After loading a program, one can ask Prolog queries about the
program. The query below asks Prolog what food‘sam' likes. The
system responds with
X = <value> if it can
prove the goal for a certain
X. The user can type the semi-colon (;) or spacebar7On
most installations, single-character commands are executed without
waiting for the RETURN key.
if (s)he wants another solution. Use the return
key if you do not want to see more answers. Prolog completes the output
with a full stop (.) if the user uses the return
key or Prolog
knows there are no more answers. If Prolog cannot find (more)
answers, it writes false. Finally, Prolog answers using an error
message to indicate the query or program contains an error.
?- likes(sam, X). X = dahl ; X = tandoori ; ... X = chips. ?-
Note that the answer written by Prolog is a valid Prolog program that, when executed, produces the same set of answers as the original program.8The SWI-Prolog top level differs in several ways from traditional Prolog top level. The current top level was designed in cooperation with Ulrich Neumerkel.