Why does SWI-Prolog only give me the first answer?

Question

I'm new to Prolog. I'm just trying simple examples to learn. I have this .pl file with these lines:

parent(pam,bob).
parent(tom,bob).
parent(tom,lio).
parent(bob,ann).
parent(bob,pat).
parent(pat,jim).

After consulting and testing, it only shows the first answer. For example:

5 ?- parent(X,Y).
X = pam,
Y = bob .

Isn't it supposed to give all the combinations that satisfy the relation parent?

Do anyone have idea what the problem is?

don't hit enter after your first results shows, use spacebar instead — Sam Segers, Dec 25 '15 at 13:26
I've added this as an answer since I've got the feeling you are not the only one running into this. — Sam Segers, Dec 25 '15 at 13:28

Sam Segers · Answer 1 · 2015-12-25T14:10:12.843

16

don't hit enter after your first results shows, use spacebar instead

[Enter] stops execution even if the backtracking is not completed yet
[Spacebar] or [;] continues with backtracking from your last result to the next result or false if there are no other results left.

edited Dec 25 '15 at 14:10

answered Dec 25 '15 at 13:28

Sam Segers

1

Can you explain what the difference between space and enter does for future readers, too? – Chris Haas Dec 25 '15 at 13:54
I thought that was already clear, but if not, I hope this will do. Thanks. – Sam Segers Dec 25 '15 at 14:20
Thanks for the answer. But it doesn't say 'false' when no answers left. Display each result for every ';' and then goes to ?-. I'm trying on SWI-Prolog 8.2.4. – Hareen Laks May 14 '21 at 11:33

1 Answers1