Scryer-prolog: dif/2 incorrect

[UWN]

?- dif(XM,-a),XM= -X,X=a.
X = a, XM = - a.

BTW, X \= Y is a safe approximation for dif/2 too. If it succeeds, there is no need for further investigation.

[UWN]

The most naive implementation is to recheck

1. X == Y -> upon success, dif/2 does not hold.
2. X \= Y -> upon success. dif/2 does hold.

whenever something "happens" with variables in subterms of X and Y.

[UWN]

The big advantage of this seemingly naive implementation is that it avoids reposting of redundant dif/2 goals:

?- X\=Y.
false.
?- dif(X,Y),X\=Y.
X = _28, Y = _52, dif(X, Y).

[mthom]

Perhaps @triska could advise on porting the dif/2 code he co-wrote for SWI to Scryer?

[UWN]

I'd say: become more conforming first.

[triska]

@mthom: Once the conformity issues are solved, I will implement a new dif/2 implementation that takes Scryer's better interface for attributed variables into account.

I am looking forward to doing this with a system where strong syntactic guarantees hold!

[UWN]

An implementation would be fine (and simple) which only traverses qua term_variables/2