package promise

  1. Overview
  2. Docs
Native implementation of a JS promise binding


Dune Dependency






Published: 19 Oct 2020



A lightweight, type-safe binding to JS promises:

Js.log(Promise.resolved("Hello"));  /* Promise { 'Hello' } */

-> => s ++ " world!")
->Promise.get(s => Js.log(s));      /* Hello world! */

As you can see on the first line, Promise.t maps directly to familiar JS promises from your JS runtime. That means...

  • You can use reason-promise directly to write JS bindings.

  • All JS tooling for promises immediately works with reason-promise.

  • Even if you do something exotic, like switch out the promise implementation at the JS level, for, say, better stack traces, reason-promise still binds to it!

There is only one exception to the rule that Promise.t maps directly to JS promises: when there is a promise nested inside another promise. JS breaks the type safety of promises in a misguided attempt to disallow nesting. reason-promise instead emulates it in a way that makes promises type-safe again. This is in contrast to BuckleScript's built-in Js.Promise, which directly exposes the JS behavior, and so is not type-safe.

In addition:



npm install reason-promise

Then, add reason-promise to your bsconfig.json:

  "bs-dependencies": [

Getting started

To quickly get a project for pasting the code examples, clone the example repo. The code is in

git clone
cd promise-example-bsb
npm install
npm run test    # To run each example.

There it also an example repo with a trivial binding to parts of node-fetch.

While reading the tutorial, it can be useful to glance at the type signatures of the functions from time to time. They provide a neat summary of what each function does and what it expects from its callback.

Creating new promises

The most basic function for creating a new promise is Promise.pending:

let (p, resolve) = Promise.pending();
Js.log(p);    /* Promise { <pending> } */

The second value returned, resolve, is a function for resolving the promise:

let (p, resolve) = Promise.pending();
Js.log(p);    /* Promise { 'Hello' } */

Promise.resolved is a helper that returns an already-resolved promise:

let p = Promise.resolved("Hello");
Js.log(p);    /* Promise { 'Hello' } */

...and Promise.exec is for wrapping functions that take callbacks:

external setTimeout: (unit => unit, int) => unit = "setTimeout";

let p = Promise.exec(resolve => setTimeout(resolve, 1000));
Js.log(p);    /* Promise { <pending> } */

/* Program then waits for one second before exiting. */

Getting values from promises

To do something once a promise is resolved, use Promise.get:

let (p, resolve) = Promise.pending();

p->Promise.get(s => Js.log(s));

resolve("Hello");   /* Prints "Hello". */

Transforming promises

Use to transform the value inside a promise:

let (p, resolve) = Promise.pending();

-> => s ++ " world")
->Promise.get(s => Js.log(s));

resolve("Hello");   /* Hello world */

To be precise, creates a new promise with the transformed value.

If the function you are using to transform the value also returns a promise, use Promise.flatMap instead of Promise.flatMap will flatten the nested promise.


If you have a chain of promise operations, and you'd like to inspect the value in the middle of the chain, use Promise.tap:

let (p, resolve) = Promise.pending();

->Promise.tap(s => Js.log("Value is now: " ++ s))
-> => s ++ " world")
->Promise.tap(s => Js.log("Value is now: " ++ s))
->Promise.get(s => Js.log(s));


Value is now: Hello
Value is now: Hello world
Hello world

Concurrent combinations

Promise.race waits for one of the promises passed to it to resolve:

external setTimeout: (unit => unit, int) => unit = "setTimeout";

let one_second = Promise.exec(resolve => setTimeout(resolve, 1000));
let five_seconds = Promise.exec(resolve => setTimeout(resolve, 5000));

Promise.race([one_second, five_seconds])
->Promise.get(() => { Js.log("Hello"); exit(0); });

/* Prints "Hello" after one second. */

Promise.all instead waits for all of the promises passed to it, concurrently:

external setTimeout: (unit => unit, int) => unit = "setTimeout";

let one_second = Promise.exec(resolve => setTimeout(resolve, 1000));
let five_seconds = Promise.exec(resolve => setTimeout(resolve, 5000));

