Use compiler integrated async phase under -Xasync

Author: retronym

.travis.yml

@@ -5,8 +5,9 @@ import: scala/scala-dev:travis/default.yml
 language: scala
 
 scala:
-  - 2.12.11
-  - 2.13.2
+  # Add when released
+  # - 2.12.12 when
+  - 2.13.3
 
 env:
   - ADOPTOPENJDK=8

README.md

@@ -1,14 +1,18 @@
 # scala-async [![Build Status](https://travis-ci.org/scala/scala-async.svg?branch=master)](https://travis-ci.org/scala/scala-async) [<img src="https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-async_2.12.svg?label=latest%20release%20for%202.12">](http://search.maven.org/#search%7Cga%7C1%7Cg%3Aorg.scala-lang.modules%20a%3Ascala-async_2.12) [<img src="https://img.shields.io/maven-central/v/org.scala-lang.modules/scala-async_2.13.svg?label=latest%20release%20for%202.13">](http://search.maven.org/#search%7Cga%7C1%7Cg%3Aorg.scala-lang.modules%20a%3Ascala-async_2.13)
 
-## Supported Scala versions
-
-This branch (version series 0.10.x) targets Scala 2.12 and 2.13. `scala-async` is no longer maintained for older versions.
-
 ## Quick start
 
 To include scala-async in an existing project use the library published on Maven Central.
 For sbt projects add the following to your build definition - build.sbt or project/Build.scala:
 
+### Use a modern Scala compiler
+
+As of scala-async 1.0, Scala 2.12.12+ or 2.13.3+ are required.
+
+### Add dependency
+
+#### SBT Example
+
 ```scala
 libraryDependencies += "org.scala-lang.modules" %% "scala-async" % "0.10.0"
 libraryDependencies += "org.scala-lang" % "scala-reflect" % scalaVersion.value % Provided
@@ -17,21 +21,51 @@ libraryDependencies += "org.scala-lang" % "scala-reflect" % scalaVersion.value %
 For Maven projects add the following to your <dependencies> (make sure to use the correct Scala version suffix
 to match your project’s Scala binary version):
 
+#### Maven Example
+
 ```scala
 <dependency>
-	<groupId>org.scala-lang.modules</groupId>
-	<artifactId>scala-async_2.12</artifactId>
-	<version>0.10.0</version>
+  <groupId>org.scala-lang.modules</groupId>
+  <artifactId>scala-async_2.13</artifactId>
+  <version>1.0.0</version>
 </dependency>
 <dependency>
-	<groupId>org.scala-lang</groupId>
-	<artifactId>scala-reflect</artifactId>
-	<version>2.12.11</version>
-	<scope>provided</scope>
+  <groupId>org.scala-lang</groupId>
+  <artifactId>scala-reflect</artifactId>
+  <version>2.13.3</version>
+  <scope>provided</scope>
 </dependency>
 ```
 
-After adding scala-async to your classpath, write your first `async` block:
+### Enable compiler support for `async`
+
+Add the `-Xasync` to the Scala compiler options.
+
+#### SBT Example
+```scala
+scalaOptions += "-Xasync"
+```
+
+#### Maven Example
+
+```xml
+<project>
+  ...
+  <plugin>
+    <groupId>net.alchim31.maven</groupId>
+    <artifactId>scala-maven-plugin</artifactId>
+    <version>4.4.0</version>
+    <configuration>
+      <args>
+        <arg>-Xasync</arg>
+      </args>
+    </configuration>
+  </plugin>
+  ...
+</project>
+```
+
+### Start coding
 
