InfiniteTraversable.hs

import Control.Applicative
import Control.Monad
import Control.Monad.IO.Class
import System.IO.Unsafe

newtype LazyIO a = LazyIO { runLazyIO :: IO a }

instance Monad LazyIO where
  return a = LazyIO (return a)
  LazyIO x >>= f = LazyIO $ unsafeInterleaveIO $ x >>= runLazyIO . f

instance Applicative LazyIO where
  pure  = return
  (<*>) = ap

instance Functor LazyIO where
  fmap = liftM

instance MonadIO LazyIO where
  liftIO = LazyIO

-- try
--
-- > runLazyIO simpleExample >>= print
simpleExample :: LazyIO ()
simpleExample = do
    a <- liftIO (print "computing a") >> return 1
    b <- liftIO (print "computing b") >> return 2
    liftIO $ print $ a + b

-- try
--
-- > runLazyIO hereWeGo >>= print
hereWeGo :: LazyIO ()
hereWeGo = do
   xs <- mapM (\x -> liftIO (print x) >> return x) [1..]
   liftIO (print (take 10 xs))