question-mark

Stuck on an issue?

Lightrun Answers was designed to reduce the constant googling that comes with debugging 3rd party libraries. It collects links to all the places you might be looking at while hunting down a tough bug.

And, if you’re still stuck at the end, we’re happy to hop on a call to see how we can help out.

`concatMap` omits values that `mergeMap` catches

See original GitHub issue

Issue Description

Hi There,

I’ve added two extensions to Wonka that potentially highlight an incompatibility between concatMap and mergeMap.

iterate A new source that represents a potentially infinite generator from a starting value and an iteration step. Analogous to Haskell’s iterate (https://hackage.haskell.org/package/base-4.14.0.0/docs/Prelude.html#v:iterate) but with optional termination.

let iterate =
    (start: 'a, next: 'a => option('a)): Wonka.Types.sourceT(int) => {
  Wonka.Types.curry(sink => {
    let state: Wonka.trampolineT(option('a)) = {
      ended: false,
      looping: false,
      pulled: false,
      current: Some(start),
    };

    sink(.
      Wonka.Types.Start(
        (. signal) =>
          switch (signal, state.looping) {
          | (Pull, false) =>
            state.pulled = true;
            state.looping = true;

            while (state.pulled && !state.ended) {
              switch (state.current) {
              | Some(x) =>
                state.current = next(x);
                state.pulled = false;
                sink(. Push(x));
              | None =>
                state.ended = true;
                sink(. End);
              };
            };
          | (Pull, true) => state.pulled = true
          | (Close, _) => state.ended = true
          },
      ),
    );
  });
};

downTo An operator for getting numbers start, start - 1, … down to a provided bound. E.g., downTo(3, 1) would produce [|3, 2, 1|] in a stream.

let downTo = (start: int, bound: int): Wonka.Types.sourceT(int) =>
  iterate(start, x => x <= bound ? None : Some(x - 1));

toArrayAsync A sink that captures all stream values in an array, wrapped up in a promise. This fixes toArray to work with asynchronous streams.

let toAsyncArray = (source: Wonka.Types.sourceT('a)): Js.Promise.t(array('a)) =>
  Js.Promise.make((~resolve, ~reject as _) => {
    let state: Wonka.toArrayStateT('a) = {
      values: Rebel.MutableQueue.make(),
      talkback: (. _) => (),
      value: None,
      ended: false,
    };

    source((. signal) =>
      switch (signal) {
      | Start(x) =>
        state.talkback = x;
        x(. Pull);
      | Push(value) =>
        Rebel.MutableQueue.add(state.values, value);
        state.talkback(. Pull);
      | End =>
        state.ended = true;
        state.talkback(. Close);
        let finalArray = Rebel.MutableQueue.toArray(state.values);
        resolve(. finalArray);
      }
    );

    ();
  });

Some tests to demonstrate how these work:

test("Wonka iterate down.", () =>
  Library.Wonkatools.downTo(3, 0)
  |> Wonka.toArray
  |> expect == [|3, 2, 1, 0|]
);

test("Wonka sync arrays dont collect", () =>
  [|1, 2, 3|]
  |> Wonka.fromArray
  |> Wonka.mergeMap((. x) => Js.Promise.resolve(x) |> Wonka.fromPromise)
  |> Wonka.toArray
  |> expect == [||]
);

testPromise("Wonka async arrays do collect", () =>
  [|1, 2, 3|]
  |> Wonka.fromArray
  |> Wonka.mergeMap((. x) => Js.Promise.resolve(x) |> Wonka.fromPromise)
  |> Library.Wonkatools.toAsyncArray
  |> Js.Promise.then_(array => array |> expect == [|1, 2, 3|] |> Js.Promise.resolve)
);

The issue

Finally, here is a description of the issue. Below is a test that times out (concatMap fails to collect any values). It times out:

Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout.Timeout - Async callback was not invoked within the 5000ms

If I replace concatMap with mergeMap, this test succeeds.

testPromise("Wonka concat problem", () =>
  Library.Wonkatools.downTo(3, 0)
  |> Wonka.concatMap((. x) => Js.Promise.resolve(x) |> Wonka.fromPromise)
  |> Library.Wonkatools.toAsyncArray
  |> Js.Promise.then_(array => array |> expect == [|3, 2, 1, 0|] |> Js.Promise.resolve)
);

Any thoughts on why concatMap is skipping these values?

Tests use bs-jest.

Issue Analytics

  • State:closed
  • Created 2 years ago
  • Comments:5 (3 by maintainers)

github_iconTop GitHub Comments

1reaction
kittencommented, Aug 16, 2020

I see! So I suppose as concatMap was waiting for the promise source to resolve, when it was pulling again the source was stuck in a looping state which didn’t allow it to push anymore, while mergeMap would eagerly pull, which allowed the promises to just resolve in order. That makes sense 😅

Glad this is resolved! I should probably add iterate to the built-in sources (Great Name btw!)

1reaction
Pet3riscommented, Aug 16, 2020

@kitten You’re a legend - everything works!

Read more comments on GitHub >

github_iconTop Results From Across the Web

Understanding RxJS map, mergeMap, switchMap and ...
RxJS comes with a 'normal' map function, but also has functions like mergeMap, switchMap and concatMap which all behave slightly different.
Read more >
RxJs Mapping: switchMap vs mergeMap vs concatMap vs ...
So the map operator is all about mapping the values of the input ... But here is the catch: this only works because...
Read more >
Observable | RxJS API Document - ReactiveX
Combines multiple Observables to create an Observable whose values are calculated from the latest values of each of its input Observables.
Read more >
concatMap vs. switchMap when observable only returns one ...
With switchMap it unsubscribes, while with concatMap it'll wait until the merged Observable completes. But if you know that for example the ...
Read more >
RxJS 5 Operators By Example - gists · GitHub
A complete list of RxJS 5 operators with easy to understand explanations and runnable examples. Table of Contents. buffer; bufferCount; bufferTime; bufferToggle ...
Read more >

github_iconTop Related Medium Post

No results found

github_iconTop Related StackOverflow Question

No results found

github_iconTroubleshoot Live Code

Lightrun enables developers to add logs, metrics and snapshots to live code - no restarts or redeploys required.
Start Free

github_iconTop Related Reddit Thread

No results found

github_iconTop Related Hackernoon Post

No results found

github_iconTop Related Tweet

No results found

github_iconTop Related Dev.to Post

No results found

github_iconTop Related Hashnode Post

No results found