Question

I’m very new to Prolog, and i’m having a problem where my variable seems to be forgotten

test(S) :-
X = 'testing',
(S = y, write(X) );
(S = n, write(X) ).

Running

test(y)

Prints out the text as expected, but

test(n)

prints out

_L160

Which i assume means the variable is uninstantsiated? Why is this happening?

I know it can spit it into two predicates like:

test(y) :- X = 'testing', write(X).
test(n) :- X = 'testing', write(X).

but my actual problem is in a much larger predicate that cant be simplified like this.

Answer 1

The disjunction (;) currently succeeds by either:

1. Assign ‘testing’ to X, y to S and write X.
   OR, if that fails, backtrace and
2. Assign n to S and write X

Add some parentheses to make it work as intended.

test(S):-
X = 'testing',
    (
        (S = y, write(X) )
    ; 
        (S = n, write(X) )
    ).