Exceptions not being caught by 'try'

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:

⟨interactive⟩: 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).