Why doesn't OCaml have typeclasses (or something similar)?

[nojb]

@bbatsov

I get that any form of type-level programming comes with a certain degree of complexity, but I also find it somewhat frustrating that basic things like printing some data for debug purposes require you to either hand-roll some print function or use ppx. The inability to overload operators is also a bit of a pain, although to a smaller extent.

I would like to emphasize that these two issues are really orthogonal: “printing some data for debug purposes” is about defining new functions, while “overloading operators” is about figuring out which existing functions to use at a given type.

Only the latter is solved by type classes/traits/implicits. For the former, PPX is used today, but it is an imperfect solution (mostly because it is syntactic and separate from the compiler). It corresponds to the “deriving” mechanism found in Haskell or Rust.

Personally, I think that in terms of sheer utility it is the latter, having a built-in deriving mechanism, that is much more useful than having type classes/implicits (which allow overloading). See also this related thread:

Ad hoc polymorphism and usability

Just to be clarify for other readers of this thread: in order to achieve the kind of usability that is found in Haskell and Rust, you need both mechanisms, type classes/modular implicits and “deriving”, simultaneously (and indeed both Haskell and Rust implement both mechanisms). The reason is that “modular implicits” will never define new functions for you. It will just make it easier to figure out which existing function to use at a given type. In order to automatically define new functions …

Cheers,
Nicolas