[Haskell-beginners] Hints on how to remove unsafePerformIO from my function?

[David McBride]

I am writing a voip server to take advantage of haskells awesome threading
and parsing ability.  At first everything was going great, but my thread
that fetches udp makes use of unsafePerformIO:

fetchUDPSIP :: TChan B.ByteString -> IO ()
fetchUDPSIP chan = do
  sock <- getUDPSocket 5060
  let results = (unstrict . repeat . getUDP) sock
  mapM_ (atomically . writeTChan chan) results
  where
    unstrict [] = []
    unstrict (x:xs) = unsafePerformIO x:unstrict xs


It fetches it from a socket, and then writes it to a TChan.  This results in
a stream of bytestrings that another thread can read from.  If I don't use
unsafePerformIO, then it tries to read all possible packets before returning
anything, and so it never writes to the TChan at all.  The problem with
doing it this way is that unsafePerformIO apparently stops every other
thread from doing anything while it is waiting for a packet.

But I can't think of a way to rewrite this function to do what I want.  I'm
kind of new to this, does anyone have any hints that could help me out?

Here's a simple version without any of the implementation details:

fetchLine = do
  let results = (unstrict . repeat) getLine
  mapM_ putStrLn results
  where
    unstrict [] = []
    unstrict (x:xs) = unsafePerformIO x:unstrict xs
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