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This manual is for GNU Bison (version 1.75, 14 October 2002), the
GNU parser generator.

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INFO-DIR-SECTION GNU programming tools
START-INFO-DIR-ENTRY
* bison: (bison). GNU parser generator (yacc replacement).
END-INFO-DIR-ENTRY

File: bison.info, Node: Symbols, Next: Rules, Prev: Grammar Outline, Up: Grammar File

Symbols, Terminal and Nonterminal
=================================

"Symbols" in Bison grammars represent the grammatical classifications
of the language.

A "terminal symbol" (also known as a "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 `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 "nonterminal symbol" stands for a class of syntactically equivalent
groupings. The symbol name is used in writing grammar rules. By
convention, it should be all lower case.

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

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

* A "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
`%token'. *Note Token Type Names: Token Decl.

* A "character token type" (or "literal character token") is written
in the grammar using the same syntax used in C for character
constants; for example, `'+'' 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 (*note Data Types of Semantic
Values: Value Type.), associativity, or precedence (*note Operator
Precedence: Precedence.).

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

All the usual escape sequences used in character literals in C can
be used in Bison as well, but you must not use the null character
as a character literal because its numeric code, zero, signifies
end-of-input (*note Calling Convention for `yylex': Calling
Convention.).

* A "literal string token" is written like a C string constant; for
example, `"<="' is a literal string token. A literal string token
doesn't need to be declared unless you need to specify its semantic
value data type (*note Value Type::), associativity, or precedence
(*note Precedence::).

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

*WARNING*: literal string tokens do not work in Yacc.

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

All the escape sequences used in string literals in C can be used
in Bison as well. A literal string token must contain two or more
characters; for a token containing just one character, use a
character token (see above).

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

If `yylex' is defined in a separate file, you need to arrange for the
token-type macro definitions to be available there. Use the `-d'
option when you run Bison, so that it will write these macro definitions
into a separate header file `NAME.tab.h' which you can include in the
other source files that need it. *Note Invoking Bison: Invocation.

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

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

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

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

File: bison.info, Node: Rules, Next: Recursion, Prev: Symbols, Up: Grammar File

Syntax of Grammar Rules
=======================

A Bison grammar rule has the following general form:

RESULT: COMPONENTS...
;

where RESULT is the nonterminal symbol that this rule describes, and
COMPONENTS are various terminal and nonterminal symbols that are put
together by this rule (*note Symbols::).

For example,

exp: exp '+' exp
;

says that two groupings of type `exp', with a `+' token in between, can
be combined into a larger grouping of type `exp'.

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

Scattered among the components can be ACTIONS that determine the
semantics of the rule. An action looks like this:

{C STATEMENTS}

Usually there is only one action and it follows the components. *Note
Actions::.

Multiple rules for the same RESULT can be written separately or can
be joined with the vertical-bar character `|' as follows:

RESULT: RULE1-COMPONENTS...
| RULE2-COMPONENTS...
...
;

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

If COMPONENTS in a rule is empty, it means that RESULT can match the
empty string. For example, here is how to define a comma-separated
sequence of zero or more `exp' groupings:

expseq: /* empty */
| expseq1
;

expseq1: exp
| expseq1 ',' exp
;

It is customary to write a comment `/* empty */' in each rule with no
components.

File: bison.info, Node: Recursion, Next: Semantics, Prev: Rules, Up: Grammar File

Recursive Rules
===============

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

expseq1: exp
| expseq1 ',' exp
;

Since the recursive use of `expseq1' is the leftmost symbol in the
right hand side, we call this "left recursion". By contrast, here the
same construct is defined using "right recursion":

expseq1: exp
| exp ',' expseq1
;

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. *Note
The Bison Parser Algorithm: Algorithm, for further explanation of this.

"Indirect" or "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:

expr: primary
| primary '+' primary
;

primary: constant
| '(' expr ')'
;

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

File: bison.info, Node: Semantics, Next: Locations, Prev: Recursion, Up: Grammar File

Defining Language Semantics
===========================

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 `X + Y' is to add the numbers
associated with X and 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.

File: bison.info, Node: Value Type, Next: Multiple Types, Up: Semantics

Data Types of Semantic Values
-----------------------------

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 (*note Reverse Polish Notation
Calculator: RPN Calc.).

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

#define YYSTYPE double

This macro definition must go in the prologue of the grammar file
(*note Outline of a Bison Grammar: Grammar Outline.).

