+@node Rules, Recursion, Symbols, Grammar File
+@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
+@var{result}: @var{components}@dots{}
+ ;
+@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}.
+
+Whitespace in rules is significant only to separate symbols. You can add
+extra whitespace 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
+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, Semantics, Rules, Grammar File
+@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 somethings. 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, Declarations, Recursion, Grammar File
+@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, Multiple Types, , Semantics
+@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
+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 C declarations section of the grammar
+file (@pxref{Grammar Outline, ,Outline of a Bison Grammar}).
+
+@node Multiple Types, Actions, Value Type, Semantics
+@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}, 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, Action Types, Multiple Types, Semantics
+@subsection Actions
+@cindex action
+@vindex $$
+@vindex $@var{n}
+
+An action accompanies a syntactic rule and contains C code to be executed
+each time an instance of that rule is recognized. The task of most actions
+is to compute a semantic value for the grouping built by the rule from the
+semantic values associated with tokens or smaller groupings.
+
+An action consists of C statements surrounded by braces, much like a
+compound statement in C. It can be placed at any position in the rule; it
+is executed at that position. Most rules have just one action at the end
+of the rule, following all the components. Actions in the middle of a rule
+are tricky and used only for special purposes (@pxref{Mid-Rule Actions, ,Actions in Mid-Rule}).
+
+The C code in an action can refer to the semantic values of the components
+matched by the rule with the construct @code{$@var{n}}, which stands for
+the value of the @var{n}th component. The semantic value for the grouping
+being constructed is @code{$$}. (Bison translates both of these constructs
+into array element references when it copies the actions into the parser
+file.)
+
+Here is a typical example:
+
+@example
+@group
+exp: @dots{}
+ | exp '+' exp
+ @{ $$ = $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{$$} so that it becomes the semantic value of
+the addition-expression just recognized by the rule. If there were a
+useful semantic value associated with the @samp{+} token, it could be
+referred to as @code{$2}.@refill
+
+@cindex default action
+If you don't specify an action for a rule, Bison supplies a default:
+@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule becomes
+the value of the whole rule. Of course, the default rule is valid only
+if the two data types match. There is no meaningful default action for
+an empty rule; every empty rule must have an explicit action unless the
+rule's value does not matter.
+
+@code{$@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}.
+
+@node Action Types, Mid-Rule Actions, Actions, Semantics
+@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,@refill
+
+@example
+@group
+exp: @dots{}
+ | exp '+' exp
+ @{ $$ = $1 + $3; @}
+@end group
+@end example
+
+@noindent
+@code{$1} and @code{$3} refer to instances of @code{exp}, so they all
+have the data type declared for the nonterminal symbol @code{exp}. If
+@code{$2} were used, it would have the data type declared for the
+terminal symbol @code{'+'}, whatever that might be.@refill
+
+Alternatively, you can specify the data type when you refer to the value,
+by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the
+reference. For example, if you have defined types as shown here:
+
+@example
+@group
+%union @{
+ int itype;
+ double dtype;
+@}
+@end group
+@end example
+
+@noindent
+then you can write @code{$<itype>1} to refer to the first subunit of the
+rule as an integer, or @code{$<dtype>1} to refer to it as a double.
+
+@node Mid-Rule Actions, , Action Types, Semantics
+@subsection Actions in Mid-Rule
+@cindex actions in mid-rule
+@cindex mid-rule actions
+
+Occasionally it is useful to put an action in the middle of a rule.
+These actions are written just like usual end-of-rule actions, but they
+are executed before the parser even recognizes the following components.
+
+A mid-rule action may refer to the components preceding it using
+@code{$@var{n}}, but it may not refer to subsequent components because
+it is run before they are parsed.
+
+The mid-rule action itself counts as one of the components of the rule.
+This makes a difference when there is another action later in the same rule
+(and usually there is another at the end): you have to count the actions
+along with the symbols when working out which number @var{n} to use in
+@code{$@var{n}}.
+
+The mid-rule action can also have a semantic value. The action can set
+its value with an assignment to @code{$$}, 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{}>} 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 Declarations, Multiple Parsers, Semantics, Grammar File
+@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
+* 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.
+* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* Start Decl:: Specifying the start symbol.
+* Pure Decl:: Requesting a reentrant parser.
+* Decl Summary:: Table of all Bison declarations.
+@end menu
+
+@node Token Decl, Precedence Decl, , Declarations
+@subsection Token Type Names
+@cindex declaring token type names
+@cindex token type names, declaring
+@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 precedence.
