<?xml version="1.0" ?>
<concept name="Expr" category="utility">
  <!--
  Copyright 2010 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)
  -->
  <param name="Expr" role="expession-type"/>
  <param name="Tag" role="tag-type"/>
  <param name="Domain" role="domain-type"/>
  <param name="N" role="mpl-integral-constant-type"/>

  <models-sentence>
    The type <arg num="1"/> must be a model of <self/>.
  </models-sentence>

  <models-sentence>
    The type <arg num="3"/> must be a model of <conceptname>Domain</conceptname>.
  </models-sentence>

  <models-sentence>
    The type <arg num="4"/> must be a model of MPL Integral Constant.
  </models-sentence>

  <description>
    <para>
      An Expr represents a tagged node in an expression tree.
      The children of the Expr must themselves satisfy the
      Expr concept. The Expr has an arity representing the
      number of children. If the number of children is zero,
      the Expr also has a value. An Expr also has an associated
      <conceptname>Domain</conceptname>.
    </para>
  </description>

  <notation variables="e">
    <sample-value>
      <type name="Expr"/>
    </sample-value>
  </notation>

  <associated-type name="proto_tag">
    <get-member-type name="proto_tag">
      <type name="Expr"/>
    </get-member-type>
    <description>
      <simpara>The tag type of the Expr.</simpara>
    </description>
  </associated-type>

  <associated-type name="proto_args">
    <get-member-type name="proto_args">
      <type name="Expr"/>
    </get-member-type>
    <description>
      <simpara>A typelist representing either the types of the child nodes, or,
        if the arity of the Expr is 0, of the value of the terminal.</simpara>
    </description>
  </associated-type>

  <associated-type name="proto_arity">
    <get-member-type name="proto_arity">
      <type name="Expr"/>
    </get-member-type>
    <description>
      <simpara>
        The arity (number of child nodes) of the Expr.
        <computeroutput>proto_arity</computeroutput> is an MPL Integral Constant.
      </simpara>
    </description>
  </associated-type>

  <associated-type name="proto_base_expr">
    <get-member-type name="proto_base_expr">
      <type name="Expr"/>
    </get-member-type>
    <description>
      <simpara>
        A typedef for an instantiation of
        <classname alt="boost::proto::expr"><code>proto::expr&lt;&gt;</code></classname>
        that is equivalent to Expr. Expression types are equivalent if they have the
        same <code>proto_tag</code>, <code>proto_args</code>, and <code>proto_arity</code>.
      </simpara>
    </description>
  </associated-type>

  <associated-type name="proto_derived_expr">
    <get-member-type name="proto_derived_expr">
      <type name="Expr"/>
    </get-member-type>
    <description>
      <simpara>
        A typedef for <code>Expr</code>.
      </simpara>
    </description>
  </associated-type>

  <associated-type name="proto_domain">
    <get-member-type name="proto_domain">
      <type name="Expr"/>
    </get-member-type>
    <description>
      <simpara>
        The Domain of the Expr. <computeroutput>proto_domain</computeroutput>
        models <conceptname>Domain</conceptname>.
      </simpara>
    </description>
  </associated-type>

  <associated-type name="proto_childN">
    <get-member-type name="proto_childN">
      <type name="Expr"/>
    </get-member-type>
    <description>
      <simpara>The type of the Nth child of Expr. Requires 
        <code>0 == N::value || N::value &lt; proto_arity::value</code></simpara>
    </description>
  </associated-type>

  <valid-expression name="Get N-th Child">
    <apply-function name="boost::proto::child&lt; N &gt;">
      <sample-value>
        <type name="Expr"/>
      </sample-value>
    </apply-function>
    <return-type>
      <require-same-type testable="yes">
        <type name="proto_childN"/>
      </require-same-type>
    </return-type>
    <semantics>Extracts the Nth child from this Expr.
      Requires <computeroutput>N::value &lt; proto_arity::value</computeroutput>.</semantics>
  </valid-expression>

  <valid-expression name="Get Terminal Value">
    <apply-function name="boost::proto::value">
      <sample-value>
        <type name="Expr"/>
      </sample-value>
    </apply-function>
    <return-type>
      <require-same-type testable="yes">
        <type name="proto_child0"/>
      </require-same-type>
    </return-type>
    <semantics>
      Extracts the value from a terminal Expr.
      Requires <computeroutput>0 == proto_arity::value</computeroutput>.
    </semantics>
  </valid-expression>

  <valid-expression name="Get Base">
    <apply-method name="proto_base">
      <sample-value>
        <type name="Expr"/>
      </sample-value>
    </apply-method>
    <return-type>
      <require-same-type testable="yes">
        <type name="proto_base_expr"/>
      </require-same-type>
    </return-type>
    <semantics>
        Returns an object of type
        <classname alt="boost::proto::expr"><code>proto::expr&lt;&gt;</code></classname>
        that is equivalent to <code>e</code>.
    </semantics>
  </valid-expression>

  <example-model>
    <type name="boost::proto::literal&lt; int &gt;"/>
  </example-model>

</concept>
