!Converted with LaTeX2HTML 95.1 (Fri Jan 20 1995) by Nikos Drakos (email@example.com), CBLU, University of Leeds >
Don't use cuts indiscriminately. Here is an example taken
from O'Keefe, ``The Craft of Prolog''.
The plan is to define a predicate,
match1/2, which will take as its
first argument a pattern for a sequence (with some ``holes'' in it)
and take as its second argument a complete sequence of symbols.
The predicate should succeed if the pattern can be matched to the
sequence. For example:
| ?- match1([i,X,my,Y], [i,like,my,lecturing,job]). X = [like], Y = [lecturing,job]One way to define it is as follows:
match1(, ). match1([H|T], [F|R]):- nonvar(H), !, H = F, match1(T, R). match1([V|T], [F|R]):- match1(T, R), V = [F]. match1([V|T], [F|R]):- match1([New|T], R), V = [F|New].This works, but is not as fast as the following definition, named
match2/2, which has no cuts but distinguishes the type of symbol in the pattern list using
Xis a word, and
Xis a segment of the sequence.
match2(, ). match2([H|T], [F|R]):- match2(H, T, F, R). match2(w(Word), Items, Word, Words):- match2(Items, Words). match2(s([Word|Seg]), Items, Word, Words0):- append(Seg, Words1, Words0), match2(Items, Words1).The behaviour of this predicate is as follows:
| ?- match2([w(i),s(X),w(my),s(Y)], [i,like,my,lecturing,job]). X = [like], Y = [lecturing,job]