[Martin Meyer]

Hi Thomas and all,

the method to program for the functionality of the former reference variables described in tutorial http://wiki.visual-prolog.com/index.php ... _a_Project works well. However a little drawback is, that predicate unify/2 has to be declared as nondeterm rather than determ.

Is there some way to trick the compiler, so that unify/2 can be declared as determ while it still leaves the backtrack point to undo the binding?

If there is no such trick, can some 1st language construct in future VIP versions be invented to improve the issue? As the compiler can see, that backtracking into unify/2 will not produce an alternative solution, but only cause side effects, it might allow the predicate to be declared as "determ [besidesSideEffects]".

Best regards,
Martin

[Thomas Linder Puls]

Nondeterm means it can fail and it can leave a backtrack points. Failing is obvious, and the backtrack point is necessary to undo the binding of variables when backtracking across the previous binding, i.e. the side effect you are mentioning.

The answer is no, in both cases. The backtrack point is necessary (to free the variables), so the predicate must be nondeterm.

determ-execpt-for side-effects would only act as a comment, because backtracking will takel place to invoke perform the side effects. I.e. determ-except-for-side-effects still have to leave a backtrack point (and hence can be backtracked to).

[Martin Meyer]

Yes - and thx for answering, Thomas!

The determ-execpt-for-side-effects was just a quick idea of mine. Certainly you have much better ideas. Might also be, that's best to leave the issue as it is. I only intended to bring the topic to your attention.

Another idea, which comes to my mind, is "logical-determ". It would be internally handled as a nondeterm, for which an exception is raised, when the predicate produces more than one solution.

Many regards,
Martin

[Martin Meyer]

I have experimented a little and found a way to do the trick!! In this code bindTo is a procedure, which leaves a backtrack point:

Code: Select all

interface refVar
    open core
 
    properties
        bindTo : predicate{integer Val} (o).
 
    predicates
        tryGetValue : ()
            -> integer Value
            determ.
 
end interface refVar
 
%------
 
class refVar : refVar
end class refVar
 
%------
 
implement refVar
    open core
 
    facts
        value : integer := erroneous.
        bindTo : predicate{integer} := uncheckedConvert(predicate{integer}, bindTo_mt).
 
    predicates
        bindTo_mt : (integer Val)
            multi (i).
    clauses
        bindTo_mt(Val):-
            value := Val.
        bindTo_mt(_):-
            value := erroneous,
            fail.
 
    clauses
        tryGetValue() = value:-
            not(isErroneous(value)).
 
end implement refVar
 
%------
 
implement main
 
    class predicates
        displayVal : (refVar RefVar)
            procedure (i).
    clauses
        displayVal(RefVar):-
            if Val = RefVar:tryGetValue() then
                stdIo::write("bound to ", Val, "\n")
            else
                stdIo::write("unbound\n")
            end if.
 
    class predicates
        test : (refVar RefVar)
            procedure (i).
    clauses
        test(RefVar):-
            RefVar:bindTo(123),
            displayVal(RefVar),
            fail.
        test(RefVar):-
            displayVal(RefVar).
 
    clauses
        run():-
            console::init(),
            test(refVar::new()).
 
end implement main
 
goal
    mainExe::run(main::run).

[Thomas Linder Puls]

OK, using an uncheckedConvert it is possible.