It has been proven that you can express certain things with ML-style functors that are difficult or impossible to express with standard Haskell-style type class

It has been proven that you can express certain things with ML-style functors that are difficult or impossible to express with standard Haskell-style type classes. But almost everybody uses GHC and turns on lots of type system extensions in their projects, which is going to make the issue a lot murkier, and on top of that ML-style functors require a bit more finger work and explicitness. Ultimately, it's going to come down to preference. They're roughly equivalent the way Python and Ruby are roughly equivalent, or Common Lisp and Scheme. From any other linguistic paradigm, yeah, they're extremely close. But from one to the other, it feels like there is a tremendous gulf. A lot of the benefits tmhedberg is talking about come from I/O being a monad in Haskell and the lack of "assignables," which constrains your ability to mix side-effects and state changes with pure code. Of course, you can defeat it if you really try (MVars and unsafePerformIO) but it's not the kind of thing you would do accidentally. And you could achieve a lot of the same benefits in OCaml by writing monadic standard libraries and never using refs. So, if by "expressive" you mean the type system allows you to express complex relationships, Haskell is more expressive. But if by "expressive" you mean the language allows you to express calculations in more ways, OCaml is more expressive. In my experience, "expressiveness" is usually a foil for some other attribute and doesn't usually illuminate the debate much. In the end, they're just tools, albeit with overlapping purposes. OCaml is easier to learn and compiles faster, Haskell raises your confidence in the code and has cooler tricks. Both are fascinating and stunningly better than lots of other options. If I had to choose which one to learn today, I'd look at some example code in both and ask myself which one I want to be staring at for the next six months.