gnu1987/bison-1.19a/bison.info-4

This is Info file bison.info, produced by Makeinfo version 1.67 from
the input file ./bison.texinfo.

   This file documents the Bison parser generator.

   Copyright (C) 1988, 1989, 1990 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.

   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.


File: bison.info,  Node: Semantic Tokens,  Next: Lexical Tie-ins,  Prev: Context Dependency,  Up: Context Dependency

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:

     foo (x);

   This looks like a function call statement, but if `foo' is a typedef
name, then this is actually a declaration of `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,
`IDENTIFIER' and `TYPENAME'.  When `yylex' finds an identifier, it
looks up the current declaration of the identifier in order to decide
which token type to return: `TYPENAME' if the identifier is declared as
a typedef, `IDENTIFIER' otherwise.

   The grammar rules can then express the context dependency by the
choice of token type to recognize.  `IDENTIFIER' is accepted as an
expression, but `TYPENAME' is not.  `TYPENAME' can start a declaration,
but `IDENTIFIER' cannot.  In contexts where the meaning of the
identifier is *not* significant, such as in declarations that can
shadow a typedef name, either `TYPENAME' or `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:

     typedef int foo, bar, lose;
     static foo (bar);        /* redeclare `bar' as static variable */
     static int foo (lose);   /* redeclare `foo' as function */

   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:

     initdcl:
               declarator maybeasm '='
               init
             | declarator maybeasm
             ;
     
     notype_initdcl:
               notype_declarator maybeasm '='
               init
             | notype_declarator maybeasm
             ;

Here `initdcl' can redeclare a typedef name, but `notype_initdcl'
cannot.  The distinction between `declarator' and `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.


File: bison.info,  Node: Lexical Tie-ins,  Next: Tie-in Recovery,  Prev: Semantic Tokens,  Up: Context Dependency

Lexical Tie-ins
===============

   One way to handle context-dependency is the "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 `hex (HEX-EXPR)'.  After the keyword `hex' comes an
expression in parentheses in which all integers are hexadecimal.  In
particular, the token `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:

     %{
     int hexflag;
     %}
     %%
     ...
     expr:   IDENTIFIER
             | constant
             | HEX '('
                     { hexflag = 1; }
               expr ')'
                     { hexflag = 0;
                        $$ = $4; }
             | expr '+' expr
                     { $$ = make_sum ($1, $3); }
             ...
             ;
     
     constant:
               INTEGER
             | STRING
             ;

Here we assume that `yylex' looks at the value of `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 `hexflag' shown in the C declarations section of
the parser file is needed to make it accessible to the actions (*note C
Declarations::.).  You must also write the code in `yylex' to obey the
flag.


File: bison.info,  Node: Tie-in Recovery,  Prev: Lexical Tie-ins,  Up: Context Dependency

Lexical Tie-ins and Error Recovery
==================================

   Lexical tie-ins make strict demands on any error recovery rules you
have.  *Note 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:

     stmt:   expr ';'
             | IF '(' expr ')' stmt { ... }
             ...
             error ';'
                     { hexflag = 0; }
             ;

   If there is a syntax error in the middle of a `hex (EXPR)'
construct, this error rule will apply, and then the action for the
completed `hex (EXPR)' will never run.  So `hexflag' would remain set
for the entire rest of the input, or until the next `hex' keyword,
causing identifiers to be misinterpreted as integers.

   To avoid this problem the error recovery rule itself clears
`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:

     expr:   ...
             | '(' expr ')'
                     { $$ = $2; }
             | '(' error ')'
             ...

   If this rule acts within the `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 `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
`hex' construct or might not, depending on circumstances?  There is no
way you can write the action to determine whether a `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.


File: bison.info,  Node: Debugging,  Next: Invocation,  Prev: Context Dependency,  Up: Top

Debugging Your Parser
*********************

   If a Bison grammar compiles properly but doesn't do what you want
when it runs, the `yydebug' parser-trace feature can help you figure
out why.

   To enable compilation of trace facilities, you must define the macro
`YYDEBUG' when you compile the parser.  You could use `-DYYDEBUG=1' as
a compiler option or you could put `#define YYDEBUG 1' in the C
declarations section of the grammar file (*note C Declarations::.).
Alternatively, use the `-t' option when you run Bison (*note
Invocation::.).  We always define `YYDEBUG' so that debugging is always
possible.

