# Pastebin ljFJ4GK1

import Control.Monad.Except
import Control.Monad.Cont

-- liftEither is available in mtl >=2.2.2
one :: Either e a -> ExceptT e (ContT r IO) a
one = liftEither

two :: Either e a -> ExceptT e (ContT r IO) a
two   = _                           -- GHC asks for: `Either e a -> ExceptT e (ContT r IO) a`
two e = _                           -- Name the parameter; GHC asks for: `ExceptT e (ContT r IO) a`
two e = ExceptT _                   -- Introduce `ExceptT` constructor; GHC asks for: `(ContT r IO) (Either e a)`
two e = ExceptT $ ContT _           -- Introduce `ContT` constructor; GHC asks for: (Either e a -> IO r) -> IO r
two e = ExceptT $ ContT $ \k -> _   -- GHC asked for a function, so introduce lambda; GHC now asks for: `IO r`
two e = ExceptT $ ContT $ \k -> k _ -- The only thing that returns `IO r` is `k`; GHC now asks for: `Either e a`
two e = ExceptT $ ContT $ \k -> k e -- `e` is the only value of type `Either e a` available to us