Add IO#to conversion method

core/shared/src/main/scala/cats/effect/Effect.scala

@@ -52,4 +52,10 @@ trait Effect[F[_]] extends Async[F] with LiftIO[F] {
       })
     }
   }
+
+  override def liftIO[A](ioa: IO[A]): F[A] = {
+    // Implementation for `IO#to` depends on the `Async` type class,
+    // and not on `Effect`, so this shouldn't create a cyclic dependency
+    ioa.to[F](this)
+  }
 }

core/shared/src/main/scala/cats/effect/IO.scala

@@ -19,6 +19,7 @@ package effect
 
 import cats.effect.internals.{AndThen, IOPlatform, NonFatal}
 import scala.annotation.tailrec
+import scala.annotation.unchecked.{uncheckedVariance => uV}
 import scala.concurrent.{ExecutionContext, Future, Promise}
 import scala.concurrent.duration._
 import scala.util.{Left, Right}
@@ -380,6 +381,20 @@ sealed abstract class IO[+A] {
     p.future
   }
 
+  /**
+   * Converts the source `IO` into any `F` type that implements
+   * the [[cats.effect.Async Async]] type class.
+   */
+  final def to[F[_]](implicit F: cats.effect.Async[F]): F[A @uV] =
+    F.suspend {
+      unsafeStep match {
+        case Pure(a) => F.pure(a)
+        case RaiseError(ex) => F.raiseError(ex)
+        case async =>
+          F.async(async.unsafeRunAsync)
+      }
+    }
+
   override def toString = this match {
     case Pure(a) => s"IO($a)"
     case RaiseError(e) => s"IO(throw $e)"
@@ -426,7 +441,7 @@ private[effect] trait IOInstances extends IOLowPriorityInstances {
 
     override def shift[A](ioa: IO[A])(implicit ec: ExecutionContext) = ioa.shift
 
-    def liftIO[A](ioa: IO[A]) = ioa
+    override def liftIO[A](ioa: IO[A]) = ioa
   }
 
   implicit def ioMonoid[A: Monoid]: Monoid[IO[A]] = new IOSemigroup[A] with Monoid[IO[A]] {

laws/shared/src/test/scala/cats/effect/IOTests.scala

@@ -357,6 +357,7 @@ object IOTests {
   /** Implementation for testing default methods. */
   val ioEffectDefaults = new Effect[IO] {
     private val ref = implicitly[Effect[IO]]
+
     def async[A](k: ((Either[Throwable, A]) => Unit) => Unit): IO[A] =
       ref.async(k)
     def raiseError[A](e: Throwable): IO[A] =
@@ -373,7 +374,5 @@ object IOTests {
       ref.runAsync(fa)(cb)
     def suspend[A](thunk: =>IO[A]): IO[A] =
       ref.suspend(thunk)
-    def liftIO[A](ioa: IO[A]): IO[A] =
-      ref.liftIO(ioa)
   }
 }