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)) 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 ); 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