removed all uses of undefined from code base

It's a code smell, can lead to hard to diagnose error messages.

Assistant/Pairing.hs

@@ -81,6 +81,8 @@ data PairingInProgress = PairingInProgress
 	}
 	deriving (Show)
 
+data AddrClass = IPv4AddrClass | IPv6AddrClass
+
 data SomeAddr = IPv4Addr HostAddress
 {- My Android build of the Network library does not currently have IPV6
  - support. -}

Assistant/Pairing/Network.hs

@@ -33,9 +33,9 @@ pairingPort = 55556
 {- Goal: Reach all hosts on the same network segment.
  - Method: Use same address that avahi uses. Other broadcast addresses seem
  - to not be let through some routers. -}
-multicastAddress :: SomeAddr -> HostName
-multicastAddress (IPv4Addr _) = "224.0.0.251"
-multicastAddress (IPv6Addr _) = "ff02::fb"
+multicastAddress :: AddrClass -> HostName
+multicastAddress IPv4AddrClass = "224.0.0.251"
+multicastAddress IPv6AddrClass = "ff02::fb"
 
 {- Multicasts a message repeatedly on all interfaces, with a 2 second
  - delay between each transmission. The message is repeated forever
@@ -62,7 +62,7 @@ multicastPairMsg repeats secret pairdata stage = go M.empty repeats
 	sendinterface cache i = void $ tryIO $
 		withSocketsDo $ bracket setup cleanup use
 	  where
-		setup = multicastSender (multicastAddress i) pairingPort
+		setup = multicastSender (multicastAddress IPv4AddrClass) pairingPort
 		cleanup (sock, _) = sClose sock -- FIXME does not work
 		use (sock, addr) = do
 			setInterface sock (showAddr i)

Assistant/Threads/Committer.hs

@@ -196,7 +196,7 @@ maxCommitSize :: Int
 maxCommitSize = 5000
 
 {- Decide if now is a good time to make a commit.
- - Note that the list of changes has an undefined order.
+ - Note that the list of changes has a random order.
  -
  - Current strategy: If there have been 10 changes within the past second,
  - a batch activity is taking place, so wait for later.

Assistant/Threads/PairListener.hs

@@ -31,7 +31,7 @@ pairListenerThread urlrenderer = namedThread "PairListener" $ do
   where
 	{- Note this can crash if there's no network interface,
 	 - or only one like lo that doesn't support multicast. -}
-	getsock = multicastReceiver (multicastAddress $ IPv4Addr undefined) pairingPort
+	getsock = multicastReceiver (multicastAddress IPv4AddrClass) pairingPort
 		
 	go reqs cache sock = liftIO (getmsg sock []) >>= \msg -> case readish msg of
 		Nothing -> go reqs cache sock

Assistant/Threads/XMPPPusher.hs

@@ -78,4 +78,5 @@ selectNextPush lastpushedto l = go [] l
 		(Pushing clientid _)
 			| Just clientid /= lastpushedto -> (m, rejected ++ ms)
 		_ -> go (m:rejected) ms
-	go [] [] = undefined
+	go [] [] = error "empty push queue"
+