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

Forgot reply to all ---------- Forwarded message ---------- From: Sergey Mironov Date: 2011/6/16 Subject: Re: [Haskell-cafe] [iteratee] how to do nothing .. properly To: John Lato 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 :

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 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 :

...

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 wrote:

...

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

...

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>

------------------------------

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

End of Haskell-Cafe Digest, Vol 94, Issue 3 *******************************************