Boost C++ Libraries

As you know from the Actors in Detail section, Actions are what brings life to a Phoenix expression tree.

When dealing with a Phoenix expression tree, it gets evaluated top-down. Example:

_1 + 3 * _2

Can be visualized as an AST in the following way:

In terms of actions this means:

Every time a rule is matched, an action will be called. The action determines how the Phoenix AST will be traversed.

Writing an Action

As mentioned in Actors in Detail actions are Proto Primitive Transforms for convenience Phoenix provides an abstraction to this:

template <typename Fun>
struct call;

This is similar to proto::call but does more. It calls the Fun function object passed as template parameter with the Context and the children of the expression associated with the rule.

Lets have an (simplified) example on how to write an evaluation action for rule::plus:

struct plus_eval
{
    typedef int result_type;

    template <typename Lhs, typename Rhs, typename Context>
    result_type operator()(Lhs const& lhs, Rhs const &rhs, Context & ctx)
    {
        return eval(lhs, ctx) + eval(rhs, ctx);
    }
};

template <>
struct default_actions::when<rule::plus>
    : call<plus_eval>
{};

That's it. When evaluating a plus expression, the plus_eval callable gets called with the left hand side and right hand side expression and the associated Context.

But there is more: As Actions can be full fletched Proto Transforms, you can in fact use any proto expression you can imagine as the action. Phoenix predifines a set of callables and transform to deal with the Context information passed along and of course every Phoenix expression can be used as a Phoenix grammar or Proto Pass Through Transform.

Phoenix is equipped with a predefined set of expressions, rules and actions to make all the stuff work you learned in the Starter Kit and Modules sections. See the next section for more details!