Kyle Murphy
Remember that in most cases, once something is "inside" of a monad (IO in this case), it's very difficult, impossible, to get it out again.
I think, this is very misleading. You never get "out of a monad", you get "into" it, and you can do that whenever you want. Remember that the monadic world is inside of the pure world, not the pure world inside of the monadic world. In fact, the monadic world is part of it. The name bound by '<-' is something you can refer to in the pure world. There is nothing difficult about using the result of a monadic computation in the pure world. Just give it a name and refer to it, and that's it.
The do syntax just adds a convenient abstraction so that you don't have to construct complex chains of >>= and >> operations by hand. In our limited example this is fairly trivial, but consider something like:
main = do something <- foo blah <- bar something moreblah <- baz something blah putStrLn moreblah putStrLn "really long chain now"
Pretty straightforward: main = foo >>= \something -> bar something >>= \blah -> baz something blah >>= \moreblah -> putStrLn moreblah >> putStrLn "really long chain now"
which would be translated to:
main = foo >>= (\something -> bar something >>= (\blah -> baz something blah >>= (\moreblah -> putStrLn moreblah >> (putStrLn "really long chain now"))))
You could just as well write: main = do { something <- foo; blah <- bar something; moreblah <- baz something blah; putStrLn moreblah; putStrLn "really long chain now" } You are complicating things artifically. For example there is no reason to have all those parentheses, because (>>=) is associative by definition. The do-notation doesn't make nasty things nice. It makes nice things nicer. Don't be afraid to use raw monadic combinators, because sometimes they are much better: main = getArgs >>= mapM_ (readFile >=> putStr) Much more concise than: main = do files <- getArgs forM_ files $ \file -> do content <- readFile file putStr content Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/