Add suspend strawman (#17164)

Add a test that combines various experiments using our hypothetical
Suspension abstractions.

Author: odersky

tests/pos/suspend-strawman/choices.scala

@@ -0,0 +1,28 @@
+import scala.util.boundary, boundary.Label
+import runtime.suspend
+
+trait Choice:
+  def choose[A](choices: A*): A
+
+// the handler
+def choices[T](body: Choice ?=> T): Seq[T] =
+  boundary[Seq[T]]:
+    given Choice with
+      def choose[A](choices: A*): A =
+        suspend[A, Seq[T]](s => choices.flatMap(s.resume))
+    Seq(body)
+
+def choose[A](choices: A*)(using c: Choice): A = c.choose(choices*)
+
+def TestChoices: Seq[Int] =
+  choices:
+    def x = choose(1, -2, -3)
+    def y = choose("ab", "cde")
+    val xx = x;
+    xx + (
+      if xx > 0 then
+        val z = choose(xx / 2, xx * 2)
+        y.length * z
+      else y.length
+    )
+

tests/pos/suspend-strawman/generators.scala

@@ -0,0 +1,67 @@
+import scala.util.boundary
+import scala.compiletime.uninitialized
+import runtime.suspend
+
+trait CanProduce[-T]:
+  def produce(x: T): Unit
+
+object generate:
+
+  def produce[T](x: T)(using cp: CanProduce[T]): Unit = cp.produce(x)
+
+  def apply[T](body: CanProduce[T] ?=> Unit): Iterator[T] = new:
+    var nextKnown: Boolean = false
+    var nextElem: Option[T] = uninitialized
+
+    var step: () => Unit = () =>
+      boundary[Unit]:
+        given CanProduce[T] with
+          def produce(x: T): Unit =
+            nextElem = Some(x)
+            suspend[Unit, Unit]: k =>
+              step = () => k.resume(())
+        body
+      nextElem = None
+
+    def hasNext: Boolean =
+      if !nextKnown then { step(); nextKnown = true }
+      nextElem.isDefined
+
+    def next: T =
+      require(hasNext)
+      nextKnown = false
+      nextElem.get
+end generate
+
+enum Tree[T]:
+  case Leaf(x: T)
+  case Inner(xs: List[Tree[T]])
+
+def leafs[T](t: Tree[T]): Iterator[T] =
+  generate:
+    def recur(t: Tree[T]): Unit = t match
+      case Tree.Leaf(x) => generate.produce(x)
+      case Tree.Inner(xs) => xs.foreach(recur)
+    recur(t)
+
+object Variant2:
+  trait Generator[T]:
+    def nextOption: Option[T]
+
+  def generate[T](body: CanProduce[T] ?=> Unit): Generator[T] = new:
+
+    def nextOption: Option[T] =
+      step()
+
+    var step: () => Option[T] = () =>
+      boundary:
+        given CanProduce[T] with
+          def produce(x: T): Unit =
+            suspend[Unit, Option[T]]: k =>
+              step = () => k.resume(())
+              Some(x)
+        body
+        None
+  end generate
+
+

tests/pos/suspend-strawman/monadic-reflect.scala

@@ -0,0 +1,56 @@
+import scala.util.boundary
+import runtime.suspend
+
+trait Monad[F[_]]:
+
+  /** The unit value for a monad */
+  def pure[A](x: A): F[A]
+
+  extension [A](x: F[A])
+    /** The fundamental composition operation */
+    def flatMap[B](f: A => F[B]): F[B]
+
+    /** The `map` operation can now be defined in terms of `flatMap` */
+    def map[B](f: A => B) = x.flatMap(f.andThen(pure))
+
+end Monad
+
+trait CanReflect[M[_]]:
+  def reflect[R](mr: M[R]): R
+
+trait Monadic[M[_]: Monad]:
+
+  /**
+   * Embedding of pure values into the monad M
+   */
+  def pure[A](a: A): M[A]
+
+  /**
+   * Sequencing of monadic values
+   *
+   * Implementations are required to implement sequencing in a stack-safe
+   * way, that is they either need to implement trampolining on their own
+   * or implement `sequence` as a tail recursive function.
+   *
+   * Actually the type X can be different for every call to f...
+   * It is a type aligned sequence, but for simplicity we do not enforce this
+   * here.
+   */
+  def sequence[X, R](init: M[X])(f: X => Either[M[X], M[R]]): M[R]
+
+  /**
+   * Helper to summon and use an instance of CanReflect[M]
+   */
+  def reflect[R](mr: M[R])(using r: CanReflect[M]): R = r.reflect(mr)
+
+  /**
+   * Reify a computation into a monadic value
+   */
+  def reify[R](prog: CanReflect[M] ?=> R): M[R] =
+    boundary [M[R]]:
+      given CanReflect[M] with
+        def reflect[R2](mr: M[R2]): R2 =
+          suspend [R2, M[R]] (k => mr.flatMap(k.resume))
+      pure(prog)
+
+end Monadic

tests/pos/suspend-strawman/runtime.scala

@@ -0,0 +1,11 @@
+package runtime
+import scala.util.boundary, boundary.Label
+
+/** A hypothetical API for suspensions. Not yet implemented.
+ *  Suspension contain a delimited contination, which can be
+ *  invoked with `resume`
+ */
+class Suspension[-T, +R]:
+  def resume(arg: T): R = ???
+
+def suspend[T, R](body: Suspension[T, R] => R)(using Label[R]): T = ???

tests/pos/suspend-strawman/simple-futures.scala

@@ -0,0 +1,53 @@
+package simpleFutures
+
+import scala.collection.mutable.ListBuffer
+import scala.util.boundary, boundary.Label
+import runtime.suspend
+
+object Scheduler:
+  def schedule(task: Runnable): Unit = ???
+
+trait Async:
+  def await[T](f: Future[T]): T
+
+class Future[+T](body: Async ?=> T):
+  private var result: Option[T] = None
+  private var waiting: ListBuffer[T => Unit] = ListBuffer()
+  private def addWaiting(k: T => Unit): Unit = waiting += k
+
+  def await(using a: Async): T = a.await(this)
+
+  private def complete(): Unit =
+    Future.async:
+      val value = body
+      val result = Some(value)
+      for k <- waiting do
+        Scheduler.schedule(() => k(value))
+      waiting.clear()
+
+  Scheduler.schedule(() => complete())
+
+object Future:
+
+  // a handler for Async
+  def async(body: Async ?=> Unit): Unit =
+    boundary [Unit]:
+      given Async with
+        def await[T](f: Future[T]): T = f.result match
+          case Some(x) => x
+          case None => suspend[T, Unit](s => f.addWaiting(s.resume))
+      body
+
+end Future
+
+def Test(x: Future[Int], xs: List[Future[Int]]) =
+  Future:
+    x.await + xs.map(_.await).sum
+
+
+
+
+
+
+
+