.. 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

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

.. rst-class:: align-right

built-in method

.. index:: pair: clause/2; Built-in method .. _methods_clause_2:

clause/2

Description

::

clause(Head, Body)

Enumerates, by backtracking, the clauses of a dynamic predicate.

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

This method may be used to enumerate 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

::

clause(+callable, ?body) - zero_or_more

Errors

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

Examples

| To retrieve a matching clause of a local dynamic predicate or a dynamic predicate in :term:this: | clause(Head, Body) | To retrieve a matching clause of a public or protected dynamic predicate in :term:self: | ::clause(Head, Body) | To retrieve a matching clause of a public dynamic predicate in an explicit object: | Object::clause(Head, Body)

.. seealso::

:ref:methods_abolish_1, :ref:methods_asserta_1, :ref:methods_assertz_1, :ref:methods_retract_1, :ref:methods_retractall_1 :ref:directives_dynamic_0, :ref:directives_dynamic_1, :ref:directives_uses_2, :ref:directives_use_module_2