prolog-dif

I can't come up with a situation where I would need it. Elegant systems provide false/0 as a declarative synonym for the imperative fail/0 . An example where it is useful is when you manually want to force backtracking for side-effects, like: ?- between(1,3,N), format("line ~w\n", [N]), false. line 1 line 2 line 3 Instead of false/0 , you can also use any goal that fails, for example a bit shorter: ?- between(1,3,N), format("line ~w\n", [N]), 0=1. line 1 line 2 line 3 Thus, false/0 is not strictly needed but quite nice. EDIT : I sometimes see beginners who want to state for example "my

Standard term order (ISO/IEC 13211-1 7.2 Term order) is defined over all terms — including variables. While there are good uses for this — think of the implementation of setof/3 , this makes many otherwise clean and logical uses of the built-ins in 8.4 Term comparison a declarative nightmare with imps (short form for imperative constructs) all around. 8.4 Term comparison features: 8.4 Term comparison 8.4.1 (@=<)/2, (==)/2, (\==)/2, (@<)/2, (@>)/2, (@>=)/2. 8.4.2 compare/3 . 8.4.3 sort/2 . 8.4.4 keysort/2 . To give an example, consider: ?- X @< a. true. This succeeds, because 7.2 Term order An

I need to find the first duplicate value in a list. prep(3,[1,3,5,3,5]). Should be true. prep(5,[1,3,5,3,5]). Should be false. I thought checking for equality with the current value and the previous list members until I find a duplicate, if it finds one it will test for equality with X but I have no idea how to do that in Prolog! I appreciate any help! Thanks 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).

If I want to make sure that two variables do not instantiate to the same term, what is the preferred way to do it? Let's say I need to find directed edges in a graph, and a node cannot have an edge to itself: node(a, x, y). node(b, z, x). node(c, y, y). (the edges here are a -> c, b -> a, but not c -> c) The following works: edge(A, B) :- node(A, _, X), node(B, X, _), A \== B. This works too [swi-prolog]: edge(A, B) :- dif(A, B), node(A, _, X), node(B, X, _). This does not work, apparently (because neither A nor B are instantiated yet?): edge(A, B) :- A \== B, node(A, _, X), node(B, X, _). I

I'm having some trouble understanding why my code in prolog does something based on the order I put my rules in. Here is my database: parent(tom, bob). parent(tom, liz). parent(mary, bob). parent(mary, liz). male(tom). male(bob). female(mary). female(liz). And here are the rules: %difference(X, Y) ==> Predicate to check if two people X and Y are not the same person. difference(X, Y) :- \==(X, Y). father(X, Y) :- male(X), parent(X, Y), difference(X, Y). mother(X, Y) :- female(X), parent(X, Y), difference(X, Y). sibling(X, Y) :- difference(X, Y), mother(M, X), mother(M, Y), father(F, X), father