.. This file is part of Logtalk https://logtalk.org/ SPDX-FileCopyrightText: 1998-2024 Paulo Moura <pmoura@logtalk.org> SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

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.. rst-class:: align-right

built-in method

.. index:: pair: (\+)/1; Built-in method .. _methods_not_1:

`(\+)/1`

Description

\+ Goal

Not-provable meta-predicate. True iff call(Goal) is false. This built-in meta-predicate is declared as a private method and thus cannot be used as a message to an object.

.. warning::

The argument is always compiled (for improved performance). As a consequence, when the argument is a control construct (e.g., a conjunction), any meta-variables will be wrapped with the equivalent to the call/1 control construct. Note that these semantics differ from the ISO Prolog Core standard specification for the (\+)/1 built-in predicate. For example, assuming a conforming system:

| ?- X = !, \+ (member(Y,[1,2,3]), X, write(Y), fail). 1

X = !

But in Logtalk X is compiled into a meta-call, which is not cut-transparent:

yes | ?- logtalk << (X = !, \+ (member(Y,[1,2,3]), X, write(Y), fail)). 123

X = !

Note that the ISO Prolog Core standard doesn't specify a cut-transparent alternative to the call/1 control construct.

Meta-predicate template

\+ 0

Modes and number of proofs

\+ +callable - zero_or_one

Errors

| Goal is a variable: | instantiation_error | Goal is neither a variable nor a callable term: | type_error(callable, Goal)

Examples

.. seealso::

:ref:methods_call_N, :ref:methods_ignore_1, :ref:methods_once_1