Unification

Unification aims to make the two sides of an expression match. For example, if we input

[1, 2, 3] = List.

we get the output

List = [1, 2, 3].

This is because Prolog has tried to match the right hand side to the left. The solution in this case implies that List must be the same value as the value on the left, i.e. [1, 2, 3].

Prolog can unify in both directions. For example, if we input

[A, B, 3] = [1, 2, C].

we get

A = 1,
B = 2,
C = 3.

Falsehood

If there is no logically correct assignment of variables in an expression, Prolog will simply return false, as the expression cannot make sense. e.g.

[A, 2, 4] = [1, B, 3]

3 cannot equal 4, and so this is a logically impossible expression.