"callbacks" in Haskell

[Tom Pledger]

Robert Vollmert writes:
 | Hello,
 | 
 | I've been having a little trouble writing a module that waits for and
 | handles IO events, e.g. by reading from a pipe. It seemed natural to
 | use some form of callbacks here, though that may very well be the
 | wrong approach. I'd be happy to hear of alternatives.
 | 
 | Anyway, I got a callback-based approach to work to some extent, but
 | am not happy with the callback's type.
 :

Can the event and handler inhabit the same monad, say IO?  If so, is
the following the sort of thing you had in mind?

    waitAndHandleOneEvent      :: IO e -> (e -> IO ()) -> IO ()
    waitAndHandleOneEvent ioe h = forkIO (do e <- ioe
                                             h e)

But then, because ioe and h always get combined the same way (ioe >>=
h), we can generalise waitAndHandleOneEvent to

    waitAndDoOneThing   :: IO () -> IO ()
    waitAndDoOneThing io = forkIO io

which is equivalent to using forkIO directly, as in

    do ...
       let ioe :: IO MyEvent
           ioe  = ...
           h   :: MyEvent -> IO ()
           h e  = ...
       forkIO (ioe >>= h)
       ...

HTH.
Tom