Sometimes seeing the same concept from different perspectives makes it easier to understand. For a scala treatment of the same topic see
https://underscore.io/blog/posts/2017/06/02/uniting-church-a...
Also in Scala; an interesting solution to the expression problem:
https://i.cs.hku.hk/~bruno/papers/Modularity2016.pdf
Its a paper, but its very light on dense academic prose and very heavy on copy pastable code :)