[SOLVED] Prolog instantiation error when using two "is/2" predicates

Prolog instantiation error when using two "is/2" predicates

I'm new to Prolog and wanted to try some basic examples. I saw multiple questions about the instantiation error that Prolog throws, but I couldn't find a solution for my example.

My rule is for updating a location after a forward action is executed in a certain direction. I just removed the heading for simplification, it keeps failing:

% loc(x, y, t)
loc(1, 1, 1) :- !.  % location at time t = 1
loc(X2, Y, T2) :-  % X1 incremented by 1
    T2 > 1,
    T1 is T2 - 1,
    forward(T1),  % a forward action was executed at the previous time step
    X1 is X2 - 1,
    loc(X1, Y, T1).

As I said, this throws the dreaded error:

ERROR: Arguments are not sufficiently instantiated

Can anyone point me in the right direction?

Solution

It looks like X2 is an output argument. If this is indeed the case, the code should be modified as follows:

loc(1, 1, 1) :- !. 
loc(X2, Y, T2) :- 
    T2 > 1,
    T1 is T2 - 1,
    loc(X1, Y, T1),
    X2 is X1 + 1.

Example:

?- loc(X,Y,3).
X = 3,
Y = 1.