Here is a pure version using dif/2 which implements sound inequality. dif/2 is offered by B-Prolog, YAP-Prolog, SICStus-Prolog and SWI-Prolog. firstdup(E, [E|L]) :- member(E, L). firstdup(E, [N|L]) :- non_member(N, L), firstdup(E, L). member(E, [E|_L]). member(E, [_X|L]) :- member(E, L). non_member(_E, []). non_member(E, [F|Fs]) :- dif(E, F), non_member(E, Fs). The advantages are that it can also … Read more
For elegance and didactic reasons alone, dif/2 is clearly preferable here and also in the vast majority of other cases, since as you already note “a lot of unnecessary unifications might take place” otherwise, and also because dif/2 is a pure and nicely declarative predicate that can be used in all directions and at any … Read more
The actual reason for your problem is (\==)/2 within different/2. It succeeds simply too often. Replace it with dif/2 and you will get the expected behavior. dif/2 is available in many Prolog systems like SICStus, YAP, B, SWI. You can also define a safe approximation in ISO-Prolog like so: iso_dif(X, Y) :- X \== Y, … Read more
In place of not(X = Y) you need to write \+ X = Y or X \= Y. But consider to use dif(X,Y) instead. dif/2 is present in B, SWI, YAP, SICStus. To see the difference: ?- X = b, dif(a, X). X = b. ?- X = b, \+ a = X. X = … Read more
Well for one thing, the name should be more declarative, like equality_truth/2.