File: bison.info, Node: Multiple Types, Next: Actions, Prev: Value Type, Up: Semantics

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 `int' or `long', while a string constant needs type `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:

* Specify the entire collection of possible data types, with the
`%union' Bison declaration (*note The Collection of Value Types:
Union Decl.).

* Choose one of those types for each symbol (terminal or
nonterminal) for which semantic values are used. This is done for
tokens with the `%token' Bison declaration (*note Token Type
Names: Token Decl.) and for groupings with the `%type' Bison
declaration (*note Nonterminal Symbols: Type Decl.).

File: bison.info, Node: Actions, Next: Action Types, Prev: Multiple Types, Up: Semantics

Actions
-------

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 (*note Actions in
Mid-Rule: Mid-Rule Actions.).

The C code in an action can refer to the semantic values of the
components matched by the rule with the construct `$N', which stands for
the value of the Nth component. The semantic value for the grouping
being constructed is `$$'. (Bison translates both of these constructs
into array element references when it copies the actions into the parser
file.)

Here is a typical example:

exp: ...
| exp '+' exp
{ $$ = $1 + $3; }

This rule constructs an `exp' from two smaller `exp' groupings
connected by a plus-sign token. In the action, `$1' and `$3' refer to
the semantic values of the two component `exp' groupings, which are the
first and third symbols on the right hand side of the rule. The sum is
stored into `$$' 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 `+' token, it could be
referred to as `$2'.

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

a-or-b: 'a'|'b' { a_or_b_found = 1; };

If you don't specify an action for a rule, Bison supplies a default:
`$$ = $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.

`$N' with N zero or negative is allowed for reference to tokens and
groupings on the stack _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:

foo: expr bar '+' expr { ... }
| expr bar '-' expr { ... }
;

bar: /* empty */
{ previous_expr = $0; }
;

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

File: bison.info, Node: Action Types, Next: Mid-Rule Actions, Prev: Actions, Up: Semantics

Data Types of Values in Actions
-------------------------------

If you have chosen a single data type for semantic values, the `$$'
and `$N' constructs always have that data type.

If you have used `%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 `$$' or `$N', its data type is determined by which symbol it refers
to in the rule. In this example,

exp: ...
| exp '+' exp
{ $$ = $1 + $3; }

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

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

%union {
int itype;
double dtype;
}

then you can write `$<itype>1' to refer to the first subunit of the
rule as an integer, or `$<dtype>1' to refer to it as a double.

File: bison.info, Node: Mid-Rule Actions, Prev: Action Types, Up: Semantics

Actions in Mid-Rule
-------------------

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
`$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
N to use in `$N'.

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

There is no way to set the value of the entire rule with a mid-rule
action, because assignments to `$$' 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 `let'
statement that looks like `let (VARIABLE) STATEMENT' and serves to
create a variable named VARIABLE temporarily for the duration of
STATEMENT. To parse this construct, we must put VARIABLE into the
symbol table while STATEMENT is parsed, then remove it afterward. Here
is how it is done:

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

As soon as `let (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
`context' in the data-type union. Then it calls `declare_variable' to
add the new variable to that list. Once the first action is finished,
the embedded statement `stmt' can be parsed. Note that the mid-rule
action is component number 5, so the `stmt' is component number 6.

After the embedded statement is parsed, its semantic value becomes
the value of the entire `let'-statement. Then the semantic value from
the earlier action is used to restore the prior list of variables. This
removes the temporary `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:

compound: '{' declarations statements '}'
| '{' statements '}'
;

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

compound: { prepare_for_local_variables (); }
'{' declarations statements '}'
| '{' statements '}'
;

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 "look-ahead" token at this time, since the parser is still deciding
what to do about it. *Note Look-Ahead Tokens: Look-Ahead.)

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

compound: { prepare_for_local_variables (); }
'{' declarations statements '}'
| { prepare_for_local_variables (); }
'{' statements '}'
;

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:

compound: '{' { prepare_for_local_variables (); }
declarations statements '}'
| '{' statements '}'
;

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:

subroutine: /* empty */
{ prepare_for_local_variables (); }
;

compound: subroutine
'{' declarations statements '}'
| subroutine
'{' statements '}'
;

Now Bison can execute the action in the rule for `subroutine' without
deciding which rule for `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.

