Prolog: Can't increase value of variable inside complex term

Question

So I'm working on a prolog problem where a state is defined in a complex term, When I try to increase the value of x inside this complex term nothing happens for example

CurrentState(left, x, y).



test = CurrentState(left, x,y),
newX = x + 1,
write(newX).

Is there a workaround for this?

Answer 1

Facts and predicates need to start with a lowercase letter.

Variables need to start with an uppercase letter.

= is not assignment, and Prolog predicates aren't function calls, and you never use test anyway.

Even then, what do you want x + 1 to do when you haven't given any value for x? It makes no more sense than left + 1 written like that.

It would all be more like:

currentState(left, 5, 2).   % fact

test(NewX) :-
    currentState(Direction, X, Y),   % search for the fact, fill the variables.
    NewX is X + 1,                   % new variable for new value.
    write(NewX).

Then

?- test(NewX).
6
NewX = 6

After that you still don't want to be storing the state as a fact in the Prolog database and removing/asserting it.