Hi, While I support GHC head for "doctest", I encountered the following bug. "doctest" uses a GHCi subprocess to evaluate an expression represented in String. Stderr from GHCi is merged into stdout by hDuplicateTo in the GHCi side. Even evaluating an error expression, for instance "1 `div` 0", the line buffering does not work. "doctest" waits for output from GHCi forever. This does not happen if stderr is not merged into stdout. The following code demonstrates this bug. Running it with GHC head waits forever. Running it with GHC 7.6.3 prints: "*** Exception: divide by zero" "3" If you change "1 `div` 0" into "1 `div` 0\nprint 10", this code run by GHC head prints: "*** Exception: divide by zero" "10" This is a serious behavior change for "doctest". I hope this will be fixed. --Kazu module Main where import System.Process import System.IO myProc :: CreateProcess myProc = (proc "ghc" ["-v0", "--interactive", "-ignore-dot-ghci"]) { std_in = CreatePipe , std_out = CreatePipe , std_err = Inherit } setMode :: Handle -> IO () setMode hdl = do hSetBinaryMode hdl False hSetBuffering hdl LineBuffering newInterpreter :: IO (Handle, Handle) newInterpreter = do (Just stdin_, Just stdout_, _, _) <- createProcess myProc setMode stdin_ setMode stdout_ hPutStrLn stdin_ "import System.IO" hPutStrLn stdin_ "import GHC.IO.Handle" hPutStrLn stdin_ "hDuplicateTo stdout stderr" hFlush stdin_ return (stdin_, stdout_) eval :: Handle -> Handle -> String -> IO String eval hin hout expr = do hPutStrLn hin expr hFlush hin hGetLine hout main :: IO () main = do (stdin_, stdout_) <- newInterpreter eval stdin_ stdout_ "1 `div` 0" >>= print eval stdin_ stdout_ "1 + 2" >>= print return ()