Unicode console I/O in Haskell on Windows

It seems rather difficult to get console I/O to work with Unicode characters in Haskell under Windows. Here is the tale of woe:

  1. (Preliminary.) Before you even consider doing Unicode I/O in the console under Windows, you need to make sure that you're using a console font that can render the characters you want. The raster fonts (the default) have infinitely poor coverage (and don't allow copy-pasting of characters they can't represent), and the TrueType options MS provides (Consolas, Lucida Console) have not-great coverage (though these will allow copy-pasting of characters they cannot represent). You might consider installing DejaVu Sans Mono (follow the instructions at the bottom here; you may have to reboot before it works). Until this is sorted, no apps will be able to do much Unicode I/O; not just Haskell.
  2. Having done this, you will notice that some apps will be able to do console I/O under Windows. But getting it to work remains quite complicated. There are basically two ways to write to the console under Windows. (What follows is true for any language, not just Haskell; don't worry, Haskell will enter the picture in a bit!)...
  3. Option A is to use the usual c-library style byte-based i/o functions; the hope is that the OS will interpret these bytes according to some encoding which can encode all the weird and wonderful characters you want. For instance, using the equivalent technique on Mac OS X, where the standard system encoding is usually UTF8, this works great; you send out UTF8 output, you see pretty symbols.
  4. On Windows, it works less well. The default encoding that Windows expects will generally not be an encoding covering all the Unicode symbols. So if you want to see pretty symbols this way, one way or another, you need to change the encoding. One possibility would be for your program to use the SetConsoleCP win32 command. (So then you need to bind to the Win32 library.) Or, if you'd rather not do that, you can expect your program's user to change the code page for you (they would then have to call the chcp command before they run your program).
  5. Option B is to use the Unicode-aware win32 console API commands like WriteConsoleW. Here you send UTF16 directly to Windows, which renders it happily: there's no danger of an encoding mismatch because Windows always expects UTF16 with these functions.

Unfortunately, neither of these options works very well from Haskell. First, there are no libraries that I know of that use Option B, so that's not very easy. This leaves option A. If you use Haskell's I/O library (putStrLn and so on), this is what the library will do. In modern versions of Haskell, it will carefully ask Windows what the current code page is, and output your strings in the proper encoding. There are two problems with this approach:

  • One is not a showstopper, but it is annoying. As mentioned above, the default encoding will almost never encode the characters you want: you need the user to change to an encoding that does. Thus your user needs to chcp cp65001 before they run your program (you may find it distasteful to force your users to do this). Or, you need to bind to SetConsoleCP and do the equivalent inside your program (and then use hSetEncoding so that the Haskell libraries will send output using the new encoding), which means you need to wrap the relevant part of the win32 libraries to make them Haskell-visible.
  • Much more seriously, there is a bug in Windows (resolution: won't fix) which leads to a bug in Haskell which means that if you have selected any code page like cp65001 which can cover all of Unicode, Haskell's I/O routines will malfunction and fail. So essentially, even if you (or your user) set the encoding properly to some encoding that covers all the wonderful Unicode characters, and then 'do everything right' in telling Haskell to output things using that encoding, you still lose.

The bug listed above is still unresolved and listed as low priority; the basic conclusion there is that Option A (in my classification above) is unworkable and one needs to switch to Option B to get reliable results. It is not clear what the timeframe will be for this to be resolved, as it looks like some considerable work.

The question is: in the meantime, can anyone suggest a workaround to allow the use of Unicode console I/O in Haskell under Windows?

See also this Python bug tracker database entry, grappling with the same problem in Python 3 (fix proposed, but not yet accepted into the codebase), and this Stack Overflow answer, giving a workaround for this problem in Python (based on 'option B' in my classification).


  • I thought I would answer my own question, and list as one possible answer, the following, which is what I'm actually doing at the moment. It is quite possible that one can do better, which is why I'm asking the question! But I thought it would make sense to make the following available to people. It's basically a translation from Python to Haskell of this python workaround for the same issue. It uses 'option B' mentioned in the question.

