Values

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#### Values

    #include <boost/phoenix/core/value.hpp>

Whenever we see a constant in a partially applied function, an

    expression::value<T>::type

(where T is the type of the constant) is automatically created for us. For instance:

    add(arg1, 6)

Passing a second argument, `6`, an `expression::value<T>::type` is implicitly created behind the scenes. This is also equivalent to `add(arg1, val(6))`.

    val(v)

generates an `expression::value<T>::type` where `T` is the type of `x`. In most cases, there's no need to explicitly use `val`, but, as we'll see later on, there are situations where this is unavoidable.

### Evaluating a Value

Like arguments, values are also actors. As such, values can be evaluated. Invoking a value gives the value's identity. Example:

    cout << val(3)() << val("Hello World")();

prints out "3 Hello World".