control construct

[]/1

Description

[Object::Message]
[{Proxy}::Message]

This control construct allows the programmer to send a message to an object while preserving the original sender and meta-call context. It is mainly used in the definition of object handlers for unknown messages. This functionality is usually known as delegation but be aware that this is an overloaded word that can mean different things in different object-oriented programming languages.

To prevent using of this control construct to break object encapsulation, an attempt to delegate a message to the original sender results in an error. The remaining error conditions are the same as the (::)/2 control construct.

Note that, despite the correct functor for this control construct being (traditionally) '.'/2, we refer to it as []/1 simply to emphasize that the syntax is a list with a single element.

Modes and number of proofs

[+object_identifier::+callable] - zero_or_more
[{+object_identifier}::+callable] - zero_or_more

Errors

Object is a variable:

instantiation_error

Object is neither a variable nor an object identifier:

type_error(object_identifier, Object)

Object does not exist:

existence_error(object, Object)

Object and the original sender are the same object:

permission_error(access, object, Sender)

Proxy is a variable:

instantiation_error

Proxy is neither a variable nor an object identifier:

type_error(object_identifier, Proxy)

Proxy, with predicate indicator Name/Arity, does not exist in the user pseudo-object:

existence_error(procedure, Name/Arity)

Message is a variable:

instantiation_error

Message is neither a variable nor a callable term:

type_error(callable, Message)

Message, with predicate indicator Name/Arity, is declared private:

permission_error(access, private_predicate, Name/Arity)

Message, with predicate indicator Name/Arity, is declared protected:

permission_error(access, protected_predicate, Name/Arity)

Message, with predicate indicator Name/Arity, is not declared:

existence_error(predicate_declaration, Name/Arity)

Examples

% delegate unknown messages to the "backup" object:
forward(Message) :-
    [backup::Message].

See also

(::)/2, (::)/1, (^^)/1, forward/1