File: bison.info, Node: Locations, Next: Declarations, Prev: Semantics, Up: Grammar File

Tracking Locations
==================

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

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

* Menu:

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

File: bison.info, Node: Location Type, Next: Actions and Locations, Up: Locations

Data Type of Locations
----------------------

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

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

struct
{
int first_line;
int first_column;
int last_line;
int last_column;
}

File: bison.info, Node: Actions and Locations, Next: Location Default Action, Prev: Location Type, Up: Locations

Actions and Locations
---------------------

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

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

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

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

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

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

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

File: bison.info, Node: Location Default Action, Prev: Actions and Locations, Up: Locations

Default Action for Locations
----------------------------

Actually, actions are not the best place to compute locations. Since
locations are much more general than semantic values, there is room in
the output parser to redefine the default action to take for each rule.
The `YYLLOC_DEFAULT' macro is invoked each time a rule is matched,
before the associated action is run.

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

The `YYLLOC_DEFAULT' macro takes three parameters. The first one is
the location of the grouping (the result of the computation). The
second one is an array holding locations of all right hand side
elements of the rule being matched. The last one is the size of the
right hand side rule.

By default, it is defined this way for simple LALR(1) parsers:

#define YYLLOC_DEFAULT(Current, Rhs, N) \
Current.first_line = Rhs[1].first_line; \
Current.first_column = Rhs[1].first_column; \
Current.last_line = Rhs[N].last_line; \
Current.last_column = Rhs[N].last_column;

and like this for GLR parsers:

#define YYLLOC_DEFAULT(Current, Rhs, N) \
Current.first_line = YYRHSLOC(Rhs,1).first_line; \
Current.first_column = YYRHSLOC(Rhs,1).first_column; \
Current.last_line = YYRHSLOC(Rhs,N).last_line; \
Current.last_column = YYRHSLOC(Rhs,N).last_column;

When defining `YYLLOC_DEFAULT', you should consider that:

* All arguments are free of side-effects. However, only the first
one (the result) should be modified by `YYLLOC_DEFAULT'.

* For consistency with semantic actions, valid indexes for the
location array range from 1 to N.

File: bison.info, Node: Declarations, Next: Multiple Parsers, Prev: Locations, Up: Grammar File

Bison Declarations
==================

The "Bison declarations" section of a Bison grammar defines the
symbols used in formulating the grammar and the data types of semantic
values. *Note Symbols::.

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

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 (*note Languages and Context-Free Grammars:
Language and Grammar.).

* 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.

File: bison.info, Node: Token Decl, Next: Precedence Decl, Up: Declarations

Token Type Names
----------------

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

%token NAME

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

Alternatively, you can use `%left', `%right', or `%nonassoc' instead
of `%token', if you wish to specify associativity and precedence.
*Note Operator Precedence: Precedence Decl.

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

%token NUM 300

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

In the event that the stack type is a union, you must augment the
`%token' or other token declaration to include the data type
alternative delimited by angle-brackets (*note More Than One Value
Type: Multiple Types.).

For example:

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

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

%token arrow "=>"

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

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

Once you equate the literal string and the token name, you can use them
interchangeably in further declarations or the grammar rules. The
`yylex' function can use the token name or the literal string to obtain
the token type code number (*note Calling Convention::).

File: bison.info, Node: Precedence Decl, Next: Union Decl, Prev: Token Decl, Up: Declarations

Operator Precedence
-------------------

Use the `%left', `%right' or `%nonassoc' declaration to declare a
token and specify its precedence and associativity, all at once. These
are called "precedence declarations". *Note Operator Precedence:
Precedence, for general information on operator precedence.

The syntax of a precedence declaration is the same as that of
`%token': either

%left SYMBOLS...

%left <TYPE> SYMBOLS...

And indeed any of these declarations serves the purposes of `%token'.
But in addition, they specify the associativity and relative precedence
for all the SYMBOLS:

* The associativity of an operator OP determines how repeated uses
of the operator nest: whether `X OP Y OP Z' is parsed by grouping
X with Y first or by grouping Y with Z first. `%left' specifies
left-associativity (grouping X with Y first) and `%right'
specifies right-associativity (grouping Y with Z first).
`%nonassoc' specifies no associativity, which means that `X OP Y
OP Z' is considered a syntax error.

* 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.

File: bison.info, Node: Union Decl, Next: Type Decl, Prev: Precedence Decl, Up: Declarations

The Collection of Value Types
-----------------------------

The `%union' declaration specifies the entire collection of possible
data types for semantic values. The keyword `%union' is followed by a
pair of braces containing the same thing that goes inside a `union' in
C.

