[/==============================================================================
    Copyright (C) 2001-2010 Joel de Guzman
    Copyright (C) 2001-2005 Dan Marsden
    Copyright (C) 2001-2010 Thomas Heller

    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)
===============================================================================/]

[section Core]

Actors are composed to create more complex actors in a tree-like hierarchy. The
primitives are atomic entities that are like the leaves in the tree. Phoenix is
extensible. New primitives can be added anytime. Right out of the box, there are
only a few primitives, these are all defined in the Core module.

This section shall deal with these preset primitives.

[section 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 `v`. 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.

[h2 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".

[endsect]

[section References]

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

Values are immutable constants. Attempting to modify a value will result in a
compile time error. When we want the function to modify the parameter, we use a
reference instead. For instance, imagine a lazy function `add_assign`:

    void add_assign(T& x, T y) { x += y; } // pseudo code

Here, we want the first function argument, x, to be mutable. Obviously, we
cannot write:

    add_assign(1, 2) // error first argument is immutable

In C++, we can pass in a reference to a variable as the first argument in our
example above. Yet, by default, the library forces arguments passed to partially
applied functions to be immutable values (see [link phoenix.modules.core.values
Values]). To achieve our intent, we use:

    expression::reference<T>::type

This is similar to `expression::value<T>::type` before but instead holds a reference to a
variable.

We normally don't instantiate `expression::reference<T>::type` objects directly. Instead we
use:

    ref(v)

For example (where `i` is an `int` variable):

    add_assign(ref(i), 2)

[heading Evaluating a Reference]

References are actors. Hence, references can be evaluated. Such invocation gives
the reference's identity. Example:

    int i = 3;
    char const* s = "Hello World";
    cout << ref(i)() << ref(s)();

prints out "3 Hello World"


[heading Constant References]

Another free function
    
    cref(cv)
    
may also be used. `cref(cv)` creates an
`expression::reference<T const>::type` object. This is similar to `expression::value<T>::type` but
when the data to be passed as argument to a function is heavy and expensive to
copy by value, the `cref(cv)` offers a lighter alternative.

[endsect]

[section Arguments]

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

We use an instance of:

    expression::argument<N>::type

to represent the Nth function argument. The argument placeholder acts as an
imaginary data-bin where a function argument will be placed.

[heading Predefined Arguments]

There are a few predefined instances of `expression::argument<N>::type` named
`arg1`..`argN`, and its __bll__ counterpart `_1`..`_N`. (where N is a predefined
maximum).

Here are some sample preset definitions of `arg1`..`argN`

    namespace placeholders
    {
        expression::argument<1>::type const arg1 = {};
        expression::argument<2>::type const arg2 = {};
        expression::argument<3>::type const arg3 = {};
    }

and its __bll__ `_1`..`_N` style counterparts:


    namespace placeholders
    {
        expression::argument<1>::type const _1 = {};
        expression::argument<2>::type const _2 = {};
        expression::argument<3>::type const _3 = {};
    }

[note You can set `BOOST_PHOENIX_ARG_LIMIT`, the predefined maximum
placeholder index. By default, `BOOST_PHOENIX_ARG_LIMIT` is set to `BOOST_PHOENIX_LIMIT`
(See [link phoenix.actor Actor]).]

[heading User Defined Arguments]

When appropriate, you can define your own `argument` names. For example:

    expression::argument<1>::type x; // note one based index

`x` may now be used as a parameter to a lazy function:

    add(x, 6)

which is equivalent to:

    add(arg1, 6)

[/note
When dealing with argument placeholders the question arises whether you can call
member functions on an `argument` actor.

This is possible by supplying a custom `actor` which has a member
generator function. See [link phoenix.advanced_topics.extending_actors Extending Actors]
for more details.
]

[heading Evaluating an Argument]

An argument, when evaluated, selects the Nth argument from the those passed
in by the client.

For example:

    char        c = 'A';
    int         i = 123;
    const char* s = "Hello World";

    cout << arg1(c) << endl;        //  Get the 1st argument: c
    cout << arg1(i, s) << endl;     //  Get the 1st argument: i
    cout << arg2(i, s) << endl;     //  Get the 2nd argument: s

will print out:

    A
    123
    Hello World

[heading Extra Arguments]

In C and C++, a function can have extra arguments that are not at all used by
the function body itself. These extra arguments are simply ignored.

Phoenix also allows extra arguments to be passed. For example, recall our
original `add` function:

    add(arg1, arg2)

We know now that partially applying this function results to a function that
expects 2 arguments. However, the library is a bit more lenient and allows the
caller to supply more arguments than is actually required. Thus, `add` actually
allows 2 /or more/ arguments. For instance, with:

    add(arg1, arg2)(x, y, z)

the third argument `z` is ignored. Taking this further, in-between arguments are
also ignored. Example:

    add(arg1, arg5)(a, b, c, d, e)

Here, arguments b, c, and d are ignored. The function `add` takes in the first
argument (`arg1`) and the fifth argument (`arg5`).

[note There are a few reasons why enforcing strict arity is not
desirable. A case in point is the callback function. Typical callback functions
provide more information than is actually needed. Lambda functions are often
used as callbacks.]

[endsect]

[section Nothing]

    #include <boost/phoenix/core/nothing.hpp>

Finally, the `expression::null<mpl::void_>::type` does nothing; (a "bum", if you will :-) ).
There's a sole `expression::null<mpl::void_>::type` instance named "nothing". This actor is
actually useful in situations where we don't want to do anything. (See
[link phoenix.modules.statement.for_statement for_ Statement] for example).

[endsect]

[endsect]
