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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

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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