existentials.hs

{-# LANGUAGE ExistentialQuantification, LambdaCase #-}

import Data.IORef
import Control.Monad

data Mover = forall a. Mover (IORef [a]) (IORef [a])

doMove :: Mover -> IO ()
doMove (Mover left right) = do
    readIORef left >>= \case
        [] -> return ()
        (x:xs) -> do
            writeIORef left xs
            modifyIORef right (x:)

main = do
    a1 <- newIORef ["foo", "bar"]
    a2 <- newIORef []
    b1 <- newIORef [1, 2, 6]
    b2 <- newIORef []

    let movers :: [Mover]
        movers = [Mover a1 a2, Mover b1 b2]

    forM_ movers doMove

    readIORef a1 >>= putStrLn . show
    readIORef a2 >>= putStrLn . show
    readIORef b1 >>= putStrLn . show
    readIORef b2 >>= putStrLn . show