///////////////////////////////////////////////////////////////////////////////
/// \file callable.hpp
/// Definintion of callable_context\<\>, an evaluation context for
/// proto::eval() that explodes 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.
//
// Copyright 2008 Eric Niebler. Distributed under the Boost
// Software License, Version 1.0. (See accompanying file
// LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
#ifndef BOOST_PROTO_CONTEXT_CALLABLE_HPP_EAN_06_23_2007
#define BOOST_PROTO_CONTEXT_CALLABLE_HPP_EAN_06_23_2007
#include <boost/config.hpp>
#include <boost/detail/workaround.hpp>
#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/iteration/iterate.hpp>
#include <boost/preprocessor/facilities/intercept.hpp>
#include <boost/preprocessor/repetition/repeat.hpp>
#include <boost/preprocessor/repetition/enum_params.hpp>
#include <boost/preprocessor/repetition/enum_trailing.hpp>
#include <boost/preprocessor/repetition/enum_trailing_params.hpp>
#include <boost/preprocessor/arithmetic/inc.hpp>
#include <boost/preprocessor/selection/max.hpp>
#include <boost/mpl/if.hpp>
#include <boost/mpl/bool.hpp>
#include <boost/utility/result_of.hpp>
#include <boost/type_traits/remove_cv.hpp>
#include <boost/proto/proto_fwd.hpp>
#include <boost/proto/traits.hpp> // for child_c
namespace boost { namespace proto
{
namespace detail
{
template<typename Context>
struct callable_context_wrapper
: remove_cv<Context>::type
{
callable_context_wrapper();
typedef private_type_ fun_type(...);
operator fun_type *() const;
private:
callable_context_wrapper &operator =(callable_context_wrapper const &);
};
template<typename T>
yes_type check_is_expr_handled(T const &);
no_type check_is_expr_handled(private_type_ const &);
template<typename Expr, typename Context, long Arity = Expr::proto_arity_c>
struct is_expr_handled;
template<typename Expr, typename Context>
struct is_expr_handled<Expr, Context, 0>
{
static callable_context_wrapper<Context> &sctx_;
static Expr &sexpr_;
static typename Expr::proto_tag &stag_;
static const bool value =
sizeof(yes_type) ==
sizeof(
detail::check_is_expr_handled(
(sctx_(stag_, proto::value(sexpr_)), 0)
)
);
typedef mpl::bool_<value> type;
};
}
namespace context
{
/// \brief 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.
///
/// <tt>callable_eval\<\></tt> requires that \c Context is a
/// PolymorphicFunctionObject that can be invoked with \c Expr's
/// tag and children as expressions, as follows:
///
/// \code
/// context(Expr::proto_tag(), child_c<0>(expr), child_c<1>(expr), ...)
/// \endcode
template<
typename Expr
, typename Context
, long Arity // = Expr::proto_arity_c
>
struct callable_eval
{};
/// \brief 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.
///
/// <tt>callable_eval\<\></tt> requires that \c Context is a
/// PolymorphicFunctionObject that can be invoked with \c Expr's
/// tag and children as expressions, as follows:
///
/// \code
/// context(Expr::proto_tag(), value(expr))
/// \endcode
template<typename Expr, typename Context>
struct callable_eval<Expr, Context, 0>
{
typedef typename proto::result_of::value<Expr const &>::type value_type;
typedef
typename BOOST_PROTO_RESULT_OF<
Context(typename Expr::proto_tag, value_type)
>::type
result_type;
/// \param expr The current expression
/// \param context The callable evaluation context
/// \return <tt>context(Expr::proto_tag(), value(expr))</tt>
result_type operator ()(Expr &expr, Context &context) const
{
return context(typename Expr::proto_tag(), proto::value(expr));
}
};
/// \brief An evaluation context adaptor that makes authoring a
/// context a simple matter of writing function overloads, rather
/// then writing template specializations.
///
/// <tt>callable_context\<\></tt> 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, \c DefaultCtx.
///
/// <tt>callable_context\<\></tt> is defined simply as:
///
/// \code
/// 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
/// , callable_eval<Expr, ThisContext>
/// , typename DefaultCtx::template eval<Expr, Context>
/// >::type
/// {};
/// };
/// \endcode
///
/// The Boolean metafunction <tt>is_expr_handled_\<\></tt> uses
/// metaprogramming tricks to determine whether \c Context has
/// an overloaded function call operator that accepts the
/// fanned-out constituents of an expression of type \c Expr.
/// If so, the handling of the expression is dispatched to
/// <tt>callable_eval\<\></tt>. If not, it is dispatched to
/// the user-specified \c DefaultCtx.
///
/// Below is an example of how to use <tt>callable_context\<\></tt>:
///
/// \code
/// // An evaluation context that increments all
/// // integer terminals in-place.
/// struct increment_ints
/// : callable_context<
/// increment_ints const // derived context
/// , null_context const // fall-back context
/// >
/// {
/// typedef void result_type;
///
/// // Handle int terminals here:
/// void operator()(proto::tag::terminal, int &i) const
/// {
/// ++i;
/// }
/// };
/// \endcode
///
/// With \c increment_ints, we can do the following:
///
/// \code
/// literal<int> i = 0, j = 10;
/// proto::eval( i - j * 3.14, increment_ints() );
///
/// assert( i.get() == 1 && j.get() == 11 );
/// \endcode
template<
typename Context
, typename DefaultCtx // = default_context
>
struct callable_context
{
/// A BinaryFunction that accepts an \c Expr and a
/// \c Context, and either fans out the expression and passes
/// it to the context, or else hands off the expression to
/// \c DefaultCtx.
///
/// If \c Context is a PolymorphicFunctionObject such that
/// it can be invoked with the tag and children of \c Expr,
/// as <tt>ctx(Expr::proto_tag(), child_c\<0\>(expr), child_c\<1\>(expr)...)</tt>,
/// then <tt>eval\<Expr, ThisContext\></tt> inherits from
/// <tt>callable_eval\<Expr, ThisContext\></tt>. Otherwise,
/// <tt>eval\<Expr, ThisContext\></tt> inherits from
/// <tt>DefaultCtx::eval\<Expr, Context\></tt>.
template<typename Expr, typename ThisContext = Context>
struct eval
: mpl::if_c<
detail::is_expr_handled<Expr, Context>::value
, callable_eval<Expr, ThisContext>
, typename DefaultCtx::template eval<Expr, Context>
>::type
{};
};
}
#include <boost/proto/context/detail/callable_eval.hpp>
}}
#endif