{- An extended version of the representation of a monad transformer as a class, 
   see Mark P. Jones's article in Meijers/Jeuring (Eds.): 
   "Advanced Functional Programming...", Berlin/Heidelberg 1995
                                                                  -}
module MonadT
where

class MonadT t where
    lift  :: Monad m => m a -> t m a
    start :: Monad m => t m a -> m a -- performs the calcuation represented by (
t m a) in m
    
{-- Liftable is intended to map "lift" to an ADT which encapsulates
    the characteristic monadic operations. 
    mapLift === fmap (lift .) if l could be declared as a Functor.
                                                                             --}
class Liftable l where 
    mapLift :: (MonadT t, Monad m) => l m  -> l (t m)