[Haskell-cafe] [iteratee] how to do nothing .. properly

[Sergey Mironov]

I am glad to help! Looks like upgrading to 0.8.5.0 also fixes initial
problem that involved me into testing!

I'll take the opportunity and ask another thing about iteratee: Is it
expected behavior that throwErr consumes all data in current chunk? I
wish it to stop in place and let after-checkErr code to continue the
parsing. Well, I already found solution (or workaround?) - I wrap
Iteratee with ErrorT monad and use ErrorT's raiseError instead of
throwErr. Is it correct?

Here is example code

instance Exception Int

iter4 = do
    I.dropWhile (/= 3)
    h<-I.head
    throwErr $ toException $ (-4::Int)  -- doesn't meter what exactly to throw
    return h

-- catch the error with checkErr
iter5 = do
    (_,b)<-countBytes $ I.checkErr $ iter4
    s <- I.stream2list
    return (b,s)

print5 = enumPure1Chunk [1..10] (iter5) >>= run >>= print


Thanks a lot!
Sergey

2011/6/2 John Lato <jwlato at gmail.com>:
> Hi Sergey,
>
> I've got an explanation; quite surprisingly it's a bug in enumPure1Chunk.
> Even though it is an odd case, I'm surprised that it hasn't come up before
> now since enumPure1Chunk appears frequently.
>
> I've just uploaded 0.8.5.0 which has the fix.  There's now an additional
> Monoid constraint on enumPure1Chunk, unfortunately.
>
> Thanks very much for reporting this.
>
> John L
>
> On Thu, Jun 2, 2011 at 10:02 AM, Sergey Mironov <ierton at gmail.com> wrote:
>>
>> Ok. I've checked iteratee-0.8.3.0 and 0.8.4.0. Results are same.
>>
>> Sergey
>>
>> 2011/6/2 John Lato <jwlato at gmail.com>:
>> > Hi Sergey,
>> > I can't explain this; maybe it's a bug in enumWith?  I'll look into it.
>> > Thanks,
>> > John
>> >
>> >>
>> >> Message: 20
>> >>
>> >> Date: Thu, 2 Jun 2011 02:46:32 +0400
>> >> From: Sergey Mironov <ierton at gmail.com>
>> >> Subject: [Haskell-cafe] [iteratee] how to do nothing .. properly
>> >> To: haskell-cafe at haskell.org
>> >> Message-ID: <BANLkTimMFRWgH9Nopt-eua+L7jQcGq+u=g at mail.gmail.com>
>> >> Content-Type: text/plain; charset=ISO-8859-1
>> >>
>> >> Hi. Would anybody explain a situation with iter6 and iter7 below?
>> >> Strange thing - first one consumes no intput, while second consumes it
>> >> all, while all the difference is peek  which should do no processing
>> >> (just copy next item in stream and return to user).
>> >> What I am trying to do - is to write an iteratee consuing no input,
>> >> but returning a constant I give to it. I thought (return a) should do
>> >> it, but it seems I was wrong as return actually consumes all unparsed
>> >> stream. iter6 experience tells me that (peek>>return a) is what I
>> >> need, but it's completely confusing and not what I expected.
>> >>
>> >> Thanks,
>> >> Sergey
>> >>
>> >>  import Data.Iteratee as I
>> >>  import Data.Iteratee.IO
>> >>  import Control.Monad
>> >>  import Control.Exception
>> >>  import Data.ByteString
>> >>  import Data.Char
>> >>  import Data.String
>> >>
>> >>  -- countBytes :: (..., Num b) => Iteratee s m a -> Iteratee s m (a, b)
>> >>  countBytes i = enumWith i I.length
>> >>
>> >>  iter6 = do
>> >>     h <- countBytes $ (peek >> return 0)
>> >>     s <- I.stream2list
>> >>     return (h,s)
>> >>
>> >>  iter7 = do
>> >>     h <- countBytes $ (return 0)
>> >>     s <- I.stream2list
>> >>     return (h,s)
>> >>
>> >>  print6 = enumPure1Chunk [1..10] (iter6) >>= run >>= print
>> >>  print7 = enumPure1Chunk [1..10] (iter7) >>= run >>= print
>> >>
>> >>
>> >> Here is example ghci session
>> >>
>> >> *Main> print6
>> >> ((0,0),[1,2,3,4,5,6,7,8,9,10])
>> >> -- read 0 items, returns 0
>> >> *Main> print7
>> >> ((0,10),[])
>> >> -- read 10 items (???) returns 0
>> >> *Main>
>> >>
>> >>
>> >>
>> >> ------------------------------
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>> >
>> >
>
>