|
Sometimes we require matching without unification. Prolog has a
built-in predicate "==". This matches two terms as before
but, to succeed, the two terms have to already have identical values.
Now for some examples. Obviously two identical atoms will match.
| ?- munich == munich.
yes
An atom and an uninstantiated variable will not match, and an atom
and an instantiated variable will match only if the variable is instantiated
to a value exactly the same as the atom:
| ?- russia == Country.
no
| ?- Country = russia, Country == russia.
Country = russia ?
What happens if both terms are uninstantiated variables? The key point
is whether or not the variables are distinct:
| ?- Variable1 == Variable2.
no
If the two variables have been previously unified, then (although
they may not be instantiated (ie have a value)), the expression will
succeed.
| ?- Variable1 = Variable2, Variable1 == Variable2.
Variable2 = Variable1 ?
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