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

[Daniel Schüssler]

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