From bce5de8e37c4cd0d23515781f73547d3a2d2e888 Mon Sep 17 00:00:00 2001
From: "P. Oscar Boykin" <oscar.boykin@gmail.com>
Date: Sat, 2 Sep 2017 13:57:56 -1000
Subject: [PATCH] Improve Eval.memoize

---
 core/src/main/scala/cats/Eval.scala | 140 ++++++++++++++++++----------
 1 file changed, 93 insertions(+), 47 deletions(-)

diff --git a/core/src/main/scala/cats/Eval.scala b/core/src/main/scala/cats/Eval.scala
index 4361e9903b..8641e1ba5c 100644
--- a/core/src/main/scala/cats/Eval.scala
+++ b/core/src/main/scala/cats/Eval.scala
@@ -253,34 +253,32 @@ object Eval extends EvalInstances {
    * they will be automatically created when needed.
    */
   sealed abstract class Call[A](val thunk: () => Eval[A]) extends Eval[A] {
-    def memoize: Eval[A] = new Later(() => value)
-    def value: A = Call.loop(this).value
+    def memoize: Eval[A] = Memoize(this)
+    def value: A = evaluate(this)
   }
 
-  object Call {
-
-    /**
-     * Collapse the call stack for eager evaluations.
-     */
-    @tailrec private def loop[A](fa: Eval[A]): Eval[A] = fa match {
-      case call: Eval.Call[A] =>
-        loop(call.thunk())
-      case compute: Eval.Compute[A] =>
-        new Eval.Compute[A] {
-          type Start = compute.Start
-          val start: () => Eval[Start] = () => compute.start()
-          val run: Start => Eval[A] = s => loop1(compute.run(s))
-        }
-      case other => other
-    }
-
-    /**
-     * Alias for loop that can be called in a non-tail position
-     * from an otherwise tailrec-optimized loop.
-     */
-    private def loop1[A](fa: Eval[A]): Eval[A] = loop(fa)
+  /**
+   * Collapse the call stack for eager evaluations.
+   * returns a non Call Eval node
+   */
+  @tailrec private def doCall[A](fa: Eval[A]): Eval[A] = fa match {
+    case call: Eval.Call[A] =>
+      doCall(call.thunk())
+    case compute: Eval.Compute[A] =>
+      new Eval.Compute[A] {
+        type Start = compute.Start
+        val start: () => Eval[Start] = () => compute.start()
+        val run: Start => Eval[A] = s => doCall1(compute.run(s))
+      }
+    case other => other
   }
 
+  /**
+   * Alias for doCall that can be called in a non-tail position
+   * from an otherwise tailrec-optimized doCall.
+   */
+  private def doCall1[A](fa: Eval[A]): Eval[A] = doCall(fa)
+
   /**
    * Compute is a type of Eval[A] that is used to chain computations
    * involving .map and .flatMap. Along with Eval#flatMap it
@@ -299,30 +297,78 @@ object Eval extends EvalInstances {
     val start: () => Eval[Start]
     val run: Start => Eval[A]
 
-    def memoize: Eval[A] = Later(value)
-
-    def value: A = {
-      type L = Eval[Any]
-      type C = Any => Eval[Any]
-      @tailrec def loop(curr: L, fs: List[C]): Any =
-        curr match {
-          case c: Compute[_] =>
-            c.start() match {
-              case cc: Compute[_] =>
-                loop(
-                  cc.start().asInstanceOf[L],
-                  cc.run.asInstanceOf[C] :: c.run.asInstanceOf[C] :: fs)
-              case xx =>
-                loop(c.run(xx.value), fs)
-            }
-          case x =>
-            fs match {
-              case f :: fs => loop(f(x.value), fs)
-              case Nil => x.value
-            }
-        }
-      loop(this.asInstanceOf[L], Nil).asInstanceOf[A]
+    def memoize: Eval[A] = Memoize(this)
+
+    def value: A = evaluate(this)
+  }
+
+  private case class Memoize[A](eval: Eval[A]) extends Eval[A] {
+    var result: Option[A] = None
+    def memoize: Eval[A] = this
+    def value: A =
+      result match {
+        case Some(a) => a
+        case None =>
+          val a = evaluate(this)
+          result = Some(a)
+          a
+      }
+  }
+
+  private def evaluate[A](e: Eval[A]): A = {
+    type L = Eval[Any]
+    type C = Any => Eval[Any]
+
+    /**
+     * Here we implement memoization in a way
+     * that only callers to memoize pay a price.
+     * We insert a side effecting fn into the pipeline
+     * the updates the memo when it is not yet set
+     */
+    val memos = collection.mutable.Map.empty[L, Any]
+    def addToMemo(m: Memoize[Any]): C = { a: Any =>
+      memos.put(m.eval, a)
+      m.result = Some(a)
+      Now(a)
     }
+    @tailrec def loop(curr: L, fs: List[C]): Any =
+      curr match {
+        case c: Compute[_] =>
+          c.start() match {
+            case cc: Compute[_] =>
+              loop(
+                cc.start().asInstanceOf[L],
+                cc.run.asInstanceOf[C] :: c.run.asInstanceOf[C] :: fs)
+            case xx =>
+              loop(c.run(xx.value), fs)
+          }
+        case call: Call[_] =>
+          loop(doCall(call), fs)
+        case m@Memoize(eval) =>
+          m.result match {
+            case Some(a) =>
+              fs match {
+                case f :: fs => loop(f(a), fs)
+                case Nil => a
+              }
+            case None =>
+              memos.get(eval) match {
+                case Some(a) =>
+                  fs match {
+                    case f :: fs => loop(f(a), fs)
+                    case Nil => a
+                  }
+                case None =>
+                  loop(eval, addToMemo(m) :: fs)
+              }
+          }
+        case x =>
+          fs match {
+            case f :: fs => loop(f(x.value), fs)
+            case Nil => x.value
+          }
+      }
+    loop(e.asInstanceOf[L], Nil).asInstanceOf[A]
   }
 }