Do you sometimes need to `set (span even)` or `set group`?  When is that 
helpful?

On Sunday, March 6, 2016 at 4:14:21 PM UTC, Gabriel Gonzalez wrote:
>
> I would prefer to keep them the way they are, because the full `Lens` 
> functionality isn’t used just for `zoom` but also for other utilities like 
> `over` or `set`.  It’s easier to just warn users that a lens is improper 
> than to create parallel versions of every utility that a person might want 
> to use with these `Lens`es.
>
> On Mar 6, 2016, at 6:06 AM, Thomas Wilkinson <thomas.s....@gmail.com 
> <javascript:>> wrote:
>
> I am slightly freaked out by the `Lens`es provided by `Pipes.Parse` being 
> 'improper'.  Could they not be `Getter`s instead?
>
>     type Splitter a m = forall x. Getter (Producer a m x) (Producer a m 
> (Producer a m x))
>     -- Should have   " join . view sp == id "     for any sp :: Splitter a 
> m  and  p :: Producer a m r
>
>     span :: Monad m => (a -> Bool) -> Splitter a m
>     splitAt :: Monad m => Int -> Splitter a m
>     groupBy :: Monad m => (a -> a -> Bool) -> Splitter a m
>     group :: (Monad m, Eq a) => Splitter a m
>
>     -- would replace `zoom`
>     split :: Splitter a m -> Parser a m r -> Parser a m r
>     split sp st = StateT $ \p -> fmap (fmap join) (runStateT st (p^.sp))
>
> This means that users would sometimes have to use `zoom` and sometimes 
> have to use `split`, which might be annoying.  But it feels better than 
> having fake `Lens`es somehow.
>
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