   The trace facility uses `stderr', so you must add
`#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 `yydebug'.
You can do this by making the C code do it (in `main', perhaps), or you
can alter the value with a C debugger.

   Each step taken by the parser when `yydebug' is nonzero produces a
line or two of trace information, written on `stderr'.  The trace
messages tell you these things:

   * Each time the parser calls `yylex', what kind of token was read.

   * Each time a token is shifted, the depth and complete contents of
     the state stack (*note Parser States::.).

   * Each time a rule is reduced, which rule it is, and the complete
     contents of the state stack afterward.

   To make sense of this information, it helps to refer to the listing
file produced by the Bison `-v' option (*note Invocation::.).  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.

   The debugging information normally gives the token type of each token
read, but not its semantic value.  You can optionally define a macro
named `YYPRINT' to provide a way to print the value.  If you define
`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 `yylval').

   Here is an example of `YYPRINT' suitable for the multi-function
calculator (*note Mfcalc Decl::.):

     #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);
     }


File: bison.info,  Node: Invocation,  Next: Table of Symbols,  Prev: Debugging,  Up: Top

Invoking Bison
**************

   The usual way to invoke Bison is as follows:

     bison INFILE

   Here INFILE is the grammar file name, which usually ends in `.y'.
The parser file's name is made by replacing the `.y' with `.tab.c'.
Thus, the `bison foo.y' filename yields `foo.tab.c', and the `bison
hack/foo.y' filename yields `hack/foo.tab.c'.

* 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.


File: bison.info,  Node: Bison Options,  Next: Option Cross Key,  Up: Invocation

Bison Options
=============

   Bison supports both traditional single-letter options and mnemonic
long option names.  Long option names are indicated with `--' instead of
`-'.  Abbreviations for option names are allowed as long as they are
unique.  When a long option takes an argument, like `--file-prefix',
connect the option name and the argument with `='.

   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.

`-b FILE-PREFIX'
`--file-prefix=PREFIX'
     Specify a prefix to use for all Bison output file names.  The
     names are chosen as if the input file were named `PREFIX.c'.

`-d'
`--defines'
     Write an extra output file containing macro definitions for the
     token type names defined in the grammar and the semantic value type
     `YYSTYPE', as well as a few `extern' variable declarations.

     If the parser output file is named `NAME.c' then this file is
     named `NAME.h'.

     This output file is essential if you wish to put the definition of
     `yylex' in a separate source file, because `yylex' needs to be
     able to refer to token type codes and the variable `yylval'.
     *Note Token Values::.

`-l'
`--no-lines'
     Don't put any `#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.

`-o OUTFILE'
`--output-file=OUTFILE'
     Specify the name OUTFILE for the parser file.

     The other output files' names are constructed from OUTFILE as
     described under the `-v' and `-d' switches.

`-p PREFIX'
`--name-prefix=PREFIX'
     Rename the external symbols used in the parser so that they start
     with PREFIX instead of `yy'.  The precise list of symbols renamed
     is `yyparse', `yylex', `yyerror', `yylval', `yychar' and `yydebug'.

     For example, if you use `-p c', the names become `cparse', `clex',
     and so on.

     *Note Multiple Parsers::.

`-t'
`--debug'
     Output a definition of the macro `YYDEBUG' into the parser file,
     so that the debugging facilities are compiled.  *Note Debugging::.

`-v'
`--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 `.tab.c' or `.c' from the
     parser output file name, and adding `.output' instead.

     Therefore, if the input file is `foo.y', then the parser file is
     called `foo.tab.c' by default.  As a consequence, the verbose
     output file is called `foo.output'.

`-V'
`--version'
     Print the version number of Bison.

`-y'
`--yacc'
`--fixed-output-files'
     Equivalent to `-o y.tab.c'; the parser output file is called
     `y.tab.c', and the other outputs are called `y.output' and
     `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:

          bison -y $*


File: bison.info,  Node: Option Cross Key,  Next: VMS Invocation,  Prev: Bison Options,  Up: Invocation

Option Cross Key
================

   Here is a list of options, alphabetized by long option, to help you
find the corresponding short option.

