How should a scala cats IOApp acquire an ExecutionContext?

Question

I've recently converted my app to inheriting cats's IOApp as described here . I read in that documentation:

the Timer[IO] dependency is already provided by IOApp, so on top of the JVM there's no longer a need for an implicit ExecutionContext to be in scope

However, I'm interacting with several other libraries (ie http4s) that do require an ExecutionContext . Is there a recommended way to acquire one in this type of application? Does the good old import scala.concurrent.ExecutionContext.Implicits.global play nice with that provided Timer[IO] ?

Answer 1

Try extending trait IOApp.WithContext . For global ExecutionContext

import cats.effect._
import scala.concurrent.ExecutionContext

object Main extends IOApp.WithContext {
  implicit val ec = ExecutionContext.global

  override protected def executionContextResource: Resource[SyncIO, ExecutionContext] =
    Resource.liftF(SyncIO(ec))

  def run(args: List[String]): IO[ExitCode] = {

    implicitly[Timer[IO]]
    implicitly[ContextShift[IO]]
    implicitly[ExecutionContext]

    IO.pure(ExitCode.Success)
  }
}

or for ExecutionContext from a thread pool with fixed number of threads

import java.util.concurrent.{Executors, TimeUnit}
import cats.effect._
import scala.concurrent.ExecutionContext

object Main extends IOApp.WithContext {
  override protected def executionContextResource: Resource[SyncIO, ExecutionContext] =
    Resource.make(SyncIO(Executors.newFixedThreadPool(8)))(pool => SyncIO {
      pool.shutdown()
      pool.awaitTermination(10, TimeUnit.SECONDS)
    }).map(ExecutionContext.fromExecutorService)

  def run(args: List[String]): IO[ExitCode] = {
    executionContextResource.use { implicit ec =>

      implicitly[Timer[IO]]
      implicitly[ContextShift[IO]]
      implicitly[ExecutionContext]

      SyncIO.pure(ExitCode.Success)
    }.toIO
  }
}

https://github.com/typelevel/cats-effect/issues/337

https://github.com/typelevel/cats-effect/pull/344