### Re: Seeking Control.Lens Combinator

Sebastiaan Visser <haskell <at> fvisser.nl>

2013-10-05 14:14:20 GMT

Charles,
I know you specifically asked for a Control.Lens combinator and I don't have one for you, but I take the
opportunity to show you how easy this is using fclabels:
tupleUp :: f :-> a
-> f :-> b
-> f :-> (a, b)
tupleUp a b = point $
(,) <$> L.fst >- a -- L.fst, L.snd are lenses from Data.Label.Base
<*> L.snd >- b
In fclabels you can use the Applicative instance (and Alternative for multi-constructor cases) to derive
views like this. If you look at the structure it'll become clear how easily this scales up to different
types of views.
This particular example uses total monomorphic lenses, but also works for partial lenses and polymorphic updates.
Maybe there is a similar way in Control.Lens to do this.
Sebastiaan
On Oct 5, 2013, at 1:23 AM, Charlie Paul <charlieap <at> gmail.com> wrote:
> Hello,
>
> I'm looking for a combinator along the lines of
> (&&&) :: Lens' a b -> Lens' a b' -> Lens' a (b,b')
> I can see how it could lead to lenses that don't follow the laws, but
> for Lenses which are somehow independent (like _1 and _2), it works
> perfectly well. Is there a way in lens to specify this "independence"?

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