boost/lambda/closures.hpp

/*=============================================================================
    Adaptable closures

    Phoenix V0.9
    Copyright (c) 2001-2002 Joel de Guzman

    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)

    URL: http://spirit.sourceforge.net/

==============================================================================*/
#ifndef PHOENIX_CLOSURES_HPP
#define PHOENIX_CLOSURES_HPP

///////////////////////////////////////////////////////////////////////////////
#include "boost/lambda/core.hpp"
///////////////////////////////////////////////////////////////////////////////
namespace boost {
namespace lambda {

///////////////////////////////////////////////////////////////////////////////
//
//  Adaptable closures
//
//      The framework will not be complete without some form of closures
//      support. Closures encapsulate a stack frame where local
//      variables are created upon entering a function and destructed
//      upon exiting. Closures provide an environment for local
//      variables to reside. Closures can hold heterogeneous types.
//
//      Phoenix closures are true hardware stack based closures. At the
//      very least, closures enable true reentrancy in lambda functions.
//      A closure provides access to a function stack frame where local
//      variables reside. Modeled after Pascal nested stack frames,
//      closures can be nested just like nested functions where code in
//      inner closures may access local variables from in-scope outer
//      closures (accessing inner scopes from outer scopes is an error
//      and will cause a run-time assertion failure).
//
//      There are three (3) interacting classes:
//
//      1) closure:
//
//      At the point of declaration, a closure does not yet create a
//      stack frame nor instantiate any variables. A closure declaration
//      declares the types and names[note] of the local variables. The
//      closure class is meant to be subclassed. It is the
//      responsibility of a closure subclass to supply the names for
//      each of the local variable in the closure. Example:
//
//          struct my_closure : closure<int, string, double> {
//
//              member1 num;        // names the 1st (int) local variable
//              member2 message;    // names the 2nd (string) local variable
//              member3 real;       // names the 3rd (double) local variable
//          };
//
//          my_closure clos;
//
//      Now that we have a closure 'clos', its local variables can be
//      accessed lazily using the dot notation. Each qualified local
//      variable can be used just like any primitive actor (see
//      primitives.hpp). Examples:
//
//          clos.num = 30
//          clos.message = arg1
//          clos.real = clos.num * 1e6
//
//      The examples above are lazily evaluated. As usual, these
//      expressions return composite actors that will be evaluated
//      through a second function call invocation (see operators.hpp).
//      Each of the members (clos.xxx) is an actor. As such, applying
//      the operator() will reveal its identity:
//
//          clos.num() // will return the current value of clos.num
//
//      *** [note] Acknowledgement: Juan Carlos Arevalo-Baeza (JCAB)
//      introduced and initilally implemented the closure member names
//      that uses the dot notation.
//
//      2) closure_member
//
//      The named local variables of closure 'clos' above are actually
//      closure members. The closure_member class is an actor and
//      conforms to its conceptual interface. member1..memberN are
//      predefined typedefs that correspond to each of the listed types
//      in the closure template parameters.
//
//      3) closure_frame
//
//      When a closure member is finally evaluated, it should refer to
//      an actual instance of the variable in the hardware stack.
//      Without doing so, the process is not complete and the evaluated
//      member will result to an assertion failure. Remember that the
//      closure is just a declaration. The local variables that a
//      closure refers to must still be instantiated.
//
//      The closure_frame class does the actual instantiation of the
//      local variables and links these variables with the closure and
//      all its members. There can be multiple instances of
//      closure_frames typically situated in the stack inside a
//      function. Each closure_frame instance initiates a stack frame
//      with a new set of closure local variables. Example:
//
//          void foo()
//          {
//              closure_frame<my_closure> frame(clos);
//              /* do something */
//          }
//
//      where 'clos' is an instance of our closure 'my_closure' above.
//      Take note that the usage above precludes locally declared
//      classes. If my_closure is a locally declared type, we can still
//      use its self_type as a paramater to closure_frame:
//
//          closure_frame<my_closure::self_type> frame(clos);
//
//      Upon instantiation, the closure_frame links the local variables
//      to the closure. The previous link to another closure_frame
//      instance created before is saved. Upon destruction, the
//      closure_frame unlinks itself from the closure and relinks the
//      preceding closure_frame prior to this instance.
//
//      The local variables in the closure 'clos' above is default
//      constructed in the stack inside function 'foo'. Once 'foo' is
//      exited, all of these local variables are destructed. In some
//      cases, default construction is not desirable and we need to
//      initialize the local closure variables with some values. This
//      can be done by passing in the initializers in a compatible
//      tuple. A compatible tuple is one with the same number of
//      elements as the destination and where each element from the
//      destination can be constructed from each corresponding element
//      in the source. Example:
//
//          tuple<int, char const*, int> init(123, "Hello", 1000);
//          closure_frame<my_closure> frame(clos, init);
//
//      Here now, our closure_frame's variables are initialized with
//      int: 123, char const*: "Hello" and int: 1000.
//
///////////////////////////////////////////////////////////////////////////////


