[Python-Dev] type categories

[Martijn Faassen]

Andrew Koenig wrote:
> >> The category names look like general purpose interface names. The
> >> addition of interfaces has been discussed quite a bit. While many
> >> people are interested in having interfaces added to Python, there
> >> are many design issues that will have to be resolved before it
> >> happens.
> 
> Oren> Nope. Type categories are fundamentally different from
> Oren> interfaces.  An interface must be declared by the type while a
> Oren> category can be an observation about an existing type.
> 
> Why?  That is, why can't you imagine making a claim that type
> X meets interface Y, even though the author of neither X nor Y
> made that claim?

That's entirely possible, and as Guido mentioned earlier in the thread,
the Zope 3 interface package allows that. I think that still currently
doesn't work with built-in types yet, but that's an implementation detail,
not a fundamental problem.

(it's in Interface.Implements, the implements() function)
 
> However, now that you bring it up... One difference I see between
> interfaces and categories is that I can imagine categories carrying
> semantic information to the human reader of the code that is not
> actually expressed in the category itself.  As a simple example,
> I can imagine a PartialOrdering category that I might like as part
> of the specification for an argument to a sort function.

But isn't that exactly what interfaces are? Of course you may not want
to make all interfaces explicit as it is too much programming overhead;
that's in part what's nice about a dynamically typed language. However,
an interface does carry semantic information to the human reader of the
code that is not actually expressed in the category itself. By making
interfaces explicit the human reader can also write code that introspects
interface information.

Or do you mean sometimes it is not useful to make interfaces explicit
at all, as you're never going to introspect on them anyway? I'd say
they may still be useful as documentation, in which case they seem to work
like your 'category'. Or of course you can not specify them at all.

Regards,

Martijn