[Haskell-cafe] [Conduit] weird action of leftover.

[Michael Snoyman]

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 at 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
>
> --
> 竹密岂妨流水过
> 山高哪阻野云飞
>
> And for G+, please use magiclouds#gmail.com.
>
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