Hi,
I found a workaround for this problem.
https://github.com/sol/doctest-haskell/issues/57
--Kazu
> 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 ()
>
>
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