On 19 July 2010 16:07, David McBride <dmcbride@neondsl.com> wrote:

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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