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   Copyright 1998-2021 Paulo Moura <pmoura@logtalk.org>
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.. index:: pair: asserta/1; Built-in method
.. _methods_asserta_1:

``asserta/1``
=============

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

::

   asserta(Head)
   asserta((Head:-Body))

Asserts a clause as the first one for an object dynamic predicate. If the
predicate is not previously declared (using a scope directive), then a
dynamic predicate declaration is added to the object (assuming that we are
asserting locally or that the
:ref:`dynamic_declarations <flag_dynamic_declarations>` compiler flag was
set to ``allow`` when the object was created or compiled).

When the predicate indicator for ``Head`` is declared in a
:ref:`directives_uses_2` or :ref:`directives_use_module_2` directive, the
clause is asserted 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 assert clauses for predicates that are not
declared dynamic for dynamic objects provided that the predicates are
declared in :term:`this`. This allows easy initialization of dynamically
created objects when writing constructors.

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

::

   asserta(+clause) - one

Errors
------

| ``Head`` is a variable:
|     ``instantiation_error``
| ``Head`` is a neither a variable nor a callable term:
|     ``type_error(callable, Head)``
| ``Body`` cannot be converted to a goal:
|     ``type_error(callable, Body)``
| 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``, does not match a declared predicate and the target object was created or compiled with support for dynamic declaration of predicates turned off:
|     ``permission_error(create, predicate_declaration, Name/Arity)``

Examples
--------

| To assert a clause as the first one for a local dynamic predicate or a dynamic predicate in :term:`this`:
|     ``asserta(Clause)``
| To assert a clause as the first one for any public or protected dynamic predicate in :term:`self`:
|     ``::asserta(Clause)``
| To assert a clause as the first one for any public dynamic predicate in an explicit object:
|     ``Object::asserta(Clause)``

.. seealso::

   :ref:`methods_abolish_1`,
   :ref:`methods_assertz_1`,
   :ref:`methods_clause_2`,
   :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`
