Async / Await

[mrispoli24]

I've read there's not async / await in reason. What's the best way to handle something like fetching from an API like a headless CMS? Are js promises the best way to handle this or is there a better way? How about features like generators / async generators?

[baransu]

Monadic bind operator was recently introduced into OCaml but afaik there is no Reason support for that yet. In the meanwhile, you can use community reasonml-labs/bs-let which allows you to use that not only for Promises but also for other "monadic" data types like Option or Result.

module Option = {
  let let_ = Belt.Option.flatMap;
};

type address = {street: option(string)};

type personalInfo = {address: option(address)};

type user = {info: option(personalInfo)};

// Get the user's street name from a bunch of nested options. If anything is
// None, return None.
let getStreet = (maybeUser: option(user)): option(string) => {
  let%Option user = maybeUser;
  // Notice that info isn't an option anymore once we use let%Option!
  let%Option info = user.info;
  let%Option address = info.address;
  let%Option street = address.street;
  Some(street->Js.String.toUpperCase);
};

[fhammerschmidt]

While you can get pretty far with Js.Promise, the API is very unpleasant both from a JS developer and a functional developer perspective. The problem is that JavaScripts Promise specification is not monadic which makes it hard to write an elegant API for.

But imho @aantron recently did a pretty good job with the promise library and I think when you tie it with pipes, you can get a long way without async/await (or rather monadic let).

That being said, the PR including monadic let support just got merged, so it will be very soon possible to use an async/await-ish syntax in Reason.

[aantron]

Thanks for the ping :) Coincidentally, I just opened aantron/promise#52 about async/await using the new let in Promise about this, at prompting of @mrmurphy.