     --debug                               -t
     --defines                             -d
     --file-prefix=PREFIX                  -b FILE-PREFIX
     --fixed-output-files --yacc           -y
     --name-prefix                         -p
     --no-lines                            -l
     --output-file=OUTFILE                 -o OUTFILE
     --verbose                             -v
     --version                             -V


File: bison.info,  Node: VMS Invocation,  Prev: Option Cross Key,  Up: Invocation

Invoking Bison under 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 `/' for the leading `--', and substitute a `_'
for each `-' in the name of the long option.  For example, the
following invocation under VMS:

     bison /debug/name_prefix=bar foo.y

   is equivalent to the following command under POSIX.

     bison --debug --name-prefix=bar foo.y

   The VMS filesystem does not permit filenames such as `foo.tab.c'.
In the above example, the output file would instead be named
`foo_tab.c'.


File: bison.info,  Node: Table of Symbols,  Next: Glossary,  Prev: Invocation,  Up: Top

Bison Symbols
*************

`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 `error' becomes the current look-ahead
     token.  Actions corresponding to `error' are then executed, and
     the look-ahead token is reset to the token that originally caused
     the violation.  *Note Error Recovery::.

`YYABORT'
     Macro to pretend that an unrecoverable syntax error has occurred,
     by making `yyparse' return 1 immediately.  The error reporting
     function `yyerror' is not called.  *Note Parser Function::.

`YYACCEPT'
     Macro to pretend that a complete utterance of the language has been
     read, by making `yyparse' return 0 immediately.  *Note Parser
     Function::.

`YYBACKUP'
     Macro to discard a value from the parser stack and fake a
     look-ahead token.  *Note Action Features::.

`YYERROR'
     Macro to pretend that a syntax error has just been detected: call
     `yyerror' and then perform normal error recovery if possible
     (*note Error Recovery::.), or (if recovery is impossible) make
     `yyparse' return 1.  *Note Error Recovery::.

`YYINITDEPTH'
     Macro for specifying the initial size of the parser stack.  *Note
     Stack Overflow::.

`YYLTYPE'
     Macro for the data type of `yylloc'; a structure with four
     members.  *Note Token Positions::.

`YYMAXDEPTH'
     Macro for specifying the maximum size of the parser stack.  *Note
     Stack Overflow::.

`YYRECOVERING'
     Macro whose value indicates whether the parser is recovering from a
     syntax error.  *Note Action Features::.

`YYSTYPE'
     Macro for the data type of semantic values; `int' by default.
     *Note Value Type::.

`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 `yyparse'.)  Error-recovery rule actions may
     examine this variable.  *Note Action Features::.

`yyclearin'
     Macro used in error-recovery rule actions.  It clears the previous
     look-ahead token.  *Note Error Recovery::.

`yydebug'
     External integer variable set to zero by default.  If `yydebug' is
     given a nonzero value, the parser will output information on input
     symbols and parser action.  *Note Debugging::.

`yyerrok'
     Macro to cause parser to recover immediately to its normal mode
     after a parse error.  *Note Error Recovery::.

`yyerror'
     User-supplied function to be called by `yyparse' on error.  The
     function receives one argument, a pointer to a character string
     containing an error message.  *Note Error Reporting::.

`yylex'
     User-supplied lexical analyzer function, called with no arguments
     to get the next token.  *Note Lexical::.

`yylval'
     External variable in which `yylex' should place the semantic value
     associated with a token.  (In a pure parser, it is a local
     variable within `yyparse', and its address is passed to `yylex'.)
     *Note Token Values::.

`yylloc'
     External variable in which `yylex' should place the line and
     column numbers associated with a token.  (In a pure parser, it is a
     local variable within `yyparse', and its address is passed to
     `yylex'.)  You can ignore this variable if you don't use the `@'
     feature in the grammar actions.  *Note Token Positions::.

`yynerrs'
     Global variable which Bison increments each time there is a parse
     error.  (In a pure parser, it is a local variable within
     `yyparse'.)  *Note Error Reporting::.

`yyparse'
     The parser function produced by Bison; call this function to start
     parsing.  *Note Parser Function::.

`%left'
     Bison declaration to assign left associativity to token(s).  *Note
     Precedence Decl::.

`%nonassoc'
     Bison declaration to assign nonassociativity to token(s).  *Note
     Precedence Decl::.

