[Python-3000] Generic functions vs. OO

[Guido van Rossum]

[With apologies for having a monologue with myself here.]

On 11/24/06, I wrote:
> I'd like to take the opportunity to present another example of what
> I'd like to be able to do with abilities and generic functions.
[...]

Thinking about my own example some more, I think it would actually
work just as well using ABCs instead of abilities. (The GFs are still
needed.) It seems the use case for making abilities/interfaces
separate from ABCs hinges on the assumption that there are classes
that implement certain protocols without realizing it, IOW emergent
protocols, plus that it's not practical to change __bases__. This
seems to be exacerbated by there not being any standard
abilities/interfaces defined, while some of the standard types are
just *begging* to be classified by a container hierarchy. Plus, the
standard types won't let you alter their __bases__ at all.

I wonder if a bunch of well thought-out standard ABCs, applied to the
standard data types, and perhaps more support for setting __bases__,
wouldn't address most concerns.

Note: I still like GFs for operations, especially binary operators,
because they solve the problem of two-sided dispatch much better than
__add__ and __radd__ ever can. (For unary operators I'm not sure it
buys us that much; I'm pretty happy with __len__ and __iter__.)

-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)