%define Summary - Bison 2.5

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3.7.13 %define Summary

There are many features of Bison's behavior that can be controlled by assigning the feature a single value. For historical reasons, some such features are assigned values by dedicated directives, such as %start, which assigns the start symbol. However, newer such features are associated with variables, which are assigned by the %define directive:

— Directive: %define variable
— Directive: %define variable value
— Directive: %define variable "value"

Define variable to value.
value must be placed in quotation marks if it contains any character other than a letter, underscore, period, or non-initial dash or digit. Omitting "value" entirely is always equivalent to specifying "".
It is an error if a variable is defined by %define multiple times, but see -D name[=value].

The rest of this section summarizes variables and values that %define accepts.

Some variables take Boolean values. In this case, Bison will complain if the variable definition does not meet one of the following four conditions:

value is true
value is omitted (or "" is specified). This is equivalent to true.
value is false.
variable is never defined. In this case, Bison selects a default value.

What variables are accepted, as well as their meanings and default values, depend on the selected target language and/or the parser skeleton (see %language, see %skeleton). Unaccepted variables produce an error. Some of the accepted variables are:

api.pure
- Language(s): C
- Purpose: Request a pure (reentrant) parser program. See A Pure (Reentrant) Parser.
- Accepted Values: Boolean
- Default Value: false
api.push-pull
- Language(s): C (deterministic parsers only)
- Purpose: Request a pull parser, a push parser, or both. See A Push Parser. (The current push parsing interface is experimental and may evolve. More user feedback will help to stabilize it.)
- Accepted Values: pull, push, both
- Default Value: pull
lr.default-reductions
- Language(s): all
- Purpose: Specify the kind of states that are permitted to contain default reductions. See Default Reductions. (The ability to specify where default reductions should be used is experimental. More user feedback will help to stabilize it.)
- Accepted Values: most, consistent, accepting
- Default Value:
  - accepting if lr.type is canonical-lr.
  - most otherwise.
lr.keep-unreachable-states
- Language(s): all
- Purpose: Request that Bison allow unreachable parser states to remain in the parser tables. See Unreachable States.
- Accepted Values: Boolean
- Default Value: false
lr.type
- Language(s): all
- Purpose: Specify the type of parser tables within the LR(1) family. See LR Table Construction. (This feature is experimental. More user feedback will help to stabilize it.)
- Accepted Values: lalr, ielr, canonical-lr
- Default Value: lalr
namespace
- Languages(s): C++
- Purpose: Specify the namespace for the parser class. For example, if you specify:
```
               %define namespace "foo::bar"
```
  Bison uses foo::bar verbatim in references such as:
```
               foo::bar::parser::semantic_type
```
  However, to open a namespace, Bison removes any leading :: and then splits on any remaining occurrences:
```
               namespace foo { namespace bar {
                 class position;
                 class location;
               } }
```
- Accepted Values: Any absolute or relative C++ namespace reference without a trailing "::". For example, "foo" or "::foo::bar".
- Default Value: The value specified by %name-prefix, which defaults to yy. This usage of %name-prefix is for backward compatibility and can be confusing since %name-prefix also specifies the textual prefix for the lexical analyzer function. Thus, if you specify %name-prefix, it is best to also specify %define namespace so that %name-prefix only affects the lexical analyzer function. For example, if you specify:
```
               %define namespace "foo"
               %name-prefix "bar::"
```
  The parser namespace is foo and yylex is referenced as bar::lex.
parse.lac
- Languages(s): C (deterministic parsers only)
- Purpose: Enable LAC (lookahead correction) to improve syntax error handling. See LAC.
- Accepted Values: none, full
- Default Value: none