proto/doc/reference/transform/when.xml

<?xml version="1.0" encoding="utf-8"?>
<header name="boost/proto/transform/when.hpp">
  <para>
    Definition of the
    <computeroutput>
      <classname alt="boost::proto::when">proto::when&lt;&gt;</classname>
    </computeroutput> and
    <computeroutput>
      <classname alt="boost::proto::otherwise">proto::otherwise&lt;&gt;</classname>
    </computeroutput> transforms.
  </para>
  <namespace name="boost">
    <namespace name="proto">
      <struct name="when">
        <template>
          <template-type-parameter name="Grammar"/>
          <template-type-parameter name="PrimitiveTransform">
            <default>Grammar</default>
          </template-type-parameter>
        </template>
        <purpose>A grammar element and a <conceptname>PrimitiveTransform</conceptname> that associates
          a transform with the grammar.</purpose>
        <description>
          <para>
            Use <computeroutput>proto::when&lt;&gt;</computeroutput> to override a grammar's default
            transform with a custom transform. It is for used when composing larger transforms by
            associating smaller transforms with individual rules in your grammar, as in the following
            transform which counts the number of terminals in an expression.
            <programlisting>// Count the terminals in an expression tree.
// Must be invoked with initial state == mpl::int_&lt;0&gt;().
struct CountLeaves :
  <classname>proto::or_</classname>&lt;
    proto::when&lt;<classname>proto::terminal</classname>&lt;<classname>proto::_</classname>&gt;, mpl::next&lt;<classname>proto::_state</classname>&gt;()&gt;,
    proto::otherwise&lt;<classname>proto::fold</classname>&lt;<classname>proto::_</classname>, <classname>proto::_state</classname>, CountLeaves&gt; &gt;
  &gt;
{};</programlisting>
          </para>
          <para>
            In <computeroutput>proto::when&lt;G, T&gt;</computeroutput>, when <computeroutput>T</computeroutput>
            is a class type it is a <conceptname>PrimitiveTransform</conceptname> and the following equivalencies hold:
          </para>
          <itemizedlist>
            <listitem>
              <para>
                <computeroutput>boost::result_of&lt;proto::when&lt;G,T&gt;(E,S,V)&gt;::type</computeroutput> is the same as
                <computeroutput>boost::result_of&lt;T(E,S,V)&gt;::type</computeroutput>.
              </para>
            </listitem>
            <listitem>
              <para>
                <computeroutput>proto::when&lt;G,T&gt;()(e,s,d)</computeroutput> is the same as
                <computeroutput>T()(e,s,d)</computeroutput>.
              </para>
            </listitem>
          </itemizedlist>
        </description>
        <inherit><type>PrimitiveTransform</type></inherit>
        <typedef name="proto_grammar">
          <type>typename Grammar::proto_grammar</type>
        </typedef>
      </struct>

      <struct-specialization name="when">
        <template>
          <template-type-parameter name="Grammar"/>
          <template-type-parameter name="Fun"/>
        </template>
        <specialization>
          <template-arg>Grammar</template-arg>
          <template-arg>Fun *</template-arg>
        </specialization>
        <inherit><type><classname>proto::when</classname>&lt; Grammar, Fun &gt;</type></inherit>
        <purpose>A specialization that treats function pointer <conceptname>Transform</conceptname>s as if they
          were function type <conceptname>Transform</conceptname>s.</purpose>
        <description>
          <para>
            This specialization requires that <computeroutput>Fun</computeroutput> is actually a function type.
          </para>
          <para>
            This specialization is required for nested transforms such as
            <computeroutput>proto::when&lt;G, T0(T1(_))&gt;</computeroutput>. In C++, functions that are used
            as parameters to other functions automatically decay to funtion pointer types. In other words, the
            type <computeroutput>T0(T1(_))</computeroutput> is indistinguishable from
            <computeroutput>T0(T1(*)(_))</computeroutput>. This specialization is required to handle these
            nested function pointer type transforms properly.
          </para>
        </description>
      </struct-specialization>

