[Python-Dev] type categories

Guido van Rossum guido@python.org
Wed, 14 Aug 2002 09:09:19 -0400

> Type categories are fundamentally different from interfaces.  An 
> interface must be declared by the type while a category can be an 
> observation about an existing type. 

Yup.  (In Python these have often been called "protocols".  Jim Fulton
calls them "lore protocols".)

> Two types that are defined independently in different libraries may
> in fact fit under the same category because they implement the same
> protocol.  With named interfaces they may in fact be compatible but
> they will not expose the same explicit interface. Requiring them to
> import the interface from a common source starts to sound more like
> Java than Python and would introduce dependencies and interface
> version issues in a language that is wonderfully free from such
> arbitrary complexities.

Hm, I'm not sure if you can solve the version incompatibility problem
by ignoring it. :-)

> Python is a dymanic language. It deserves a dynamic type category
> system, not static interfaces that must be declared. It's fine to
> write a class and somehow say "I intend this class to be in category
> X, please warn me if I write a method that will make it
> incompatible". But I don't want declarations to be a *requirement*
> for being considered compatible with a protocol. I have noticed that
> a lots of protocols are defined retroactively by observation of the
> behavior of existing code. There shoudln't be any need to go tag
> someone else's code as conforming to a protocol or put a wrapper
> around it just to be able to use it.

Are you familiar with Zope's Interface package?  It solves this
problem (nicely, IMO) by allowing you to place an interface
declaration inside a class but also allowing you to make calls to an
interface registry that declare interfaces for pre-existing classes.

> A category is defined mathematically by a membership predicate. So
> what we need for type categories is a system for writing predicates
> about types.

Now I think you've lost me.  How can a category on the one hand be
observed after the fact and on the other hand defined by a rigorous
mathematical definition?  How could a program tell by looking at a
class whether it really is an implementation of a given protocol?

> Standard Python expressions should not be used for defining a
> category membership predicate. A Python expression is not a pure
> function. This makes it impossible to cache the results of which
> type belongs to what category for efficiency. Another problem is
> that many different expressions may be equivalent but if two
> independently defined categories use equivalent predicates they
> should *be* the same category.  They should be merged at runtime
> just like interned strings.

Again you've lost me.  I expect there's something here that you assume
well-known.  Can you please clarify this?  What on earth do you mean
by "A Python expression is not a pure function" ?

> About a year ago I worked on a system for predicates having a
> canonical representation for security applications. . While I was
> working on it I realized that it would be perfect for implementing a
> type category system for Python. It would be useful at runtime for
> error detection and runtime queries of protocols. It would also be
> useful at compile time for early detection of some errors and
> possibly for optimization. By implementing an optional strict mode
> the early error detection could be improved to the point where it's
> effectively a static type system.

So let's see a proposal already.  I can't guess what you are proposing
from this description except that you think highly of your own
invention.  I wouldn't expect you to mention it otherwise, so that's 
bits of information. :-)

> Just a quick example of the usefulness of canonical predicates: if I
> calculate the intersection of two predicates and reduce it to
> canonical form it will reduce to the FALSE predicate if no input
> will satisfy both predicates. It will be equal to one of the
> predicate if it is contained by the other.
> I spent countless hours thinking about these issues, probably more than 
> most people on this list...

How presumptuous.

> I think I have the foundation for a powerful yet unobtrusive type
> category system. Unfortunately it will take me some time to put it
> in writing and I don't have enough free time (who does?)

I say vaporware. :-)

Tell us about it when you have time.

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