I think it's more than that - it's also that type classes allow a type of polymorphism that is not the subtyping polymorphism that mainstream OO depends so heav

I think it's more than that - it's also that type classes allow a type of polymorphism that is not the subtyping polymorphism that mainstream OO depends so heavily on. ML attacks from a different direction with its parameterized modules, but with a similar result. That's why I feel like something's missing from F# in this regard, in comparison to other statically typed functional languages.