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

[Sergey Mironov]

Forgot reply to all

---------- Forwarded message ----------
From: Sergey Mironov <ierton at gmail.com>
Date: 2011/6/16
Subject: Re: [Haskell-cafe] [iteratee] how to do nothing .. properly
To: John Lato <jwlato at gmail.com>


Thanks for explanations! /ErrorT String (Iteratee s m) a/ definitely
does the work. But wrapping Enumeratee is harder since /ErrorT String
(Enumeratee s s' m) a/ doesn't compiles. Maybe most users actually
don't need to wrap Enumeratee, will check it. Heh, throwRecoverableErr
name tells me "use me, I am all you need":) Well, one just need to
grok the interface.. Anyway, iteratees are much deeper and powerful
than I expected when switched to it from Data.Binary. I'll better open
new threads for asking more questions.

Again thanks,
Sergey

PS
sorry for delayed answer, had vacation without email access

2011/6/5 John Lato <jwlato at gmail.com>:
> Yes, this is expected.  'throwErr' is only meant to be used when the error
> should be non-recoverable, and the stream would often be invalid then, so
> throwErr doesn't take any steps to preserve it.  You could retain the rest
> of the stream with getChunk and use throwRecoverableErr though.
>
> Wrapping an iteratee with ErrorT is fine, and I use this approach often.  I
> typically would use an explicit Either rather than ErrorT, but the two
> approaches are exactly the same.
>
> Note that wrapping the other way, Iteratee s (ErrorT e m), can sometimes
> cause problems, and is best avoided unless you really know what you're
> doing.  This may change in a future release.
>
> John
>
> On Thu, Jun 2, 2011 at 4:27 PM, Sergey Mironov <ierton at gmail.com> wrote:
>>
>> 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>
>> >> >>
>> >> >>
>> >> >>
>> >> >> ------------------------------
>> >> >>
>> >> >> _______________________________________________
>> >> >> Haskell-Cafe mailing list
>> >> >> Haskell-Cafe at haskell.org
>> >> >> http://www.haskell.org/mailman/listinfo/haskell-cafe
>> >> >>
>> >> >>
>> >> >> End of Haskell-Cafe Digest, Vol 94, Issue 3
>> >> >> *******************************************
>> >> >
>> >> >
>> >
>> >
>
>