Exceptions not being caught by 'try' - exception

Question

I'm using hedis and trying to handle the case of the server going dead. According to the documentation:
Connection to the server lost:
In case of a lost connection, command functions throw a ConnectionLostException. It can only be caught outside of runRedis.
So I would assume I want to catch the ConnectionLostException. However, whilst I can seem to catch it correctly, it seems to also bubble up to the top, and I'm not sure why. Here's some code (just running in GHCI):
:set -XOverloadedStrings
import Database.Redis
import Control.Exception
conn <- connect defaultConnectInfo
runRedis conn $ ping
Now, if I kill the redis server between making the connection and running the command, I get the result I expect:
&langle;interactive&rangle;: ConnectionLost
*** Exception: ConnectionLost
So instead I try to do the following (I added >>= evaluate in order to attempt to force evaluation of the error, but it made no difference):
let tryR = try :: IO a -> IO (Either ConnectionLostException a)
tryR . (>>= evaluate) . runRedis conn $ ping
This gives me:
Left Con: ConnectionLost
nectionLost
So I'm getting the Left result as expected, but halfway through the exception is also presumably being caught and displayed by GHCI. Is this a problem with things not being evaluated?

Answer 1

Like John hinted, there seems to be somthing that prints this message to stderr.
Consider this example:
{-# LANGUAGE OverloadedStrings #-}
import Control.Concurrent (threadDelay)
import Control.Exception
import Control.Monad
import Database.Redis
import System.Mem
tryR :: IO a -> IO (Either ConnectionLostException a)
tryR = try
main :: IO ()
main = do
conn <- connect defaultConnectInfo
loop conn
putStrLn $ "exiting gracefully after counting up some numbers"
performGC
forM_ [1..10] $ \i -> do
print i
threadDelay 10000 -- 0.05 seconds
where
loop conn = do
e <- tryR . (>>= evaluate) . runRedis conn $ ping
case e of
Right x -> do print x
threadDelay 1000000
loop conn
Left err -> do putStrLn $ "tryR caught exception: " ++ show err
It prints:
Right Pong
Right Pong <-------------- after this I Ctrl-C the redis server
tryR caught exception: ConnectionLost
exiting gracefully after counting up some numbers
1
test: ConnectionLost
2
3
4
5
6
7
8
9
10
This looks like something in the stack is printing this test: ConnectionLost (or test.hs: ConnectionLost if you use GHCI/runghc) asynchronously.
If that's GHC, that is probably a bug, but chances are high it is done by hedis or one of its dependencies (I haven't found it in hedis itself yet).