Here is circular part of my facts (define relation between people):
connection(harry, ron).
connection(ron, harry).
connection(hermione, harry).
I want to find out if there is a connection, either directly or through other people using recursion.
This code runs into an infinite loop in the above situation:
connected(A, B) :-
connection(A, B).
connected(A, B) :-
connection(A, C),
connected(C, B).
I want the recursion to stop when it encounters previous searches. For example:
?- connected(hermione, X).
X = harry ;
X = ron.
Thank you!
Prolog's execution mechanism is quite un-intuitive. It combines recursion (as you know it from other languages) with backtracking. But there is a nice way out. You can consider smaller programs instead of your original program.
To better understand the actual loop, here is the minimal program fragment called a failure-slice that is the cause for non-termination. There is some extra falsework in it: Some goals false. With these goals the number of inferences is reduced (or stays the same). Thus, when the resulting program is looping, this is also a reason why your original program loops.
connection(harry, ron). connection(ron, harry).connection(herminone, harry) :- false.connected(A, B) :- false, connection(A, B). connected(A, B) :- connection(A, C), connected(C, B), false. ?- connected(A, B).
The query still loops. Note that it fails (and thus terminates) when asked the right persons (you know who, I suppose):
?- connected(tom, B).
false.
To solve this problem, the simplest way is to use closure/3 like so:
connected(A, B) :-
closure(connection, A, B).