let%Everything ReasonML

let%Anything is a really cool module for ReasonML,

Which allows you to create a module and use as with let%Module, lets take a look how to use.

Since buckleScript have upgraded to 7++ by default and I am using bs-let for demo:

Create a module and method that you like, with let_.

Here I create a very basic module of MapEmpty with let_, that takes a parameter and returns a callback which will be used as promise.

module MapEmpty = {
  let let_ = (params, cb) => params->Belt.Option.getWithDefault("")->cb;
};

So we can use this module as %Module

 {
  let a = None;

  let%MapEmpty b = a;

  Js.log(b);
};

You can find very cool introduction on EggHead of how you can use with async/await to replace promise in ReasonML.

Although EggHead is using promise from got module, I like to use bs-fetch so that you don't have to write binding. Lets install bs-fetch

yarn add bs-fetch

And remember to add bs-fetch to your bsconfig file

Now we want to create a Fetch module that will call a ajax and return the json data

Module Fetch = {
  let perform = url =>
    Js.Promise.(
      Fetch.fetch(url)
      |> then_(Fetch.Response.json)
      |> then_(json => json |> resolve)
    );
}

and create the async method

Module Async = {
  let let_ = (prop, cb) => Js.Promise.then_(cb, prop);
}

so now you can use it like the way you want like Javascript async/await

{
  let%Async res = Promise.perform("http://www.your-json.com/xxx.json");

  Js.log(res); // here you will received the json
  Js.Promise.resolve();
};