New fix for #1861: Add fall-back to CPS for all unsupported uses of await

This is a re-implementation of a previous fix. It is more modular, since
it enables the definition of a CPS-based fall-back as a subclass of `AsyncBase`.
Thus, it's possible to define fall-back-enabled subclasses of `AsyncBase`
targetting not only Scala Futures.

1 files changed, 31 insertions, 0 deletions

diff --git a/src/main/scala/scala/async/AsyncWithCPSFallback.scala b/src/main/scala/scala/async/AsyncWithCPSFallback.scala
new file mode 100644
index 0000000..39e43a3
--- /dev/null
+++ b/src/main/scala/scala/async/AsyncWithCPSFallback.scala
@@ -0,0 +1,31 @@
+package scala.async
+
+import scala.language.experimental.macros
+
+import scala.reflect.macros.Context
+import scala.util.continuations._
+
+object AsyncWithCPSFallback extends AsyncBase {
+
+  import scala.concurrent.{Future, ExecutionContext}
+  import ExecutionContext.Implicits.global
+
+  lazy val futureSystem = ScalaConcurrentFutureSystem
+  type FS = ScalaConcurrentFutureSystem.type
+
+  /* Fall-back for `await` when it is called at an unsupported position.
+   */
+  override def awaitFallback[T, U](awaitable: futureSystem.Fut[T], p: futureSystem.Prom[U]): T @cpsParam[U, Unit] =
+    shift {
+      (k: (T => U)) =>
+        awaitable onComplete {
+          case tr => p.success(k(tr.get))
+        }
+    }
+
+  override def fallbackEnabled = true
+
+  def async[T](body: T) = macro asyncImpl[T]
+
+  override def asyncImpl[T: c.WeakTypeTag](c: Context)(body: c.Expr[T]): c.Expr[Future[T]] = super.asyncImpl[T](c)(body)
+}