[Haskell-cafe] Stacked return

[Nickolay Kudasov]

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 <KAction at gnu.org>

>
> 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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