Promise
Reason has built-in support for JavaScript promises through BuckleScript. 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 how to create a JS promise on the Reason 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 (they have a dot). make
returns the created promise.
Usage
Using the pipe operator (see "Composition operators" in the Pervasives module):
RElet 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!