Is do-notation specific to "base:GHC.Base.Monad"?
- by yairchu
The idea that the standard Monad class is flawed and that it should actually extend Functor or Pointed is floating around.
I'm not necessarily claiming that it is the right thing to do, but suppose that one was trying to do it:
import Prelude hiding (Monad(..))
class Functor m => Monad m where
return :: a -> m a
join :: m (m a) -> m a
join = (>>= id)
(>>=) :: m a -> (a -> m b) -> m b
a >>= t = join (fmap t a)
(>>) :: m a -> m b -> m b
a >> b = a >>= const b
So far so good, but then when trying to use do-notation:
whileM :: Monad m => m Bool -> m ()
whileM iteration = do
done <- iteration
if done
then return ()
else whileM iteration
The compiler complains:
Could not deduce (base:GHC.Base.Monad m) from the context (Monad m)
Question:
Does do-notation work only for base:GHC.Base.Monad? Is there a way to make it work with an alternative Monad class?
Extra context:
What I really want to do is replace base:Control.Arrow.Arrow with a "generalized" Arrow class:
{-# LANGUAGE TypeFamilies #-}
class Category a => Arrow a where
type Pair a :: * -> * -> *
arr :: (b -> c) -> a b c
first :: a b c -> a (Pair a b d) (Pair a c d)
second :: a b c -> a (Pair a d b) (Pair a d c)
(***) :: a b c -> a b' c' -> a (Pair a b b') (Pair a c c')
(&&&) :: a b c -> a b c' -> a b (Pair a c c')
And then use the Arrow's proc-notation with my Arrow class, but that fails like in the example above of do-notation and Monad.
I'll use mostly Either as my pair type constructor and not the (,) type constructor as with the current Arrow class. This might allow to make the code of my toy RTS game (cabal install DefendTheKind) much prettier.