 ```scala
 import scala.concurrent.ExecutionContext.Implicits.global
@@ -93,6 +127,22 @@ def combined: Future[Int] = async {
 }
 ```
 
+## Limitations
+
+### `await` must be directly in the control flow of the async expression
+
+The `await` cannot be nested under a local method, object, class or lambda:  
+
+```
+async {
+  List(1).foreach { x => await(f(x) } // invali
+}
+```
+
+### `await` must be not be nested within `try` / `catch` / `finally`.
+
+This implementation restriction may be lifted in future versions.
+
 ## Comparison with direct use of `Future` API
 
 This computation could also be expressed by directly using the
@@ -119,53 +169,3 @@ The `async` approach has two advantages over the use of
      required at each generator (`<-`) in the for-comprehension.
      This reduces the size of generated code, and can avoid boxing
      of intermediate results.
-
-## Comparison with CPS plugin
-
-The existing continuations (CPS) plugin for Scala can also be used
-to provide a syntactic layer like `async`. This approach has been
-used in Akka's [Dataflow Concurrency](http://doc.akka.io/docs/akka/2.3-M1/scala/dataflow.html)
-(now deprecated in favour of this library).
-
-CPS-based rewriting of asynchronous code also produces a closure
-for each suspension. It can also lead to type errors that are
-difficult to understand.
-
-## How it works
-
- - The `async` macro analyses the block of code, looking for control
-   structures and locations of `await` calls. It then breaks the code
-   into 'chunks'. Each chunk contains a linear sequence of statements
-   that concludes with a branching decision, or with the registration
-   of a subsequent state handler as the continuation.
- - Before this analysis and transformation, the program is normalized
-   into a form amenable to this manipulation. This is called the
-   "A Normal Form" (ANF), and roughly means that:
-     - `if` and `match` constructs are only used as statements;
-       they cannot be used as an expression.
-     - calls to `await` are not allowed in compound expressions.
- - Identify vals, vars and defs that are accessed from multiple
-   states. These will be lifted out to fields in the state machine
-   object.
- - Synthesize a class that holds:
-   - an integer representing the current state ID.
-   - the lifted definitions.
-   - an `apply(value: Try[Any]): Unit` method that will be
-     called on completion of each future. The behavior of
-     this method is determined by the current state. It records
-     the downcast result of the future in a field, and calls the
-     `resume()` method.
-   - the `resume(): Unit` method that switches on the current state
-     and runs the users code for one 'chunk', and either:
-       a) registers the state machine as the handler for the next future
-       b) completes the result Promise of the `async` block, if at the terminal state.
-   - an `apply(): Unit` method that starts the computation.
-
-## Limitations
-
- - See the [neg](https://github.com/scala/async/tree/master/src/test/scala/scala/async/neg) test cases
-   for constructs that are not allowed in an `async` block.
- - See the [issue list](https://github.com/scala/async/issues?state=open) for which of these restrictions are planned
-   to be dropped in the future.
- - See [#32](https://github.com/scala/async/issues/32) for why `await` is not possible in closures, and for suggestions on
-   ways to structure the code to work around this limitation.

build.sbt

@@ -4,13 +4,13 @@ ScalaModulePlugin.scalaModuleOsgiSettings
 name := "scala-async"
 
 libraryDependencies += "org.scala-lang" % "scala-reflect" % scalaVersion.value % "provided"
-libraryDependencies += "org.scala-lang" % "scala-compiler" % scalaVersion.value % "test" // for ToolBox
 libraryDependencies += "junit" % "junit" % "4.12" % "test"
 libraryDependencies += "com.novocode" % "junit-interface" % "0.11" % "test"
 
 ScalaModulePlugin.enableOptimizer
 testOptions += Tests.Argument(TestFrameworks.JUnit, "-q", "-v", "-s")
 scalacOptions in Test ++= Seq("-Yrangepos")
+scalacOptions ++= List("-deprecation" , "-Xasync")
 
 parallelExecution in Global := false
 

pending/run/fallback0/MinimalScalaTest.scala

@@ -13,8 +13,9 @@
 package scala.async
 
 import scala.language.experimental.macros
-import scala.concurrent.{Future, ExecutionContext}
+import scala.concurrent.{ExecutionContext, Future}
 import scala.annotation.compileTimeOnly
+import scala.reflect.macros.whitebox
 
 /**
  * Async blocks provide a direct means to work with [[scala.concurrent.Future]].
@@ -50,14 +51,42 @@ object Async {
    * Run the block of code `body` asynchronously. `body` may contain calls to `await` when the results of
    * a `Future` are needed; this is translated into non-blocking code.
    */
-  def async[T](body: => T)(implicit execContext: ExecutionContext): Future[T] = macro internal.ScalaConcurrentAsync.asyncImpl[T]