    The basic idea is that you create a module IOUtil.hs, with the following content, which you can import into your code:

    {-# LANGUAGE ForeignFunctionInterface #-}
    {-# LANGUAGE CPP #-}
    {-# LANGUAGE NoImplicitPrelude #-}
    module IOUtil (
      IOUtil.putChar, IOUtil.putStr, IOUtil.putStrLn, IOUtil.print,
      IOUtil.getChar, IOUtil.getLine, IOUtil.getContents, IOUtil.readIO,
      ePutChar, ePutStr, ePutStrLn, ePrint,
      trace, traceIO
      ) where
    #ifdef mingw32_HOST_OS
    import Foreign.C.Types (CWchar)
    import Foreign
    import Prelude hiding (getContents, putStr, putStrLn) --(IO, Read, Show, String)
    --import qualified System.IO
    import qualified System.IO (getContents)
    import System.IO hiding (getContents, putStr, putStrLn)
    import Data.Char (ord)
     {- <>
        HANDLE WINAPI GetStdHandle(DWORD nStdHandle);
        returns INVALID_HANDLE_VALUE, NULL, or a valid handle -}
    foreign import stdcall unsafe "GetStdHandle" win32GetStdHandle :: DWORD -> IO (HANDLE)
    std_OUTPUT_HANDLE = -11 :: DWORD  -- all DWORD arithmetic is performed modulo 2^n
    std_ERROR_HANDLE  = -12 :: DWORD
     {- <>
        DWORD WINAPI GetFileType(HANDLE hFile); -}
    foreign import stdcall unsafe "GetFileType" win32GetFileType :: HANDLE -> IO (DWORD)
    _FILE_TYPE_CHAR   = 0x0002 :: DWORD
    _FILE_TYPE_REMOTE = 0x8000 :: DWORD
     {- <>
        BOOL WINAPI GetConsoleMode(HANDLE hConsole, LPDWORD lpMode); -}
    foreign import stdcall unsafe "GetConsoleMode" win32GetConsoleMode :: HANDLE -> LPDWORD -> IO (BOOL)
    _INVALID_HANDLE_VALUE = (intPtrToPtr $ -1) :: HANDLE
    is_a_console :: HANDLE -> IO (Bool)
    is_a_console handle
      = if (handle == _INVALID_HANDLE_VALUE) then return False
          else do ft <- win32GetFileType handle
                  if ((ft .&. complement _FILE_TYPE_REMOTE) /= _FILE_TYPE_CHAR) then return False
                    else do ptr <- malloc
                            cm  <- win32GetConsoleMode handle ptr
                            free ptr
                            return cm
    real_stdout :: IO (Bool)
    real_stdout = is_a_console =<< win32GetStdHandle std_OUTPUT_HANDLE
    real_stderr :: IO (Bool)
    real_stderr = is_a_console =<< win32GetStdHandle std_ERROR_HANDLE
     {- BOOL WINAPI WriteConsoleW(HANDLE hOutput, LPWSTR lpBuffer, DWORD nChars,
                                  LPDWORD lpCharsWritten, LPVOID lpReserved); -}
    foreign import stdcall unsafe "WriteConsoleW" win32WriteConsoleW
    data ConsoleInfo = ConsoleInfo Int (Ptr CWchar) (Ptr DWORD) HANDLE
    writeConsole :: ConsoleInfo -> [Char] -> IO ()
    writeConsole (ConsoleInfo bufsize buf written handle) string
      = let fillbuf :: Int -> [Char] -> IO ()
            fillbuf i [] = emptybuf buf i []
            fillbuf i remain@(first:rest)
              | i + 1 < bufsize && ordf <= 0xffff = do pokeElemOff buf i asWord
                                                       fillbuf (i+1) rest
              | i + 1 < bufsize && ordf >  0xffff = do pokeElemOff buf i word1
                                                       pokeElemOff buf (i+1) word2
                                                       fillbuf (i+2) rest
              | otherwise                         = emptybuf buf i remain
              where ordf   = ord first
                    asWord = fromInteger (toInteger ordf) :: CWchar
                    sub    = ordf - 0x10000
                    word1' = ((shiftR sub 10) .&. 0x3ff) + 0xD800
                    word2' = (sub .&. 0x3FF)             + 0xDC00
                    word1  = fromInteger . toInteger $ word1'
                    word2  = fromInteger . toInteger $ word2'
            emptybuf :: (Ptr CWchar) -> Int -> [Char] -> IO ()
            emptybuf _ 0 []     = return ()
            emptybuf _ 0 remain = fillbuf 0 remain
            emptybuf ptr nLeft remain
              = do let nLeft'    = fromInteger . toInteger $ nLeft
                   ret          <- win32WriteConsoleW handle ptr nLeft' written nullPtr
                   nWritten     <- peek written
                   let nWritten' = fromInteger . toInteger $ nWritten
                   if ret && (nWritten > 0)
                      then emptybuf (ptr `plusPtr` (nWritten' * szWChar)) (nLeft - nWritten') remain
                      else fail "WriteConsoleW failed.\n"
        in  fillbuf 0 string
    szWChar = sizeOf (0 :: CWchar)
    makeConsoleInfo :: DWORD -> Handle -> IO (Either ConsoleInfo Handle)
    makeConsoleInfo nStdHandle fallback
      = do handle     <- win32GetStdHandle nStdHandle
           is_console <- is_a_console handle
           let bufsize = 10000
           if not is_console then return $ Right fallback
             else do buf     <- mallocBytes (szWChar * bufsize)
                     written <- malloc
                     return . Left $ ConsoleInfo bufsize buf written handle
    {-# NOINLINE stdoutConsoleInfo #-}
    stdoutConsoleInfo :: Either ConsoleInfo Handle
    stdoutConsoleInfo = unsafePerformIO $ makeConsoleInfo std_OUTPUT_HANDLE stdout
    {-# NOINLINE stderrConsoleInfo #-}
    stderrConsoleInfo :: Either ConsoleInfo Handle
    stderrConsoleInfo = unsafePerformIO $ makeConsoleInfo std_ERROR_HANDLE stderr
    interact     :: (String -> String) -> IO ()
    interact f   = do s <- getContents
                      putStr (f s)
    conPutChar ci  = writeConsole ci . replicate 1
    conPutStr      = writeConsole
    conPutStrLn ci = writeConsole ci . ( ++ "\n")
    putChar      :: Char -> IO ()
    putChar      = (either conPutChar  hPutChar ) stdoutConsoleInfo
    putStr       :: String -> IO ()
    putStr       = (either conPutStr   hPutStr  ) stdoutConsoleInfo
    putStrLn     :: String -> IO ()
    putStrLn     = (either conPutStrLn hPutStrLn) stdoutConsoleInfo
    print        :: Show a => a -> IO ()
    print        = putStrLn . show
    getChar      = System.IO.getChar
    getLine      = System.IO.getLine
    getContents  = System.IO.getContents
    readIO       :: Read a => String -> IO a
    readIO       = System.IO.readIO
    readLn       :: Read a => IO a
    readLn       = System.IO.readLn
    ePutChar     :: Char -> IO ()
    ePutChar     = (either conPutChar  hPutChar ) stderrConsoleInfo
    ePutStr     :: String -> IO ()
    ePutStr      = (either conPutStr   hPutStr  ) stderrConsoleInfo
    ePutStrLn   :: String -> IO ()
    ePutStrLn    = (either conPutStrLn hPutStrLn) stderrConsoleInfo
    ePrint       :: Show a => a -> IO ()
    ePrint       = ePutStrLn . show
    import qualified System.IO
    import Prelude (IO, Read, Show, String)
    interact     = System.IO.interact
    putChar      = System.IO.putChar
    putStr       = System.IO.putStr
    putStrLn     = System.IO.putStrLn
    getChar      = System.IO.getChar
    getLine      = System.IO.getLine
    getContents  = System.IO.getContents
    ePutChar     = System.IO.hPutChar System.IO.stderr
    ePutStr      = System.IO.hPutStr System.IO.stderr
    ePutStrLn    = System.IO.hPutStrLn System.IO.stderr
    print        :: Show a => a -> IO ()
    print        = System.IO.print
    readIO       :: Read a => String -> IO a
    readIO       = System.IO.readIO
    readLn       :: Read a => IO a
    readLn       = System.IO.readLn
    ePrint       :: Show a => a -> IO ()
    ePrint       = System.IO.hPrint System.IO.stderr
    trace :: String -> a -> a
    trace string expr = unsafePerformIO $ do
        traceIO string
        return expr
    traceIO :: String -> IO ()
    traceIO = ePutStrLn

    then, you use the I/O functions therein contained instead of the standard library ones. They will detect whether output is redirected; if not (i.e. if we're writing to a 'real' console) then we'll bypass the usual Haskell I/O functions and write directly to the win32 console using WriteConsoleW, the unicode-aware win32 console function. On non-windows platforms, conditional compilation means that the functions here just call the standard-library ones.

    If you need to print to stderr, you should use (e.g.) ePutStrLn, not hPutStrLn stderr; we don't define a hPutStrLn. (Defining one is an exercise for the reader!)