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<?xml version="1.0" encoding="utf-8"?>
<!--
Copyright 2012 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)
-->
<header name="boost/proto/transform/fold_tree.hpp">
<para>
Contains definition of the
<computeroutput>
<classname alt="boost::proto::fold_tree">proto::fold_tree<></classname>
</computeroutput> and
<computeroutput>
<classname alt="boost::proto::reverse_fold_tree">proto::reverse_fold_tree<></classname>
</computeroutput>
transforms.
</para>
<namespace name="boost">
<namespace name="proto">
<struct name="fold_tree">
<template>
<template-type-parameter name="Sequence"/>
<template-type-parameter name="State0"/>
<template-type-parameter name="Fun"/>
</template>
<inherit><classname>proto::transform</classname>< fold_tree<Sequence, State0, Fun> ></inherit>
<purpose>A <conceptname>PrimitiveTransform</conceptname> that recursively applies the
<computeroutput><classname>proto::fold</classname><></computeroutput> transform to sub-trees
that all share a common tag type.</purpose>
<description>
<para>
<computeroutput>proto::fold_tree<></computeroutput> is useful for flattening trees into lists;
for example, you might use <computeroutput>proto::fold_tree<></computeroutput> to flatten an
expression tree like <computeroutput>a | b | c</computeroutput> into a Fusion list like
<computeroutput>cons(c, cons(b, cons(a)))</computeroutput>.
</para>
<para>
<computeroutput>proto::fold_tree<></computeroutput> is easily understood in terms of a
<computeroutput>recurse_if_<></computeroutput> helper, defined as follows:
<programlisting> template<typename Tag, typename Fun>
struct recurse_if_ :
<classname>proto::if_</classname><
// If the current node has type type "Tag" ...
boost::is_same<<classname>proto::tag_of</classname><<classname>proto::_</classname>>, Tag>(),
// ... recurse, otherwise ...
<classname>proto::fold</classname><<classname>proto::_</classname>, <classname>proto::_state</classname>, recurse_if_<Tag, Fun> >,
// ... apply the Fun transform.
Fun
>
{};</programlisting>
</para>
<para>
With <computeroutput>recurse_if_<></computeroutput> as defined above,
<computeroutput>proto::fold_tree<Sequence, State0, Fun>()(expr, state, data)</computeroutput>
is equivalent to:
<programlisting><classname>proto::fold</classname><
Sequence,
State0,
recurse_if_<typename Expr::proto_tag, Fun>
>()(expr, state, data).</programlisting>
It has the effect of folding a tree front-to-back, recursing into child nodes that share a
tag type with the parent node.
</para>
</description>
<struct name="impl">
<template>
<template-type-parameter name="Expr"/>
<template-type-parameter name="State"/>
<template-type-parameter name="Data"/>
</template>
<inherit>
<type>
<classname>proto::fold</classname><Sequence, State0, recurse_if_<typename Expr::proto_tag, Fun> >
::template impl<Expr, State, Data></type>
</inherit>
</struct>
</struct>
<struct name="reverse_fold_tree">
<template>
<template-type-parameter name="Sequence"/>
<template-type-parameter name="State0"/>
<template-type-parameter name="Fun"/>
</template>
<inherit><classname>proto::transform</classname>< reverse_fold_tree<Sequence, State0, Fun> ></inherit>
<purpose>A <conceptname>PrimitiveTransform</conceptname> that recursively applies the
<computeroutput><classname>proto::reverse_fold<></classname></computeroutput> transform to
sub-trees that all share a common tag type.</purpose>
<description>
<para>
<computeroutput>proto::reverse_fold_tree<></computeroutput> is useful for flattening trees
into lists; for example, you might use <computeroutput>proto::reverse_fold_tree<></computeroutput>
to flatten an expression tree like <computeroutput>a | b | c</computeroutput> into a Fusion list like
<computeroutput>cons(a, cons(b, cons(c)))</computeroutput>.
</para>
<para>
<computeroutput>proto::reverse_fold_tree<></computeroutput> is easily understood in terms of
a <computeroutput>recurse_if_<></computeroutput> helper, defined as follows:
<programlisting> template<typename Tag, typename Fun>
struct recurse_if_ :
<classname>proto::if_</classname><
// If the current node has type type "Tag" ...
boost::is_same<<classname>proto::tag_of</classname><<classname>proto::_</classname>>, Tag>(),
// ... recurse, otherwise ...
<classname>proto::reverse_fold</classname><<classname>proto::_</classname>, <classname>proto::_state</classname>, recurse_if_<Tag, Fun> >,
// ... apply the Fun transform.
Fun
>
{};</programlisting>
</para>
<para>
With <computeroutput>recurse_if_<></computeroutput> as defined above,
<computeroutput>proto::reverse_fold_tree<Sequence, State0, Fun>()(expr, state, data)</computeroutput>
is equivalent to:
<programlisting><classname>proto::reverse_fold</classname><
Sequence,
State0,
recurse_if_<typename Expr::proto_tag, Fun>
>()(expr, state, data).</programlisting>
It has the effect of folding a tree back-to-front, recursing into child nodes that share a
tag type with the parent node.
</para>
</description>
<struct name="impl">
<template>
<template-type-parameter name="Expr"/>
<template-type-parameter name="State"/>
<template-type-parameter name="Data"/>
</template>
<inherit>
<type>
<classname>proto::reverse_fold</classname><Sequence, State0, recurse_if_<typename Expr::proto_tag, Fun> >
::template impl<Expr, State, Data></type>
</inherit>
</struct>
</struct>
</namespace>
</namespace>
</header>
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