Promise returned undefined instead of value (10.0.1 & 10.1.1)

[El-Fitz]

Thank you for filing! Check list:

• Is it a bug? Usage questions should often be asked in the forum instead.
• Concise, focused, friendly issue title & description.
• A minimal, reproducible example.
• OS and browser versions, if relevant.
• Is it already fixed in master?

===========================================

Hi!

I've noticed, in some conditions, some promises returning "undefined" instead of the value they should return.
I got it working by using a different syntax, but I thought you might be interested.

I've got the following code

// Adapter.res

@genType
type repositoryResult = string

@genType
type serviceResult = string

type adapter = {
  firstAction: Inputs.firstAction => promise<serviceResult>,
  otherAction: Inputs.otherAction => promise<array<serviceResult>>,
}

let mapResult = (repositoryResult: repositoryResult) => repositoryResult

@genType
let make = (repository: repository): adapter => {
  firstAction: (input: Inputs.firstAction) =>
    input->repository.doSomething->then(result => result->mapResult->resolve),
  otherAction: (input: Inputs.otherAction) =>
    input->repository.otherAction->then(outputs => {
      // outputs the expected values
      Js.log2("outputs", outputs)
      Belt.Array.map(outputs, mapResult)->resolve;
  }),
}

Which compiles to

// Adapter.mjs

// Generated by ReScript, PLEASE EDIT WITH CARE

import * as Curry from "rescript/lib/es6/curry.js";
import * as Belt_Array from "rescript/lib/es6/belt_Array.js";
import * as Js_promise2 from "rescript/lib/es6/js_promise2.js";

var Inputs = {};

function mapResult(repositoryResult) {
  return repositoryResult;
}

function make(repository) {
  return {
          firstAction: (function (input) {
              return Js_promise2.then(Curry._1(repository.firstAction, input), (function (result) {
                            return Promise.resolve(result);
                          }));
            }),
          otherAction: (function (input) {
              return Js_promise2.then(Curry._1(repository.otherAction, input), (function (summaries) {
                            console.log("outputs", outputs);
                            return Promise.resolve(Belt_Array.map(outputs, mapResult));
                          }));
            })
        };
}

export {
  Inputs ,
  mapResult ,
  make ,
}
/* No side effect */

In another module, I use it like this:

// UseCase.res

let doSomething = (
  dependencies: dependencies,
  {input, idealChunkLength, idealLength, idealCount}: params,
) =>
  input
  ->ChunkingUtil.chunkInput(idealChunkLength)
  ->Belt.Array.map(({paragraphs}) => textBlocksToText(paragraphs))
  ->(
    chunks => 
      Adapter.make(dependencies.textSummarisationAdapterRepository).otherAction({
        chunks chunks,
        idealLength,
        idealCount,
      })->then(results => {
        // outputs undefined
        Js.log2("Results: ", results)
        mergeOutputs(results)->resolve
      })
  )

Interestingly, Js.log2("outputs", outputs) in Adapter.res prints the expected values.
But Js.log2("Results: ", results) prints undefined (and the rest of the logic falls apart afterwards accordingly).
And it was the case using both 10.0.1 (with@ryyppy/rescript-promise) and 10.1.1 (with Js.Promise2)

However, when updating the otherAction in Adapter.res to use async / await syntax, like below, it all works as expected, and both Js.log2 output the expected values.

@genType
let make = (repository: repository): adapter => {
  ...
  otherAction: async (input: Inputs.otherAction) => {
    let summaries = await input->repository.otherAction
    Js.log2("outputs", outputs)
    Belt.Array.map(outputs, mapResult);
  }
}

which compiles to

function make(repository) {
  return {
          ...
          otherAction: (async function (input) {
              var outputs = await Curry._1(repository.otherAction, input);
              console.log("outputs", outputs);
              return Belt_Array.map(outputs, mapResult);
            })
}

[cristianoc]

Would you be able to give a self-contained example (can be compiled without any additional code), with only the minimal code strictly needed to observe the difference. (E.g. firtstAction is not actually used so could be omitted), perhaps with 2 functions otherAction and otherAction2 to show the difference when calling one vs the other.

[El-Fitz]

Sure! I'll try to that over the weekend

[eshurakov]

I'm also experiencing something similar (ReScript 10.1.2).

Example code:

Js.Promise2.resolve("hello")
->Js.Promise2.then(v => {
  Js.log(v)
  Js.Promise2.resolve("hello 2")
})
->Js.Promise2.then(v => {
  Js.log(v)
  Js.Promise2.resolve("hello 3")
})
->ignore

When run it prints

hello
undefined

And I expect it to print:

hello
hello 2

Compiled code:

var Js_promise2 = require("rescript/lib/js/js_promise2.js");

Js_promise2.then(Js_promise2.then(Promise.resolve("hello"), (function (v) {
  console.log(v);
  return Promise.resolve("hello 2");
})), (function (v) {
        console.log(v);
        return Promise.resolve("hello 3");
      }));

If I try to log the result of calling the Js_promise2.then

console.log(Js_promise2.then(Promise.resolve("hello"), (function (v) {
  return Promise.resolve("hello 2");
})))

I get undefined.

---

I also get undefined if I run the code sample from here: https://rescript-lang.org/docs/manual/latest/promise#access-the-contents-and-transform-a-promise

[cristianoc]

Silly mistake. Fix in #5996 will be in the next 10.3 bug fix release.