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On page 141 of "Yet another Haskell Tutorial" (9.7 Monad Transformers)<br><br>mapTreeM action (Leaf a) = do<br> lift (putStrLn ("Leaf" ++ show a))<br> b <- action a<br> return (Leaf b)<br> <br>mapTreeM :: (MonadTrans t, Monad (t IO), Show a) => (a -> t IO a1) -> Tree a -> t IO (Tree a1)<br><br>Why does the type signature of mapTreeM look like this?<br>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?<br><br>I have no idea about the explanation in the book...is there anyone can give me some hints about this?<br>Thank you in advance!<br><br /><hr />立刻下载 MSN 保护盾，保障Messenger 安全稳定！ <a href='http://im.live.cn/safe/' target='_new'>现在就下载！</a></body>
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