Re: [Haskell-cafe] Stacked return

Hi Dmitry, Perhaps you just want monad transformers [1, 2]. If you're not familiar with them, you should probably read [3]. With transformers you'd be able to do this (and more): $ return 2 :: IO Int $ return 2 :: ListT Maybe Int -- this works like Maybe [Int] $ return 2 :: MaybeT [] Int -- this works like [Maybe Int] $ runListT $ return 2 :: Maybe [Int] Just [2] [1] http://hackage.haskell.org/package/transformers [2] http://hackage.haskell.org/package/mtl [3] http://web.cecs.pdx.edu/~mpj/pubs/springschool.html 2013/11/23 Dmitry Bogatov

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.

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