For example:

%union {
double val;
symrec *tptr;
}

This says that the two alternative types are `double' and `symrec *'.
They are given names `val' and `tptr'; these names are used in the
`%token' and `%type' declarations to pick one of the types for a
terminal or nonterminal symbol (*note Nonterminal Symbols: Type Decl.).

Note that, unlike making a `union' declaration in C, you do not write
a semicolon after the closing brace.

File: bison.info, Node: Type Decl, Next: Expect Decl, Prev: Union Decl, Up: Declarations

Nonterminal Symbols
-------------------

When you use `%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 `%type' declaration, like this:

%type <TYPE> NONTERMINAL...

Here NONTERMINAL is the name of a nonterminal symbol, and TYPE is the
name given in the `%union' to the alternative that you want (*note The
Collection of Value Types: Union Decl.). You can give any number of
nonterminal symbols in the same `%type' declaration, if they have the
same value type. Use spaces to separate the symbol names.

You can also declare the value type of a terminal symbol. To do
this, use the same `<TYPE>' construction in a declaration for the
terminal symbol. All kinds of token declarations allow `<TYPE>'.

File: bison.info, Node: Expect Decl, Next: Start Decl, Prev: Type Decl, Up: Declarations

Suppressing Conflict Warnings
-----------------------------

Bison normally warns if there are any conflicts in the grammar
(*note Shift/Reduce Conflicts: Shift/Reduce.), 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 `%expect' declaration.

The declaration looks like this:

%expect N

Here N is a decimal integer. The declaration says there should be
no warning if there are N shift/reduce conflicts and no reduce/reduce
conflicts. An error, instead of the usual warning, is given if there
are either more or fewer conflicts, or if there are any reduce/reduce
conflicts.

In general, using `%expect' involves these steps:

* Compile your grammar without `%expect'. Use the `-v' option to
get a verbose list of where the conflicts occur. Bison will also
print the number of conflicts.

* 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.

* Add an `%expect' declaration, copying the number N from the number
which Bison printed.

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.

File: bison.info, Node: Start Decl, Next: Pure Decl, Prev: Expect Decl, Up: Declarations

The Start-Symbol
----------------

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 `%start'
declaration as follows:

%start SYMBOL

File: bison.info, Node: Pure Decl, Next: Decl Summary, Prev: Start Decl, Up: Declarations

A Pure (Reentrant) Parser
-------------------------

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

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

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

%pure-parser

The result is that the communication variables `yylval' and `yylloc'
become local variables in `yyparse', and a different calling convention
is used for the lexical analyzer function `yylex'. *Note Calling
Conventions for Pure Parsers: Pure Calling, for the details of this.
The variable `yynerrs' also becomes local in `yyparse' (*note The Error
Reporting Function `yyerror': Error Reporting.). The convention for
calling `yyparse' itself is unchanged.

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

File: bison.info, Node: Decl Summary, Prev: Pure Decl, Up: Declarations

Bison Declaration Summary
-------------------------

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

`%union'
Declare the collection of data types that semantic values may have
(*note The Collection of Value Types: Union Decl.).

`%token'
Declare a terminal symbol (token type name) with no precedence or
associativity specified (*note Token Type Names: Token Decl.).

`%right'
Declare a terminal symbol (token type name) that is
right-associative (*note Operator Precedence: Precedence Decl.).

`%left'
Declare a terminal symbol (token type name) that is
left-associative (*note Operator Precedence: Precedence Decl.).

`%nonassoc'
Declare a terminal symbol (token type name) that is nonassociative
(using it in a way that would be associative is a syntax error)
(*note Operator Precedence: Precedence Decl.).

`%type'
Declare the type of semantic values for a nonterminal symbol
(*note Nonterminal Symbols: Type Decl.).

`%start'
Specify the grammar's start symbol (*note The Start-Symbol: Start
Decl.).

`%expect'
Declare the expected number of shift-reduce conflicts (*note
Suppressing Conflict Warnings: Expect Decl.).

In order to change the behavior of `bison', use the following
directives:

`%debug'
In the parser file, define the macro `YYDEBUG' to 1 if it is not
already defined, so that the debugging facilities are compiled.
*Note Tracing Your Parser: Tracing.

`%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 Semantic Values of Tokens: Token Values.

`%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.y'.

