lihaoyi/scala.rx

An experimental library for Functional Reactive Programming in Scala

Roland Reckel

@rreckel

Does an "official" latest version of scala.rx exist? I often try some snippets of code in ScalaFiddle but there the only version is 0.3.2

Anyway I can test the code with the latest master and keep you informed.

Roland Reckel

@rreckel

Hi again,
I am sorry to report that the latest master does not resolve the loop in flatMap with toRx.
The following script can be executed in ammonite:

import $ivy.`com.lihaoyi::scalarx:0.4.0-SNAPSHOT`, rx._, rx.async._
import Ctx.Owner.Unsafe._

import scala.concurrent._
import scala.concurrent.duration._
import scala.concurrent.ExecutionContext.Implicits.global

object FlatMap {

  def test() = {
    def request(id: Int): Future[String] = Future {
      Thread.sleep(500)
      println(s"Request: $id")
      id.toString
    }

    val id = Var[Int](0)
    val idString = id.flatMap(i => request(i).toRx("..."))

    id() = 10
    id() = 20
    println("End...")
    Await.result(Future{Thread.sleep(3000); println("Waited long enough")}, 30 seconds)
  }
}

FlatMap.test()

and produces the following output:

End...
Request: 0
Request: 10
Request: 20
Request: 20
Request: 20
Request: 20
Request: 20
Waited long enough
Request: 20

Felix Dietze

@fdietze

@rreckel Could you please open a PR with a failing test case?

Nick Childers

@Voltir

I've been trying to tackle this problem - it's not the same issue as the flatmap problem. It also seems really hard to solve in a satisfying way.

Making futures compose nicely in Rx contexts is hard because the re-evaluation step recreates the future, creating the cycle of infinite loop

Roland Reckel

@rreckel

Call me crazy, but would it be possible to create a "toRx" function for Monix Task? Would that be easier perhaps, as those are lazy evaluated?

I guess I will have to look at the scala.rx code a bit.

Nick Childers

@Voltir

Sure - I think that's possible

Perhaps something akin to how sttp does its effect encoding https://github.com/softwaremill/sttp

I wouldn't want to add a dependency to monix in the core repo though

Roland Reckel

@rreckel

You are right.... adding a dependency is not good.

Nick Childers

@Voltir

What gives me hope is that toRx does appear to work if the definition is outside of the flatmap - I think if I can make the flatmap macro generate the equivalent of that, we might finally have a decent solution

Minghao Liu

@molikto

@  val a = Var(1)  

a: Var[Int] = Var@70(1)

@  

@ val b = Var(2)  
b: Var[Int] = Var@6b(2)

@  val c = Rx { println("c triggered "+ ( a() + b()))}  
c triggered 3
c: Rx.Dynamic[Unit] = Rx@2e(())


@  

@ a.kill() 


@ b() = 3 
c triggered 4


@ a() = 5 
c triggered 8

so seems kill() doesn't work on Var?

Nick Childers

@Voltir

@molikto ah, thats interesting - what happens if you dont do the b() = 3 step?

kill on a Var just clears the downstream listeners - Vars dont really have an owner. What I think happens is when you trigger a re-evaluation of c via b() = 3, the dependency c has on a is just recreated. I guess "kill" is kind of a misleading name for what actually happens on a Var.

Peng Li

@freewind

Hi guys, I have some questions when testing scala.rx code, and post a question here: https://stackoverflow.com/questions/49073908/how-to-test-vars-of-scala-rx-with-scalatest

Thanks for helping

Nick Childers

@Voltir

@freewind i took a stab at answering it, let me know if you want any clarification

Peng Li

@freewind

@Voltir Thanks for your answer! It gives me some idea and I changed my test using scalatest's AsyncTestSuite and the code is:

test("watchOrders") {
  var result = List.empty[Order]
  val stream = client.watchOrders()
  val promise = Promise[Assertion]()
  stream.fold(List.empty[Order]) {
    case (acc, Some(elem)) =>
      val result = elem :: acc
      println("result size: " + result.size)
      if (result.size == 4) promise.success({
        assert(validateTheOrders(result))
      })
      result
  }
  promise.future
}

The good new is if the validateTheOrders(result) is passed, the test will pass and terminate as expected

But if the assertion is false, it will never stop, and the result size printed in console will be:

result size: 1
result size: 2
result size: 3
result size: 4
result size: 4
result size: 4
result size: 4
...

