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

[John A. De Goes]

It's not OK and it's an artifact of the weak-typing and ill-defined semantics that pervade iteratee libraries. It's possible to do a lot of bad stuff, including binding with an iteratee yielding a remainder without consuming input.

Regards,

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

On Apr 19, 2011, at 6:27 AM, Daniel Schüssler wrote:

> Hello,
> 
> for reference, said instance is:
> 
>> instance Monad m => Monad (Iteratee a m) where
>> 	return x = yield x (Chunks [])
>> 
>> 	m0 >>= f = ($ m0) $ fix $
>> 		\bind m -> Iteratee $ runIteratee m >>= \r1 ->
>> 			case r1 of
>> 				Continue k -> return (Continue (bind . k))
>> 				Error err -> return (Error err)
>> 				Yield x (Chunks []) -> runIteratee (f x)
>> 				Yield x extra -> runIteratee (f x) >>= \r2 ->
>> 					case r2 of
>> 						Continue k -> runIteratee (k extra)
>> 						Error err -> return (Error err)
>> 						Yield x' _ -> return (Yield x' extra)
> 
> 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)?
> 
> Cheers,
> Daniel
> 
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