[Python-3000] Abilities / Interfaces

[Phillip J. Eby]

At 10:43 AM 11/22/2006 -0800, Guido van Rossum wrote:
>There is a dangerous amount of content-free rhetoric in this thread.

Actually, I think that some of my statements are being misunderstood as 
such, when I'm intending only to describe *how* interfaces (or inspection) 
can be *derived* from generic functions.  When I say that these other 
concepts are "unnecessary", I mean that they can be derived from generics, 
and that the reverse is not true.

This isn't rhetoric, IMO, and it's not about religion or purity, but about 
practicalities.  Often, an interface consists of just one operation, so 
treating interfaces as fundamental introduces additional "unnecessary" 
complexity in those cases, whereas being able to treat that one-operation 
interface as a single generic function removes the need for interfaces -- 
even as a *concept* to be understood, let alone implemented.  That's the 
power of Python's existing generics: they don't rely on you "implementing 
the iterable interface", but just on defining the iter() *operation*.


>As I've tried to mention through examples, there are plenty of
>situations where a carefully placed if-then is a lot more practical
>than adding a new method to some remote class (or several).

Absolutely.  I am saying only that the inspection can be implemented *in 
terms of* generic functions.


>But even the best generic function API I've seen is a lot
>more verbose than this -- there seems to be a separate set-up
>involved.

Actually, my 'defop' proposal would allow you to do this:

     def sendmail(...):
         # default case

     defop sendmail(...):
         # special case, w/additional type info in arg signature

The first statement creates a function called 'sendmail', and the 
subsequent statement overloads it.  Of course, I suppose 'defop' could also 
be spelled differently to emphasize the difference, e.g.:

     def sendmail(...):
         # default case

     overload sendmail(...):
         # special case, w/additional type info in arg signature