Search code examples
prologfailure-slicetransitive-closure

Prolog "Out of local stack" Error

Here is my simple Prolog program:

friend(X,Y):-
   knows(X,Y).
friend(X,Z):-
   friend(X,Y),
   friend(Y,Z).

knows(brian,tom).
knows(tom,peter).

If I type the following query

friend(brian,peter).

Prolog will give the following output:

?- friend(brian,peter).
true

If a further type a semicolon, Prolog will say:

ERROR: Out of local stack

What am I doing wrong here?

Solution

  • The error is in the second clause. It should be instead:

    friend(X,Z):-
   knows(X,Y),
   friend(Y,Z).

    Otherwise, when you ask Prolog for more solutions, you end up having the friend/2predicate recursively calling itself without first establishing a knows/2intermediate relation. You can learn more about the bug in your program by tracing the calls to the friend/2 predicate. Try:

    ?- trace, friend(brian,peter).