[/==============================================================================
    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 Function]

The `function` class template provides a mechanism for implementing lazily
evaluated functions. Syntactically, a lazy function looks like an ordinary C/C++
function. The function call looks familiar and feels the same as ordinary C++
functions. However, unlike ordinary functions, the actual function execution is
deferred.

    #include <boost/phoenix/function.hpp>

Unlike ordinary function pointers or functor objects that need to be explicitly
bound through the bind function (see [link phoenix.modules.bind Bind]),
the argument types of these functions are automatically lazily bound.

In order to create a lazy function, we need to implement a model of the
__PFO__ concept. For a function that takes `N` arguments, a model of __PFO__ must
provide:

* An `operator()` that takes `N` arguments, and implements
the function logic. This is also true for ordinary function pointers.
* A nested metafunction `result<Signature>` or nested typedef `result_type`, following the __boost_result_of__ Protocol

[/
* A nested metafunction `result<A1, ... AN>` that takes the types of the `N` arguments to
the function and returns the result type of the function. (There is a special case for function
objects that accept no arguments. Such nullary functors are only required to define a typedef
`result_type` that reflects the return type of its `operator()`).
]

For example, the following type implements the FunctionEval concept, in order to provide a
lazy factorial function:

    struct factorial_impl
    {
        template <typename Sig>
        struct result;
        
        template <typename This, typename Arg>
        struct result<This(Arg const &)>
        {
            typedef Arg type;
        };

        template <typename Arg>
        Arg operator()(Arg const & n) const
        {
            return (n <= 0) ? 1 : n * (*this)(n-1);
        }
    };

(See [@../../example/factorial.cpp factorial.cpp])

[/note The type of Arg is either a const-reference or non-const-reference
(depending on whether your argument to the actor evaluation is a const-ref or
non-const-ref).]

Having implemented the `factorial_impl` type, we can declare and instantiate a lazy
`factorial` function this way:

    function<factorial_impl> factorial;

Invoking a lazy function such as `factorial` does not immediately execute the function
object `factorial_impl`. Instead, an [link phoenix.actor actor] object is
created and returned to the caller. Example:

    factorial(arg1)

does nothing more than return an actor. A second function call will invoke
the actual factorial function. Example:

    std::cout << factorial(arg1)(4);

will print out "24".

Take note that in certain cases (e.g. for function objects with state), an
instance of the model of __PFO__ may be passed on to the constructor. Example:

    function<factorial_impl> factorial(ftor);

where ftor is an instance of factorial_impl (this is not necessary in this case
as `factorial_impl` does not require any state).

[important Take care though when using function objects with state because they are
often copied repeatedly, and state may change in one of the copies, rather than the
original.]

[section Adapting Functions]

If you want to adapt already existing functions or function objects it will become
a repetetive task. Therefor the following boilerplate macros are provided to help
you adapt already exsiting functions, thus reducing the need to
[link phoenix.modules.bind] functions.

[section BOOST_PHOENIX_ADAPT_FUNCTION_NULLARY]

[heading Description]
`BOOST_PHOENIX_ADAPT_FUNCTION_NULLARY` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary nullary function a lazy
function.

[note These macros generate no global objects. The resulting lazy functions are real functions
      that create the lazy function expression object]

[heading Synopsis]

    BOOST_PHOENIX_ADAPT_FUNCTION_NULLARY(
        RETURN_TYPE
      , LAZY_FUNCTION
      , FUNCTION
    )

[heading Semantics]

The above macro generates all necessary code to have a nullary lazy function
`LAZY_FUNCTION` which calls the nullary `FUNCTION` that has the return type
`RETURN_TYPE`

[heading Header]

    #include <boost/phoenix/function/adapt_function.hpp>

[heading Example]

    namespace demo
    {
        int foo()
        {
            return 42;
        }
    }

    BOOST_PHOENIX_ADAPT_FUNCTION_NULLARY(int, foo, demo::foo)

    int main()
    {
        using boost::phoenix::placeholders::_1;

        assert((_1 + foo())(1) == 43);
    }

[endsect]

[section BOOST_PHOENIX_ADAPT_FUNCTION]

[heading Description]
`BOOST_PHOENIX_ADAPT_FUNCTION` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary function a lazy function.

[heading Synopsis]

    BOOST_PHOENIX_ADAPT_FUNCTION(
        RETURN_TYPE
      , LAZY_FUNCTION
      , FUNCTION
      , FUNCTION_ARITY
    )

[heading Semantics]

The above macro generates all necessary code to have a lazy function
`LAZY_FUNCTION` which calls `FUNCTION` that has the return type
`RETURN_TYPE` with `FUNCTION_ARITY` number of arguments.

