A bug of linebuffering

[Kazu Yamamoto ( 山本和彦 )]

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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