The interactive gprolog interpreter runs queries against a loaded list of predicates, that is why you get the existence_error exception. You will have to load your predicates into it, either by using an IDE that does the loading for you or doing it manually. Here’s one approach: | ?- [user]. compiling user for byte code… … Read more
(Note: this answer is somewhat of a guess) Consider how Prolog determines whether f(a,c) is true or not. It checks the first rule, f(a,b), and doesn’t find a match, but the second rule, f(a,c) matches. Therefore, f(a,c) is true. Furthermore, since there are no more rules for f, there is no point in allowing a … Read more
Prior to the WAM, there was the ZIP by Clocksin. Its design is still very interesting. SWI-Prolog uses it. And also B-Prolog has slowly migrated from a WAM design towards the ZIP. Of course, on that way many new innovations were developed. Another alternative is the VAM. A comparison as of 1993 is: http://www.complang.tuwien.ac.at/ulrich/papers/PDF/binwam-nov93.pdf In … Read more
I am writing this answer, because the best answer yet was in a comment by lurker. I want to have it show up as an actual answer. Your code is not working, because you’re doing R1 is R+1 when R isn’t instantiated in the case not_number([X|T], R). Your recursive case is strung a little backwards. … Read more
First try! The following code is based on the meta-predicates tfilter/3 and tpartition/4, the monotone if-then-else control construct if_/3, the reified unary logical connective not_t/3, and the reified term equality predicate (=)/3: different([],[_|_]). different([X|Xs0],Ys0) :- tpartition(=(X),Ys0,Es,Ys1), if_(Es=[], true, (tfilter(not_t(=(X)),Xs0,Xs1),different(Xs1,Ys1))). Sample query: ?- different([A,B],[X,Y]). A=Y , dif(B,Y), X=Y ; A=X , B=X , dif(X,Y) ; A=X … Read more
I don’t know what do you mean by “not equal” (does not unify?), but you could try these: X \= Y not(X = Y) \+ (X = Y)
The difference between CLIPS and Prolog/Datalog is that CLIPS is a “production rule system” that operates by forward chaining: given a set of facts and rules, it will try to make every possible derivation of new facts and store those in memory. A query is then answered by checking whether it matches something in the … Read more
I’m assuming you are using swi interactively and trying to enter the fact gives you an error like so: 1 ?- like(x, y). ERROR: toplevel: Undefined procedure: like/2 (DWIM could not correct goal) Since the fact does not exist in the database. If this is the case, try asserting the fact first: 2 ?- assert(like(x,y)). … Read more
In SWI-Prolog: aggregate_all(count, is_man(X), Count).
Correct, this is a warning to enforce best practices, which is to put all related clauses together in the source file. Other than that, the proximity of clauses to each other in the source file does not matter, as long as their relative order does not change.