ExistentialTypesExample from Universal and Existential Types blog post

ExistentialExample.re

/* link to playground: https://bit.ly/2uP4SgO */
module type Animal = {
  type t;
  let make: (string) => t;
  let sprint: (t) => string;
};

module Animal: Animal = {
  type t = string;
  let make = (name) => name;
  let sprint = a => {j|$a is running!|j};
};

let zebra = Animal.make("Zebra");
Js.log(Animal.sprint(zebra));