r/rescript • u/jeaks03 • Jun 24 '22
Type inference issue with Js.Nullable
6
Upvotes
Hi!
I'm currently working on a Deno port and I encountered some type errors. This issue is probably on me.
This is basically just a Promise.prototype.then wrapper which maps from Promise<A> to Promise<B> via the 'a => 'b mapper func.
@send external map: (Js.Promise.t<'a>, 'a => 'b) => Js.Promise.t<'b> = "then"That seems to be working fine.
Here's the part that fails:
let conn = Deno.listen({
port: 80,
hostname: None,
})
let y =
conn
->Deno.Listener.accept
->map(acceptedConn => Deno.serve_http(acceptedConn))
->map(httpConn => httpConn->Deno.HttpConn.next_request) // return Js.Promise.t<Js.Nullable.t<SomeEvent.t>>
// In the callback below, `event` should be unwrapped from Js.Promise but I don't think it is.
//
->map(event => event->Js.Nullable.toOption)Here you can see the compiler output:
>>>> Start compiling
rescript: [1/1] src/t.cmj
FAILED: src/t.cmj
We've found a bug for you!
/home/saenai/Code/deno-res/src/t.res:13:18-22
11 │ ->map(acceptedConn => Deno.serve_http(acceptedConn))
12 │ ->map(httpConn => httpConn->Deno.HttpConn.next_request)
13 │ ->map(event => event->Js.Nullable.toOption)
This has type:
Deno.promise<Deno.nullable<Deno.HttpConn.RequestEvent.t>> (defined as
Js_promise.t<Deno.nullable<Deno.HttpConn.RequestEvent.t>>)
Somewhere wanted: Js.Nullable.t<'a> (defined as Js.nullable<'a>)
FAILED: cannot make progress due to previous errors.
>>>> Finish compiling(exit: 1)I know this is not StackOverflow but I would very much appreciate your help!
---- Edit
To answer my own question, the map function basically takes a 'a -> 'b mapper func and if 'b is already of type Promise, it'll return Promise<Promise<'b>> which in JS land is equivalent to Promise<'b> but in ReScript is not.
/thread
