getExecDir = try_size 2048 -- plenty, PATH_MAX is 512 under Win32.
  where
    try_size size = allocaArray (fromIntegral size) $ \buf -> do
        ret <- c_GetModuleFileName nullPtr buf size
        case ret of
          0 -> return Nothing
          _ | ret < size -> fmap (Just . dropFileName) $ peekCWString buf
            | otherwise  -> try_size (size * 2)
getExecDir = try_size 2048 -- plenty, PATH_MAX is 512 under Win32.
  where
    try_size size = allocaArray (fromIntegral size) $ \buf -> do
        ret <- c_GetModuleFileName nullPtr buf size
        case ret of
          0 -> return Nothing
          _ | ret < size -> fmap (Just . dropFileName) $ peekCWString buf
            | otherwise  -> try_size (size * 2)
getExecDir = try_size 2048 -- plenty, PATH_MAX is 512 under Win32.
  where
    try_size size = allocaArray (fromIntegral size) $ \buf -> do
        ret <- c_GetModuleFileName nullPtr buf size
        case ret of
          0 -> return Nothing
          _ | ret < size -> fmap (Just . dropFileName) $ peekCWString buf
            | otherwise  -> try_size (size * 2)