Execute two fs2 tasks concurrently (non-determenistically)

Question

With Scalaz Task I make this with scalaz.Nondeterminism.both:

Nondeterminism[Task]
 .both(
   Task.now("Hello"),
   Task.now("world")
 )

or with Nondeterminism[Task].gatherUnordered().

How can I do the same thing with fs2 0.9.x version tasks?

Answer 1

I'm assuming you're on fs2 version 0.9.x. To execute several Tasks in parallel, you can simply call Task.start. Here's an example from the docs:

for {
   f <- Task.start { expensiveTask1 }
   // at this point, `expensive1` is evaluating in background

   g <- Task.start { expensiveTask2 }
   // now both `expensiveTask2` and `expensiveTask1` are running

   result1 <- f
   // we have forced `f`, so now only `expensiveTask2` may be running

   result2 <- g
   // we have forced `g`, so now nothing is running and we have both results

 } yield (result1 + result2)

So in your case it would look like this:

for {
  ta <- Task.start(Task.now("Hello"))
  tb <- Task.start(Task.now("World"))
  a <- ta
  b <- tb
} yield (a, b)

Note that in the future it might be possible to do something like this with much less boilerplate. There's a PR in the works to add a Parallel type class, which would allow us to write something like this:

(taskA, taskB).parMapN((a, b) => ...)