[Python-3000] my take on "typeclasses"

[Talin]

Guido van Rossum wrote:
> On 5/11/06, Talin <talin at acm.org> wrote:
> 
>> The word 'sequence' here doesn't mean an exclusive category. In a sense,
>> all sequences are really mappings - they "map" a range of integers to
>> values.
> 
> 
> No, the requirements for sequence-ness are stronger. The set of ints
> mapped must be exactly range(len(x)). And iterating over the sequence
> is required to return the values in numerical order of the index set
> (unlike iterating over a dict whose keys happen to be a contiguous set
> of ints starting at 0). And for mutable sequences, insertions and
> deletions move all the other keys around -- very different from a
> mutable mapping. And sequences are expected to implement slicing,
> concatenation and repetition.
> 
> In general, there are two possible taxonomies of container types for
> Python: minimal and maximal. The minimal taxonomy attempts to describe
> the minimal features that a  specimen of container ought to have in
> order to be considered a member of a given species of container. It is
> useful for determination but not so useful for prediction of behavior.
> The maximal taxonomy attempts to describe all the features shared by
> (most) members of a species; it's harder to use for determination, but
> more useful for describing expected behavior.
> 
> I think I'm more comfortable with a maximal taxonomy. In a maximal
> taxonomy, I'd describe a large set of invariants, attributes,
> behavior, etc., and say e.g. "this is how a file behaves". A
> particular class can then claim to be a file by explicitly declaring
> this (how that's spelled is a different issue -- it doesn't need to be
> done by inheritance from an abstract base class or interface, it could
> also be an arbitrary property, agreed upon by convention, or an
> external registry of all file types, for example).
> 
> For the purposes of generic/overloaded functions, if we can overload
> on such abstract container species, the declared container species (or
> the "best" one, if multiple species can be declared) should determine
> the implementation chosen -- and damn the torpedoes if the object
> declares it's a file but doesn't implement isatty(). (I.e. I don't
> care, it's still a file, just like a three-legged dog is still a dog.)
> 

OK, this sounds right to me. At least, it sounds like what you are 
describing is what I am asking for :)

-- Talin