How to give a type declaration for the funtion that return a promise?

I’m writing a type declaration for koa framework, but I don’t know how to declare a funtion which has promise type.The lib code is here:

app.use(function (context, next) {
next().then(() => {

Now in my project, the type code is

type middleware = ?
@send external use: (app, middleware) => unit = “use”

what do I need to write for type middleware, Can anybody help me?

type middleware = unit => Js.Promise.t<unit>
@send external use: (app, middleware) => unit = "use"

More docs here: Async & Promise | ReScript Language Manual

Emmm…,like this

type context = {mutable body: string}
type middleware = (context, unit => Js.Promise.t<unit>) => unit
@send external use: (app, middleware) => unit = "use"

then my rescript is:

app->use((context, next) => {

But I got err that say:

FAILED: examples/index.cmj

  We've found a bug for you!

   9 │ // you can use middleware like this
  10 │ app->use((context, next) => {
  11 │   next()->Js.Promise.then_();
  12 │ })
  13 │ 

  This has type: Js.Promise.t<unit> (defined as Js_promise.t<unit>)
  Somewhere wanted: 'a => Js.Promise.t<'b>

FAILED: cannot make progress due to previous errors.

You’ve added a context argument to the definition, but when you call the next function you don’t pass the context.

Should be something like: next(context)->Js.Promise.then_();

I changed next()->Js.Promise.then_() to next()->then()with @ryyppy/rescript-promise lib, everything is Ok!!!
Thank you for your help! :hugs: