PROPOSAL: Make Applicative a superclass of Monad

Ashley Yakeley ashley at
Tue Jun 24 21:16:54 EDT 2008

I had proposed this to the GHC Trac, but it was pointed out that it 
would break Haskell 98. That proposal has been closed.

Make Applicative (in Control.Applicative) a superclass of Monad (in 
Control.Monad). Rename members of Applicative and other functions, to 
avoid unnecessary duplication. Generalise types of certain existing 
functions, as appropriate.

For example:

class Functor f => Applicative f where
   return :: a -> f a
   ap :: f (a -> b) -> f a -> f b
   (>>) :: f a -> f b -> f b
   (>>) = liftA2 (const id)

liftA2 :: Applicative f => (a -> b -> c) -> f a -> f b -> f c
liftA2 f a b = ap (fmap f a) b
-- etc.

class Applicative m => Monad m where
   (>>=) :: m a -> (a -> m b) -> m b
   fail :: String -> m a
   fail s = error s

Ashley Yakeley

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