Re: [Haskell-cafe] class default method proposal

11 Dec 2007

      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