A Monad that can suspend side effects into the F[_] context and supports lazy and potentially asynchronous evaluation.

import cats.effect.{Async, IO, SyncIO}

trait Effect[F[_]] extends Async[F] {
  def runAsync[A](fa: F[A])(cb: Either[Throwable, A] => IO[Unit]): SyncIO[Unit]

This type class is describing data types that:

  • Implement the Async algebra.
  • Implement a lawful Effect.runAsync operation that triggers the evaluation (in the context of SyncIO).

Note: this is the safe and generic version of IO.unsafeRunAsync (aka Haskell’s unsafePerformIO).


It represents the intention to evaluate a given effect in the context of F[_] asynchronously giving you back a SyncIO[A]. Eg.:

import cats.effect.{Effect, SyncIO, IO}

val task = IO("Hello World!")

val ioa: SyncIO[Unit] =
  Effect[IO].runAsync(task) {
    case Right(value) => IO(println(value))
    case Left(error)  => IO.raiseError(error)