+@xref{Precedence Decl, ,Operator Precedence}.
+
+You can explicitly specify the numeric code for a token type by appending
+an integer value in the field immediately following the token name:
+
+@example
+%token NUM 300
+@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 ASCII 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
+
+@node Precedence Decl, Union Decl, Token Decl, Declarations
+@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, Type Decl, Precedence Decl, Declarations
+@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 a
+pair of braces 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}).
+
+Note that, unlike making a @code{union} declaration in C, you do not write
+a semicolon after the closing brace.
+
+@node Type Decl, Expect Decl, Union Decl, Declarations
+@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.
+
+@node Expect Decl, Start Decl, Type Decl, Declarations
+@subsection Suppressing Conflict Warnings
+@cindex suppressing conflict warnings
+@cindex preventing warnings about conflicts
+@cindex warnings, preventing
+@cindex conflicts, suppressing warnings of
+@findex %expect
+
+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 no
+warning if there are @var{n} shift/reduce conflicts and no reduce/reduce
+conflicts. The usual warning is given if there are either more or fewer
+conflicts, or if there are any reduce/reduce conflicts.
+
+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.
+@end itemize
+
+Now Bison will stop annoying you about the conflicts you have checked, but
+it will warn you again if changes in the grammar result in additional
+conflicts.
+
+@node Start Decl, Pure Decl, Expect Decl, Declarations
+@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, Decl Summary, Start Decl, Declarations
+@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.
+
+The Bison parser is not normally a reentrant program, because it uses
+statically allocated variables for communication with @code{yylex}. These
+variables include @code{yylval} and @code{yylloc}.
+
+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 effect is that the two communication variables become local
+variables in @code{yyparse}, and a different calling convention is used for
+the lexical analyzer function @code{yylex}. @xref{Pure Calling, ,Calling 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.
+
+@node Decl Summary, , Pure Decl, Declarations
+@subsection Bison Declaration Summary
+@cindex Bison declaration summary
+@cindex declaration summary
+@cindex summary, Bison declaration
+
+Here is a summary of all Bison declarations:
+
+@table @code
+@item %union
+Declare the collection of data types that semantic values may have
+(@pxref{Union Decl, ,The Collection of Value Types}).
+
+@item %token
+Declare a terminal symbol (token type name) with no precedence
+or associativity specified (@pxref{Token Decl, ,Token Type Names}).
+
+@item %right
+Declare a terminal symbol (token type name) that is right-associative
+(@pxref{Precedence Decl, ,Operator Precedence}).
+
+@item %left
+Declare a terminal symbol (token type name) that is left-associative
+(@pxref{Precedence Decl, ,Operator Precedence}).
+
+@item %nonassoc
+Declare a terminal symbol (token type name) that is nonassociative
+(using it in a way that would be associative is a syntax error)
+(@pxref{Precedence Decl, ,Operator Precedence}).
+
+@item %type
+Declare the type of semantic values for a nonterminal symbol
+(@pxref{Type Decl, ,Nonterminal Symbols}).
+
+@item %start
+Specify the grammar's start symbol (@pxref{Start Decl, ,The Start-Symbol}).
+
+@item %expect
+Declare the expected number of shift-reduce conflicts
+(@pxref{Expect Decl, ,Suppressing Conflict Warnings}).
+
+@item %pure_parser
+Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}).
+@end table
+
+@node Multiple Parsers, , Declarations, Grammar File
+@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{yylval}, @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, Algorithm, Grammar File, Top
+@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 additional
+C code 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.
+@end menu
+
+@node Parser Function, Lexical, , Interface
+@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.
+
+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 (return is due to a syntax error).
+
+In an action, you can cause immediate return from @code{yyparse} by using
+these macros:
+
+@table @code
+@item YYACCEPT
+@findex YYACCEPT
+Return immediately with value 0 (to report success).
+
+@item YYABORT
+@findex YYABORT
+Return immediately with value 1 (to report failure).
+@end table
+
+@node Lexical, Error Reporting, Parser Function, Interface
+@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}.@refill
+
+@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 Positions:: How @code{yylex} must return the text position
+ (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, Token Values, , Lexical
+@subsection Calling Convention for @code{yylex}
+
+The value that @code{yylex} returns must be the numeric code for the type
+of token it has just found, or 0 for 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. The null character
+must not be used this way, because its code is zero and that is what
+signifies end-of-input.
