-Xasync when used with js.Promise potentially generates invalid code

[aappddeevv]

This is a bit experimental and I'm not sure I fully understand the -Xasync FSM code generation, but while it works for Option and Future (via the tests) invalid code seems to be generated for scala.js+js.Promise because it is assumed you can extract the value out. A couple of notes from retronym on scala/contributors made me think this might work but I'm not sure it does under scala.js or any effect that cannot be queried.

Here's a repo with the code https://github.com/aappddeevv/scalajs-promise-async. The issue is that even for the simplest case (sbt tests/testOnly ttg.scalajs.async.SmokeTest) the code generated would never work even if my implementation is wrong.

We can't extract a value directly from a js.Promise so the impl must always work with js.Promise . However, the code generated in case 3 is clearly doing a cast via $uI which will always fail.

It's highly likely the problem is my code, but at first blush it seems like this won't ever work for js.Promise.

// code for adding 3 Int js.Promises together.
class $c_Lttg_scalajs_async_SmokeTest$stateMachine$async$1 extends $c_Lttg_scalajs_async_PromiseStateMachine {
  constructor() {
    super();
    this.Lttg_scalajs_async_SmokeTest$stateMachine$async$1__f_result1 = 0;
    this.Lttg_scalajs_async_SmokeTest$stateMachine$async$1__f_result2 = 0;
    $ct_Lttg_scalajs_async_PromiseStateMachine__(this)
  };
  apply__sjs_js_Thenable__V(tr$async) {
    while (true) {
      try {
        switch (this.Lttg_scalajs_async_PromiseStateMachine__f_state$async) {
          case 0: {
            const awaitable$async = Promise.resolve(1);
            tr$async = this.getCompleted__sjs_js_Promise__sjs_js_Promise(awaitable$async);
            this.Lttg_scalajs_async_PromiseStateMachine__f_state$async = 1;
            if ((tr$async !== null)) {
              /*<skip>*/
            } else {
              this.onComplete__sjs_js_Thenable__V(awaitable$async);
              return (void 0)
            };
            break
          }
          case 1: {
            const tryGetResult$async = this.tryGet__sjs_js_Promise__O(tr$async);
            if ((this === tryGetResult$async)) {
              return (void 0)
            };
            this.Lttg_scalajs_async_SmokeTest$stateMachine$async$1__f_result1 = $uI(tryGetResult$async);
            const awaitable$async$2 = Promise.resolve(2);
            tr$async = this.getCompleted__sjs_js_Promise__sjs_js_Promise(awaitable$async$2);
            this.Lttg_scalajs_async_PromiseStateMachine__f_state$async = 2;
            if ((tr$async !== null)) {
              /*<skip>*/
            } else {
              this.onComplete__sjs_js_Thenable__V(awaitable$async$2);
              return (void 0)
            };
            break
          }
          case 2: {
            const tryGetResult$async$2 = this.tryGet__sjs_js_Promise__O(tr$async);
            if ((this === tryGetResult$async$2)) {
              return (void 0)
            };
            this.Lttg_scalajs_async_SmokeTest$stateMachine$async$1__f_result2 = $uI(tryGetResult$async$2);
            const awaitable$async$3 = Promise.resolve(3);
            tr$async = this.getCompleted__sjs_js_Promise__sjs_js_Promise(awaitable$async$3);
            this.Lttg_scalajs_async_PromiseStateMachine__f_state$async = 3;
            if ((tr$async !== null)) {
              /*<skip>*/
            } else {
              this.onComplete__sjs_js_Thenable__V(awaitable$async$3);
              return (void 0)
            };
            break
          }
          case 3: {
            const tryGetResult$async$3 = this.tryGet__sjs_js_Promise__O(tr$async);
            if ((this === tryGetResult$async$3)) {
              return (void 0)
            };
            $uI(tryGetResult$async$3);
            this.completeSuccess__O__V(((this.Lttg_scalajs_async_SmokeTest$stateMachine$async$1__f_result1 + this.Lttg_scalajs_async_SmokeTest$stateMachine$async$1__f_result2) | 0));
            return (void 0);
            break
          }
          default: {
            const i = this.Lttg_scalajs_async_PromiseStateMachine__f_state$async;
            throw $ct_jl_IllegalStateException__T__(new $c_jl_IllegalStateException(), ("" + i))
          }
        }
      } catch (e) {
        const e$2 = $m_sjsr_package$().wrapJavaScriptException__O__jl_Throwable(e);
        this.completeFailure__jl_Throwable__V(e$2);
        return (void 0)
      }
    }
  };
  apply__O__O(v1) {
    this.apply__sjs_js_Thenable__V(v1)
  };
}

[sjrd]

What is the issue that we can address in Scala.js, here? It's not like we can add the ability to extract the value of a js.Promise.

[aappddeevv]

If you see something that I did wrong, then perhaps there is a way around the issue. I'm fine concluding that it is not possible in scala.js using -Xasync but I did not want to make any assumptions without further review of the issue. There is another cps'ish plugin brewing for dotty so perhaps that's going to provide a better path if -Xasync won't work.

[sjrd]

My hypothesis is that you should take a Try[AnyRef] as parameter of tryGet and implement it like for Future. Your getCompleted method should return a Try[AnyRef] that is always null, indicating that the status is not know yet. And in onComplete, you should translate the parameters passed in the callback to a Try[AnyRef].

[aappddeevv]

I'll think through that.

The challenge I have in returning null from getCompleted is that it short circuits in case 0 javascript code and returns the first result immediately, which is wrong, i.e., the test returns the result of the first promise which is 1 not 3. That return statement is not there for the Option version as the FSM seems to expand it different when onComplete is ???, but that's not an option for this.

X ttg.scalajs.async.SmokeTest.js.Promise 1 3ms 
  utest.AssertionError: println("Macro success, running assert"); assert(result == 3)
  result: Int = 1
    utest.asserts.Asserts$.assertImpl(tests-test-fastopt.js:15603)
    ttg.scalajs.async.SmokeTest$.ttg$scalajs$async$SmokeTest$$$anonfun$tests$3(tests-test-fastopt.js:22473)
    <jscode>.{anonymous}()(tests-test-fastopt.js:22452)
    <jscode>.processTicksAndRejections(task_queues.js:93)

[aappddeevv]

I'll also try dotty-cps-async and see if that works. If one of them works, then I think that's good enough.

[aappddeevv]

Alright, dotty-cps-async seems to work. I'll publish a short blog on this topic and ask others for help around -Xasync to see if it can work. So we can close this.