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Negation and Control in Prolog by Lee Naish (auth.)

By Lee Naish (auth.)

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Because no walt declaration has ones in the first and third arguments, the call delays and X and Y are reset and marked. Append without wait declarations can efficiently join and split lists, but when it is part of a larger program, it can cause problems, as the following procedure illustrates: append3(A, B, D, E) : - append(A, B, C), append(C, D, E). Without wait declarations on append this program works for joining lists, but not for splitting them. Consider the goal below. []). Successive solutions to the first call to append are found by backtracking.

After the positions of the first N queens have been decided, there is one call to safe delayed, and N calls to nodiag delayed (one for each queen). When a new queen is added, these calls are woken. Each of the calls to nodiag checks if the new queen can be taken. The call to safe creates a call to nodiag, for the new queen, and another call to safe. When all eight queens have been successfully placed, perm binds the end of the list to [ ] and all the delayed calls succeed. Because perm constructs the list X before the next queen position is chosen (by delete), there is some unnecessary delaying of calls.

Some calls to system predicates delay because the presence of variables makes the result of a call difficult or impossible to compute. In other cases, delaying a :all until some variables are bound can increase efficiency or prevent an infinite loop. Whatever the reason, the offending variables are marked, to indicate that the call is waiting for them to be bou:ld. If a marked variable is bound at any stage, all the calls waiting for it are woken up. 4). At any point in a PROLOG computation, there is a current goal clause.

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