Adding an expression

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### Adding an expression

This is not a toy example. This is actually part of the library. Remember the `while` lazy statement? Putting together everything we've learned so far, we eill present it here in its entirety (verbatim):

    BOOST_PHOENIX_DEFINE_EXPRESSION(
        (boost)(phoenix)(while_)
      , (meta_grammar)           // Cond
        (meta_grammar)           // Do
    )

    namespace boost { namespace phoenix
    {
        struct while_eval
        {
            typedef void result_type;

            template <typename Cond, typename Do, typename Context>
            result_type
            operator()(Cond const& cond, Do const& do_, Context & ctx) const
            {
                while(eval(cond, ctx))
                {
                    eval(do_, ctx);
                }
            }
        };

        template <typename Dummy>
        struct default_actions::when<rule::while_, Dummy>
            : call<while_eval, Dummy>
        {};

        template <typename Cond>
        struct while_gen
        {
            while_gen(Cond const& cond) : cond(cond) {}

            template <typename Do>
            typename expression::while_<Cond, Do>::type const
            operator[](Do const& do_) const
            {
                return expression::while_<Cond, Do>::make(cond, do_);
            }

            Cond const& cond;
        };

        template <typename Cond>
        while_gen<Cond> const
        while_(Cond const& cond)
        {
            return while_gen<Cond>(cond);
        }
    }}

`while_eval` is an example of how to evaluate an expression. It gets called in the `rule::while` action. `while_gen` and `while_` are the expression template front ends. Let's break this apart to undestand what's happening. Let's start at the bottom. It's easier that way.

When you write:

    while_(cond)

we generate an instance of `while_gen<Cond>`, where `Cond` is the type of `cond`. `cond` can be an arbitrarily complex actor expression. The `while_gen` template class has an `operator[]` accepting another expression. If we write:

    while_(cond)
    [
        do_
    ]

it will generate a proper composite with the type:

    expression::while_<Cond, Do>::type

where `Cond` is the type of `cond` and `Do` is the type of `do_`. Notice how we are using Phoenix's Expression mechanism here

    template <typename Do>
    typename expression::while_<Cond, Do>::type const
    operator[](Do const& do_) const
    {
        return expression::while_<Cond, Do>::make(cond, do_);
    }

Finally, the `while_eval` does its thing:

    while(eval(cond, ctx))
    {
        eval(do_, ctx);
    }

`cond` and `do_`, at this point, are instances of Actor. `cond` and `do_` are the Actors passed as parameters by `call`, ctx is the Context