If you ever wondered what a real world application of the recursion operators as defined in Bananas, Lenses, Envelopes and Barbed Wire [1], you should definitely check out HNix[2].

They use these patterns to process their AST, The way they associate the location informations [3] by composing it to the "location-less" AST is pretty amazing. If we ignore the Fix point functor "hack", this is a one liner.

I'm so pumped by this project :) !

[1]: https://maartenfokkinga.github.io/utwente/mmf91m.pdf
[2]: https://github.com/haskell-nix/hnix
[3]: https://github.com/haskell-nix/hnix/blob/master/src/Nix/Expr/Types/Annotated.hs#L106