[Python-3000] Fw: typeclasses, duck-typing

[Ben.Young at risk.sungard.com]

python-3000-bounces+python=theyoungfamily.co.uk at python.org wrote on 
10/05/2006 17:39:57:

> At 11:21 AM 5/10/2006 +0100, Ben.Young at risk.sungard.com wrote:
> >Have you taken a look at ConceptGCC? (
> >http://www.osl.iu.edu/~dgregor/ConceptGCC/)
> >
> >It's basically a first cut at implementing the Concepts which will be
> >availiable in the next version of C++. They basically provide full 
compile
> >time "duck typing" as well as adaptation for types that don't exactly 
fit
> >the concept. (e.g if the type doesn't have a length method, here's how 
you
> >provide it.
> >
> >It's a very powerfull idea, and probably well worth looking at for
> >inspiration.
> 
> FWIW, I skimmed through the tutorial; it appears that a "Concept" is 
> roughly equivalent to a typeclass in Haskell, and a "Model" is roughly 
> equivalent to a typeclass instance

Hi Phillip,

I don't know much about Haskell, but this sounds roughly right. As I 
understand it, a Concept describes what interface/capabilties a type 
should have, while a Model describes a mapping of types onto Concepts for 
types that don't fit automatically.

For instance, a Model can be used to give all raw pointer types a 
value_type typedef member to allow them to fit the Iterator Concept.

> 
> One of the papers on the site mentions something called "concept-based 
> overloading", which if I'm reading it correctly, is the idea that's at 
the 
> heart of this thread; i.e., it should be possible to say that a 
particular 
> implementation of one operation is dependent upon the availability of an 

> implementation of some other operation.
> 

Yes, C++ previously had compilicated ways of "overloading" template 
implementations based on things like boosts enable_if (you don't want to 
know!), but Concepts allow this to be done in a clear and obvious way.

A dynamic version of concepts (I guess a combo of multi-methods and 
adaptation) would be great for Python, but it's hard to see how it could 
be done simply and efficiently.

Cheers,
Ben

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