`%locations'
Generate the code processing the locations (*note Special Features
for Use in Actions: Action Features.). This mode is enabled as
soon as the grammar uses the special `@N' tokens, but if your
grammar does not use it, using `%locations' allows for more
accurate parse error messages.

`%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', `yynerrs', `yylval', `yychar',
`yydebug', and possible `yylloc'. For example, if you use
`%name-prefix="c_"', the names become `c_parse', `c_lex', and so
on. *Note Multiple Parsers in the Same Program: Multiple Parsers.

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

This option also tells Bison to write the C code for the grammar
actions into a file named `FILENAME.act', in the form of a
brace-surrounded body fit for a `switch' statement.

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

`%output="FILENAME"'
Specify the FILENAME for the parser file.

`%pure-parser'
Request a pure (reentrant) parser program (*note A Pure
(Reentrant) Parser: Pure Decl.).

`%token-table'
Generate an array of token names in the parser file. The name of
the array is `yytname'; `yytname[I]' is the name of the token
whose internal Bison token code number is I. The first three
elements of `yytname' are always `"$end"', `"error"', and
`"$undefined"'; after these come the symbols defined in the
grammar file.

For single-character literal tokens and literal string tokens, the
name in the table includes the single-quote or double-quote
characters: for example, `"'+'"' is a single-character literal and
`"\"<=\""' is a literal string token. All the characters of the
literal string token appear verbatim in the string found in the
table; even double-quote characters are not escaped. For example,
if the token consists of three characters `*"*', its string in
`yytname' contains `"*"*"'. (In C, that would be written as
`"\"*\"*\""').

When you specify `%token-table', Bison also generates macro
definitions for macros `YYNTOKENS', `YYNNTS', and `YYNRULES', and
`YYNSTATES':

`YYNTOKENS'
The highest token number, plus one.

`YYNNTS'
The number of nonterminal symbols.

`YYNRULES'
The number of grammar rules,

`YYNSTATES'
The number of parser states (*note Parser States::).

`%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. *Note Understanding Your Parser: Understanding, for
more information.

`%yacc'
Pretend the option `--yacc' was given, i.e., imitate Yacc,
including its naming conventions. *Note Bison Options::, for more.

File: bison.info, Node: Multiple Parsers, Prev: Declarations, Up: Grammar File

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 `yyparse', `yylval', and so
on.

The easy way to do this is to use the option `-p PREFIX' (*note
Invoking Bison: Invocation.). This renames the interface functions and
variables of the Bison parser to start with PREFIX instead of `yy'.
You can use this to give each parser distinct names that do not
conflict.

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

*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, `YYSTYPE' is not
renamed, but defining this in different ways in different parsers causes
no trouble (*note Data Types of Semantic Values: Value Type.).

The `-p' option works by adding macro definitions to the beginning
of the parser source file, defining `yyparse' as `PREFIXparse', and so
on. This effectively substitutes one name for the other in the entire
parser file.

File: bison.info, Node: Interface, Next: Algorithm, Prev: Grammar File, Up: Top

Parser C-Language Interface
***************************

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

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

* Menu:

* Parser Function:: How to call `yyparse' and what it returns.
* Lexical:: You must supply a function `yylex'
which reads tokens.
* Error Reporting:: You must supply a function `yyerror'.
* Action Features:: Special features for use in actions.

File: bison.info, Node: Parser Function, Next: Lexical, Up: Interface

The Parser Function `yyparse'
=============================

You call the function `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 `yyparse' to return immediately without
reading further.

The value returned by `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 `yyparse' by using
these macros:

`YYACCEPT'
Return immediately with value 0 (to report success).

`YYABORT'
Return immediately with value 1 (to report failure).

File: bison.info, Node: Lexical, Next: Error Reporting, Prev: Parser Function, Up: Interface

The Lexical Analyzer Function `yylex'
=====================================

The "lexical analyzer" function, `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 `yyparse' can
call it. The function is sometimes referred to as a lexical scanner.

In simple programs, `yylex' is often defined at the end of the Bison
grammar file. If `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 `-d' option when you run Bison, so that it
will write these macro definitions into a separate header file
`NAME.tab.h' which you can include in the other source files that need
it. *Note Invoking Bison: Invocation.

* Menu:

* Calling Convention:: How `yyparse' calls `yylex'.
* Token Values:: How `yylex' must return the semantic value
of the token it has read.
* Token Positions:: How `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 (*note A Pure (Reentrant) Parser: Pure Decl.).