root/tags/lgt2310/library/assignvars.lgt

/*
This file contains an adaptation to Logtalk of code for logical assignment 
of Prolog terms developed by Nobukuni Kino. For more information, please 
consult the URL http://www.kprolog.com/en/logical_assignment/

As a derivative work, this file is licensed under the Open Software License 
version 2.1 (http://opensource.org/licenses/osl-2.1.php).
*/


:-op(100, xfx, '<=').
:-op(100, xfx, '=>').


:- category(assignvars).

    :- info([
        version is 1.0,
        author is 'Nobukuni Kino and Paulo Moura',
        date is 2005/1/7,
        comment is 'Assignable variables (supporting logical, backtracable assignement of non-variable terms).']).

    :- public(assignable/1).
    :- mode(assignable(-assignvar), one).
    :- info(assignable/1, [
        comment is 'Makes Variable an assignable variable. Initial state will be empty.',
        argnames is ['Variable'],
        exceptions is [
            'Variable is not a variable' - type_error(variable, 'Variable')]]).

    :- public(assignable/2).
    :- mode(assignable(-assignvar, @nonvar), one).
    :- info(assignable/2, [
        comment is 'Makes Variable an assignable variable and sets its initial state to Value.',
        argnames is ['Variable', 'Value'],
        exceptions is [
            'Variable is not a variable' - type_error(variable, 'Variable'),
            'Value is not instantiated' - instantiation_error]]).

    :- public((<=)/2).
    :- mode(<=(?assignvar, @nonvar), one).
    :- info((<=)/2, [
        comment is 'Sets the state of the assignable variable Variable to Value (initializing the variable if needed).',
        argnames is ['Variable', 'Value'],
        exceptions is [
            'Value is not instantiated' - instantiation_error]]).

    :- public((=>)/2).
    :- mode(=>(+assignvar, ?nonvar), zero_or_one).
    :- info((=>)/2, [
        comment is 'Unifies Value with the current state of the assignable variable Variable.',
        argnames is ['Variable', 'Value'],
        exceptions is [
            'Variable is not instantiated' - instantiation_error]]).

    :-op(100, xfx, <=).
    :-op(100, xfx, =>).

    assignable(Assig) :-
        nonvar(Assig),
        self(Self),
        sender(Sender),
        throw(error(type_error(variable, Assig), Self::assignable(Assig), Sender)).
    assignable([_| _]).

    assignable(Assig, Init) :-
        nonvar(Assig),
        self(Self),
        sender(Sender),
        throw(error(type_error(variable, Assig), Self::assignable(Assig, Init), Sender)).
    assignable(Assig, Init) :-
        var(Init),
        self(Self),
        sender(Sender),
        throw(error(instantiation_error, Self::assignable(Assig, Init), Sender)).
    assignable([_, Init| _], Init).

    Assig <= Value :-
        var(Value),
        self(Self),
        sender(Sender),
        throw(error(instantiation_error, Self::Assig <= Value, Sender)).

    [_| Tail] <= Value :-
        nonvar(Tail) ->
            Tail <= Value
            ;
            Tail = [Value| _].

    Assig => Value :-
        var(Assig),
        self(Self),
        sender(Sender),
        throw(error(instantiation_error, Self::Assig => Value, Sender)).

    [Current| Tail] => Value :-
        nonvar(Tail) ->
            Tail => Value
            ;
            Current = Value.

:- end_category.