DEC-10 Prolog, C-Prolog, and Quintus Prolog provide two inequality operations:
=\=/2 is reasonably straightforward. Either it is true, and will
remain true, or it is false, and will remain false, or the Prolog
system will report an error if it cannot determine which. Thus
1 =\= 2 is true,
1 =\= 1.0 is false, and
_ =\= 3 will result in an error exception.
\==/2 is not really a logical relation, but a meta-logical relation
var/1. It tests whether two terms are exactly
identical, down to having the same variables in the same places.
It either succeeds or fails; and if it fails it will continue to
do so, but if it succeeds it may fail later on. For example,
| ?- X \== Y, % succeeds | X = Y, % succeeds, unifying X and Y | X \== Y. % FAILS, now that X and Y are unified no
It is safe to use
\==/2 to test the equality of terms
when you know in advance that the terms will be ground by the time
the test is made.