A spiral approach to learning: you understand, then you learn more, and then you are more confused than ever.

I recall that a function in the IO Monad just combines actions to make a big action list, and doesn't actually do the side-effect-laden "work" until it is later triggered, normally because the program is defined to evaluate main.

Depending on the time of day and which example I pick, I can sometimes follow what's "really happening" in the definition of >>=.  But here are some specific questions:

In <http://www.haskell.org/haskellwiki/Monads_as_computation>

What is a for-each loop really? It's something which performs some action based on each
element of a list. So we might imagine a function with the type:

forM :: (Monad m) => [a] -> (a -> m b) -> m [b]

(as an added bonus, we'll have it collect the results of each iteration).

We can write this with sequence and map:

forM xs f = sequence (map f xs)

we apply the function to each element of the list to construct the action for that
iteration, and then sequence the actions together into a single computation.

So map by itself produces a [m b].  Why does it need to be turned into m [b]?  What does the 'sequence' accomplish, other than restructuring the results that already exist?

The reason I asked about (#⋯#) is because I wanted to see what IO was really doing, to see what the difference was between using >>= initially and then somehow "cranking" it later.

<http://hackage.haskell.org/package/base-4.6.0.1/docs/src/GHC-Base.html#%3E%3E%3D> lists

instance  Monad IO  where
    {-# INLINE return #-}
    {-# INLINE (>>)   #-}
    {-# INLINE (>>=)  #-}
    m >> k    = m >>= \ _ -> k
    return    = returnIO
    (>>=)     = bindIO
    fail s    = failIO s

returnIO :: a -> IO a
returnIO x = IO $ \ s -> (# s, x #)

bindIO :: IO a -> (a -> IO b) -> IO b
bindIO (IO m) k = IO $ \ s -> case m s of (# new_s, a #) -> unIO (k a) new_s

thenIO :: IO a -> IO b -> IO b
thenIO (IO m) k = IO $ \ s -> case m s of (# new_s, _ #) -> unIO k new_s

unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #))
unIO (IO a) = a

where bindIO is the function of interest.  In a chain of commands, getLine might be the 'k' argument to bindIO.  Somewhere there's a real machine function called to do the reading from the file, right?




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