Add Async#asyncCheckAttempt for #3087

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

@@ -1072,10 +1072,81 @@ object IO extends IOCompanionPlatform with IOLowPriorityImplicits {
   def defer[A](thunk: => IO[A]): IO[A] =
     delay(thunk).flatten
 
+  /**
+   * Suspends an asynchronous side effect with optional immediate result in `IO`.
+   *
+   * The given function `k` will be invoked during evaluation of the `IO` to:
+   *   - check if result is already available;
+   *   - "schedule" the asynchronous callback, where the callback of type `Either[Throwable, A]
+   *     \=> Unit` is the parameter passed to that function. Only the ''first'' invocation of
+   *     the callback will be effective! All subsequent invocations will be silently dropped.
+   *
+   * The process of registering the callback itself is suspended in `IO` (the outer `IO` of
+   * `IO[Either[Option[IO[Unit]], A]]`).
+   *
+   * The effect returns `Either[Option[IO[Unit]], A]` where:
+   *   - right side `A` is an immediate result of computation (callback invocation will be
+   *     dropped);
+   *   - left side `Option[IO[Unit]] `is an optional finalizer to be run in the event that the
+   *     fiber running `asyncCheckAttempt(k)` is canceled.
+   *
+   * For example, here is a simplified version of `IO.fromCompletableFuture`:
+   *
+   * {{{
+   * def fromCompletableFuture[A](fut: IO[CompletableFuture[A]]): IO[A] = {
+   *   fut.flatMap { cf =>
+   *     IO.asyncCheckAttempt { cb =>
+   *       if (cf.isDone) {
+   *         //Register immediately available result of the completable future or handle an error
+   *         IO(cf.get)
+   *           .map(Right(_))
+   *           .handleError { e =>
+   *             cb(Left(e))
+   *             Left(None)
+   *           }
+   *       } else {
+   *         IO {
+   *           //Invoke the callback with the result of the completable future
+   *           val stage = cf.handle[Unit] {
+   *             case (a, null) => cb(Right(a))
+   *             case (_, e) => cb(Left(e))
+   *           }
+   *
+   *           //Cancel the completable future if the fiber is canceled
+   *           Left(Some(IO(stage.cancel(false)).void))
+   *         }
+   *       }
+   *     }
+   *   }
+   * }
+   * }}}
+   *
+   * Note that `asyncCheckAttempt` is uncancelable during its registration.
+   *
+   * @see
+   *   [[async]] for a simplified variant without an option for immediate result
+   */
+  def asyncCheckAttempt[A](
+      k: (Either[Throwable, A] => Unit) => IO[Either[Option[IO[Unit]], A]]): IO[A] = {
+    val body = new Cont[IO, A, A] {
+      def apply[G[_]](implicit G: MonadCancel[G, Throwable]) = { (resume, get, lift) =>
+        G.uncancelable { poll =>
+          lift(k(resume)) flatMap {
+            case Right(a) => G.pure(a)
+            case Left(Some(fin)) => G.onCancel(poll(get), lift(fin))
+            case Left(None) => poll(get)
+          }
+        }
+      }
+    }
+
+    IOCont(body, Tracing.calculateTracingEvent(k))
+  }
+
   /**
    * Suspends an asynchronous side effect in `IO`.
    *
-   * The given function will be invoked during evaluation of the `IO` to "schedule" the
+   * The given function `k` will be invoked during evaluation of the `IO` to "schedule" the
    * asynchronous callback, where the callback of type `Either[Throwable, A] => Unit` is the
    * parameter passed to that function. Only the ''first'' invocation of the callback will be
    * effective! All subsequent invocations will be silently dropped.
@@ -1107,8 +1178,13 @@ object IO extends IOCompanionPlatform with IOLowPriorityImplicits {
    * }
    * }}}
    *
+   * Note that `async` is uncancelable during its registration.
+   *
    * @see
    *   [[async_]] for a simplified variant without a finalizer
+   * @see
+   *   [[asyncCheckAttempt]] for more generic version providing an optional immediate result of
+   *   computation
    */
   def async[A](k: (Either[Throwable, A] => Unit) => IO[Option[IO[Unit]]]): IO[A] = {
     val body = new Cont[IO, A, A] {
@@ -1128,7 +1204,7 @@ object IO extends IOCompanionPlatform with IOLowPriorityImplicits {
   /**
    * Suspends an asynchronous side effect in `IO`.
    *
-   * The given function will be invoked during evaluation of the `IO` to "schedule" the
+   * The given function `k` will be invoked during evaluation of the `IO` to "schedule" the
    * asynchronous callback, where the callback is the parameter passed to that function. Only
    * the ''first'' invocation of the callback will be effective! All subsequent invocations will
    * be silently dropped.
@@ -1155,8 +1231,13 @@ object IO extends IOCompanionPlatform with IOLowPriorityImplicits {
    * This function can be thought of as a safer, lexically-constrained version of `Promise`,
    * where `IO` is like a safer, lazy version of `Future`.
    *
+   * Also, note that `async` is uncancelable during its registration.
+   *
    * @see
-   *   [[async]]
+   *   [[async]] for more generic version providing a finalizer
+   * @see
+   *   [[asyncCheckAttempt]] for more generic version providing an optional immediate result of
+   *   computation and a finalizer
    */
   def async_[A](k: (Either[Throwable, A] => Unit) => Unit): IO[A] = {
     val body = new Cont[IO, A, A] {
@@ -1610,6 +1691,10 @@ object IO extends IOCompanionPlatform with IOLowPriorityImplicits {
   private[this] val _asyncForIO: kernel.Async[IO] = new kernel.Async[IO]
     with StackSafeMonad[IO] {
 