Promise.all([one_second, five_seconds])
->Promise.get(_ => { Js.log("Hello"); exit(0); });

/* Prints "Hello" after five seconds. */

For convenience, there are several variants of Promise.all:

Handling errors with Result

Promises that can fail are represented using the standard library's Result, and its constructors Ok and Error:

open Belt.Result;

->Promise.getOk(s => Js.log(s));      /* Hello */

Promise.getOk waits for p to have a value, and runs its function only if that value is Ok(_). If you instead resolve the promise with Error(_), there will be no output:

open Belt.Result;

->Promise.getOk(s => Js.log(s));      /* Program just exits. */

You can wait for either kind of value by calling Promise.getOk and Promise.getError:

open Belt.Result;

let () = {
  let p = Promise.resolved(Error("Failed"));
  p->Promise.getOk(s => Js.log(s));
  p->Promise.getError(s => Js.log("Error: " ++ s));
};                                    /* Error: Failed */

...or respond to all outcomes using the ordinary Promise.get:

open Belt.Result;

->Promise.get(result =>
  switch (result) {
  | Ok(s) => Js.log(s);
  | Error(s) => Js.log("Error: " ++ s);
  });                                 /* Error: Failed */

The full set of functions for handling results is:

There are also similar functions for working with Option:

In addition, there is also a set of variants of Promise.all for results, which propagate any Error(_) as soon as it is received:

If you'd like instead to fully wait for all the promises to resolve with either Ok(_) or Error(_), you can use the ordinary Promise.all and its variants.

Advanced: Rejection

As you can see from Handling errors, Promise doesn't use rejection for errors — but JavaScript promises do. In order to support bindings to JavaScript libraries, which often return promises that can be rejected, Promise provides the Promise.Js helper module.

Promise.Js works the same way as Promise. It similarly has:

However, because Promise.Js uses JS rejection for error handling rather than Result or Option,

Underneath, Promise and Promise.Js have the same implementation:

type Promise.t('a) = Promise.Js.t('a, never);

That is, Promise is really Promise.Js that has no rejection type, and no exposed helpers for rejection.

There are several helpers for converting between Promise and Promise.Js:

Promise.Js.catch can also perform a conversion to Promise, if you simply convert a rejection to a resolution. In the next example, note the final line is no longer using Promise.Js, but Promise:

->Promise.Js.catch(s => Promise.resolved("Error: " ++ s))
->Promise.get(s => Js.log(s));        /* Error: Failed */

There are also two functions for converting between Promise.Js and the current promise binding in the BuckleScript standard libarary, Js.Promise:

Because both libraries are bindings for the same exact kind of value, these are both no-op identity functions that only change the type.

Advanced: Bindings

Refer to the example node-fetch binding repo.

When you want to bind a JS function that returns a promise, you can use Promise directly in its return value:

/* A mock JS library. */
[%%bs.raw {|
function delay(value, milliseconds) {
  return new Promise(function(resolve) {
    setTimeout(function() { resolve(value); }, milliseconds)

/* Our binding. */
external delay: ('a, int) => Promise.t('a) = "delay";

/* Usage. */
delay("Hello", 1000)
->Promise.get(s => Js.log(s));

/* Prints "Hello" after one second. */

If the promise can be rejected, you should use Promise.Js instead, and convert to Promise as quickly as possible, with intelligent handling of rejection. Here is one way to do that:

/* Mock JS library. */
[%%bs.raw {|
function delayReject(value, milliseconds) {
  return new Promise(function(resolve, reject) {
    setTimeout(function() { reject(value); }, milliseconds)

/* Binding. */
external delayRejectRaw: ('a, int) => Promise.Js.t(_, 'a) = "delayReject";
let delayReject = (value, milliseconds) =>
  delayRejectRaw(value, milliseconds)

/* Usage. */
delayReject("Hello", 1000)
->Promise.getError(s => Js.log(s));

/* Prints "Hello" after one second. */

Note that this binding has two steps: there is a raw binding, and then an extra wrapper that converts rejections into Results. If the potential rejections are messy, this is a good place to insert additional logic for converting them to nice Reason values :)

When passing a promise to JS, it is generally safe to use Promise rather than Promise.Js:

/* Mock JS library. */
[%%bs.raw {|
function log(p) {
  p.then(function (v) { console.log(v); });

/* Binding. */
external log: Promise.t('a) => unit = "log";

/* Usage. */
log(Promise.resolved("Hello"));       /* Hello */

Discussion: Why JS promises are unsafe

The JS function Promise.resolve has a special case, which is triggered when you try to resolve a promise with another, nested promise. Unfortunately, this special case makes it impossible to assign Promise.resolve a consistent type in Reason (and most type systems).

