| causes the goal to succeed only once.
!/0
!/0 (Cut) succeeds exactly once. Unexplored branches
of the search-tree get cutted. If Cut is part of a goal then all
choicepoints of that goal are removed. That means that all bindings are
fixed.
If Cut is set on the right place (mostly at the beginning of the
body) then the complete goal can succeed only once.
The use of Cut is also important when building loops with repeat/0 because !/0 prevents that after leaving a loop this loop will be revisited by backtracing.
Using !/0 makes programs difficult to read. Therefore Cuts
should be used as rarely as possible.
Examples
Assume that the database contains:
colour(blue).
colour(red).
colour(yellow).
clothing(blouse).
clothing(trousers).
clothing(socks).
| colour(X),!. |
Succeeds once with substitution X <- blue. |
| clothing(X),!,colour(Y). |
Succeeds three times with substitution
X <- blouse, Y <- blue.
X <- blouse, Y <- red.
X <- blouse, Y <- yellow. |
Standard
This predicate is part of the ISO-Prolog Standard.
See also
abort/0,
break/0,
call/1,
fcall/1,
catch/3,
','(conjunction)/2,
';'(disjunction)/2,
fail/0,
true/0,
halt/0/1,
'->'(if-then)/2,
';'(if-then-else)/2,
'\+'(not provable)/1,
once/1,
repeat/0,
throw/1.
|
