I've been playing around with Control.Concurrent.Chan today. Something seems a bit off:

-- Bug.hs
import Control.Concurrent (forkIO, yield)
import Control.Concurrent.Chan
import Data.List (elem)
import Control.Exception
import Control.Concurrent.MVar

data Ex = Ex deriving Show
instance Exception Ex

main = do
  ch <- newChan
  sync1 <- newEmptyMVar
  sync2 <- newEmptyMVar
  forkIO $ do {writeList2Chan ch [1..3*10^6 :: Int]; putMVar sync1 ()}
  yield
  writeChan ch (-12)
  cont <- getChanContents ch

  tid <- forkIO $ do
           evaluate (last cont)
           putMVar sync2 ()

  yield
  throwTo tid Ex


  print (elem (3*10^6) cont)
  takeMVar sync1
  tryTakeMVar sync2

When I run this single-threaded (+RTS -N1), it prints

Bug: Ex
Bug: Ex

One of the thunks in the lazy list gets overwritten by the (asynchronous) exception. This seems a bit surprising; is it the way it should be? Does hGetContents do this too?