There and back again

gistfile1.hs

import Data.Profunctor
import qualified Data.Set as S
import Control.Monad.Trans (liftIO)
import Control.Monad.Trans.State

type MyMonad r = StateT (S.Set Int) IO r

runMyMonad :: MyMonad r -> IO r
runMyMonad m = evalStateT m S.empty

newtype ID = ID Int 

create :: Int -> MyMonad ID 
create x = do
    modify (S.insert x)
    liftIO ∘ print $ x 
    get >>= liftIO ∘ print
    return (ID x)

destroy :: ID -> MyMonad ()
destroy (ID x) = do
    modify (S.delete x)
    liftIO ∘ print $ x 
    get >>= liftIO ∘ print

wrap :: Int -> MyMonad () -> MyMonad ()
wrap x a = dimap create (>>= destroy) (λb -> b >>= λy -> a >> return y) x

testFold :: [Int] -> MyMonad() -> MyMonad ()
testFold xs a = foldr wrap a xs

testAction :: MyMonad ()
testAction = liftIO (putStrLn "Done")