Peng Li

@freewind

@Voltir I just make it work inspired with the eventually method from haoyi's utest. Now I'm still using scalatest's SyncTestSuit, and the code is like:

  private def waitUntil[T](stream: Rx[T])(cond: T => Boolean, maxWait: Duration): T = {
    val promise = Promise[T]()
    Executors.newSingleThreadScheduledExecutor().scheduleAtFixedRate(() => {
      val list = stream.now
      if (cond(list)) {
        promise.success(list)
      }
    }, 0L, 100L, TimeUnit.MILLISECONDS)
    Await.result(promise.future, maxWait)
  }

  test("watchOrders") {
    val stream = client.watchOrders()
    val listRx: Rx[List[Order]] = stream.fold(List.empty[Order]) {
      case (acc, Some(elem)) => elem :: acc
      case (acc, _) => acc
    }
    val list = waitUntil(listRx)(_.size >= 4, 10.seconds)
    assert(validateTheOrders(list))
  }

It's working great now

Nick Childers

@Voltir

@/all scala.rx 0.4.0 was published

scalway

@scalway

@Voltir where? I cannot find sources.

Oh... there is still old documentation & there is no github release :).

moritz bust

@busti

Why do I have to have an implicit Owner in scope when using rx.fold()() in a class / trait, but not when I use it elsewhere? Can I avoid that somehow?

scalway

@scalway

https://github.com/lihaoyi/scala.rx#ownership-context

This is how scala.rx fights against memory leaks

moritz bust

@busti

Well yes, but if I understand it correctly, the context is generated automagically via macros most of the time? When I use fold in a repl, I do not need to declare an ownership context...

Is it okay to just include an implicit val ctx = Ctx.Owner.save() in my class? I have seen that somewhere...
But unfortunately .save() lacks documentation.

moritz bust

@busti

In this example:

class Foo {
  val a = Var(0)

  val b = a.fold(1)(_ * _)
}

how do I obtain / define / pass an Ownership Context?
It just is not very clear to me.

Nick Childers

@Voltir

Pass it in as an implicit class param on Foo

moritz bust

@busti

@Voltir But where does that implicit come from? I am not using any rx's elsewhere...

Nick Childers

@Voltir

so, if you have some object:

object Main {
  val foo = new Foo()
}

you will get a context for free

(magic macro stuff happens to make that possible)

if you are inside a class, then you can again make that outer class take an implicit ctx

if you know for sure that Foo (or the class that wraps it) is going to be instantiated a finite number of times, then its safe to use the "unsafe" owner ctx

moritz bust

@busti

That is what I wanted to know, thanks a lot!

sidnt

@sidnt

Weird: I've done import rx._ but my code says not found: value Obs in this code piece for providing the implicit conversion.

sidnt

@sidnt

Oh. So this:
https://stackoverflow.com/questions/35752639/scala-rx-with-scalatags-example-compilation-error

moritz bust

@busti

How would I merge two rx's of the same type into a single one, that updates whenever either of the source Rx's updates?

Sakib Hadžiavdić

@sake92

Hi all!, I'm struggling with this snippet: https://github.com/lihaoyi/hands-on-scala-js/blob/master/examples/demos/src/main/scala/advanced/BasicRx.scala#L68-L79

It's using "scalarx" % "0.2.8", but latest is "0.4.0"

This is what I got so far:

implicit def rxFrag[T](r: Rx[T])(implicit dataCtx:  Ctx.Data, ev: T => Frag): Frag = {
    import Ctx.Owner.Unsafe._

    def rSafe: dom.Node = span(r()).render

    var last = rSafe
    r.triggerLater {
      val newLast = rSafe
      js.Dynamic.global.last = last
      last.parentNode.replaceChild(newLast, last)
      last = newLast
    }

    last
  }

but now even the simplest code requires this implicit Ctx.Data...

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