+    override def asyncCheckAttempt[A](
+        k: (Either[Throwable, A] => Unit) => IO[Either[Option[IO[Unit]], A]]): IO[A] =
+      IO.asyncCheckAttempt(k)
+
     override def async[A](k: (Either[Throwable, A] => Unit) => IO[Option[IO[Unit]]]): IO[A] =
       IO.async(k)
 

kernel/shared/src/main/scala/cats/effect/kernel/Async.scala

@@ -62,25 +62,39 @@ import java.util.concurrent.atomic.AtomicReference
 trait Async[F[_]] extends AsyncPlatform[F] with Sync[F] with Temporal[F] {
 
   /**
-   * The asynchronous FFI.
+   * Suspends an asynchronous side effect with optional immediate result in `F`.
    *
-   * `k` takes a callback of type `Either[Throwable, A] => Unit` to signal the result of the
-   * asynchronous computation. The execution of `async(k)` is semantically blocked until the
-   * callback is invoked.
+   * The given function `k` will be invoked during evaluation of `F` to:
+   *   - check if result is already available;
+   *   - "schedule" the asynchronous callback, where the callback of type `Either[Throwable, A]
+   *     \=> Unit` is the parameter passed to that function. Only the ''first'' invocation of
+   *     the callback will be effective! All subsequent invocations will be silently dropped.
    *
-   * `k` returns an `Option[F[Unit]]` which is an optional finalizer to be run in the event that
-   * the fiber running `async(k)` is canceled. If passed `k` is `None`, then created effect will
-   * be uncancelable.
+   * The process of registering the callback itself is suspended in `F` (the outer `F` of
+   * `F[Either[Option[G[Unit]], A]]`).
    *
-   * Also, note that `async` is uncancelable during its registration.
+   * The effect returns `Either[Option[F[Unit]], A]` where:
+   *   - right side `A` is an immediate result of computation (callback invocation will be
+   *     dropped);
+   *   - left side `Option[F[Unit]] `is an optional finalizer to be run in the event that the
+   *     fiber running `asyncCheckAttempt(k)` is canceled.
+   *
+   * Also, note that `asyncCheckAttempt` is uncancelable during its registration.
+   *
+   * @see
+   *   [[async]] for a simplified variant without an option for immediate result
+   * @see
+   *   [[async_]] for a simplified variant without an option for immediate result or finalizer
    */
-  def async[A](k: (Either[Throwable, A] => Unit) => F[Option[F[Unit]]]): F[A] = {
+  def asyncCheckAttempt[A](
+      k: (Either[Throwable, A] => Unit) => F[Either[Option[F[Unit]], A]]): F[A] = {
     val body = new Cont[F, A, A] {
       def apply[G[_]](implicit G: MonadCancel[G, Throwable]) = { (resume, get, lift) =>
         G.uncancelable { poll =>
           lift(k(resume)) flatMap {
-            case Some(fin) => G.onCancel(poll(get), lift(fin))
-            case None => poll(get)
+            case Right(a) => G.pure(a)
+            case Left(Some(fin)) => G.onCancel(poll(get), lift(fin))
+            case Left(None) => poll(get)
           }
         }
       }
@@ -90,9 +104,48 @@ trait Async[F[_]] extends AsyncPlatform[F] with Sync[F] with Temporal[F] {
   }
 
