# implements_protocol/2-3

## Description

    implements_protocol(Object, Protocol)
    implements_protocol(Category, Protocol)

    implements_protocol(Object, Protocol, Scope)
    implements_protocol(Category, Protocol, Scope)

Enumerates, by backtracking, all pairs of entities such that an object or a category implements a protocol. The relation scope is represented by the atoms `public`, `protected`, and `private`. This predicate only returns direct implementation relations; it does not implement a transitive closure.

## Modes and number of proofs

    implements_protocol(?object_identifier, ?protocol_identifier) - zero_or_more
    implements_protocol(?category_identifier, ?protocol_identifier) - zero_or_more

    implements_protocol(?object_identifier, ?protocol_identifier, ?scope) - zero_or_more
    implements_protocol(?category_identifier, ?protocol_identifier, ?scope) - zero_or_more

## Errors

Object is neither a variable nor a valid object identifier:
`type_error(object_identifier, Object)`
Category is neither a variable nor a valid category identifier:
`type_error(category_identifier, Category)`
Protocol is neither a variable nor a valid protocol identifier:
`type_error(protocol_identifier, Protocol)`
Scope is neither a variable nor an atom:
`type_error(atom, Scope)`
Scope is an atom but an invalid entity scope:
`domain_error(scope, Scope)`

## Examples

    % check that the list object implements the listp protocol:
    | ?- implements_protocol(list, listp).

    % check that the list object publicly implements the listp protocol:
    | ?- implements_protocol(list, listp, public).

    % enumerate only objects that implement the listp protocol:
    | ?- current_object(Object), implements_protocol(Object, listp).

    % enumerate only categories that implement the serialization protocol:
    | ?- current_category(Category), implements_protocol(Category, serialization).

See also

current_object/1, current_protocol/1, current_category/1, conforms_to_protocol/2-3