[Python-3000] Abilities / Interfaces

[Gustavo Niemeyer]

> 2. Do we need interfaces if we have generic functions?
> 
> Phillip Eby argues that we don't need interfaces, because the right
> API for generic functions makes it easy to do what we do with
> interfaces. Let me paraphrase his argument.

I agree that we don't *need* interfaces to have generic functions,
just like we don't need them to have adaptation.  Using your
example, consider Flatten as a base class, and F as an adapter
registry containing adapters Fi from C to Flatten.  It's feasible.

Interfaces provide greater value for both approaches.  A short
usage case, shortened from the other thread.

"""
Let's say that you create a library named libfruit, and that it
implements a very nice IFruit interface.  This library is so nice
and shiny that it becomes widely used, and thus many ways to handle
a fruit are developed by third parties.  Now, someone else has a
libapple library, which unfortunately is a bit outdated, and wasn't
designed to work with the shiny libfruit library.

With interfaces, all you need to do is define an adapter from Apple
to IFruit, and instantly all the widely available frameworks will
be handling apples just fine.
"""

Without interfaces, we won't be able to say "in all places, use the
generic method registered for an IFruit whenever an Apple is seen".

Using generic functions, the same is true.  The only difference is
that with generic functions each "adapter registry" is capable of
adapting only to a single "interface", thus removing the need to
explicitly declare it.


> 3. API design -- how do we spell the various concepts? E.g.
> has_ability(x, A) asks whether object x has the ability A;
> provides_ability(C, A) asks whether class C provides the ability A to
> its instances.

I'm sure you're aware about it, but in Zope 3 terminology, these
are 'provide' and 'implement', respectively.


> - We need to come up with syntax for the common case, declaring a
> class that claims to provide some abilities. Some thoughts: putting
(...)

Have you considered class decorators?

  @implements(FooBar)
  class Baz:
     ...

-- 
Gustavo Niemeyer
http://niemeyer.net