Simon Peyton-Jones wrote:
Is this the most up-to-date description of the proposal? http://repetae.net/recent/out/classalias.html
Has anyone looked at my (confusingly named and horribly written) variant? http://haskell.org/haskellwiki/Superclass_defaults My idea is to split class aliases into two separate things: 1. Superclass defaults: allow a class declaration to contain defaults for methods from superclasses. Allow an instance declaration to be for multiple classes at once, using the most specific defaults: instance (FooBar a, Foo a, Bar a) -- pick the defaults from FooBar, since it is a subclass -- of both Foo and Bar. 2. Class aliases: simply an alias for zero or more classes. class alias FooAndBar a = (Foo a, Bar a) In a context FooAndBar a is simply replaced by (Foo a, Bar a). For instantiation purposes the class alias could override the default methods as if it was a subclass of Foo and Bar. No new methods can be added. Alternatively class aliases could be just macros, like type synonyms. Part 1 applies for instance to the Functor/Applicative/Monad hierarchy, fmap could have a default in terms of (>>=). Part 2 is useful for splitting classes up into smaller bits. Twan