[Haskell-cafe] Stacked return

[Antonio]

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances,
  FlexibleContexts, OverlappingInstances, TypeFamilies #-}


class Monad m => Ret a m b where
    returnN :: a -> m b

instance (Monad m, a ~ b) => Ret a m b where
    returnN = return

instance (Monad m, Monad n, Ret a m b) => Ret a n (m b) where
    returnN = return . returnN


boo :: [[[Maybe [Either () [Int]]]]]
boo = returnN 0


Dmitry Bogatov <KAction at gnu.org> writes:

> Hi, list!
>
> I want to write function, that will stack `return` as much times, as
> necessery. In code, I want
> <$> magicLift 2 :: IO Int
> <$> magicLift 2 :: Maybe [Int]
> both be valid.
>
> My best approach is following (not work)
>
>     {-# LANGUAGE FlexibleInstances #-}
>     {-# LANGUAGE UndecidableInstances #-}
>
>     class Monad m => MonadS m where
>         liftS :: a -> m a
>
>     instance (Monad m) => MonadS m where
>         liftS = return
>
> but
> <$> :t liftS 2
> liftS 2 :: (Monad m, Num a) => m a
>
> What would you suggest?
>
> --
> Best regards, Dmitry Bogatov <KAction at gnu.org>,
> Free Software supporter and netiquette guardian.
> 	git clone git://kaction.name/rc-files.git --depth 1
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