[Haskell] Monads vs. continuations

[Gregory Woodhouse]

Newbie alert:

I have some superficial familiarity with continuations as they occur  
in traditional denotational semantics, but certainly not a deep  
understanding. I also have a special interest in distributed and/or  
concurrent processing, so it only seemed natural to me that the monad  
concept was one I'd want to confront head on. Now, I warn you: I am  
quite new to Haskell (though I have some prior exposure to Scheme and  
a background in mathematics). Now, I've just started reading through  
Thompson's text and came across this example:

goUntilEmpty :: IO ()
goUntilEmpty
= do line <- getLine
          if (line == [])
            then return ()
           else (do putStrLn line
                          goUntilEmpty)

Okay, this seems sensible enough. Loosely speaking, I see this code  
as getting a line, checking to see if it's empty. If it is, it just  
quits (returning the "empty" value). Otherwise, it prints line, and  
invokes itself through a *new* do statement. That seems awfully like  
using a continuation to track the input stream as part of the  
environment. But it seems obvious to me that here is something I'm  
not understanding here. I think of the do as providing an appropriate  
continuation (one in which the line just read is gone) to pass to the  
next call.

Okay, maybe I'm taking things a bit too literally here, but I seem to  
recall that a monad is an algebraic object with right and left  
identity and an associative composition. I understand the monad here  
takes a value (()) and returns an object IO (), and do becomes a  
functor of sorts, taking ordinary functions and mapping them to new  
functions having their codomain in a new category (an instance of a  
monad?) This is where it seems to me that I must be getting the  
terminology wrong. Can someone help me out here?


===
Gregory Woodhouse
gregory.woodhouse at sbcglobal.net

"The most incomprehensible thing about
the world is that it is at all comprehensible."
  --Albert Einstein (1879-1955)