      <struct-specialization name="when">
        <template>
          <template-type-parameter name="Grammar"/>
          <template-type-parameter name="R"/>
          <template-type-parameter name="A" pack="1"/>
        </template>
        <specialization>
          <template-arg>Grammar</template-arg>
          <template-arg>R(A...)</template-arg>
        </specialization>
        <inherit><type><classname>proto::transform</classname>&lt; when&lt;Grammar, R(A...)&gt; &gt;</type></inherit>
        <purpose>A grammar element and a <conceptname>Transform</conceptname> that associates a
          transform with the grammar. </purpose>
        <description>
          <para>
            Use <computeroutput>proto::when&lt;&gt;</computeroutput> to override a grammar's default
            transform with a custom transform. It is for use when composing larger transforms by associating
            smaller transforms with individual rules in your grammar.
          </para>
          <para>
            The <computeroutput>when&lt;G, R(A...)&gt;</computeroutput> form accepts either a
            <conceptname>CallableTransform</conceptname> or an <conceptname>ObjectTransform</conceptname> as its
            second parameter. <computeroutput>proto::when&lt;&gt;</computeroutput> uses
            <computeroutput><classname>proto::is_callable</classname>&lt;R&gt;::value</computeroutput> to
            distinguish between the two, and uses
            <computeroutput><classname>proto::call&lt;&gt;</classname></computeroutput> to evaluate
            <conceptname>CallableTransform</conceptname>s and
            <computeroutput><classname>proto::make&lt;&gt;</classname></computeroutput> to evaluate
            <conceptname>ObjectTransform</conceptname>s.
          </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::transform_impl</classname>&lt; Expr, State, Data &gt;</type></inherit>
          <typedef name="call_">
            <purpose>For exposition only</purpose>
            <type><classname>proto::call</classname>&lt;R(A...)&gt;</type>
          </typedef>
          <typedef name="make_">
            <purpose>For exposition only</purpose>
            <type><classname>proto::make</classname>&lt;R(A...)&gt;</type>
          </typedef>
          <typedef name="which">
            <purpose>For exposition only</purpose>
            <type>typename mpl::if_&lt;<classname>proto::is_callable</classname>&lt;R&gt;,call_,make_&gt;::type</type>
          </typedef>
          <typedef name="result_type">
            <type>typename boost::result_of&lt;which(Expr, State, Data)&gt;::type</type>
          </typedef>
          <method-group name="public member functions">
            <method name="operator()" cv="const">
              <type>result_type</type>
              <parameter name="expr">
                <paramtype>typename impl::expr_param</paramtype>
                <description>
                  <para>The current expression </para>
                </description>
              </parameter>
              <parameter name="state">
                <paramtype>typename impl::state_param</paramtype>
                <description>
                  <para>The current state </para>
                </description>
              </parameter>
              <parameter name="data">
                <paramtype>typename impl::data_param</paramtype>
                <description>
                  <para>An arbitrary data </para>
                </description>
              </parameter>
              <description>
                <para>
                  Evaluate <computeroutput>R(A...)</computeroutput> as a transform either with
                  <computeroutput><classname>proto::call&lt;&gt;</classname></computeroutput> or with
                  <computeroutput><classname>proto::make&lt;&gt;</classname></computeroutput> depending
                  on whether <computeroutput><classname>proto::is_callable</classname>&lt;R&gt;::value</computeroutput>
                  is <computeroutput>true</computeroutput> or <computeroutput>false</computeroutput>.
                </para>
              </description>
              <requires>
                <para>
                  <computeroutput><classname>proto::matches</classname>&lt;Expr, Grammar&gt;::value</computeroutput>
                  is <computeroutput>true</computeroutput>.
                </para>
              </requires>
              <returns>
                <para>
                  <computeroutput>which()(expr, state, data)</computeroutput>
                </para>
              </returns>
            </method>
          </method-group>
        </struct>
        <typedef name="proto_grammar">
          <type>typename Grammar::proto_grammar</type>
        </typedef>
      </struct-specialization>

      <struct-specialization name="when">
        <template>
          <template-type-parameter name="Grammar"/>
        </template>
        <specialization>
          <template-arg>Grammar</template-arg>
          <template-arg><classname>proto::external_transform</classname></template-arg>
        </specialization>
        <inherit><type>
  <classname>proto::transform</classname>&lt; when&lt;Grammar, <classname>proto::external_transform</classname>&gt; &gt;</type></inherit>
        <purpose>A grammar element that associates an externally-specified transform with the grammar.
          The transform is looked up in the Data parameter using the Grammar as a key.</purpose>
        <description>
          <para>
            Use <computeroutput>proto::when&lt;&gt;</computeroutput> to override a grammar's default
            transform with a custom transform. It is for use when composing larger transforms by associating
            smaller transforms with individual rules in your grammar.
          </para>
          <para>
            The <computeroutput>when&lt;G, <classname>proto::external_transform</classname>&gt;</computeroutput>
            indicates that the associated transform is not yet known. It should be looked up when the transform
            is about to be applied. It is found by looking it up in the passed-in Data parameter, which
            behaves like a compile-time map from grammar types to transform types. The map is indexed using
            <computeroutput>Grammar</computeroutput> as a key. The associated value type is used as the transform
            to apply. In this way, the same grammar can be used to define multiple evaluating strategies that
            can be added post-hoc.
          </para>
          <para>
            See <computeroutput><classname>proto::external_transforms</classname></computeroutput> for an example.
          </para>
        </description>
        <struct name="impl">
          <template>
            <template-type-parameter name="Expr"/>
            <template-type-parameter name="State"/>
            <template-type-parameter name="Data"/>
          </template>
          <inherit><type>
    boost::remove_reference&lt; Data &gt;::type
      ::template when&lt; Grammar &gt;
        ::template impl&lt; Expr, State, Data &gt;</type></inherit>
        </struct>
        <typedef name="proto_grammar">
          <type>typename Grammar::proto_grammar</type>
        </typedef>
      </struct-specialization>
      
