Re: [Haskell-cafe] Stacked return

{-# 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 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 , Free Software supporter and netiquette guardian. git clone git://kaction.name/rc-files.git --depth 1 GPG: 54B7F00D Html mail and proprietary format attachments are forwarded to /dev/null. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe