Whereas ordinary directed lenses go from a source to a view, symmetric lenses relate two different views of an internal source. I’ve been thinking about how to smoosh together symmetric lenses with polymorphic lenses, which are the sort used in functional programming. My actual goal is to invent “symmetric open games” (without relying on non-well-founded recursion like I did in this paper). But with the right definition, I think symmetric polymorphic lenses could be a useful tool to programmers as well.

Normally when I’m playing with an idea like this I mock it up in Haskell and play around with it to check it works, before committing to too much hard work on the proofs. But this would be very difficult (and maybe impossible) to write in Haskell, so the idea of this post is to give enough detail that some folks could try it in Agda or Idris, and give me feedback on whether it feels like something useful. (In the meantime, I will try an extremely fast and loose Haskell version by pretending that pullbacks and pushouts are products and coproducts, and see what happens.)