| This is a proposal for a language extension which will hopefully mitigate the | issues holding back evolution of the standard prelude as well as provide | useful class abstraction capabilities in general.

A short summary would be "type synonyms for class constraints". You'd definitely want the syntax to look as much like a type synonym decl as possible.

I've considered this before, but never done anything about it because superclasses are so close. Specifically, what is the difference between

(i) class (C a, D a) => CD a and (ii) class alias CD a = (C a, D a)

Note that (i) is Haskell 98.

* In both cases one can write f :: (CD a) => ... instead of the more voluminous f :: (C a, D a)

* However with (i), for each type T one must write instance C T where { ...meths for C... } instance D T where { ...meths for D... } instance CD T where {}

whereas with (ii) one can write instance CD T where { ...meths for C... ...meths for D... }

I believe that this latter is the sole difference. Am I right? [Implementation aspects aside.... with (i) GHC will pass one dictionary CD containing a pair of dictionaries, one for C and one for D.]

If so, than rather than invent a whole new mechanism, why not simply extend the existing superclass mechanism to allow a single instance decl to declare instances for several classes? For example, one add to Haskell 98 the following: an instance declaration for a class CD with superclasses C and D may give the instances for its superclasses C and D

[One could quibble about details. E.g Should the class decl for CD *say* whether the instance decl *must* contain decls for the superclass methods? Or can one vary it on a instance-by-instance basis, which might be more flexible?]

Anyway, my main point it: would a smaller change not suffice?

Simon