Suppose that we want to test the predicate double/2 to see if it works for its intended inputs.
double(X,Y):-To do this, we write a test predicate:Y is 2*X.
test:-Here is a transcription of executing the query test:read(X),
double(X,Y),
write(Y),
nl.
?- test.Note that, since we are using read/1 which only accepts valid Prolog terms terminated by a ``.'' followed by Return (in this case), we have to enter input integers as 2.!
: 2.
4
yes
Now to make this into a loop. The easy way is to recursively call test/0. We would prefer, however, to put in a test so that we can abort the loop. This requires an end-of-input marker.
test:-When we input the end-of-input marker (-1) we backtrack to read/1 which fails (for this Prolog implementation!) and test/0 fails as there are no other clauses. We could always add a second clause (after ---not before) which guaranteed that the goal test succeeded once the end-of-input marker was met.read(X),
\+(X = -1),double(X,Y),
write(Y),
nl,
test.
Note that it is up to us to make sure that read/1 is never asked to process non-integer inputs. We could always define and use our own read_integer/1 to catch non-integer input.