module Main (main) where

import Control.Monad
import Control.Monad.Reader
import Control.Monad.State.Lazy
import Control.Monad.Trans.Maybe
import Data.Maybe (catMaybes)

liftMaybeT :: Monad m => m a -> MaybeT m a
liftMaybeT act = MaybeT $ Just `liftM` act

type ResultM = MaybeT (State [()])
-- type ResultM = MaybeT (Reader [()])

runComputation :: ResultM a -> (Maybe a)
runComputation r = (flip evalState $ [(), undefined]) . runMaybeT $ r
-- runComputation r = (flip runReader $ [(), undefined]) . runMaybeT $ r

runFunction :: ResultM ()
runFunction = do
  units <- get
  -- units <- ask
  units' <- liftMaybeT . fmap catMaybes . traverse runMaybeT $ fmap do_return units
  msum $ fmap (const $ return ()) units'
  where
    do_return () = return ()

main :: IO ()
main = do
  putStrLn "Running once"
  putStrLn . show . runComputation $ do
    runFunction
  putStrLn "Running twice"
  putStrLn . show . runComputation $ do
    runFunction
    runFunction
  putStrLn "Done"
  return ()