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once +Goal

Succeeds if Goal succeeds, and removes all its alternatives --- equivalent to call((Goal, !))

Goal: Goal.

Description

Used to find a single solution for Goal, alternative solutions are ignored (cut).

Note that !/0 does not cut through once/1.

Modes and Determinism

once(+) is det

Modules

This predicate is sensitive to its module context (tool predicate, see @/2).

Fail Conditions

Fails if Goal fails

Exceptions

(4) instantiation fault: Goal is not instantiated.
(5) type error: Goal is neither an atom nor a compound term.

Examples

Success:
      [eclipse]: once member(X, [1,2,3]).
      X = 1       % only first solution is bound.
      yes.

Fail:
      [eclipse]: once 1=2.
      no.

Error:
      once Goal.                     (Error 4).
      once "ls".                     (Error 5).
      once 1.0.                      (Error 5).