%
% An almost portable implementation of logial assignment.
% Distributed from http://www.kprolog.com/en/logical_assignment/
% 
% Copyright(C) 2003-2005 Nobukuni Kino, KLS Research, Inc.
%                        kino@kprolog.com
%
% Licensed under the Open Software License version 2.1
%	         http://opensource.org/licenses/osl-2.1.php
%
% 12/31/2004	First distribution
% 1/2/2005	assignable/2 is corrected(issued by Paulo Moura) 

% :-module assignable.
% :-public assignable/1,assignable/2.
% :-public (<=)/2,(<+)/2,(=>)/2.
:-op(700,xfy, '<=').
:-op(700,xfy, '<+').
:-op(700,xfy, '=>').

assignable([_|_]).
assignable(Assig,Init):-
	Tail=[Init|_],
	Assig=[_|Tail].
	
Assig<=Value :- poke_assig(Assig,Value).
poke_assig([X|Y],Value):-
	nonvar(Y),!,
	poke_assig(Y,Value).
poke_assig([_,Value|_],Value).
% (<+)/2 cannot have logical implementation.
Assig<+Value :- Assig<=Value.

Assig=>Value :- peek_assig(Assig,Value).
peek_assig([X|Y],Value):-
	nonvar(Y),!,
	peek_assig(Y,Value).
peek_assig([Value|_],Value).