+
+Here is an example showing these things:
+
+@example
+yylex ()
+@{
+ @dots{}
+ if (c == EOF) /* Detect end of file. */
+ 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}.
+
+@node Token Values, Token Positions, Calling Convention, Lexical
+@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 Positions, Pure Calling, Token Values, Lexical
+@subsection Textual Positions of Tokens
+
+@vindex yylloc
+If you are using the @samp{@@@var{n}}-feature (@pxref{Action Features, ,Special Features for Use in Actions}) 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. 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, , Token Positions, Lexical
+@subsection Calling 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
+yylex (lvalp, llocp)
+ 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 positions, 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.
+
+@node Error Reporting, Action Features, Lexical, Interface
+@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{parse error} or @dfn{syntax error}
+whenever it reads a token which cannot satisfy any syntax rule. A
+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 parse error, the string is normally
+@w{@code{"parse error"}}.
+
+@findex YYERROR_VERBOSE
+If you define the macro @code{YYERROR_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{"parse
+error"}}. It doesn't matter what definition you use for
+@code{YYERROR_VERBOSE}, just whether you define it.
+
+The parser can detect one other kind of error: stack overflow. This
+happens when the input contains constructions that are very deeply
+nested. It isn't likely you will encounter this, since the Bison
+parser extends its stack automatically up to a very large limit. But
+if overflow happens, @code{yyparse} calls @code{yyerror} in the usual
+fashion, except that the argument string is @w{@code{"parser stack
+overflow"}}.
+
+The following definition suffices in simple programs:
+
+@example
+@group
+yyerror (s)
+ char *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.
+
+@vindex yynerrs
+The variable @code{yynerrs} contains the number of syntax errors
+encountered 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, , Error Reporting, Interface
+@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.
+
+@table @samp
+@item $$
+Acts like a variable that contains the semantic value for the
+grouping made by the current rule. @xref{Actions}.
+
+@item $@var{n}
+Acts like a variable that contains the semantic value for the
+@var{n}th component of the current rule. @xref{Actions}.
+
+@item $<@var{typealt}>$
+Like @code{$$} but specifies alternative @var{typealt} in the union
+specified by the @code{%union} declaration. @xref{Action Types, ,Data Types of Values in Actions}.
+
+@item $<@var{typealt}>@var{n}
+Like @code{$@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}.@refill
+
+@item YYABORT;
+Return immediately from @code{yyparse}, indicating failure.
+@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+
+@item YYACCEPT;
+Return immediately from @code{yyparse}, indicating success.
+@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+
+@item 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 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.
+
+@item YYEMPTY
+@vindex YYEMPTY
+Value stored in @code{yychar} when there is no look-ahead token.
+
+@item 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}.
+
+@item 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}.
+
+@item yychar
+Variable containing the current look-ahead token. (In a pure parser,
+this is actually a local variable within @code{yyparse}.) When there is
+no look-ahead token, the value @code{YYEMPTY} is stored in the variable.
+@xref{Look-Ahead, ,Look-Ahead Tokens}.
+
+@item yyclearin;
+Discard the current look-ahead token. This is useful primarily in
+error rules. @xref{Error Recovery}.
+
+@item yyerrok;
+Resume generating error messages immediately for subsequent syntax
+errors. This is useful primarily in error rules.
+@xref{Error Recovery}.
+
+@item @@@var{n}
+@findex @@@var{n}
+Acts like a structure variable containing information on the line
+numbers and column numbers of the @var{n}th component of the current
+rule. The structure has four members, like this:
+
+@example
+struct @{
+ int first_line, last_line;
+ int first_column, last_column;
+@};
+@end example
+
+Thus, to get the starting line number of the third component, use
+@samp{@@3.first_line}.
+
+In order for the members of this structure to contain valid information,
+you must make @code{yylex} supply this information about each token.
+If you need only certain members, then @code{yylex} need only fill in
+those members.
+
+The use of this feature makes the parser noticeably slower.
+@end table
+
+@node Algorithm, Error Recovery, Interface, Top
+@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.
+* Stack Overflow:: What happens when stack gets full. How to avoid it.
