matryoshka

My experience with recursion schemes was mixed. I applied Matryoshka[1] to a problem involving an algebra of boolean operations on polygons. The implementation was elegant in so far as the essential recursion was lifted out of many separate functions and consolidated. Recursion is always a tricky thing to get right so using recursion schemes avoided an entire class of errors and the implementation was rock solid. The challenge became one of communication. Whereas looping and branching structures are considered fundamental building blocks recursion schemes are not. Team members immediately hit a wall when encountering ana/cata/hylo/*-morphisms. It's like taking a walk through your neighborhood turning a corner and finding an alien artifact. It's also a case of "the math people got there first" ie: the nomenclature hurts rather than helps u

You have a pretty narrow definition of a platform. Consider https://en.m.wikipedia.org/wiki/Computing_platform Yes, you are right, I didn't consider the checkcast instruction. Yes, F# does not have this particular limitation, but has few of its own in different places. Most notably, it lacks HKT (arguably, because of CLR awareness of generics). BTW, alternative approaches to stack-safe recursion do exist in Scala. For example, https://github.com/slamdata/matryoshka

I could never give up higher-kinded types: there are plenty of ad-hoc solutions to various effects, but a single reusable solution[1] means you can build up libraries that not only handle all the common effects, but your own custom ones as well. Even if you don't use them directly, you benefit from the ecosystem they make possible. Few other languages let you use something like treelog[2], fewer still let you fold a custom tree type while managing an effect as nicely as with matryoshka. Typeclass derivation is reflection done right, with compile-time safety and corresponding performance. I've never seen another language that made it so easy to put a database transaction boundary in exactly the right place but in a type-safe way so that you can never forget, or even have transactions implicitly applied to web endpoints that need them in a