Lazy_Reason.re

type lazy_node_t('a) =
  | Delayed(unit => 'a)
  | Value('a)
  | Exn(exn);
type lazy_t('a) = ref(lazy_node_t('a));
let lazy_from_fun = f => ref(Delayed(f));

let force = x_lazy =>
  switch (x_lazy^) {
  | Value(x) => x
  | Exn(e) => raise(e)
  | Delayed(f) =>
    try (
      {
        let r = f();
        x_lazy := Value(r);
        r;
      }
    ) {
    | e =>
      x_lazy := Exn(e);
      raise(e);
    }
  };

/* Based on http://typeocaml.com/2014/11/13/magic-of-thunk-lazy */
/* Test */
let x = () => Js.log("test");
let k = lazy_from_fun(x);
force(k)

/* what you should use: */
/* https://caml.inria.fr/pub/docs/manual-ocaml/libref/Lazy.html */
let y = () => Js.log("test");
let p = Lazy.from_fun(y);
Lazy.force(p)