It's a bug in your implementation of takeLine I believe. It doesn't take into account that lines can span multiple chunks. When you call takeLine the first time, you get "L1\n". leftover puts a chunk with exactly those contents back. When you call takeLine the second time, it gets the chunk "L1\n", and your splitAt gives you back "L1\n" and "". The "" is then leftover, and the next call to takeLine gets it.

Your takeLine needs to include logic saying "there's no newline in this chunk at all, let's get the next chunk and try that." You can look at the source to lines[1] for an example of the concept.

Michael

[1] http://haddocks.fpcomplete.com/fp/7.4.2/20130313-1/conduit/src/Data-Conduit-Binary.html#lines


On Mon, Apr 8, 2013 at 8:44 AM, Magicloud Magiclouds <magicloud.magiclouds@gmail.com> wrote:
Say I have code like below. If I comment the leftover in main, I got (Just "L1\n", Just "L2\n", Just "L3\n", Just "L4\n"). But if I did not comment the leftover, then I got (Just "L1\n", Just "L1\n", Just "", Just "L2\n").
Why is not it (Just "L1\n", Just "L1\n", Just "L2\n", Just "L3\n")?

takeLine :: (Monad m) => Consumer ByteString m (Maybe ByteString)
takeLine = do
  mBS <- await
  case mBS of
    Nothing -> return Nothing
    Just bs ->
      case DBS.elemIndex _lf bs of
        Nothing -> return $ Just bs
        Just i -> do
          let (l, ls) = DBS.splitAt (i + 1) bs
          leftover ls
          return $ Just l

main = do
  m <- runResourceT $ sourceFile "test.simple" $$ (do
    a <- takeLine
    leftover $ fromJust a
    b <- takeLine
    c <- takeLine
    d <- takeLine
    return (a, b, c, d))
  print m

--
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