You are currently looking at the v6.0 - v8.2 docs (Reason v3.6 syntax edition). You can find the latest manual page here.
(These docs are equivalent to the old BuckleScript docs before the ReScript rebrand)
Promise
ReScript has built-in support for JavaScript promises. The 3 functions you generally need are:
Js.Promise.resolve: 'a => Js.Promise.t('a)
Js.Promise.then_: ('a => Js.Promise.t('b), Js.Promise.t('a)) => Js.Promise.t('b)
Js.Promise.catch: (Js.Promise.error => Js.Promise.t('a), Js.Promise.t('a)) => Js.Promise.t('a)
Additionally, here's the type signature for creating a promise on the ReScript side:
Js.Promise.make: (
(
~resolve: (. 'a) => unit,
~reject: (. exn) => unit
) => unit
) => Js.Promise.t('a);
This type signature means that make
takes a callback that takes 2 named arguments, resolve
and reject
. Both arguments are themselves uncurried callbacks (with a dot). make
returns the created promise.
Usage
Using the pipe operator:
let myPromise = Js.Promise.make((~resolve, ~reject) => resolve(. 2));
myPromise
|> Js.Promise.then_(value => {
Js.log(value);
Js.Promise.resolve(value + 2);
})
|> Js.Promise.then_(value => {
Js.log(value);
Js.Promise.resolve(value + 3);
})
|> Js.Promise.catch(err => {
Js.log2("Failure!!", err);
Js.Promise.resolve(-2);
});
Note: we might offer a dedicated syntax for JS promises (async/await) in the future. Stay tuned!