for continuations

1 files changed, 53 insertions, 0 deletions

diff --git a/src/library/scala/Responder.scala b/src/library/scala/Responder.scala
new file mode 100644
index 0000000000..a86f1d30b8
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+++ b/src/library/scala/Responder.scala
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+package scala
+
+/** contains utility methods to build responders
+ *  @see class Responder
+ *  @since revision 6897 (will be 2.1.1)
+ */
+object Responder {
+
+  /** creates a responder that answer continuations with the constant a */
+  def constant[a](x: a) = new Responder[a] {
+    def answer(k: a => unit) = k(x)
+  }
+
+  /** executes x and returns true, useful as syntactic convenience in
+  	* for comprehensions
+	*/
+  def exec[a](x: => unit): boolean = { x; true  }
+
+  /** runs a responder, returning an optional result
+  */
+  def run[a](r: Responder[a]): Option[a] = {
+    var result: Option[a] = None
+    r.answer(x => result = Some(x))
+    result
+  }
+}
+
+/** instances of responder are the building blocks of small programs
+ *  written in continuation passing style. By using responder classes
+ *  in for comprehensions, one can embed domain-specific languages in
+ *  Scala while giving the impression that programs in these DSLs are
+ *  written in direct style.
+ *  @since revision 6897 (will be 2.1.1)
+ */
+abstract class Responder[a] {
+
+  def answer(k: a => unit): unit
+
+  def map[b](f: a => b) = new Responder[b] {
+    def answer(k: b => unit): unit =
+      Responder.this.answer(x => k(f(x)))
+  }
+
+  def flatMap[b](f: a => Responder[b]) = new Responder[b] {
+    def answer(k: b => unit): unit =
+      Responder.this.answer(x => f(x).answer(k))
+  }
+
+  def filter(p: a => boolean) = new Responder[a] {
+    def answer(k: a => unit): unit =
+      Responder.this.answer(x => if (p(x)) k(x) else ())
+  }
+}