Page 214 - DCAP506_ARTIFICIAL_INTELLIGENCE

Artificial Intelligence

Notes
Did u know? The similar variable has to be instantiated with the similar value all through
an expression. The only exception to this rule is the unidentified variable _, which is
measured to be unique whenever it appears.

Example: We provide some examples. The terms is_bigger(X, dog) and
is_bigger(elephant, dog) match, since the variable X can be instantiated with the atom elephant.
We could check this in the Prolog interpreter by submitting the equivalent query to which
Prolog would respond by listing the suitable variable instantiations:
?- is_bigger(X, dog) = is_bigger(elephant, dog).

X = elephant
Yes

The following is an example for a query that doesn’t succeed, because X cannot match with 1 and
2 at the same time.

?- p(X, 2, 2) = p(1, Y, X).
No
If, however, rather than X we use the anonymous variable _, matching is possible, since every
occurrence of _ represents a different variable. All through matching Y is instantiated with 2:
?- p(_, 2, 2) = p(1, Y, _).

Y = 2
Yes

Another example for matching:
?- f(a, g(X, Y)) = f(X, Z), Z = g(W, h(X)).
X = a

Y = h(a)
Z = g(a, h(a))

W = a
Yes

Until now so good but what happens, if matching is probable even though no specific variable
instantiation has to be imposed. Consider the following query:

?- X = my_functor(Y).
X = my_functor(_G177)

Y = _G177
Yes
In this example matching succeeds, since X could be a compound term with the function
my_function and a non-specified single argument. Y could be any valid Prolog term, but it has

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