monadTransformers.hs

import Control.Monad.Trans.Maybe
import Control.Monad.Trans.Either

safeHead :: [a] -> Maybe a
safeHead [] = Nothing
safeHead xs = Just $ head xs

maybeTHead :: MaybeT IO Char
maybeTHead = MaybeT $ do
  str <- getUserInput
  let head1 = safeHead str
  return head1

getUserInput = getLine

returnHeads :: IO (Maybe Char)
returnHeads = do
  heads <- runMaybeT $ maybeTHead
  return heads

maybeHead :: [a] -> Maybe (Either String a)
maybeHead list = do 
  runEitherT $ eitherSafeHead list
  safeHead []

eitherSafeHead :: [a] -> EitherT String Maybe a
eitherSafeHead list = EitherT $ do 
  let result = safeHead list
  case result of 
    Nothing -> return $ Left "empty!!"
    Just x  -> return $ Right x