This is a pre-release of mezzolens, a library of first-order functional

references based purely on profunctors. I wrote it because

https://www.reddit.com/user/Faucelme asked for one. You can find it at

https://hackage.haskell.org/package/mezzolensThe main purpose of this pre-release is to provide background for framing

the following question:

In a pure profunctor lens libray, how do you write choosing and beside in

such a way that it supports all the following types?

choosing :: Lens ta tb a b -> Lens sa sb a b -> Lens (Either ta sa) (Either sa sb) a b

choosing :: Traversal ta tb a b -> Travesal sa sb a b -> Traversal (Either ta sa) (Either sa sb) a b

choosing :: SEC ta tb a b -> SEC sa sb a b -> SEC (Either ta sa) (Either sa sb) a b

beside :: Traversal ta tb a b -> Travesal sa sb a b -> Traversal (Either ta sa) (Either sa sb) a b

beside :: SEC ta tb a b -> SEC sa sb a b -> SEC (Either ta sa) (Either sa sb) a b

One sufficent solution for choosing would be to write a single function

that combines

lensVL :: (forall f. Functor f => (a -> f b) -> ta -> f tb) -> Lens ta tb a b

traversal :: (forall f. Applicative f => (a -> f b) -> ta -> f tb) -> Traversal ta tb a b

into one.

The obvious approach is to write an suitable adaptor that transforms any

(strong) profunctor into a functor, and if the profunctor is wandering, it

produces an applicative functor. But I haven't been able to devise such a

suitable adaptor.

--

Russell O'Connor <

http://r6.ca/>

``All talk about `theft,''' the general counsel of the American Graphophone

Company wrote, ``is the merest claptrap, for there exists no property in

ideas musical, literary or artistic, except as defined by statute.''

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