Question

I’ve thrown a glance at Prolog, and this is my first little try at writing it: it directly bugs at my first queries. (in the example below i asked for word(‘test’).)

I meant to state:

• word/1
• alias/2
• alias is commutative
• if a variable is an alias of another then it is a word
• A, C, and D are word
• B is an alias of A.

this is the knowledge base when I do a listing. in SWI-Prolog console.

?- listing.
word(A) :-
    alias(A, B).
word('A').
word('C').
word('D').

alias(A, B) :-
        alias(B, A).
alias('A', 'B').

Yes
3 ?- word('test').
ERROR: Out of local stack
Exception: (59,743) alias(_L147, test) ?

Any clue what’s going wrong with my knwoledge base?

Answer 1

alias(A, B) :-
        alias(B, A).
alias('A', 'B').

this is an infinite loop.
for example, if you call alias(2,4) then the trace would be something like:

alias(2,4)
alias(4,2)
alias(2,4)
alias(4,2)
alias(2,4)
alias(4,2)
alias(2,4)
alias(4,2)
....