Re: freeze/2 and nb_setarg/3 interaction

Michael,

> Somehow nb_setarg/3 is being undone on backtracking when freeze/2 is
> involved.

I could be misunderstanding this, but I don't think freeze/2 is the
culprit here. I think nb_setarg/3 (and setarg/3, see below) are
fundamentally dangerous (broken, even) when the arg being assigned to
is a variable. Consider the following:

correct2 :-
    X = [_|_],
    (   nb_setarg(1, X, a),
        writeln(X),
        fail
    ;   writeln(X)
    ).

broken2 :-
    X = [H|_],
    (   H = a,
        nb_setarg(1, X, a),
        writeln(X),
        fail
    ;   writeln(X)
    ).


?- correct2.
[a|_G2435]
[a|_G2435]
true.

?- broken2.
[a|_G2271]
[_G2270|_G2271]
true.


Clearly, adding in the "H = a" goal has caused the binding of that
argument to be undone on backtracking. Maybe your bug is an instance
of this problem?

This is reminiscent of a setarg/3 bug I reported a while ago:

https://lists.iai.uni-bonn.de/pipermail/swi-prolog/2013/011602.html

In any case, maybe Jan's answer there - that this is basically
expected and unfixable - can shed some light on your problem.

Hope that helps!

Michael