`%prec'
     Bison declaration to assign a precedence to a specific rule.
     *Note Contextual Precedence::.

`%pure_parser'
     Bison declaration to request a pure (reentrant) parser.  *Note
     Pure Decl::.

`%right'
     Bison declaration to assign right associativity to token(s).
     *Note Precedence Decl::.

`%start'
     Bison declaration to specify the start symbol.  *Note Start Decl::.

`%token'
     Bison declaration to declare token(s) without specifying
     precedence.  *Note Token Decl::.

`%type'
     Bison declaration to declare nonterminals.  *Note Type Decl::.

`%union'
     Bison declaration to specify several possible data types for
     semantic values.  *Note Union Decl::.

   These are the punctuation and delimiters used in Bison input:

`%%'
     Delimiter used to separate the grammar rule section from the Bison
     declarations section or the additional C code section.  *Note
     Grammar Layout::.

`%{ %}'
     All code listed between `%{' and `%}' is copied directly to the
     output file uninterpreted.  Such code forms the "C declarations"
     section of the input file.  *Note Grammar Outline::.

`/*...*/'
     Comment delimiters, as in C.

`:'
     Separates a rule's result from its components.  *Note Rules::.

`;'
     Terminates a rule.  *Note Rules::.

`|'
     Separates alternate rules for the same result nonterminal.  *Note
     Rules::.


File: bison.info,  Node: Glossary,  Next: Index,  Prev: Table of Symbols,  Up: Top

Glossary
********

Backus-Naur Form (BNF)
     Formal method of specifying context-free grammars.  BNF was first
     used in the `ALGOL-60' report, 1963.  *Note Language and Grammar::.

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 *anywhere* an
     expression is permitted.  *Note Language and Grammar::.

Dynamic allocation
     Allocation of memory that occurs during execution, rather than at
     compile time or on entry to a function.

Empty string
     Analogous to the empty set in set theory, the empty string is a
     character string of length zero.

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.  *Note Algorithm::.

Grouping
     A language construct that is (in general) grammatically divisible;
     for example, `expression' or `declaration' in C.  *Note Language
     and Grammar::.

Infix operator
     An arithmetic operator that is placed between the operands on
     which it performs some operation.

Input stream
     A continuous flow of data between devices or programs.

Language construct
     One of the typical usage schemas of the language.  For example,
     one of the constructs of the C language is the `if' statement.
     *Note Language and Grammar::.

Left associativity
     Operators having left associativity are analyzed from left to
     right: `a+b+c' first computes `a+b' and then combines with `c'.
     *Note Precedence::.

Left recursion
     A rule whose result symbol is also its first component symbol; for
     example, `expseq1 : expseq1 ',' exp;'.  *Note Recursion::.

Left-to-right parsing
     Parsing a sentence of a language by analyzing it token by token
     from left to right.  *Note Algorithm::.

Lexical analyzer (scanner)
     A function that reads an input stream and returns tokens one by
     one.  *Note Lexical::.

Lexical tie-in
     A flag, set by actions in the grammar rules, which alters the way
     tokens are parsed.  *Note Lexical Tie-ins::.

Look-ahead token
     A token already read but not yet shifted.  *Note Look-Ahead::.

LALR(1)
     The class of context-free grammars that Bison (like most other
     parser generators) can handle; a subset of LR(1).  *Note
     Mysterious Reduce/Reduce Conflicts: Mystery Conflicts.

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.

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.  *Note Symbols::.

Parse error
     An error encountered during parsing of an input stream due to
     invalid syntax.  *Note Error Recovery::.

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.

Postfix operator
     An arithmetic operator that is placed after the operands upon
     which it performs some operation.

Reduction
     Replacing a string of nonterminals and/or terminals with a single
     nonterminal, according to a grammar rule.  *Note Algorithm::.

Reentrant
     A reentrant subprogram is a subprogram which can be in invoked any
     number of times in parallel, without interference between the
     various invocations.  *Note Pure Decl::.

Reverse polish notation
     A language in which all operators are postfix operators.

Right recursion
     A rule whose result symbol is also its last component symbol; for
     example, `expseq1: exp ',' expseq1;'.  *Note Recursion::.

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.  *Note Semantics::.

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.  *Note Algorithm::.

