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Take the following code:

import Control.Concurrent
import Control.Concurrent.MVar
import Control.Exception

main :: IO ()
main = do
    var <- newEmptyMVar
    _ <- forkIO $ do
        res <- try $ newEmptyMVar >>= takeMVar
        putMVar var res
        putStrLn "Successfully filled var"

    res <- readMVar var `onException` do
        putStrLn "Received exception, delaying..."
        threadDelay 1000000
        res <- readMVar var
        putStrLn $ "After delay, res is: " ++ show res

    print (res :: Either SomeException Int)
    putStrLn "Exiting..."

On line 9, an MVar deadlock exception is thrown, which is caught by try. That result is then put into the var MVar. On line 13, I readMVar to get this value. I would expect it to return the exception previously thrown, as a Left value. Instead, a new exception is thrown. I say new, because if I switch over to using STM instead of an MVar, I get a different exception (STM transaction instead of MVar deadlock).

With the program as above, my output is:

Successfully filled var
Received exception, delaying...
After delay, res is: Left thread blocked indefinitely in an MVar operation
async14.hs: thread blocked indefinitely in an MVar operation

I originally filed this as an issue against the async package: https://github.com/simonmar/async/issues/14

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture