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fail_if(+Goal)

Succeeds if Goal cannot be satisfied. Uses negation as failure (synonym of not/1 and \+/1).
Goal
Atom or compound term.

This built-in predicate is obsolete!

Description

Used to fail if Goal succeeds. Uses the standard Prolog form of negation as failure.

To check whether a call Goal succeeds without binding variables, the call fail_if( fail_if( Goal)) can be used.

Not that !/0 does not cut through fail_if/1. Unlike not/1 and \+/1, fail_if/1 is a protected predicate and cannot be redefined.

Modes and Determinism

Fail Conditions

Fails if Goal succeeds

Examples

Success:
      fail_if(fail).
      fail_if(1 == 2).
      fail_if(X == 1).
      fail_if(fail_if(X = 1)).
          % does not bind X

Fail:
      fail_if(X = 1).
      fail_if(true).
      fail_if(3 == 3).

See Also

\+ / 1, not / 1, ~ / 1