+@end menu
+
+@node Look-Ahead, Shift/Reduce, , Algorithm
+@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
+The current look-ahead token is stored in the variable @code{yychar}.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
+@node Shift/Reduce, Precedence, Look-Ahead, Algorithm
+@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, Contextual Precedence, Shift/Reduce, Algorithm
+@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, Using Precedence, , 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 addition 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 sum. 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, Precedence Examples, Why Precedence, 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, How Precedence, Using Precedence, Precedence
+@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, , Precedence Examples, 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, Parser States, Precedence, Algorithm
+@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.@refill
+
+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
+
+@node Parser States, Reduce/Reduce, Contextual Precedence, Algorithm
+@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, Mystery Conflicts, Parser States, Algorithm
+@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, Stack Overflow, Reduce/Reduce, Algorithm
+@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 LR(1).
+
+@cindex LR(1)
+@cindex LALR(1)
+However, Bison, like most parser generators, cannot actually handle all
+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 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 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
+
+@node Stack Overflow, , Mystery Conflicts, Algorithm
+@section Stack Overflow, and How to Avoid It
+@cindex stack overflow
+@cindex parser stack overflow
+@cindex overflow of parser stack
+
+The Bison parser stack can overflow if too many tokens are shifted and
+not reduced. When this happens, the parser function @code{yyparse}
+returns a nonzero value, pausing only to call @code{yyerror} to report
+the overflow.
+
+@vindex YYMAXDEPTH
+By defining the macro @code{YYMAXDEPTH}, you can control how deep the
+parser stack can become before a stack overflow occurs. 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.
+It must be a constant expression whose value is known at compile time.
+
+The stack space allowed is not necessarily allocated. If you specify a
+large value for @code{YYMAXDEPTH}, the parser actually 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.
+
+@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}. This value too must be a compile-time
+constant integer. The default is 200.
+
+@node Error Recovery, Context Dependency, Algorithm, Top
+@chapter Error Recovery
+@cindex error recovery
+@cindex recovery from errors
+
+It is not usually acceptable to have a program terminate on a parse
+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.
+
+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.
+
+For example, suppose that on a parse 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, Debugging, Error Recovery, Top
+@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, Lexical Tie-ins, , Context Dependency
+@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 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, lose;
+static foo (bar); /* @r{redeclare @code{bar} as static variable} */
+static int foo (lose); /* @r{redeclare @code{foo} as function} */
+@end example
+
+Unfortunately, the name being declared is separated from the declaration
+construct itself by a complicated syntactic structure---the ``declarator''.
+
+As a result, the part of 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, Tie-in Recovery, Semantic Tokens, Context Dependency
+@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;
+%@}
+%%
+@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 C declarations section of
+the parser file is needed to make it accessible to the actions
+(@pxref{C Declarations, ,The C Declarations Section}). You must also write the code in @code{yylex}
+to obey the flag.
+
+@node Tie-in Recovery, , Lexical Tie-ins, Context Dependency
+@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.
+
+@node Debugging, Invocation, Context Dependency, Top
+@chapter Debugging Your Parser
+@findex YYDEBUG
+@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.
+
+To enable compilation of trace facilities, you must define the macro
+@code{YYDEBUG} when you compile the parser. You could use
+@samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define
+YYDEBUG 1} in the C declarations section of the grammar file
+(@pxref{C Declarations, ,The C Declarations Section}). Alternatively, use the @samp{-t} option when
+you run Bison (@pxref{Invocation, ,Invoking Bison}). We always define @code{YYDEBUG} so that
+debugging is always possible.
+
+The trace facility uses @code{stderr}, so you must add @w{@code{#include
+<stdio.h>}} to the C declarations section unless it is already there.
+
+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
+#define YYPRINT(file, type, value) yyprint (file, type, value)
+
+static void
+yyprint (file, type, 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
+
+@node Invocation, Table of Symbols, Debugging, Top
+@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}. Thus, the @samp{bison foo.y} filename yields
+@file{foo.tab.c}, and the @samp{bison hack/foo.y} filename yields
+@file{hack/foo.tab.c}.@refill
+
+@menu
+* Bison Options:: All the options described in detail,
+ in alphabetical order by short options.
+* Option Cross Key:: Alphabetical list of long options.
+* VMS Invocation:: Bison command syntax on VMS.
+@end menu
+
+@node Bison Options, Option Cross Key, , Invocation
+@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.
+
+@table @samp
+@item -b @var{file-prefix}
+@itemx --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}.c}.
+
+@item -d
+@itemx --defines
+Write an extra output file containing macro definitions for the token
+type names defined in the grammar and the semantic value type
+@code{YYSTYPE}, as well as a few @code{extern} variable declarations.
