[Python-Dev] type categories
24 Aug 2002 11:59:07 -0400
David> It sounds to my C++ ear like you're trying to make this
David> analogous to runtime polymorphism in C++. I think Python's
David> polymorphism is a lot closer to what we do at compile-time in
David> C++, and it should stay that way: no inheritance relationship
David> needed... at least, not on the surface. Here's why: people
David> inevitably discover new type categories in the objects and
David> types they're already using. In C++ this happened when Stepanov
David> et al discovered that built-in pointers matched his mental
David> model of random-access iterators. A similar thing will happen
David> in Python when you make all numbers inherit from Number but
David> someone wants to impose the real mathematical categories (or
David> heck: Integer vs. Fractional) on them.
David> What Stepanov's crew did did was to invent iterator traits,
David> which decouple the type's category from the type itself. Each
David> category is represented by a class, and those classes do have
David> an inherticance relationship (i.e. every random_access_iterator
David> IS-A bidirectional_iterator).
In other words, there *is* an inheritance relationship in C++'s
compile-time polymorphism, and iterator traits are one way of
expressing that polymorphism.
So we have two desirable properties:
1) Guido's suggestion that interface specifications are
close enough to classes that they should be classes,
and should be inherited like classes, possibly with
a way of hiding that inheritance for special cases;
2) Dave's suggestion that people other than a class
author might wish to make claims about the interface
that the class supports.
I now remember that in one of my earlier messages, I said something
related to (2) as well.
Is there a way of merging these two ideas?
Andrew Koenig, firstname.lastname@example.org, http://www.research.att.com/info/ark