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   SPDX-FileCopyrightText: 1998-2026 Paulo Moura <pmoura@logtalk.org>
   SPDX-License-Identifier: Apache-2.0

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   you may not use this file except in compliance with the License.
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.. rst-class:: align-right

**built-in method**

.. index:: pair: retract/1; Built-in method
.. _methods_retract_1:

``retract/1``
=============

Description
-----------

::

   retract(Head)
   retract((Head:-Body))

Retracts a clause for an object dynamic predicate. On backtracking, the
predicate retracts the next matching clause.

When the predicate indicator for ``Head`` is declared in a
:ref:`directives_uses_2` or :ref:`directives_use_module_2` directive,
the clause is retracted in the referenced object or module. When the
backend Prolog compiler supports a module system, the predicate
argument can also be module qualified.

This method may be used to retract clauses for predicates that are not
declared dynamic for dynamic objects provided that the predicates are
declared in :term:`this`.

Modes and number of proofs
--------------------------

::

   retract(+clause) - zero_or_more

Errors
------

| ``Head`` is a variable:
|     ``instantiation_error``
| ``Head`` is neither a variable nor a callable term:
|     ``type_error(callable, Head)``
| The predicate indicator of ``Head``, ``Name/Arity``, is that of a private predicate:
|     ``permission_error(modify, private_predicate, Name/Arity)``
| The predicate indicator of ``Head``, ``Name/Arity``, is that of a protected predicate:
|     ``permission_error(modify, protected_predicate, Name/Arity)``
| The predicate indicator of ``Head``, ``Name/Arity``, is that of a static predicate:
|     ``permission_error(modify, static_predicate, Name/Arity)``
| The predicate indicator of ``Head``, ``Name/Arity``, is not declared:
|     ``existence_error(predicate_declaration, Name/Arity)``

Examples
--------

| To retract a matching clause of a dynamic predicate in :term:`this`:
|     ``retract(Clause)``
| To retract a matching clause of a public or protected dynamic predicate in :term:`self`:
|     ``::retract(Clause)``
| To retract a matching clause of a public dynamic predicate in an explicit object:
|     ``Object::retract(Clause)``

.. seealso::

   :ref:`methods_abolish_1`,
   :ref:`methods_asserta_1`,
   :ref:`methods_assertz_1`,
   :ref:`methods_clause_2`,
   :ref:`methods_retractall_1`,
   :ref:`directives_dynamic_0`,
   :ref:`directives_dynamic_1`,
   :ref:`directives_uses_2`,
   :ref:`directives_use_module_2`
