The Future is Now

Future is a lightweight, functional alternative to Js.Promise for ReScript, designed to make async code more composable and maintainable.

Compatibility

• Use [email protected] for ReasonML or ReScript ≤ v10.
• Use [email protected] for ReScript ≥ v11.

Key Benefits of Using Future Over Promises:

• Lighter weight – Only ~25 lines of implementation.
• Simpler – Futures only resolve to a single type (as opposed to resolve and reject types), giving you more flexibility on your error handling.
• More robust – Futures have sound typing (unlike JS promises).

Installation

First make sure you have rescript >= 11.1.X. Then install with npm:

$ npm install --save reason-future

Then add "reason-future" to your rescript.json dev dependencies:

{
  ...
  "bs-dependencies": [
    "reason-future"
  ]
}

Basic Usage

To create a task, use Future.make. It provides a single resolve function, similar to how Promises work but without a reject:

let futureGreeting = Future.make(resolve => resolve("hi"));

To get the value of a future, use Future.get:

let futureGreeting = Future.make(resolve => resolve("hi"));
futureGreeting->Future.get(x => Js.log("Got value: " ++ x));

/* Alternatively: */

Future.make(resolve => resolve("hi"))
->Future.get(x => Js.log("Got value: " ++ x));

Future.get only retrieves the future value. If you want to transform it to a different value, then you should use Future.map:

/* Shortcut for: let future_A = Future.make(resolve => resolve(99)); */
let future_A = Future.value(99);

let future_B = future_A->Future.map(n => n + 1);


future_A->Future.get(n => Js.log(n)); /* logs: 99 */

future_B->Future.get(n => Js.log(n)); /* logs: 100 */

And finally, if you map a future and return another future, you probably want to flatMap instead:

let futureNum = Future.value(50);

let ft_a = futureNum->Future.map(n => Future.value(n + 10));
let ft_b = futureNum->Future.flatMap(n => Future.value(n + 20));

/* ft_a has type future(future(int)) – probably not what you want. */
/* ft_b has type future(int) */

API

Core functions. Note: _ represents the future itself as inserted by -> (the pipe operator).

• Future.make(resolver) - Create a new, potentially-async future.
• Future.value(x) - Create a new future with a plain value (synchronous).
• Future.map(_,fn) - Transform a future value into another value
• Future.flatMap(_,fn) - Transform a future value into another future value
• Future.get(_,fn) - Get the value of a future
• Future.tap(_,fn) - Do something with the value of a future without changing it. Returns the same future so you can continue using it in a pipeline. Convenient for side effects such as console logging.
• Future.all(_,fn) - Turn a list of futures into a future of a list. Used when you want to wait for a collection of futures to complete before doing something (equivalent to Promise.all in Javascript).

Result

Convenience functions when working with a future Result. Note: _ represents the future itself as inserted by -> (the pipe operator).

Note 2: The terms Result.Ok and Result.Error in this context are expected to be read as Ok and Error.

• Future.mapOk(_,fn) - Transform a future value into another value, but only if the value is an Result.Ok. Similar to Promise.prototype.then
• Future.mapError(_,fn) - Transform a future value into another value, but only if the value is an Result.Error. Similar to Promise.prototype.catch
• Future.tapOk(_,fn) - Do something with the value of a future without changing it, but only if the value is a Ok. Returns the same future. Convenience for side effects such as console logging.
• Future.tapError(_,fn) - Same as tapOk but for Result.Error

The following are more situational:

• Future.flatMapOk(_, fn) - Transform a future Result.Ok into a future Result. Flattens the inner future.
• Future.flatMapError(_, fn) - Transform a future Result.Error into a future Result. Flattens the inner future.

FutureJs

Convenience functions for interop with JavaScript land.

• FutureJs.fromPromise(promise, errorTransformer)
  • promise is the RescriptCore.Promise.t('a) that will be transformed into a Future.t(result('a, 'e))
  • errorTransformer allows you to determine how Promise.error objects will be transformed before they are returned wrapped within a Error. This allows you to implement the error handling method which best meets your application's needs.
• FutureJs.toPromise(future)
  • future is any Future.t('a) which is transformed into a RescriptCore.Promise.t('a). Always resolves, never rejects the promise.
• FutureJs.resultToPromise(future)
  • future is the Future.t(result('a, 'e)) which is transformed into a RescriptCore.Promise.t('a). Resolves the promise on Ok result and rejects on Error result.

Example use:

/*
  This error handler is super simple; you will probably want
  to write something more sophisticated in your app.
*/
let handleError = Js.String.make;

somePromiseGetter()
->FutureJs.fromPromise(handleError)
->Future.tap(value => Js.log2("It worked!", value))
->Future.tapError(err => Js.log2("uh on", err));

See Composible Error Handling in OCaml for several strategies that you may employ.

Stack Safety

By default this library is not stack safe, you will recieve a 'Maximum call stack size exceeded' error if you recurse too deeply. You can opt into stack safety by passing an optional parameter to the constructors of trampoline. This has a slight overhead. For example:

let stackSafe = Future.make(~executor=`trampoline, resolver);
let stackSafeValue = Future.value(~executor=`trampoline, "Value");

TODO

• [ ] Implement cancellation tokens
• [x] Interop with Js.Promise
• [x] flatMapOk / flatMapError (with composable error handling)

Build

npm run build

Build + Watch

npm run start

Test

npm test

Editor

If you use vscode, Press Windows + Shift + B it will build automatically