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!

Cut - succeeds and removes all choice points between cut and parent goal.

Description

The cut operation succeeds immediately when first encountered as a goal. The cut commits all the choices made since the parent goal was invoked, and causes any other alternatives to be discarded.

Note that:

!/0 cuts through ,/2, ;/2 and the right hand side of ->/2, i.e. cuts that occur in these positions affect the whole clause and subsequent alternatives. It does NOT cut through call/1, not/1, once/1, the left hand side of ->/2 (the condition), or other meta-calling constructs - such cuts only have a local effect.

Modes and Determinism

Examples

Success:
      [eclipse]: [user].
       or(A -> B, C) :- call(A), !, call(B).
       or(_ -> _, C) :- call(C).
       user compiled 412 bytes in 0.02 seconds
      [eclipse]: or(write(a)->fail, write(k)).
      a
      no.
      [eclipse]: or(fail->fail,write(k)).
      k
      yes.

      [eclipse]: [user].
       echo :- repeat, read(X), echo(X), !.
       echo(end_of_file).
       echo(X) :- writeln(X), fail.
       user compiled 404 bytes in 0.02 seconds
      yes.    % if the cut is left out, backtracking occurs.
      [eclipse]: echo.
       f(1,2).
      f(1,2)
       end_of_file.
      yes.

See Also

fail / 0, once / 1, repeat / 0, ; / 2