Respond to an unknown method call: Difference between revisions

=={{header|Logtalk}}==

There are two ways to handle unknown messages. From the sender side, we can catch the exception that is generated when an object doesn't understand a message:

<lang logtalk>

:- object(foo).

:- public(try/0).

try :-

catch(bar::message, Error, handler(Error)).

handler(error(existence_error(predicate_declaration,message/0),_)) :-

% handle the unknown message

...

:- end_object.

</lang>

From the receiver side, an object can implement the built-in protocol "forwarding", defining the handler for messages that it doesn't understands (e.g. by forwarding or delegating it to another object):

<lang logtalk>

:- object(foo,

implements(forwarding)).

forward(Message) :-

% handle the unknown message

...

:- end_object.

</lang>