[Haskell-cafe] Is there a StateT but with resulting pair swapped?

[Viktor Dukhovni]

On Thu, Feb 25, 2021 at 01:08:41PM -0800, Javran Cheng wrote:

> I'm playing with alex recently and noticed that:
> 
> newtype Alex a
>   = Alex {unAlex :: AlexState -> Either String (AlexState, a)}
> 
> which is almost StateT AlexState (Except String), and looks like I can make
> a MonadError instance out of it.
> 
> Therefore I'm curious if there's any existing package that has newtype
> StateT' s m a = StateT' (s -> m (s, a)) -- note the swapped pair here

I am not aware of any, so I gave a go at writing a prototype of a StateT
that's agnostic as to the order in which the pair elements are stored,
allowing the Alex type to be coerced to the variant that's stored in
"reverse" order as (s, a).  The use of coercion requires that all the
relevant constructors be in scope, even those you're not otherwise
explicitly using.

Running the below program (also attached):

    module Main (main) where

    import Data.Coerce (coerce)
    import Control.Monad.Identity (Identity(..))
    import Control.Monad.Trans.Except (ExceptT(..), Except, runExcept)

    import qualified GStateT as GS
    import GStateT ( GStateT(..), Swap(..) )

    -- Fake Alex with state Int
    type AlexState = Int
    newtype Alex a = Alex {unAlex :: AlexState -> Either String (AlexState, a) }

    -- Alias the swapped GStateT as StateT
    type StateT = GS.SwapStateT
    runStateT :: Monad m => StateT s m a -> s -> m (s, a)
    runStateT m = GS.runSwapStateT m

    main :: IO ()
    main = do
        let x = (Alex $ \i -> Right (i, i+1)) :: Alex Int
            y = coerce x :: StateT AlexState (Except String) Int
        print $ unAlex x 1
        print $ runExcept $ runStateT y 1
        print $ runExcept $ GS.execStateT y 1
        print $ runExcept $ GS.evalStateT y 1

prints:

    Right (1,2)
    Right (1,2)
    Right 1
    Right 2

showing that all the pieces appear to fit togher.  I've not implemented
MTL-style class instances, but those could also be added.

-- 
    Viktor.
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