I learned SML/NJ and OCaml a bit over 20 years ago and liked them, but when I tried my hand at Haskell my eyes glossed over. I get its power. But I do not like its syntax, it's hard to read. And yes, the obsession with purity.

Exactly right. I quite like haskell in theory, but in practice I quite dislike both reading and writing it.

But I like ocaml both in theory and practice (also in part due to having my eyes opened to SML about 20 years ago).

I actually preferred SML/NJ when I played with writing it, but OCaml "won" in the popularity contest. Some of the things that made OCaml "better" (objects, etc.) haven't aged well, either.

Still with OCaml finally supporting multicore and still getting active interest, I often ponder going back and starting a project in it someday. I really like what I see with MirageOS.

These days I just work in Rust and it's Ok.

Yep, right there with you. OCaml was only ever better in my view because it had developed enough libraries to be an actual pragmatic choice, unlike the other languages in that family. And yep, Rust is perfectly good too, IMO, but I do find that I rarely-to-never actually care about all the zero-cost abstractions that make it "hard" to use.

OCaml's object system is very nice, though. Structural typing with full inference is pretty awesome, and it also cleanly decouples subtyping from implementation inheritance.