Single-character literal
     A single character that is recognized and interpreted as is.
     *Note Grammar in Bison::.

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.
     *Note Start Decl::.

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.  *Note Multi-function
     Calc::.

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.  *Note Symbols::.

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.  *Note Language and Grammar::.


File: bison.info,  Node: Index,  Prev: Glossary,  Up: Top

Index
*****

* Menu:

* $$:                                    Actions.
* $N:                                    Actions.
* %expect:                               Expect Decl.
* %left:                                 Using Precedence.
* %nonassoc:                             Using Precedence.
* %prec:                                 Contextual Precedence.
* %pure_parser:                          Pure Decl.
* %right:                                Using Precedence.
* %start:                                Start Decl.
* %token:                                Token Decl.
* %type:                                 Type Decl.
* %union:                                Union Decl.
* @N:                                    Action Features.
* action:                                Actions.
* action data types:                     Action Types.
* action features summary:               Action Features.
* actions in mid-rule:                   Mid-Rule Actions.
* actions, semantic:                     Semantic Actions.
* additional C code section:             C Code.
* algorithm of parser:                   Algorithm.
* associativity:                         Why Precedence.
* Backus-Naur form:                      Language and Grammar.
* Bison declaration summary:             Decl Summary.
* Bison declarations:                    Declarations.
* Bison declarations (introduction):     Bison Declarations.
* Bison grammar:                         Grammar in Bison.
* Bison invocation:                      Invocation.
* Bison parser:                          Bison Parser.
* Bison parser algorithm:                Algorithm.
* Bison symbols, table of:               Table of Symbols.
* Bison utility:                         Bison Parser.
* BNF:                                   Language and Grammar.
* C code, section for additional:        C Code.
* C declarations section:                C Declarations.
* C-language interface:                  Interface.
* calc:                                  Infix Calc.
* calculator, infix notation:            Infix Calc.
* calculator, multi-function:            Multi-function Calc.
* calculator, simple:                    RPN Calc.
* character token:                       Symbols.
* compiling the parser:                  Rpcalc Compile.
* conflicts:                             Shift/Reduce.
* conflicts, reduce/reduce:              Reduce/Reduce.
* conflicts, suppressing warnings of:    Expect Decl.
* context-dependent precedence:          Contextual Precedence.
* context-free grammar:                  Language and Grammar.
* controlling function:                  Rpcalc Main.
* dangling else:                         Shift/Reduce.
* data types in actions:                 Action Types.
* data types of semantic values:         Value Type.
* debugging:                             Debugging.
* declaration summary:                   Decl Summary.
* declarations, Bison:                   Declarations.
* declarations, Bison (introduction):    Bison Declarations.
* declarations, C:                       C Declarations.
* declaring operator precedence:         Precedence Decl.
* declaring the start symbol:            Start Decl.
* declaring token type names:            Token Decl.
* declaring value types:                 Union Decl.
* declaring value types, nonterminals:   Type Decl.
* defining language semantics:           Semantics.
* else, dangling:                        Shift/Reduce.
* error:                                 Error Recovery.
* error recovery:                        Error Recovery.
* error recovery, simple:                Simple Error Recovery.
* error reporting function:              Error Reporting.
* error reporting routine:               Rpcalc Error.
* examples, simple:                      Examples.
* exercises:                             Exercises.
* file format:                           Grammar Layout.
* finite-state machine:                  Parser States.
* formal grammar:                        Grammar in Bison.
* format of grammar file:                Grammar Layout.
* glossary:                              Glossary.
* grammar file:                          Grammar Layout.
* grammar rule syntax:                   Rules.
* grammar rules section:                 Grammar Rules.
* grammar, Bison:                        Grammar in Bison.
* grammar, context-free:                 Language and Grammar.
* grouping, syntactic:                   Language and Grammar.
* infix notation calculator:             Infix Calc.
* interface:                             Interface.
* introduction:                          Introduction.
* invoking Bison:                        Invocation.
* invoking Bison under VMS:              VMS Invocation.
* LALR(1):                               Mystery Conflicts.
* language semantics, defining:          Semantics.
* layout of Bison grammar:               Grammar Layout.
* left recursion:                        Recursion.
