On page 141 of "Yet another Haskell Tutorial" (9.7 Monad Transformers) mapTreeM action (Leaf a) = do lift (putStrLn ("Leaf" ++ show a)) b <- action a return (Leaf b) mapTreeM :: (MonadTrans t, Monad (t IO), Show a) => (a -> t IO a1) -> Tree a -> t IO (Tree a1) Why does the type signature of mapTreeM look like this? And what does it mean by "The lift tell us that we're going to be executing a command in an enclosed monad. In this case the enclosed monad is IO"? Why does the author put lift here? How does the lift work? I have no idea about the explanation in the book...is there anyone can give me some hints about this? Thank you in advance! _________________________________________________________________ Messenger10年嘉年华，礼品大奖等你拿！ http://10.msn.com.cn