+
+If the parser output file is named @file{@var{name}.c} then this file
+is named @file{@var{name}.h}.@refill
+
+This output file is essential if you wish to put the definition of
+@code{yylex} in a separate source file, because @code{yylex} needs to
+be able to refer to token type codes and the variable
+@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.@refill
+
+@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 an independent source file in its own right.
+
+@item -o @var{outfile}
+@itemx --output-file=@var{outfile}
+Specify the name @var{outfile} for the parser file.
+
+The other output files' names are constructed from @var{outfile}
+as described under the @samp{-v} and @samp{-d} switches.
+
+@item -p @var{prefix}
+@itemx --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{yylval},
+@code{yychar} and @code{yydebug}.
+
+For example, if you use @samp{-p c}, the names become @code{cparse},
+@code{clex}, and so on.
+
+@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}.
+
+@item -t
+@itemx --debug
+Output a definition of the macro @code{YYDEBUG} into the parser file,
+so that the debugging facilities are compiled. @xref{Debugging, ,Debugging Your Parser}.
+
+@item -v
+@itemx --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.
+
+This file also describes all the conflicts, both those resolved by
+operator precedence and the unresolved ones.
+
+The file's name is made by removing @samp{.tab.c} or @samp{.c} from
+the parser output file name, and adding @samp{.output} instead.@refill
+
+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}.@refill
+
+@item -V
+@itemx --version
+Print the version number of Bison.
+
+@need 1750
+@item -y
+@itemx --yacc
+@itemx --fixed-output-files
+Equivalent to @samp{-o y.tab.c}; the parser output file is called
+@file{y.tab.c}, and the other outputs are called @file{y.output} and
+@file{y.tab.h}. The purpose of this switch is to imitate Yacc's output
+file name conventions. Thus, the following shell script can substitute
+for Yacc:@refill
+
+@example
+bison -y $*
+@end example
+@end table
+
+@node Option Cross Key, VMS Invocation, Bison Options, Invocation
+@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{ --fixed-output-files \leaderfill -y}
+\line{ --name-prefix \leaderfill -p}
+\line{ --no-lines \leaderfill -l}
+\line{ --output-file \leaderfill -o}
+\line{ --verbose \leaderfill -v}
+\line{ --version \leaderfill -V}
+\line{ --yacc \leaderfill -y}
+}
+@end tex
+
+@ifinfo
+@example
+--debug -t
+--defines -d
+--file-prefix=@var{prefix} -b @var{file-prefix}
+--fixed-output-files --yacc -y
+--name-prefix -p
+--no-lines -l
+--output-file=@var{outfile} -o @var{outfile}
+--verbose -v
+--version -V
+@end example
+@end ifinfo
+
+@node VMS Invocation, , Option Cross Key, Invocation
+@section Invoking Bison under VMS
+@cindex invoking Bison under VMS
+@cindex VMS
+
+The command line syntax for Bison on VMS is a variant of the usual
+Bison command syntax---adapted to fit VMS conventions.
+
+To find the VMS equivalent for any Bison option, start with the long
+option, and substitute a @samp{/} for the leading @samp{--}, and
+substitute a @samp{_} for each @samp{-} in the name of the long option.
+For example, the following invocation under VMS:
+
+@example
+bison /debug/name_prefix=bar foo.y
+@end example
+
+@noindent
+is equivalent to the following command under POSIX.
+
+@example
+bison --debug --name-prefix=bar foo.y
+@end example
+
+The VMS file system does not permit filenames such as
+@file{foo.tab.c}. In the above example, the output file
+would instead be named @file{foo_tab.c}.
+
+@node Table of Symbols, Glossary, Invocation, Top
+@appendix Bison Symbols
+@cindex Bison symbols, table of
+@cindex symbols in Bison, table of
+
+@table @code
+@item 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 parse 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}.
+
+@item 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}}.
+
+@item 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}}.
+
+@item 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}.
+
+@item 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}.
+
+@item YYERROR_VERBOSE
+Macro that you define with @code{#define} in the Bison declarations
+section to request verbose, specific error message strings when
+@code{yyerror} is called.
+
+@item YYINITDEPTH
+Macro for specifying the initial size of the parser stack.
+@xref{Stack Overflow}.
+
+@item YYLTYPE
+Macro for the data type of @code{yylloc}; a structure with four
+members. @xref{Token Positions, ,Textual Positions of Tokens}.
+
+@item YYMAXDEPTH
+Macro for specifying the maximum size of the parser stack.
+@xref{Stack Overflow}.
