Notes on computational linguistics.pdf - UCLA Department of ...

Stabler - Lx 185/209 2003
(8) Pitfall 1: Prolog’s proof method is not an algorithm, hence not a decision method. This is the case
because the search for a proof can fail to terminate. There are cases where G, Γ ⊢ A and G, Γ ⊨ A, but
prolog will not find the proof because it gets caught in “infinite recursion.”
Infinite recursion can occur when, in the course of proving some goal, we reach a point where we are
attempting to establish that same goal. Consider how prolog would try to prove p given the axioms:
p :- p.
p.
Prolog will use the first axiom first, each time it tries to prove p, and this procedure will never terminate.
We have the same problem with
p :- p, q.
p.
q.
And also with
p :- q, p.
p.
q.
This problem is sometimes called the “left recursion” problem, but these examples show that the problem
results whenever a proof of some goal involves proving that same goal. We will consider this
problem more carefully when it arises in parsing.
Prolog was designed this way for these simple practical reasons:
(i) it is fairly easy to choose problems for which Prolog does terminate, and
(ii) the method described above allows very fast execution!
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