create_category/4 — The Logtalk Handbook v3.47.0 documentation

# `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 backend Prolog compiler with bounded integer support. The portable solution, when creating a large number of dynamic category 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 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)]
         ).

See also

abolish_category/1, category_property/2, current_category/1, complements_object/2, extends_category/2-3, imports_category/2-3