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## Struct template external_transforms

boost::proto::external_transforms — A map from grammars to transforms, used as a way to externally associate transforms.

## Synopsis

    // In header: <boost/proto/transform/when.hpp>

    template<typename... When>
    struct external_transforms {
      // types
      typedef mpl::map< typename to_mpl_pair< When >::type... > map_type;  // For exposition only.

      // member classes/structs/unions
      template<typename Grammar>
      struct when :
         proto::otherwise< typename mpl::at< map_type, Grammar >::type >
      {
      };
    };

## Description

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 `proto::external_transform`; for example: `proto::when< some_grammar, proto::external_transform >`. Then, when invoking the grammar, you can pass an approriately-defined instance of `proto::external_transforms` as the Data parameter. When an expression matches `some_grammar`, Proto will look up the approprite transform in the Data parameter using `some_grammar` as a key.

    struct int_terminal
      : proto::terminal<int>
    {};

    struct char_terminal
      : proto::terminal<char>
    {};

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

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

    // ...

    proto::literal<int> i(1);
    proto::literal<char> c('a');
    my_transforms trx;

    // Evaluate "i+c" using my_grammar with the specified transforms:
    my_grammar()(i + c, 0, trx);