The bs-mithril-stream from kristianmandrup

Modelling mithril/stream

After taking a look at the docs ( https://mithril.js.org/stream.html#stream ), I believe Mithril streams are implemented as essentially callbacks. E.g. in https://mithril.js.org/stream.html#combining-streams there's an example:

var a = stream(5)
var b = stream(7)

var added = stream.combine(function(a, b) {
    return a() + b()
}, [a, b])

console.log(added()) // logs 12

According to the docs for combine, the a and b that get passed into the combiner function are themselves stream values. Since they get called as functions, therefore they must be functions. So based on that, I came up with the following sample set of bindings to give you an idea of the approach I would take:

/* Mithril_Stream.re */

/** Need to uncurry to prevent generating JS like x(0)
    See https://bucklescript.github.io/docs/en/function#curry-uncurry */
type t('a) = (. unit) => 'a;

/* See
   https://bucklescript.github.io/docs/en/import-export#import-a-default-value
   Test: let s = make(1); */
[@bs.module] external make: 'a => t('a) = "mithril/stream";

/* See https://bucklescript.github.io/docs/en/common-data-types#tuple
   Test:
   let s1 = make(1);
   let s2 = make(2);
   let s3 = combine2((. s1, s2) => s1(. ) + s2(. ), (s1, s2)); */
[@bs.module "mithril/stream"] external combine2: (
  (. t('a1), t('a2)) => 'b,
  (t('a1), t('a2)),
) => t('b) = "combine";

[@bs.module "mithril/stream"] external combine3: (
  (. t('a1), t('a2), t('a3)) => 'b,
  (t('a1), t('a2), t('a3)),
) => t('b) = "combine";

[@bs.module "mithril/stream"] external combine4: (
  (. t('a1), t('a2), t('a3), t('a4)) => 'b,
  (t('a1), t('a2), t('a3), t('a4)),
) => t('b) = "combine";

/* Test:
   let s = make(1);
   let t = map((. x) => x + 1, s); */
[@bs.send.pipe: t('a)] external map: ((. 'a) => 'b) => t('b) = "";

[@bs.send.pipe: t('a)] tells BuckleScript to inject a final parameter of type t('a) into the external, and generate a method call to that parameter. It's very useful but was somewhat prematurely removed from the BuckleScript doc site.

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