Question

How to get Y and Z in prolog, when I only know X?

For example:

test(X, Y, Z) :- X is Y + Z.

but error:

?- test(2, Y, Z).
ERROR: is/2: Arguments are not sufficiently instantiated

Answer 1

It’s not possible, because you can choose Y to be anything you want and them compute Z or vice versa.

Although if you know that Y and Z are from some limited set (e.g. positive integers less than 5), you can do something like:

valid_number(1).
valid_number(2).
valid_number(3).
valid_number(4).

test(X, Y, Z) :- valid_number(Y), valid_number(Z), X is Y + Z.