+
+@item YYRECOVERING
+Macro whose value indicates whether the parser is recovering from a
+syntax error. @xref{Action Features, ,Special Features for Use in Actions}.
+
+@item YYSTYPE
+Macro for the data type of semantic values; @code{int} by default.
+@xref{Value Type, ,Data Types of Semantic Values}.
+
+@item yychar
+External integer variable that contains the integer value of the
+current 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}.
+
+@item yyclearin
+Macro used in error-recovery rule actions. It clears the previous
+look-ahead token. @xref{Error Recovery}.
+
+@item 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{Debugging, ,Debugging Your Parser}.
+
+@item yyerrok
+Macro to cause parser to recover immediately to its normal mode
+after a parse error. @xref{Error Recovery}.
+
+@item yyerror
+User-supplied function to be called by @code{yyparse} on error. The
+function receives one argument, a pointer to a character string
+containing an error message. @xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+
+@item yylex
+User-supplied lexical analyzer function, called with no arguments
+to get the next token. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+
+@item 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}.
+
+@item 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 Positions, ,Textual Positions of Tokens}.
+
+@item yynerrs
+Global variable which Bison increments each time there is a parse
+error. (In a pure parser, it is a local variable within
+@code{yyparse}.) @xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+
+@item yyparse
+The parser function produced by Bison; call this function to start
+parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+
+@item %left
+Bison declaration to assign left associativity to token(s).
+@xref{Precedence Decl, ,Operator Precedence}.
+
+@item %nonassoc
+Bison declaration to assign nonassociativity to token(s).
+@xref{Precedence Decl, ,Operator Precedence}.
+
+@item %prec
+Bison declaration to assign a precedence to a specific rule.
+@xref{Contextual Precedence, ,Context-Dependent Precedence}.
+
+@item %pure_parser
+Bison declaration to request a pure (reentrant) parser.
+@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
+
+@item %right
+Bison declaration to assign right associativity to token(s).
+@xref{Precedence Decl, ,Operator Precedence}.
+
+@item %start
+Bison declaration to specify the start symbol. @xref{Start Decl, ,The Start-Symbol}.
+
+@item %token
+Bison declaration to declare token(s) without specifying precedence.
+@xref{Token Decl, ,Token Type Names}.
+
+@item %type
+Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}.
+
+@item %union
+Bison declaration to specify several possible data types for semantic
+values. @xref{Union Decl, ,The Collection of Value Types}.
+@end table
+
+These are the punctuation and delimiters used in Bison input:
+
+@table @samp
+@item %%
+Delimiter used to separate the grammar rule section from the
+Bison declarations section or the additional C code section.
+@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}.
+
+@item %@{ %@}
+All code listed between @samp{%@{} and @samp{%@}} is copied directly
+to the output file uninterpreted. Such code forms the ``C
+declarations'' section of the input file. @xref{Grammar Outline, ,Outline of a Bison Grammar}.
+
+@item /*@dots{}*/
+Comment delimiters, as in C.
+
+@item :
+Separates a rule's result from its components. @xref{Rules, ,Syntax of Grammar Rules}.
+
+@item ;
+Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
+
+@item |
+Separates alternate rules for the same result nonterminal.
+@xref{Rules, ,Syntax of Grammar Rules}.
+@end table
+
+@node Glossary, Index, Table of Symbols, Top
+@appendix Glossary
+@cindex glossary
+
+@table @asis
+@item Backus-Naur Form (BNF)
+Formal method of specifying context-free grammars. BNF was first used
+in the @cite{ALGOL-60} report, 1963. @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 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 Look-ahead token
+A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead Tokens}.
+
+@item LALR(1)
+The class of context-free grammars that Bison (like most other parser
+generators) can handle; a subset of LR(1). @xref{Mystery Conflicts, ,
+Mysterious Reduce/Reduce Conflicts}.
+
+@item 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 Parse error
+An error encountered during parsing of an input stream due to invalid
+syntax. @xref{Error Recovery}.
+
+@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 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 Index, , Glossary, Top
+@unnumbered Index
+
+@printindex cp
+
+@contents
+
+@bye
+
+
+\f
+
+@c old menu
+
+* Introduction::
+* Conditions::
+* Copying:: The 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:: Debugging Bison parsers that parse wrong.
+* Invocation:: How to run Bison (to produce the parser source file).
+* Table of Symbols:: All the keywords of the Bison language are explained.
+* Glossary:: Basic concepts are explained.
+* Index:: Cross-references to the text.
+
projects / bison.git / commitdiff
