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#### Semantic Actions with Parsers

###### Description

Semantic actions may be attached to any point in the grammar specification. They allow to call a function or function object in order to provide the value to be output by the parser attached to the semantic action. Semantic actions are associated with a parser using the syntax `p[]`, where `p` is an arbitrary parser expression.

###### Header

    // forwards to <boost/spirit/home/qi/action.hpp>
    #include <boost/spirit/include/qi_action.hpp>

Also, see Include Structure.

###### Model of

> `UnaryParser`

Notation

`a`, `p`

  Instances of a parser, `P`

`A`

  Attribute type exposed by a parser, `a`

`fa`

  A (semantic action) function with signature `void(Attrib&, Context, bool&)`. The third parameter is a boolean flag that can be set to false to force the parser to fail. Both `Context` and the boolean flag are optional. For more information see below.

`Attrib`

  The attribute obtained from the parse.

`Context`

  The type of the parser execution context. For more information see below.

###### Expression Semantics

Semantics of an expression is defined only where it differs from, or is not defined in `UnaryParser`.

Expression | Semantics
--- | ---
`p[fa]` | If `p` is successful, call semantic action, `fa`. The function or function object `fa` is provided the attribute value parsed by the parser `p`, plus some more context information and a mutable bool flag which can be used to fail parsing.

The possible signatures for functions to be used as semantic actions are:

    template <typename Attrib>
    void fa(Attrib& attr);

    template <typename Attrib, typename Context>
    void fa(Attrib& attr, Context& context);

    template <typename Attrib, typename Context>
    void fa(Attrib& attr, Context& context, bool& pass);

The function or function object is expected to return the value to generate output from by assigning it to the first parameter, `attr`. Here `Attrib` is the attribute type of the parser attached to the semantic action.

The type `Context` is the type of the parser execution context. This type is unspecified and depends on the context the parser is invoked in. The value, `context` used by semantic actions written using Boost.Phoenix to access various context dependent attributes and values. For more information about Boost.Phoenix placeholder expressions usable in semantic actions see `Nonterminal`.

The third parameter, `pass`, can be used by the semantic action to force the associated parser to fail. If pass is set to `false` the action parser will immediately return `false` as well, while not invoking `p` and not generating any output.

###### Attributes

Expression | Attribute
--- | ---
`a[fa]` | `a: A --> a[fa]: A`

###### Complexity

The complexity of the action parser is defined by the complexity of the parser the semantic action is attached to and the complexity of the function or function object used as the semantic action.

###### Example

Examples for semantic actions can be found here: Examples of Semantic Actions.