Once in SICStus Prolog?

Question

How can I use once in SICStus? In SWI Prolog, the following works:

test(X, Y, abc) :- once(X == 'true' ; Y == 'true').

but in SICStus Prolog, once seems to be unrecognized. What is the equivalent of once in SICStus? Thank you.

Note that SWI is wrong here: The standard requires a syntax error in this case. SWI is the only system that accepts this malformed term. — false, Feb 28 '17 at 18:54

score 4 · Accepted Answer · edited Feb 28 '17 at 18:54

4

The answer is:

test(X, Y, abc) :- once((X == 'true' ; Y == 'true')).

The additional parenthesis did the trick.

edited Feb 28 '17 at 18:54

false

answered Feb 28 '17 at 18:51

1 Answers1