///////////////////////////////////////////////////////////////////////////////
//
//  closure_frame class
//
///////////////////////////////////////////////////////////////////////////////
template <typename ClosureT>
class closure_frame : public ClosureT::tuple_t {

public:

    closure_frame(ClosureT& clos)
    : ClosureT::tuple_t(), save(clos.frame), frame(clos.frame)
    { clos.frame = this; }

    template <typename TupleT>
    closure_frame(ClosureT& clos, TupleT const& init)
    : ClosureT::tuple_t(init), save(clos.frame), frame(clos.frame)
    { clos.frame = this; }

    ~closure_frame()
    { frame = save; }

private:

    closure_frame(closure_frame const&);            // no copy
    closure_frame& operator=(closure_frame const&); // no assign

    closure_frame* save;
    closure_frame*& frame;
};

///////////////////////////////////////////////////////////////////////////////
//
//  closure_member class
//
///////////////////////////////////////////////////////////////////////////////
template <int N, typename ClosureT>
class closure_member {

public:

    typedef typename ClosureT::tuple_t tuple_t;

    closure_member()
    : frame(ClosureT::closure_frame_ref()) {}

    template <typename TupleT>
    struct sig {

        typedef typename detail::tuple_element_as_reference<
            N, typename ClosureT::tuple_t
        >::type type;
    };

    template <class Ret, class A, class B, class C>
    //    typename detail::tuple_element_as_reference
    //        <N, typename ClosureT::tuple_t>::type
    Ret
    call(A&, B&, C&) const
    {
        assert(frame);
        return boost::tuples::get<N>(*frame);
    }


private:

    typename ClosureT::closure_frame_t*& frame;
};

///////////////////////////////////////////////////////////////////////////////
//
//  closure class
//
///////////////////////////////////////////////////////////////////////////////
template <
    typename T0 = null_type,
    typename T1 = null_type,
    typename T2 = null_type,
    typename T3 = null_type,
    typename T4 = null_type
>
class closure {

public:

    typedef tuple<T0, T1, T2, T3, T4> tuple_t;
    typedef closure<T0, T1, T2, T3, T4> self_t;
    typedef closure_frame<self_t> closure_frame_t;

                            closure()
                            : frame(0)      { closure_frame_ref(&frame); }
    closure_frame_t&        context()       { assert(frame); return frame; }
    closure_frame_t const&  context() const { assert(frame); return frame; }

    typedef lambda_functor<closure_member<0, self_t> > member1;
    typedef lambda_functor<closure_member<1, self_t> > member2;
    typedef lambda_functor<closure_member<2, self_t> > member3;
    typedef lambda_functor<closure_member<3, self_t> > member4;
    typedef lambda_functor<closure_member<4, self_t> > member5;

private:

    closure(closure const&);            // no copy
    closure& operator=(closure const&); // no assign

    template <int N, typename ClosureT>
    friend class closure_member;

    template <typename ClosureT>
    friend class closure_frame;

    static closure_frame_t*&
    closure_frame_ref(closure_frame_t** frame_ = 0)
    {
        static closure_frame_t** frame = 0;
        if (frame_ != 0)
            frame = frame_;
        return *frame;
    }

    closure_frame_t* frame;
};

}}
   //  namespace

#endif