monads

I understand that the ST monad is something like a little brother of IO, which in turn is the state monad with added RealWorld magic. I can picture states and I can picture that RealWorld is somehow put into IO, but every time I write a type signature of ST the s of the ST monad confuses me. Take, for example, ST s (STArray s a b) . How does the s work there? Is it just used to build up some artificial data dependency between computations without being able to be referenced like states in the state monad (due to the forall )? I'm just throwing out ideas and would really appreciate someone more

As far as I know, do blocks in Haskell are just some kind of syntactic sugar for monadic bind operators. For example, one could convert main = do f <- readFile "foo.txt" print f print "Finished" to main = readFile "foo.txt" >>= print >> print "Finished" Can all do blocks be converted to bind syntax? What about, for example, this block where f is used multiple times: main = do f <- readFile "foo.txt" print $ "prefix " ++ f print $ f ++ " postfix" Assuming we are in the IO monad, it is not possible to simply execute the readFile computation twice. How can this example (if possible at all)

I wrote the following code. It is working and using the Reader monad. Could you give me some hints about code style in Haskell ? Mainly, I mean monads -- I am newbie. import Control.Monad.Reader data Tree a = Node a (Tree a) (Tree a) | Empty renumberM :: Tree a -> Reader Int (Tree Int) renumberM (Node _ l r) = ask >>= (\x -> return (Node x (runReader (local (+1) (renumberM l)) x) (runReader (local (+1) (renumberM r)) x))) renumberM Empty = return Empty renumber'' :: Tree a -> Tree Int renumber'' t = runReader (renumberM t) 0 I want to show you that your idea is an instance of a more general

UPDATE: Okay this question becomes potentially very straightforward. q <- mapM return [1..] Why does this never return? Does mapM not lazily deal with infinite lists? The code below hangs. However, if I replace line A by line B, it doesn't hang anymore. Alternatively, if I preceed line A by a "splitRandom $", it also doesn't hang. Q1 is: Is mapM not lazy? Otherwise, why does replacing line A with line B "fix this" code? Q2 is: Why does preceeding line A with splitRandom "solve" the problem? import Control.Monad.Random import Control.Applicative f :: (RandomGen g) => Rand g (Double, [Double]) f