I wrote an exercise once while I was teaching: "pure functional pacman". Really I did it to teach myself Redux, and it was a pretty fun way to learn.

Totally agree with the article: sometimes functional is the right tool for the job, sometimes OOP.

A more recent example: parsing USB descriptors in Rust. The parser would have been trivial and easy to understand if I could have multiple mutable references to nodes, but alas I kept fighting the borrow checker.

I talked with a friend about why it was so hard, and he said "do it in a functional style". That made my borrow checker worries go away, but instead I have multiple "tree pivoters" that recursively descend through input trees while building output trees. It practically broke my brain and although it is pure, it's not nearly as readable or maintainable as one with mutation would have been.