Add MapView and make View.MapValue a MapView.

Author: julienrf

collections/src/main/scala/strawman/collection/Map.scala

@@ -47,10 +47,13 @@ trait Map[K, +V]
   * @define coll map
   * @define Coll `Map`
   */
-trait MapOps[K, +V, +CC[X, Y] <: MapOps[X, Y, CC, _], +C]
+// Note: the upper bound constraint on CC is useful only to
+// erase CC to IterableOps instead of Object
+trait MapOps[K, +V, +CC[_, _] <: IterableOps[_, AnyConstr, _], +C]
   extends IterableOps[(K, V), Iterable, C]
-    with PartialFunction[K, V]
-    with Equals {
+    with PartialFunction[K, V] {
+
+  override def view: MapView[K, V] = new MapView.Id(this)
 
   /** Similar to fromIterable, but returns a Map collection type */
   protected[this] def mapFromIterable[K2, V2](it: Iterable[(K2, V2)]): CC[K2, V2]
@@ -164,7 +167,7 @@ trait MapOps[K, +V, +CC[X, Y] <: MapOps[X, Y, CC, _], +C]
     *  @return a map view which maps every key of this map
     *          to `f(this(key))`. The resulting map wraps the original map without copying any elements.
     */
-  def mapValues[W](f: V => W): View[(K, W)] = new View.MapValues(toIterable, f)
+  def mapValues[W](f: V => W): MapView[K, W] = new MapView.MapValues(this, f)
 
   /** Defines the default value computation for the map,
     *  returned when a key is not found

collections/src/main/scala/strawman/collection/MapView.scala

@@ -0,0 +1,40 @@
+package strawman.collection
+
+import scala.{Int, Option}
+
+import strawman.collection.mutable.Builder
+
+trait MapView[K, +V]
+  extends MapOps[K, V, ({ type l[X, Y] = View[(X, Y)] })#l, View[(K, V)]]
+    with View[(K, V)] {
+
+  override def view: MapView[K, V] = this
+
+  protected[this] def mapFromIterable[K2, V2](it: Iterable[(K2, V2)]): View[(K2, V2)] = fromIterable(it)
+
+  def mapFactory: MapFactory[({ type l[X, Y] = View[(X, Y)] })#l] =
+    new MapFactory[({ type l[X, Y] = View[(X, Y)] })#l] {
+      def newBuilder[X, Y](): Builder[(X, Y), View[(X, Y)]] = View.newBuilder[(X, Y)]()
+      def empty[X, Y]: View[(X, Y)] = View.empty
+      def from[X, Y](it: IterableOnce[(X, Y)]): View[(X, Y)] = View.from(it.asInstanceOf[Iterable[(X, Y)]]) // Waiting for https://github.com/scala/collection-strawman/pull/428
+    }
+
+  def empty: View[(K, V)] = View.Empty
+
+}
+
+object MapView {
+
+  class Id[K, +V](underlying: MapOps[K, V, AnyConstr, _]) extends MapView[K, V] {
+    def get(key: K): Option[V] = underlying.get(key)
+    def iterator(): Iterator[(K, V)] = underlying.iterator()
+    override def knownSize: Int = underlying.knownSize
+  }
+
+  class MapValues[K, +V, +W](underlying: MapOps[K, V, AnyConstr, _], f: V => W) extends MapView[K, W] {
+    def iterator(): Iterator[(K, W)] = underlying.iterator().map(kv => (kv._1, f(kv._2)))
+    override def knownSize: Int = underlying.knownSize
+    def get(key: K): Option[W] = underlying.get(key).map(f)
+  }
+
+}

collections/src/main/scala/strawman/collection/View.scala

@@ -195,11 +195,6 @@ object View extends IterableFactoryLike[View] {
     override def knownSize = underlying.knownSize
   }
 
-  class MapValues[K, V, W](underlying: Iterable[(K, V)], f: V => W) extends View[(K, W)] {
-    def iterator(): Iterator[(K, W)] = underlying.iterator().map(kv => (kv._1, f(kv._2)))
-    override def knownSize: Int = underlying.knownSize
-  }
-
   /** A view that flatmaps elements of the underlying collection. */
   class FlatMap[A, B](underlying: Iterable[A], f: A => IterableOnce[B]) extends View[B] {
     def iterator() = underlying.iterator().flatMap(f)

collections/src/main/scala/strawman/collection/concurrent/TrieMap.scala

@@ -962,7 +962,7 @@ final class TrieMap[K, V] private (r: AnyRef, rtupd: AtomicReferenceFieldUpdater
     if (nonReadOnly) readOnlySnapshot().filterKeys(p)
     else super.filterKeys(p)
   }
-  override def mapValues[W](f: V => W): collection.View[(K, W)] = {
+  override def mapValues[W](f: V => W): collection.MapView[K, W] = {
     if (nonReadOnly) readOnlySnapshot().mapValues(f)
     else super.mapValues(f)
   }

test/junit/src/test/scala/strawman/collection/ViewTest.scala

@@ -23,12 +23,22 @@ class ViewTest {
   }
 
   @Test
-  def seqViewReverse(): Unit = {
+  def seqView(): Unit = {
     val xs = List(1, 2, 3).view
     assertEquals(List(3, 2, 1), xs.reverse.to(List))
+    assertEquals(2, xs(1))
 //    assertEquals(xs, xs.reverse.reverse.to(List)) doesn’t compile
   }
 
+  @Test
+  def mapView(): Unit = {
+    val xs = immutable.Map(1 -> "a", 2 -> "b")
+    assertEquals("a", xs.view(1))
+    val ys = xs.mapValues(s => s.toUpperCase)
+    assertTrue(ys.contains(1))
+    assertEquals("B", ys(2))
+  }
+
   @Test
   def viewsViewIsNoOp(): Unit = {
     def check[A](it: Iterable[A]): Unit = {
@@ -38,5 +48,7 @@ class ViewTest {
     check(immutable.Set(1, 2, 3)) // View
     check(List(1, 2, 3)) // SeqView
     check(immutable.Vector(1, 2, 3)) // IndexedView
+    check(immutable.Map(1 -> "a", 2 -> "b")) // MapView
   }
+
 }