Yeah I've used both quite a lot.
Both languages are pretty similar. ReasonML isn't actually a language but more like a new syntax for Ocaml (Bucklescript on the frontend), kind of like what Coffeescript is to JS.
- Both have row polymorphism and higher kinded types. - Purescript has ad hoc polymoprhism, Ocaml doesn't but it's planned in a future release. Purescript achieves it via typeclasses, future Ocaml will achieve it via modular implicits which is more sound from a type system perspective. - Purescript has higher kinded polymorphisn, Ocaml only has a lightweight version of it via it's module system. - Purescript has better support for type-level programming although awkward, but Ocaml has good interop with Coq -it's written by the same people - Purescript is purely functional, Ocaml isn't although it emphasizes a purely functional approach - Purescript places category theory abstractions first like Haskell does as a core aspect of the language. Ocaml doesn't, although it's standard library has all of the same kinds of functions and there are category theory libraries and everything - Ocaml not just -to-JS, it's also a compiled language, and has had real industrial use as a systems language in the finance industry (Jane Street) - Ocaml's compiler is _insanely_ fast compared to any other compiler I've ever used. I absolutely appreciate it - Future Ocaml will be multicore and have a builtin algebraic effects system, basically like Haskell's extensible effects but without the performance hit or having to muck about with monads - Ocaml has GADTs, polymorphic variants, and a way to extend the language via PPX. Polymorphic variants are awesome - Ocaml's module system is really powerful and amazing
Overall both languages are great, but for almost every use case I'd go with Bucklescript/Ocaml any day. It's type system is pretty simple and easy to learn. Purescript has the same issue as Haskell which is the learning curve: higher kinded polymorphism, typeclasses, enforced purity via "the IO monad", and category theory concepts all add up and take quite a while for people to get used to. And the compiler is really fast. And you can write low level imperative code if you need to for performance. And the type system is really expressive, moreso in my opinion than Purescript's because of the poly variants. And you get like 90% of the benefit of using a language like Haskell and Purescript because of the `option` type and enforced safety via exhaustive pattern matching
I can't wait until Ocaml gets modular implicits and algebraic effects into the core language.