Simon Peyton-Jones wrote:
b) having instance declarations silently spring into existence
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
Without the automatic search, this is already possible
class Functor f where
fmap :: (a -> b) -> f a -> f b
class Functor m => Monad m where
return :: a -> m a
(>>=) :: m a -> (a -> m b) -> m b
-- aka liftM
fmapDefault :: Monad m => (a -> b) -> m a -> m b
fmapDefault f m = m >>= (return . f)
instance Monad [] where
return x = [x]
(>>=) = flip concatMap
instance Functor [] where
fmap = fmapDefault
fmap is already written for you, the instance declaration is only
boilerplate. I first saw this in Data.Traversable .
Regards,
apfelmus
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