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///////////////////////////////////////////////////////////////////////////////
/// \file fold_tree.hpp
/// Contains definition of the fold_tree<> and reverse_fold_tree<> transforms.
//
// 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_TRANSFORM_FOLD_TREE_HPP_EAN_11_05_2007
#define BOOST_PROTO_TRANSFORM_FOLD_TREE_HPP_EAN_11_05_2007
#include <boost/type_traits/is_same.hpp>
#include <boost/proto/proto_fwd.hpp>
#include <boost/proto/traits.hpp>
#include <boost/proto/matches.hpp>
#include <boost/proto/transform/fold.hpp>
#include <boost/proto/transform/impl.hpp>
namespace boost { namespace proto
{
namespace detail
{
template<typename Tag>
struct has_tag
{
template<typename Expr, typename State, typename Data, typename EnableIf = Tag>
struct impl
{
typedef mpl::false_ result_type;
};
template<typename Expr, typename State, typename Data>
struct impl<Expr, State, Data, typename Expr::proto_tag>
{
typedef mpl::true_ result_type;
};
template<typename Expr, typename State, typename Data>
struct impl<Expr &, State, Data, typename Expr::proto_tag>
{
typedef mpl::true_ result_type;
};
};
template<typename Tag, typename Fun>
struct fold_tree_
: if_<has_tag<Tag>, fold<_, _state, fold_tree_<Tag, Fun> >, Fun>
{};
template<typename Tag, typename Fun>
struct reverse_fold_tree_
: if_<has_tag<Tag>, reverse_fold<_, _state, reverse_fold_tree_<Tag, Fun> >, Fun>
{};
}
/// \brief A PrimitiveTransform that recursively applies the
/// <tt>fold\<\></tt> transform to sub-trees that all share a common
/// tag type.
///
/// <tt>fold_tree\<\></tt> is useful for flattening trees into lists;
/// for example, you might use <tt>fold_tree\<\></tt> to flatten an
/// expression tree like <tt>a | b | c</tt> into a Fusion list like
/// <tt>cons(c, cons(b, cons(a)))</tt>.
///
/// <tt>fold_tree\<\></tt> is easily understood in terms of a
/// <tt>recurse_if_\<\></tt> helper, defined as follows:
///
/// \code
/// template<typename Tag, typename Fun>
/// struct recurse_if_
/// : if_<
/// // If the current node has type "Tag" ...
/// is_same<tag_of<_>, Tag>()
/// // ... recurse, otherwise ...
/// , fold<_, _state, recurse_if_<Tag, Fun> >
/// // ... apply the Fun transform.
/// , Fun
/// >
/// {};
/// \endcode
///
/// With <tt>recurse_if_\<\></tt> as defined above,
/// <tt>fold_tree\<Sequence, State0, Fun\>()(e, s, d)</tt> is
/// equivalent to
/// <tt>fold<Sequence, State0, recurse_if_<Expr::proto_tag, Fun> >()(e, s, d).</tt>
/// It has the effect of folding a tree front-to-back, recursing into
/// child nodes that share a tag type with the parent node.
template<typename Sequence, typename State0, typename Fun>
struct fold_tree
: transform<fold_tree<Sequence, State0, Fun> >
{
template<typename Expr, typename State, typename Data>
struct impl
: fold<
Sequence
, State0
, detail::fold_tree_<typename Expr::proto_tag, Fun>
>::template impl<Expr, State, Data>
{};
template<typename Expr, typename State, typename Data>
struct impl<Expr &, State, Data>
: fold<
Sequence
, State0
, detail::fold_tree_<typename Expr::proto_tag, Fun>
>::template impl<Expr &, State, Data>
{};
};
/// \brief A PrimitiveTransform that recursively applies the
/// <tt>reverse_fold\<\></tt> transform to sub-trees that all share
/// a common tag type.
///
/// <tt>reverse_fold_tree\<\></tt> is useful for flattening trees into
/// lists; for example, you might use <tt>reverse_fold_tree\<\></tt> to
/// flatten an expression tree like <tt>a | b | c</tt> into a Fusion list
/// like <tt>cons(a, cons(b, cons(c)))</tt>.
///
/// <tt>reverse_fold_tree\<\></tt> is easily understood in terms of a
/// <tt>recurse_if_\<\></tt> helper, defined as follows:
///
/// \code
/// template<typename Tag, typename Fun>
/// struct recurse_if_
/// : if_<
/// // If the current node has type "Tag" ...
/// is_same<tag_of<_>, Tag>()
/// // ... recurse, otherwise ...
/// , reverse_fold<_, _state, recurse_if_<Tag, Fun> >
/// // ... apply the Fun transform.
/// , Fun
/// >
/// {};
/// \endcode
///
/// With <tt>recurse_if_\<\></tt> as defined above,
/// <tt>reverse_fold_tree\<Sequence, State0, Fun\>()(e, s, d)</tt> is
/// equivalent to
/// <tt>reverse_fold<Sequence, State0, recurse_if_<Expr::proto_tag, Fun> >()(e, s, d).</tt>
/// It has the effect of folding a tree back-to-front, recursing into
/// child nodes that share a tag type with the parent node.
template<typename Sequence, typename State0, typename Fun>
struct reverse_fold_tree
: transform<reverse_fold_tree<Sequence, State0, Fun> >
{
template<typename Expr, typename State, typename Data>
struct impl
: reverse_fold<
Sequence
, State0
, detail::reverse_fold_tree_<typename Expr::proto_tag, Fun>
>::template impl<Expr, State, Data>
{};
template<typename Expr, typename State, typename Data>
struct impl<Expr &, State, Data>
: reverse_fold<
Sequence
, State0
, detail::reverse_fold_tree_<typename Expr::proto_tag, Fun>
>::template impl<Expr &, State, Data>
{};
};
/// INTERNAL ONLY
///
template<typename Sequence, typename State0, typename Fun>
struct is_callable<fold_tree<Sequence, State0, Fun> >
: mpl::true_
{};
/// INTERNAL ONLY
///
template<typename Sequence, typename State0, typename Fun>
struct is_callable<reverse_fold_tree<Sequence, State0, Fun> >
: mpl::true_
{};
}}
#endif
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