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

Ok. I've checked iteratee-0.8.3.0 and 0.8.4.0. Results are same. Sergey 2011/6/2 John Lato :

Hi Sergey, I can't explain this; maybe it's a bug in enumWith?  I'll look into it. Thanks, John

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Message: 20

Date: Thu, 2 Jun 2011 02:46:32 +0400 From: Sergey Mironov Subject: [Haskell-cafe] [iteratee] how to do nothing .. properly To: haskell-cafe@haskell.org Message-ID: 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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