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
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