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

[John Lato]

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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