Simon Peyton-Jones wrote:
Concerning (b) here's a suggestion. As now, require that every instance requires an instance declaration. So, in the main example of http://haskell.org/haskellwiki/Class_system_extension_proposal, for a new data type T you'd write instance Monad T where return = ... (>>=) = ...
instance Functor T instance Applicative T
Another alternative is to allow multiple classes in an instance declaration: instance (Monad T, Functor T, Applicative T) where return = ... (>>=) = ... The advantage is that this makes it more clear where the instances come from, especially if a class has multiple sub classes with different defaults. It also eliminates tricky issues with importing. Of course this needs some (albeit very little) new syntax. I wrote a proposal a while ago, http://haskell.org/haskellwiki/Superclass_defaults Twan