How to understand/use Haskell fix function

Question

I see the following code in xmonad package:

-- | Ignore SIGPIPE to avoid termination when a pipe is full, and SIGCHLD to
-- avoid zombie processes, and clean up any extant zombie processes.
installSignalHandlers :: MonadIO m => m ()
installSignalHandlers = io $ do
    installHandler openEndedPipe Ignore Nothing
    installHandler sigCHLD Ignore Nothing
    (try :: IO a -> IO (Either SomeException a))
      $ fix $ \more -> do
        x <- getAnyProcessStatus False False
        when (isJust x) more
    return ()

It seems that this fix functions comes from Data.Function

But I can't understand how it's used here and when would someone use this fix function?

Answer 1

fix is the fundamental tool used for implementing recursion. It can always be replaced by a recursive let -- and vice versa, a recursive let can be turned into a call to fix. In this example,

fix $ \more -> do
    x <- getAnyProcessStatus False False
    when (isJust x) more

is equivalent to

let more = do
        x <- getAnyProcessStatus False False
        when (isJust x) more
in more

and is essentially implementing a loop that calls getAnyProcessStatus False False until it returns Nothing.