[Haskell-cafe] class default method proposal
Twan van Laarhoven
twanvl at gmail.com
Wed Dec 12 13:18:11 EST 2007
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,
More information about the Glasgow-haskell-users