skaller wrote:
> 
> It's hard to say, because in OO terms Haskell typeclasses
> are so weak it's surprising they're any use at all.

Not sure how you come to that conclusion. Both concepts are somewhat 
incomparable, but there are a number of axes along which type classes 
are much more expressive than OO classes.

Ignoring implementation inheritance, I claim that in combination with 
existential types, even relatively basic type classes are strictly more 
powerful than OO classes (modulo some lack of syntactic sugar).

Have a look at Oleg's paper.

> C++ classes can also provide state and inheritance.
> The state looks vaguely like dependent typing to me .. :)

I grant you (implementation) inheritance, but I don't understand the 
rest of your comment. State is orthogonal to classes. Surely you can 
define a type class that contains a, say, IORef (for per-class state). 
Likewise, you can instantiate type classes to stateful types (as the 
equivalent to per-object (mutable) state).

> OK, so now we have a much more powerful model ..

I don't think so.

> yet
> it has known limitations, in particular the so-called
> 'covariance' problem prevents OO from modelling any
> serious kind of system: for example a system
> with binary operators doesn't admit an OO representation.

Which is one of the problems type classes do not have, because qualified 
types *are* more expressive than subtyping in that respect.

AFAICS, the rest of your argument hence does not follow.

-- 
Andreas Rossberg, rossberg@ps.uni-sb.de