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

[Magicloud Magiclouds]

Thank you for the reply. I've learnt the code of "lines". So it is because
how ByteString works, that the conduit is not a stream of bytes, but
chunks, right?


On Tue, Apr 9, 2013 at 12:12 PM, Michael Snoyman <michael at snoyman.com>wrote:

> 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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>>
>


-- 
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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