Dynamic dispatch in OCaml

18 Jul 2022

In this post I will show how to implement a show function in OCaml with GADts and existentials and how to recover/implement dynamic dispatch in OCaml.

I’ve been working with OCaml in the last months. It’s a wonderful language, fully functional with opt-in imperative features. But one thing that I miss is things being printable by default.

In Haskell we have the show function that works almost everywhere and one the things that we first start looking when coding in OCaml is for some sort of show function. In Python, Ruby etc we usually resort to dynamic dipatch to work on this, like supporting a method like __repr__ or to_s.

In OCaml we can use GADts with extential types to recover this kind of behavior but there is a caveat, anyway, here is the code, contact function is where we use the show function as it’s used in Haskell.

type showable = 
  | Bool : bool -> showable
  | Int : int -> showable
  | Float : float -> showable

(* dynamic dispatch based on an existential *)
let show = function
  | Bool b -> if b then "true" else "false"
  | Int i -> string_of_int i
  | Float f -> Float.to_string f

let print t = print_endline (show t)

let concat s1 s2 =
  show s1 ^ show s2

let () =
  print (Bool true);
  print (Int 1);
  print (Float 2.);
  print_endline (concat (Bool true) (Int 10))

The type of contat is showable -> showable -> string, preatty neat uh? The problem with this approach is that, while in OOP or in Haskell we can spread the implementation of show (or whatever) where is conveninent, here we need to extend the GADT AND the show function, so it’s not very usable.

It IS dynamic dispatch in essence because the pattern match in show is done at runtime, so that we can write functions as concat for example.

It’s possible to do better with records and modules but this is a matter for another post, I have to sleep 😅

That’s it, cheers!