I wonder now much C++ matters here. I rarely reach for functional patterns when the language I'm using does not provide proper constructs for readable, declarative code like pattern matching, nominative discriminated unions, and pipe operators.

Sure, you can fake all that with other constructs or patterns. But then it really does feel more like a puzzle.

IMO, good FP is good when it is maximally declarative and organized in the linear way in which humans reason best.

Maybe the author should try F# or OCaml instead of Haskell! I love FP and have also tried and hated Haskell three or four times.