Make 'where' lazy.

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index 06c8efa8b5f657b35e99ea0d8cc5fe306e986594..1213dcf68f05515287c6c24e7241fd4b70b8b0be 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -35,22 +35,16 @@ abstract class Iterable<E> {
   Iterable mappedBy(f(E element)) => new MappedIterable(this, f);
 
   /**
-   * Returns a collection with the elements of this collection
-   * that satisfy the predicate [f].
-   *
-   * The returned collection should be of the same type as the collection
-   * creating it.
-   *
-   * An element satisfies the predicate [f] if [:f(element):]
-   * returns true.
-   */
-  Collection<E> where(bool f(E element)) {
-    // TODO(floitsch): this is just a temporary function to provide a complete
-    // skeleton. It will be changed to become lazy soon.
-    List result = <E>[];
-    for (E element in this) if (f(element)) result.add(element);
-    return result;
-  }
+    * Returns a lazy [Queryable] with all elements that satisfy the

[Anders Johnsen, 2012/11/20 06:32:21]

Queryable?

[floitsch, 2012/11/28 13:49:38]

Done.

+    * predicate [f].
+    *
+    * This method returns a view of the mapped elements. As long as the
+    * returned [Queryable] is not iterated over, the supplied function [f] will
+    * not be invoked. Iterating will not cache results, and thus iterating
+    * multiple times over the the returned [Queryable] will invoke the supplied
+    * function [f] multiple times on the same element.
+    */
+  Iterable<E> where(bool f(E element)) => new FilteredIterable<E>(this, f);
 
   /**
    * Check whether the collection contains an element equal to [element].
@@ -139,6 +133,10 @@ class MappedIterable<E> extends Iterable<E> {
   MappedIterable(this._iterable, E this._f(element));
 
   Iterator<E> get iterator => new MappedIterator<E>(_iterable.iterator, _f);
+
+  // Length related functions are independent of the mapping.
+  int get length => _iterable.length;
+  bool get isEmpty => _iterable.isEmpty;
 }
 
 class MappedIterator<E> extends Iterator<E> {