Q: Python 2.0 preliminary features?

[Chris Double]

skaller <skaller at maxtal.com.au> writes:

> 	I see. I had never thought about it this way, despite
> reading some theory based on this idea, you have put this
> very simply. Thanks!

Clos, Dylan and Cecil are examples of languages that have multiple
dispatch.

> That is, it would be possible to specialised
> the dispatcher for each message kind, depending
> on what makes sense for that kind of message.

With Dylan you can dispatch on what are called singleton objects. That
is, the exact identity of an object.

Contrived example:

// Default for all messages
define method handle-message( window, msg, wparam, lparam) ... end;

// Called when msg is the instance of the WM_CREATE object (which may
be an integer) on an instance of <edit-window>.
define method handle-message( window :: <edit-window>, 
  msg == WM_CREATE, 
  wparam, 
  lparam)
 // Do something
 ...

 // Call next most specific method
 next-method();
end;

You probably get the idea. Clos has the same thing with 'EQL'
specialisers. Dylan can also dispatch on unions of types:

define method do-something( token :: one-of(#"word", #"sentence") )
end;

Where 'one-of' is a method which returns a type that behaves for type
matching purposes as a superclass of the types of the arguments passed
to it. The above method will be selected if called with
do-something(#"word") or do-something(#"sentence") but not anything
else.

I think Cecil is even more flexible in the things it can do in
dispatch. Clos is legendary for how flexible its dispatch mechanism
is.

Chris.