问题
I have been reading through the answers and comments of my previous question and I have tried applying the given explanations on an example from Bratko (Prolog Programming for Artificial Intelligence, p. 130), but I am not sure I understand it completely. The example is described below:
I read the tree and the code as follows:
In the goal list C :- P, Q, R, !, S, T, U. Prolog will one by one try to instantiate the variables, as normal, to eventually get to true.. Let's say that a value is found for P and Q, and the first try on R fails, then Prolog can back track to the case where P and Q were found, and try another option for R if available. However, if R is found as well (leading to P, Q, R = true.), and ! succeeds as it always does, we throw away any choice points and there's nothing to back track to from that point on (not even C :- V.). What this means is that if no results can be found for S, the goal C :- P, Q, R, !, S, T, U. will fail immediately. But Prolog can still backtrack to A :- B, C, D. to find other values for B. If another match is found for B, C will be tried again anew. And so on.
Assuming that my interpretation is correct, if the goal C :- P, Q, R, !, S, T, U. succeeds or fails regardless of the value of B, how would you improve efficiency? My guess would be to re-write A :- B, C, D. as A :- B, !, C, D.
Is my interpretation correct? And what about my improvement in efficiency, given some a-priori information on C?
回答1:
Yes, your understanding is correct. To see it better, we can re-write the predicates as
a = (b & c & d)
c = (p & q & r) ~~>! (s & t & u) ; v
with & for &&:, and the rest of operators, from this answer (or if it isn't clear, see this as a pseudocode, with ~~>! passing no more than one solution through). When the cut is reached, c is committed, but a is still backtrackable.
If C in A :- B, C, D. succeeds or fails regardless of the value of B, you can also reorder the goals as
A :- C, B, D.
The cut in A :- B, !, C, D. is a red cut, it only lets B succeed once, but what if you're interested in its second result? Red cuts alter the meaning of a predicate.
来源：https://stackoverflow.com/questions/40994026/how-to-interpret-this-prolog-goal-with-a-cut-and-improve-efficiency