[Haskell-cafe] Question about the Monad instance for Iteratee (from the enumerator package)

[John A. De Goes]

This is a much cleaner definition of Iteratee and I'm happy to see it.

When are you going to move from your FTP site to Github, by the way? :)

Regards,

John A. De Goes
Twitter: @jdegoes 
LinkedIn: http://linkedin.com/in/jdegoes

On Apr 21, 2011, at 12:32 AM, oleg at okmij.org wrote:

> 
> Daniel Schuessler wrote:
> 
>> The thing I don't understand yet is the last line: Why is it OK to discard the
>> leftover input from the (f x) Iteratee and yield just the leftover input from
>> the first one (m0)?
> 
> First of all, the question is about an older version of Iteratee. For
> example, the following code 
> 	http://okmij.org/ftp/Haskell/Iteratee/Iteratee.hs
> defines Iteratee a bit differently so the question does not apply.
> 
>> data Iteratee a = IE_done a
>>               | IE_cont (Maybe ErrMsg) (Stream -> (Iteratee a,Stream))
>> 
>> instance Monad Iteratee where
>>   return = IE_done
>>   IE_done a   >>= f = f a
>>   IE_cont e k >>= f = IE_cont e (docase . k)
>>    where
>>    docase (IE_done a, stream)   = case f a of
>>                  IE_cont Nothing k -> k stream
>>                  i                 -> (i,stream)
>>    docase (i, s)  = (i >>= f, s)
> 
> No left-over is discarded any more. 
> 
> Your question is about the previous design, called `The second design'
> described in Iteratee.hs. The corresponding comment block answers your
> question, please search for ``Justification for the case IE_done x
> s >>= f''.
> 
> Please see
> 	http://okmij.org/ftp/Haskell/Iteratee/IterateeM.hs
> for the `production' case of Iteratee in a base monad. The file
> IterateeM.hs talks about one more design, and its drawbacks. 
> 
> It's all about finding the optimal trade-off, I guess.
> 
> 
> 
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe