Add WithFilter.scala to stdlib

Author: odersky

tests/pos-special/stdlib/collection/WithFilter.scala

@@ -0,0 +1,70 @@
+/*
+ * 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.collection
+
+/** A template trait that contains just the `map`, `flatMap`, `foreach` and `withFilter` methods
+  * of trait `Iterable`.
+  *
+  * @tparam A Element type (e.g. `Int`)
+  * @tparam CC Collection type constructor (e.g. `List`)
+  *
+  * @define coll collection
+  */
+@SerialVersionUID(3L)
+abstract class WithFilter[+A, +CC[_]] extends Serializable {
+
+  /** Builds a new collection by applying a function to all elements of the
+    * `filtered` outer $coll.
+    *
+    *  @param f      the function to apply to each element.
+    *  @tparam B     the element type of the returned collection.
+    *  @return       a new $coll resulting from applying
+    *                the given function `f` to each element of the filtered outer $coll
+    *                and collecting the results.
+    */
+  def map[B](f: A => B): CC[B]
+
+  /** Builds a new collection by applying a function to all elements of the
+    * `filtered` outer $coll containing this `WithFilter` instance that satisfy
+    *
+    *  @param f      the function to apply to each element.
+    *  @tparam B     the element type of the returned collection.
+    *  @return       a new $coll resulting from applying
+    *                the given collection-valued function `f` to each element
+    *                of the filtered outer $coll and
+    *                concatenating the results.
+    */
+  def flatMap[B](f: A => IterableOnce[B]): CC[B]
+
+  /** Applies a function `f` to all elements of the `filtered` outer $coll.
+    *
+    *  @param  f   the function that is applied for its side-effect to every element.
+    *              The result of function `f` is discarded.
+    *
+    *  @tparam  U  the type parameter describing the result of function `f`.
+    *              This result will always be ignored. Typically `U` is `Unit`,
+    *              but this is not necessary.
+    */
+  def foreach[U](f: A => U): Unit
+
+  /** Further refines the filter for this `filtered` $coll.
+    *
+    *  @param q   the predicate used to test elements.
+    *  @return    an object of class `WithFilter`, which supports
+    *             `map`, `flatMap`, `foreach`, and `withFilter` operations.
+    *             All these operations apply to those elements of this $coll which
+    *             also satisfy both `p` and `q` predicates.
+    */
+  def withFilter(q: A => Boolean): WithFilter[A, CC]
+
+}