The whole part about lenses.

Lenses are not a concept from category theory and to understand them no knowledge of category theory is required. Category theory just gives you generalizations (other 'optics') and better encodings, but none of that is mentioned in this post.

The use of lenses here is just one approach to solving a specific problem. It is still possible to write code with the same behaviour without lenses.

Do you have a better suggestion for updating deeply-nested record fields?

Still lenses, but have you looked at https://package.elm-lang.org/packages/bChiquet/elm-accessors...?

I hadn't seen it until you linked to it from one of your earlier comments. I have no particular attachment to elm-monocle, and I appreciate you sharing :)