      <struct name="otherwise">
        <template>
          <template-type-parameter name="Fun"/>
        </template>
        <inherit><type><classname>proto::when</classname>&lt; <classname>proto::_</classname>, Fun &gt;</type></inherit>
        <purpose>
          Syntactic sugar for <computeroutput><classname>proto::when</classname>&lt; <classname>proto::_</classname>, Fun &gt;</computeroutput>,
          for use in grammars to handle all the cases not yet handled.
        </purpose>
        <description>
          <para>
            Use <computeroutput>proto::otherwise&lt;T&gt;</computeroutput> in your grammars as a synonym for
            <computeroutput><classname>proto::when</classname>&lt; <classname>proto::_</classname>, Fun &gt;</computeroutput>
            as in the following transform which counts the number of terminals in an expression.
          </para>
          <para>
            <programlisting>// Count the terminals in an expression tree.
// Must be invoked with initial state == mpl::int_&lt;0&gt;().
struct CountLeaves :
  <classname>proto::or_</classname>&lt;
    proto::when&lt;<classname>proto::terminal</classname>&lt;<classname>proto::_</classname>&gt;, mpl::next&lt;<classname>proto::_state</classname>&gt;()&gt;,
    proto::otherwise&lt;<classname>proto::fold</classname>&lt;<classname>proto::_</classname>, <classname>proto::_state</classname>, CountLeaves&gt; &gt;
  &gt;
{};</programlisting>
          </para>
        </description>
      </struct>

      <struct name="external_transform">
        <purpose>A placeholder for use as the second parameter for <computeroutput><classname>proto::when</classname></computeroutput>
          to indicate that the rule's transform is specified externally.</purpose>
        <description>
          <para>
            See <computeroutput><classname>proto::external_transforms</classname></computeroutput> for an example.
          </para>
        </description>
      </struct>
      
      <struct name="external_transforms">
        <template>
          <template-type-parameter name="When" pack="1"/>
        </template>
        <purpose>A map from grammars to transforms, used as a way to externally associate transforms.</purpose>
        <typedef name="map_type">
          <purpose>For exposition only.</purpose>
          <type>mpl::map&lt; typename to_mpl_pair&lt; When &gt;::type... &gt;</type>
        </typedef>
        <struct name="when">
          <template>
            <template-type-parameter name="Grammar"/>
          </template>
          <inherit><type><classname>proto::otherwise</classname>&lt; typename mpl::at&lt; map_type, Grammar &gt;::type &gt;</type></inherit>
        </struct>
        <description>
          <para>
            It is sometimes desirable to define a grammar that can be customized with different sets of transforms.
            To do that, where you would normally specify a transform within a grammar, you can instead put
            <computeroutput><classname>proto::external_transform</classname></computeroutput>; for example:
            <computeroutput>proto::when&lt; some_grammar, proto::external_transform &gt;</computeroutput>. Then, when
            invoking the grammar, you can pass an approriately-defined instance of <computeroutput>proto::external_transforms</computeroutput>
            as the Data parameter. When an expression matches <computeroutput>some_grammar</computeroutput>, Proto
            will look up the approprite transform in the Data parameter using <computeroutput>some_grammar</computeroutput>
            as a key.
          </para>
          <para>
            <programlisting>struct int_terminal
  : <classname>proto::terminal</classname>&lt;int&gt;
{};

struct char_terminal
  : <classname>proto::terminal</classname>&lt;char&gt;
{};

struct my_grammar
  : <classname>proto::or_</classname>&lt;
        // The next two grammar rules are customization points.
        // The associated transforms are specified externally
        // using external_transforms below.
        <classname>proto::when</classname>&lt; int_terminal, <classname>proto::external_transform</classname> &gt;
      , <classname>proto::when</classname>&lt; char_terminal, <classname>proto::external_transform</classname> &gt;
      , <classname>proto::when</classname>&lt;
            <classname>proto::plus</classname>&lt; my_grammar, my_grammar &gt;
          , <classname>proto::fold</classname>&lt; <classname>proto::_</classname>, int(), my_grammar &gt;
        &gt;
    &gt;
{};

// Here is where the transforms are associated with the
// grammar rules above.
struct my_transforms
  : proto::external_transforms&lt;
        <classname>proto::when</classname>&lt;int_terminal, print(<classname>proto::_value</classname>)&gt;
      , <classname>proto::when</classname>&lt;char_terminal, print(<classname>proto::_value</classname>)&gt;
    &gt;
{};

// ...

<classname>proto::literal</classname>&lt;int&gt; i(1);
<classname>proto::literal</classname>&lt;char&gt; c('a');
my_transforms trx;

// Evaluate "i+c" using my_grammar with the specified transforms:
my_grammar()(i + c, 0, trx);</programlisting>
          </para>
        </description>
      </struct>    
    </namespace>
  </namespace>
</header>