[heading Header]

    #include <boost/phoenix/function/adapt_function.hpp>

[heading Example]

    namespace demo
    {
        int plus(int a, int b)
        {
            return a + b;
        }

        template <typename T>
        T
        plus(T a, T b, T c)
        {
            return a + b + c;
        }
    }

    BOOST_PHOENIX_ADAPT_FUNCTION(int, plus, demo::plus, 2)

    BOOST_PHOENIX_ADAPT_FUNCTION(
        typename remove_reference<A0>::type
      , plus
      , demo::plus
      , 3
    )

    int main()
    {
        using boost::phoenix::arg_names::arg1;
        using boost::phoenix::arg_names::arg2;

        int a = 123;
        int b = 256;

        assert(plus(arg1, arg2)(a, b) == a+b);
        assert(plus(arg1, arg2, 3)(a, b) == a+b+3);
    }

[endsect]

[/

[section BOOST_PHOENIX_ADAPT_FUNCTION_VARARG]

[heading Description]
`BOOST_PHOENIX_ADAPT_FUNCTION_VARARG` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary function a lazy
function.

[heading Synopsis]

    BOOST_PHOENIX_ADAPT_FUNCTION_VARARG(
        RETURN_TYPE
      , LAZY_FUNCTION
      , FUNCTION
    )

[heading Semantics]

[heading Header]
    
    #include <boost/phoenix/function/adapt_function.hpp>

[heading Example]

[endsect]
]

[section BOOST_PHOENIX_ADAPT_CALLABLE_NULLARY]

[heading Description]
`BOOST_PHOENIX_ADAPT_CALLABLE_NULLARY` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary nullary function object a
lazy function.

[heading Synopsis]

    BOOST_PHOENIX_ADAPT_CALLABLE_NULLARY(
        LAZY_FUNCTION
      , CALLABLE
    )

[heading Semantics]

The above macro generates all necessary code to create `LAZY_FUNCTION` which
creates a lazy function object that represents a nullary call to `CALLABLE`.
The return type is specified by `CALLABLE` conforming to the __boost_result_of__
protocol.

[heading Header]
    
    #include <boost/phoenix/function/adapt_callable.hpp>

[heading Example]

    namespace demo
    {
        struct foo
        {
            typedef int result_type;

            result_type operator()() const
            {
                return 42;
            }
        }
    }
    
    BOOST_PHOENIX_ADAPT_CALLABLE_NULLARY(foo, demo::foo)

    int main()
    {
        using boost::phoenix::placeholders::_1;

        assert((_1 + foo())(1) == 43);
    }

[endsect]

[section BOOST_PHOENIX_ADAPT_CALLABLE]

[heading Description]
`BOOST_PHOENIX_ADAPT_CALLABLE` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary function object a lazy
function.

[heading Synopsis]

    BOOST_PHOENIX_ADAPT_CALLABLE(
        LAZY_FUNCTION
      , FUNCTION_NAME
      , FUNCTION_ARITY
    )

[heading Semantics]

The above macro generates all necessary code to create `LAZY_FUNCTION` which
creates a lazy function object that represents a call to `CALLABLE` with `FUNCTION_ARITY`
arguments.
The return type is specified by `CALLABLE` conforming to the __boost_result_of__
protocol.

[heading Header]
    
    #include <boost/phoenix/function/adapt_callable.hpp>

[heading Example]

    namespace demo
    {
        struct plus
        {
            template <typename Sig>
            struct result;

            template <typename This, typename A0, typename A1>
            struct result<This(A0, A1)>
                : remove_reference<A0>
            {};

            template <typename This, typename A0, typename A1, typename A2>
            struct result<This(A0, A1, A2)>
                : remove_reference<A0>
            {};

            template <typename A0, typename A1>
            A0 operator()(A0 const & a0, A1 const & a1) const
            {
                return a0 + a1;
            }

            template <typename A0, typename A1, typename A2>
            A0 operator()(A0 const & a0, A1 const & a1, A2 const & a2) const
            {
                return a0 + a1 + a2;
            }
        };
    }

    BOOST_PHOENIX_ADAPT_CALLABLE(plus, demo::plus, 2)

    BOOST_PHOENIX_ADAPT_CALLABLE(plus, demo::plus, 3)

    int main()
    {
        using boost::phoenix::arg_names::arg1;
        using boost::phoenix::arg_names::arg2;

        int a = 123;
        int b = 256;

        assert(plus(arg1, arg2)(a, b) == a+b);
        assert(plus(arg1, arg2, 3)(a, b) == a+b+3);
    }

[endsect]

[/
[section BOOST_PHOENIX_ADAPT_CALLABLE_VARARG]

[heading Description]
`BOOST_PHOENIX_ADAPT_CALLABLE_VARARG` is a macro that can be used to generate
all the necessary boilerplate to make an arbitrary function object a lazy
function.

[heading Synopsis]

    BOOST_PHOENIX_ADAPT_CALLABLE_VARARG(
        LAZY_FUNCTION_NAME
      , FUNCTION_NAME
    )

[heading Semantics]

[heading Header]
    
    #include <boost/phoenix/function/adapt_callable.hpp>

[heading Example]

[endsect]
/]

[endsect]

[endsect]