On Dec 11, 2007 1:29 PM, apfelmus
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 .
This is pretty much how I define Functor and Applicative instances for my
monads. It is admittedly irritating to have to write out the boilerplate,
but it doesn't seem irritating enough to require a language extension to
eliminate.
--
Dave Menendez