+  def async[T](body: => T)(implicit execContext: ExecutionContext): Future[T] = macro asyncImpl[T]
 
   /**
    * Non-blocking await the on result of `awaitable`. This may only be used directly within an enclosing `async` block.
    *
    * Internally, this will register the remainder of the code in enclosing `async` block as a callback
    * in the `onComplete` handler of `awaitable`, and will *not* block a thread.
    */
-  @compileTimeOnly("`await` must be enclosed in an `async` block")
+  @compileTimeOnly("[async] `await` must be enclosed in an `async` block")
   def await[T](awaitable: Future[T]): T = ??? // No implementation here, as calls to this are translated to `onComplete` by the macro.
+
+  def asyncImpl[T: c.WeakTypeTag](c: whitebox.Context)
+                                          (body: c.Tree)
+                                          (execContext: c.Tree): c.Tree = {
+    import c.universe._
+    if (!c.compilerSettings.contains("-Xasync")) {
+      c.abort(c.macroApplication.pos, "The async requires the compiler option -Xasync (supported only by Scala 2.12.12+ / 2.13.3+)")
+    } else try {
+      val awaitSym = typeOf[Async.type].decl(TermName("await"))
+      def mark(t: DefDef): Tree = {
+        import language.reflectiveCalls
+        c.internal.asInstanceOf[{
+          def markForAsyncTransform(owner: Symbol, method: DefDef, awaitSymbol: Symbol, config: Map[String, AnyRef]): DefDef
+        }].markForAsyncTransform(c.internal.enclosingOwner, t, awaitSym, Map.empty)
+      }
+      val name = TypeName("stateMachine$async")
+      q"""
+      final class $name extends _root_.scala.async.FutureStateMachine(${execContext}) {
+        // FSM translated method
+        ${mark(q"""override def apply(tr$$async: _root_.scala.util.Try[_root_.scala.AnyRef]) = ${body}""")}
+      }
+      new $name().start() : ${c.macroApplication.tpe}
+    """
+    } catch {
+      case e: ReflectiveOperationException =>
+        c.abort(c.macroApplication.pos, "-Xasync is provided as a Scala compiler option, but the async macro is unable to call c.internal.markForAsyncTransform. " + e.getClass.getName + " " + e.getMessage)
+    }
+  }
 }

src/main/scala/scala/async/Async.scala

@@ -0,0 +1,80 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+package scala.async
+
+import java.util.Objects
+
+import scala.util.{Failure, Success, Try}
+import scala.concurrent.{ExecutionContext, Future, Promise}
+
+/** The base class for state machines generated by the `scala.async.Async.async` macro.
+ *  Not intended to be directly extended in user-written code.
+ */
+abstract class FutureStateMachine(execContext: ExecutionContext) extends Function1[Try[AnyRef], Unit] {
+  Objects.requireNonNull(execContext)
+
+  type F = scala.concurrent.Future[AnyRef]
+  type R = scala.util.Try[AnyRef]
+
+  private[this] val result$async: Promise[AnyRef] = Promise[AnyRef]();
+  private[this] var state$async: Int = 0
+
+  /** Retrieve the current value of the state variable */
+  protected def state: Int = state$async
+
+  /** Assign `i` to the state variable */
+  protected def state_=(s: Int): Unit = state$async = s
+
+  /** Complete the state machine with the given failure. */
+  // scala-async accidentally started catching NonFatal exceptions in:
+  //  https://github.com/scala/scala-async/commit/e3ff0382ae4e015fc69da8335450718951714982#diff-136ab0b6ecaee5d240cd109e2b17ccb2R411
+  // This follows the new behaviour but should we fix the regression?
+  protected def completeFailure(t: Throwable): Unit = {
+    result$async.complete(Failure(t))
+  }
+
+  /** Complete the state machine with the given value. */
+  protected def completeSuccess(value: AnyRef): Unit = {
+    result$async.complete(Success(value))
+  }
+
+  /** Register the state machine as a completion callback of the given future. */
+  protected def onComplete(f: F): Unit = {
+    f.onComplete(this)(execContext)
+  }
+
+  /** Extract the result of the given future if it is complete, or `null` if it is incomplete. */
+  protected def getCompleted(f: F): Try[AnyRef] = {
+    if (f.isCompleted) {
+      f.value.get
+    } else null
+  }
+
+  /**
+   * Extract the success value of the given future. If the state machine detects a failure it may
+   * complete the async block and return `this` as a sentinel value to indicate that the caller
+   * (the state machine dispatch loop) should immediately exit.
+   */
+  protected def tryGet(tr: R): AnyRef = tr match {
+    case Success(value) =>
+      value.asInstanceOf[AnyRef]
+    case Failure(throwable) =>
+      completeFailure(throwable)
+      this // sentinel value to indicate the dispatch loop should exit.
+  }
+
+  def start[T](): Future[T] = {
+    // This cast is safe because we know that `def apply` does not consult its argument when `state == 0`.
+    Future.unit.asInstanceOf[Future[AnyRef]].onComplete(this)(execContext)
+    result$async.future.asInstanceOf[Future[T]]
+  }
+}