unfold.re

external _unfold : ('a => Js.t 'whatever) => 'a => stream 'b = "unfold" [@@bs.module "most"];

module Unfold = {
  type t 'value 'seed =
    | Value 'value 'seed
    | Done;
};

external convertUnfoldValue : Js.t {. _done : bool} => Js.t {. seed : 'a, value : 'b} =
  "%identity";

/* Creates a stream from a generating function and a seed */
let unfold f seed =>
  _unfold
    (
      fun x =>
        switch (f x) {
        | Unfold.Done => convertUnfoldValue {"_done": true}
        | Unfold.Value value seed => {"value": value, "seed": seed}
        }
    )
    seed;

A few suggestions:

• The callback can have the type 'a => Js.t 'whatever
• You can use a more general casting function external cast : Js.t 'a => Js.t 'b = "%identity"; instead of convertUnfoldValue
• In either branch of the switch, use cast to force the value to the right type
• You can 'eta-reduce' unfold, i.e. you can get rid of the seed parameter. I.e. let unfold f seed => ... seed; is equivalent to let unfold f => ...;