   /**
-   * A convenience version of [[Async.async]] for when we don't need to perform `F[_]` effects
-   * or perform finalization in the event of cancelation (i.e. created effect will be
-   * uncancelable).
+   * Suspends an asynchronous side effect in `F`.
+   *
+   * The given function `k` will be invoked during evaluation of the `F` to "schedule" the
+   * asynchronous callback, where the callback of type `Either[Throwable, A] => Unit` is the
+   * parameter passed to that function. Only the ''first'' invocation of the callback will be
+   * effective! All subsequent invocations will be silently dropped.
+   *
+   * The process of registering the callback itself is suspended in `F` (the outer `F` of
+   * `F[Option[F[Unit]]]`).
+   *
+   * The effect returns `Option[F[Unit]]` which is an optional finalizer to be run in the event
+   * that the fiber running `async(k)` is canceled.
+   *
+   * Also, note that `async` is uncancelable during its registration.
+   *
+   * @see
+   *   [[async_]] for a simplified variant without a finalizer
+   * @see
+   *   [[asyncCheckAttempt]] for more generic version with option of providing immediate result
+   *   of computation
+   */
+  def async[A](k: (Either[Throwable, A] => Unit) => F[Option[F[Unit]]]): F[A] =
+    asyncCheckAttempt[A](cb => map(k(cb))(Left(_)))
+
+  /**
+   * Suspends an asynchronous side effect in `F`.
+   *
+   * The given function `k` will be invoked during evaluation of the `F` to "schedule" the
+   * asynchronous callback, where the callback is the parameter passed to that function. Only
+   * the ''first'' invocation of the callback will be effective! All subsequent invocations will
+   * be silently dropped.
+   *
+   * This function can be thought of as a safer, lexically-constrained version of `Promise`,
+   * where `IO` is like a safer, lazy version of `Future`.
+   *
+   * Also, note that `async` is uncancelable during its registration.
+   *
+   * @see
+   *   [[async]] for more generic version providing a finalizer
+   * @see
+   *   [[asyncCheckAttempt]] for more generic version with option of providing immediate result
+   *   of computation and finalizer
    */
   def async_[A](k: (Either[Throwable, A] => Unit) => Unit): F[A] =
     async[A](cb => as(delay(k(cb)), None))
@@ -183,9 +236,9 @@ trait Async[F[_]] extends AsyncPlatform[F] with Sync[F] with Temporal[F] {
    * NOTE: This is a very low level api, end users should use `async` instead.
    * See cats.effect.kernel.Cont for more detail.
    *
-   * If you are an implementor, and you have `async`, `Async.defaultCont`
-   * provides an implementation of `cont` in terms of `async`.
-   * Note that if you use `defaultCont` you _have_ to override `async`.
+   * If you are an implementor, and you have `async` or `asyncCheckAttempt`,
+   * `Async.defaultCont` provides an implementation of `cont` in terms of `async`.
+   * Note that if you use `defaultCont` you _have_ to override `async/asyncCheckAttempt`.
    */
   def cont[K, R](body: Cont[F, K, R]): F[R]
 }

laws/shared/src/main/scala/cats/effect/laws/AsyncLaws.scala

@@ -26,6 +26,21 @@ import scala.util.{Left, Right}
 trait AsyncLaws[F[_]] extends GenTemporalLaws[F, Throwable] with SyncLaws[F] {
   implicit val F: Async[F]
 
+  // format: off
+  def asyncCheckAttemptImmediateIsPure[A](a: A) =
+    (F.asyncCheckAttempt[A](_ => F.pure(Right(a))) <* F.unit) <-> (F.pure(a))
+  // format: on
+
+  // format: off
+  def asyncCheckAttemptSuspendedRightIsAsyncRight[A](a: A, fu: F[Unit]) =
+    (F.asyncCheckAttempt[A](k => F.delay(k(Right(a))) >> fu.as(Left(None))) <* F.unit) <-> (F.async[A](k => F.delay(k(Right(a))) >> fu.as(None)) <* F.unit)
+  // format: on
+
+  // format: off
+  def asyncCheckAttemptSuspendedLeftIsAsyncLeft[A](e: Throwable, fu: F[Unit]) =
+    (F.asyncCheckAttempt[A](k => F.delay(k(Left(e))) >> fu.as(Left(None))) <* F.unit) <-> (F.async[A](k => F.delay(k(Left(e))) >> fu.as(None)) <* F.unit)
+  // format: on
+
   // format: off
   def asyncRightIsUncancelableSequencedPure[A](a: A, fu: F[Unit]) =
     (F.async[A](k => F.delay(k(Right(a))) >> fu.as(None)) <* F.unit) <-> (F.uncancelable(_ => fu) >> F.pure(a))

laws/shared/src/main/scala/cats/effect/laws/AsyncTests.scala

@@ -239,6 +239,12 @@ trait AsyncTests[F[_]] extends GenTemporalTests[F, Throwable] with SyncTests[F]
       )
 
       val props = Seq(
+        "asyncCheckAttempt immediate is pure" -> forAll(
+          laws.asyncCheckAttemptImmediateIsPure[A] _),
+        "asyncCheckAttempt suspended right is async right" -> forAll(
+          laws.asyncCheckAttemptSuspendedRightIsAsyncRight[A] _),
+        "asyncCheckAttempt suspended left is async left" -> forAll(
+          laws.asyncCheckAttemptSuspendedLeftIsAsyncLeft[A] _),
         "async right is uncancelable sequenced pure" -> forAll(
           laws.asyncRightIsUncancelableSequencedPure[A] _),
         "async left is uncancelable sequenced raiseError" -> forAll(