An example of a concurrency bug that is upstream from the point of the concurrentMerge.

double_read_WRONG.hs

import qualified System.IO.Streams as S
import System.IO.Streams.Concurrent
import Control.Concurrent
import Data.IORef
import Data.ByteString.Char8 as B

main = do 
  cnt <- newIORef 0
  s1 <- S.makeInputStream $ do
          c <- readIORef (cnt :: IORef Int)
          threadDelay$ 100 * 1000
          writeIORef cnt (c+1)
          if c >= 100
           then return Nothing
           else return$ Just c
  s2 <- S.map (Left) s1
  s3 <- S.map (Right) s1
  m  <- concurrentMerge [s2,s3]
  hd <- S.take 10 m
  ls <- S.toList hd
  print ls