Definintion of ``proto::context::callable_context<>``, an evaluation context for `proto::eval()` that fans out each node and calls the derived context type with the expressions constituents. If the derived context doesn't have an overload that handles this node, fall back to some other context.

A BinaryFunction that accepts a Proto expression and a callable context and calls the context with the expression tag and children as arguments, effectively fanning the expression out.

`proto::context::callable_eval<>` requires that `Context` is a PolymorphicFunctionObject that can be invoked with `Expr`'s tag and children as expressions, as follows:

    context(typename Expr::proto_tag(), proto::child_c<0>(expr), ... proto::child_c<N>(expr))

typename boost::result_of< Context( typename Expr::proto_tag, typename proto::result_of::child_c<0>::type, ... typename proto::result_of::child_c<N>::type, )>::type
result_type `Expr &

The current expression

` `Context &

The callable evaluation context

`

`context(typename Expr::proto_tag(), proto::child_c<0>(expr),... proto::child_c<N>(expr))`

proto::context::default_context
An evaluation context adaptor that makes authoring a context a simple matter of writing function overloads, rather then writing template specializations.

`proto::callable_context<>` is a base class that implements the context protocol by passing fanned-out expression nodes to the derived context, making it easy to customize the handling of expression types by writing function overloads. Only those expression types needing special handling require explicit handling. All others are dispatched to a user-specified default context, `DefaultCtx`.

`proto::callable_context<>` is defined simply as:

    template<typename Context, typename DefaultCtx = default_context>
    struct callable_context {
      template<typename Expr, typename ThisContext = Context>
      struct eval :
        mpl::if_<
          is_expr_handled_<Expr, Context>, // For exposition
          proto::context::callable_eval<Expr, ThisContext>,
          typename DefaultCtx::template eval<Expr, Context>
        >::type
      {};
    };

The Boolean metafunction `is_expr_handled_<>` uses metaprogramming tricks to determine whether `Context` has an overloaded function call operator that accepts the fanned-out constituents of an expression of type `Expr`. If so, the handling of the expression is dispatched to ``proto::context::callable_eval<>``. If not, it is dispatched to the user-specified `DefaultCtx`.

Example:

    // An evaluation context that increments all
    // integer terminals in-place.
    struct increment_ints :
      proto::context::callable_context<
        increment_ints const                // derived context
        proto::context::null_context const  // fall-back context
      >
    {
        typedef void result_type;

        // Handle int terminals here:
        void operator()(proto::tag::terminal, int &i) const
        {
           ++i;
        }
    };

With `increment_ints`, we can do the following:

    proto::literal<int> i = 0, j = 10;
    proto::eval( i - j * 3.14, increment_ints() );

    assert( i.get() == 1 && j.get() == 11 );

Context

A BinaryFunction that accepts an `Expr` and a `Context`, and either fans out the expression and passes it to the context, or else hands off the expression to `DefaultCtx`.

If `Context` is a PolymorphicFunctionObject such that it can be invoked with the tag and children of `Expr`, as `ctx(typename Expr::proto_tag(), child_c<0>(expr),... child_c<N>(expr))`, then `eval<Expr, ThisContext>` inherits from ``proto::context::callable_eval`<Expr, ThisContext>`. Otherwise, `eval<Expr, ThisContext>` inherits from `DefaultCtx::eval<Expr, Context>`.

see-below