Get back to IO fiber/thread after running a future

user_121595 0
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I have some cats IO operations, and Future among then. Simplified:

IO(getValue())
  .flatMap(v => IO.fromFuture(IO(blockingProcessValue(v)))(myBlockingPoolContextShift))
  .map(moreProcessing)

So I have some value in IO, then I need to do some blocking operation using a library that returns Future, and then I need to do some more processing on the value returned from Future

Future runs on a dedicated thread pool - so far so good. The problem is after Future is completed. moreProcessing runs on the same thread the Future was running on.

Is there a way to get back to the thread getValue() was running on?

cats-effect scala-cats scala
1 Answers

After the discussion in the chat, the conclusion is that the only thing OP needs is to create a ContextShift in the application entry point using the appropriate (compute) EC and then pass it down to the class containing this method.

// Entry point

val computeEC = ???
val cs = IO.contextShift(computeEC)
val myClass = new MyClass(cs, ...)

// Inside the method on MyClass
IO(getValue())
  .flatMap(v => IO.fromFuture(IO(blockingProcessValue(v)))(myBlockingPoolContextShift))
  .flatTap(_ => cs.shift)
  .map(moreProcessing)

This Scastie showed an approach using Blocker and other techniques common in the Typelevel ecosystem but were not really suitable for OP's use case; anyways I find it useful for future readers who may have a similar problem.

user_41114040
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