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Another implementation in the logic programming language Prolog uses unification variables. A difference list is a pair OpenList-Hole, where the first element OpenList is a list containing an unbound unification variable (hole) and the second element Hole is a reference to the hole.
Method 6: Use symmetric_difference to Find the Difference Between Two Lists in Python. The elements that are either in the first set or the second set are returned using the symmetric_difference() technique. The intersection, unlike the shared items of the two sets, is not returned by this technique.
Use set. difference() to Find the Difference Between Two Lists in Python.
I'm trying to understand difference lists in Prolog, but I'm struggling to actually implement one properly, everytime I try to do it, I get a list of lists, but that's not what I want. I'm trying to implement an append predicate, but having little luck so far. Few attempts, all of which don't work.
app(X, Y, Z) :- Z = [X|Y].
?- app([a,b,c], [z], Z).
Z = [[a,b,c],z].
OR
app(X, Y, Z) :- Z = [X|Hole], Hole = Y.
Same results as the first one, (they do seem to be basically the same). I have an example in a book that does work (although it's not a predicate), and I don't understand the difference. X becomes instantiated to the proper answer [a,b,c,z], how is that much different than my second example?
X = [a,b,c|Y], Y = [z].
What am I missing? Thanks.
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