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

[Felipe Almeida Lessa]

2011/4/19 Daniel Schüssler <anotheraddress at gmx.de>:
> 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)?

On that stage no input was given to (f x), we just runned it without
anything and it already yielded something.  So whatever it yielded
must be without leftovers.

Now, that's what I get from reading the code.  I don't remember if it
is explicitly allowed or forbidden for an iteratee to generate
leftovers out of nowhere.  My guess is that it doesn't make much sense
to allow it.

Cheers,

-- 
Felipe.