[Martin Meyer]

Hi Thomas,

in new VIP 7.5 the clause

Code: Select all

run() :-
    X == 1.

throws error c609 : Variable 'X' is not completely bound. Is it a bug or has Must Unify been changed for a reason?

Regards,
Martin

[Vitaly Markov]

Code: Select all

run() :-
    X = read(),
    X == 1,
    write("Ok").

If you input 1 then "Ok". Else exception.

[Thomas Linder Puls]

Yes, must unify has been changed.

Your example clause should use a regular "=", using "==" is misleading because it indicates (to other programmers) that a "cannot unify" exception could potentially be raised, which it can't.

"=" and "==" have same semantics for terms that will always unify, but it was never the intention to let "==" play the role of "=" in such cases.

[Martin Meyer]

Thanks Thomas! I see.

Btw. the change of Must Unify does not cover flows like for example (tuple(o, o), tuple(i, i)). The clause

Code: Select all

run() :-
    tuple(X, Y) == tuple(1, 2).

is still compiling. I suppose, you liked to exclude such usages too.

Regards,
Martin

[Thomas Linder Puls]

In principle, but I am not sure it is worth the effort.