Here are the details. The code will use Js.Promise.resolve, BuckleScript's direct binding to JS's Promise.resolve.

Js.Promise.resolve takes a value, and creates a promise containing that value:

/* Promise { 1 } */

/* Promise { 'foo' } */

So, we should give it the type

Js.Promise.resolve: 'a => Js.Promise.t('a);

and, indeed, that's the type it has in BuckleScript.

Following the pattern, we would expect:

let nestedPromise = Js.Promise.resolve(1);

/* Promise { Promise { 1 } } */

But that's not what happens! Instead, the output is just

/* Promise { 1 } */

The nested promise is missing! But the type system, following the pattern, still thinks that this resulting value has type


i.e., the type of the value we were (reasonably) expecting.

When you pass nestedPromise to Js.Promise.resolve, JS unwraps nestedPromise, violating the type! There is no easy way to encode such special casing in the type system — especially since JS does it not only to nested promises, but to any would-be nested object that has a .then method.

The result is, if your program executes something like this, it will have ordinary values in places where it expects another level of promises. For example, if you do

let nestedPromise = Js.Promise.resolve(1);

->Js.Promise.then_(p => /* ... */)

you would expect p in the callback to be a promise containing 1, and the type of p is indeed Js.Promise.t(int). Instead, however, p is just the bare value 1. That means the callback will cause a runtime error as soon as it tries to use promise functions on the 1. Worse, you might store p in a data structure, and the runtime error will occur at a very distant place in the code. The type system is supposed to prevent such errors! That's part of the point of using Reason.

The same special casing occurs throughout the JS Promise API — for example, when you return a promise from the callback of then_. This means that most of the JS Promise functions can't be assigned a correct type and directly, safely be used from Reason.

Discussion: How reason-promise makes promises type-safe

The previous section shows that JS promise functions are broken. An important observation is that it is only the functions that are broken — the promise data structure is not. That means that to make JS promises type-safe, we can keep the existing JS data structure, and just provide safe replacement functions to use with it in Reason. This is good news for interop :)

To fix the functions, only the special-case flattening has to be undone. So, when you call reason-promise's Promise.resolved(value), it checks whether value is a promise or not, and...

  • If value is not a promise, reason-promise just passes it to JS's Promise.resolve, because JS will do the right thing.

  • If value is a promise, it's not safe to simply pass it to JS, because it will trigger the special-casing. So, reason-promise boxes the nested promise:

    let nestedPromise = Promise.resolved(1);
    /* Promise { PromiseBox { Promise { 1 } } } */

    This box, of course, is not a promise, so inserting it in the middle is enough to suppress the special-casing.

    Whenever you try to take the value out of this resulting structure (for example, by calling Promise.get on it), reason-promise transparently unboxes the PromiseBox and passes the nested promise to your callback — as your callback would expect.

This conditional boxing and unboxing is done throughout reason-promise. It only happens for nested promises, and anything else with a .then method. For all other values, reason-promise behaves, internally, exactly like JS Promise (though with a cleaner outer API). This is enough to make promises type-safe.

This is a simple scheme, but reason-promise includes a very thorough test suite to be extra sure that it always manages the boxing correctly.

This conditional boxing is similar to how unboxed optionals are implemented in BuckleScript. Optionals are almost always unboxed, but when BuckleScript isn't sure that the unboxing will be safe, it inserts a runtime check that boxes some values, while still keeping most values unboxed.

Dependencies (4)

  1. result
  2. reason build & >= "3.3.2"
  3. ocaml
  4. dune

Dev Dependencies





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