PROPOSAL: Make Applicative a superclass of Monad

Ashley Yakeley ashley at
Tue Jun 24 05:15:19 EDT 2008

Make Applicative (in Control.Applicative) a superclass of Monad (in 
Control.Monad). Rename members of Applicative and other functions, to 
avoid unnecessary duplication.

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

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

(>>) :: (Applicative f) => f a -> f b -> f b
(>>) = liftA2 (const id)

-- etc.

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

I do not have a patch for this change, as it involves mucking around 
with GHC.Base, and I strongly suspect also fiddling with the GHC source. 
I'd like to know if it's feasible first.

Discussion period: a month?

Ashley Yakeley

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