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.. rst-class:: align-right

**built-in predicate**

.. index:: pair: create_category/4; Built-in predicate
.. _predicates_create_category_4:

``create_category/4``
=====================

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

::

   create_category(Identifier, Relations, Directives, Clauses)

Creates a new, dynamic category. This predicate is often used as a
primitive to implement high-level category creation methods.

Note that, when opting for runtime generated category identifiers, it's
possible to run out of identifiers when using a :term:`backend Prolog compiler`
with bounded integer support. The portable solution, when creating a
large number of dynamic categories in long-running applications, is to
recycle, whenever possible, the identifiers.

When creating a new dynamic parametric category, access to the object
parameters must use the :ref:`methods_parameter_2` built-in execution
context method.

When using Logtalk multi-threading features, predicates calling this
built-in predicate may need to be declared synchronized in order to
avoid race conditions.

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

::

   create_category(?category_identifier, @list(category_relation), @list(category_directive), @list(clause)) - one

Errors
------

| ``Relations``, ``Directives``, or ``Clauses`` is a variable:
|     ``instantiation_error``
| ``Identifier`` is neither a variable nor a valid category identifier:
|     ``type_error(category_identifier, Identifier)``
| ``Identifier`` is already in use:
|     ``permission_error(modify, category, Identifier)``
|     ``permission_error(modify, object, Identifier)``
|     ``permission_error(modify, protocol, Identifier)``
| ``Relations`` is neither a variable nor a proper list:
|     ``type_error(list, Relations)``
| Repeated entity relation clause:
|     ``permission_error(repeat, entity_relation, implements/1)``
|     ``permission_error(repeat, entity_relation, extends/1)``
|     ``permission_error(repeat, entity_relation, complements/1)``
| ``Directives`` is neither a variable nor a proper list:
|     ``type_error(list, Directives)``
| ``Clauses`` is neither a variable nor a proper list:
|     ``type_error(list, Clauses)``

Examples
--------

::

   | ?- create_category(
           tolerances,
           [implements(comparing)],
           [],
           [epsilon(1e-15), (equal(X, Y) :- epsilon(E), abs(X-Y) =< E)]
        ).

.. seealso::

   :ref:`predicates_abolish_category_1`, :ref:`predicates_category_property_2`,
   :ref:`predicates_current_category_1`,
   :ref:`predicates_complements_object_2`, :ref:`predicates_extends_category_2_3`,
   :ref:`predicates_imports_category_2_3`
