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/mezzolens The 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.''