* lexical analyzer:                      Lexical.
* lexical analyzer, purpose:             Bison Parser.
* lexical analyzer, writing:             Rpcalc Lexer.
* lexical tie-in:                        Lexical Tie-ins.
* literal token:                         Symbols.
* look-ahead token:                      Look-Ahead.
* LR(1):                                 Mystery Conflicts.
* main function in simple example:       Rpcalc Main.
* mfcalc:                                Multi-function Calc.
* mid-rule actions:                      Mid-Rule Actions.
* multi-function calculator:             Multi-function Calc.
* mutual recursion:                      Recursion.
* nonterminal symbol:                    Symbols.
* operator precedence:                   Precedence.
* operator precedence, declaring:        Precedence Decl.
* options for invoking Bison:            Invocation.
* overflow of parser stack:              Stack Overflow.
* parse error:                           Error Reporting.
* parser:                                Bison Parser.
* parser stack:                          Algorithm.
* parser stack overflow:                 Stack Overflow.
* parser state:                          Parser States.
* polish notation calculator:            RPN Calc.
* precedence declarations:               Precedence Decl.
* precedence of operators:               Precedence.
* precedence, context-dependent:         Contextual Precedence.
* precedence, unary operator:            Contextual Precedence.
* preventing warnings about conflicts:   Expect Decl.
* pure parser:                           Pure Decl.
* recovery from errors:                  Error Recovery.
* recursive rule:                        Recursion.
* reduce/reduce conflict:                Reduce/Reduce.
* reduction:                             Algorithm.
* reentrant parser:                      Pure Decl.
* reverse polish notation:               RPN Calc.
* right recursion:                       Recursion.
* rpcalc:                                RPN Calc.
* rule syntax:                           Rules.
* rules section for grammar:             Grammar Rules.
* running Bison (introduction):          Rpcalc Gen.
* semantic actions:                      Semantic Actions.
* semantic value:                        Semantic Values.
* semantic value type:                   Value Type.
* shift/reduce conflicts:                Shift/Reduce.
* shifting:                              Algorithm.
* simple examples:                       Examples.
* single-character literal:              Symbols.
* stack overflow:                        Stack Overflow.
* stack, parser:                         Algorithm.
* stages in using Bison:                 Stages.
* start symbol:                          Language and Grammar.
* start symbol, declaring:               Start Decl.
* state (of parser):                     Parser States.
* summary, action features:              Action Features.
* summary, Bison declaration:            Decl Summary.
* suppressing conflict warnings:         Expect Decl.
* symbol:                                Symbols.
* symbol table example:                  Mfcalc Symtab.
* symbols (abstract):                    Language and Grammar.
* symbols in Bison, table of:            Table of Symbols.
* syntactic grouping:                    Language and Grammar.
* syntax error:                          Error Reporting.
* syntax of grammar rules:               Rules.
* terminal symbol:                       Symbols.
* token:                                 Language and Grammar.
* token type:                            Symbols.
* token type names, declaring:           Token Decl.
* tracing the parser:                    Debugging.
* unary operator precedence:             Contextual Precedence.
* using Bison:                           Stages.
* value type, semantic:                  Value Type.
* value types, declaring:                Union Decl.
* value types, nonterminals, declaring:  Type Decl.
* value, semantic:                       Semantic Values.
* VMS:                                   VMS Invocation.
* warnings, preventing:                  Expect Decl.
* writing a lexical analyzer:            Rpcalc Lexer.
* YYABORT:                               Parser Function.
* YYACCEPT:                              Parser Function.
* YYBACKUP:                              Action Features.
* yychar:                                Look-Ahead.
* yyclearin:                             Error Recovery.
* YYDEBUG:                               Debugging.
* yydebug:                               Debugging.
* YYEMPTY:                               Action Features.
* yyerrok:                               Error Recovery.
* YYERROR:                               Action Features.
* yyerror:                               Error Reporting.
* YYINITDEPTH:                           Stack Overflow.
* yylex:                                 Lexical.
* yylloc:                                Token Positions.
* YYLTYPE:                               Token Positions.
* yylval:                                Token Values.
* YYMAXDEPTH:                            Stack Overflow.
* yynerrs:                               Error Reporting.
* yyparse:                               Parser Function.
* YYPRINT:                               Debugging.
* YYRECOVERING:                          Error Recovery.
* |:                                     Rules.