made streams and views more lazy by always skip...

made streams and views more lazy by always skipping builder

1 files changed, 24 insertions, 11 deletions

diff --git a/src/library/scala/collection/immutable/Stream.scala b/src/library/scala/collection/immutable/Stream.scala
index 523fda9465..9f609796f5 100644
--- a/src/library/scala/collection/immutable/Stream.scala
+++ b/src/library/scala/collection/immutable/Stream.scala
@@ -117,8 +117,9 @@ self =>
   override def ++[B >: A, That](that: Traversable[B])(implicit bf: BuilderFactory[B, That, Stream[A]]): That = {
     def loop(these: Stream[A]): Stream[B] =
       if (these.isEmpty) that.toStream else new Stream.Cons(these.head, loop(these.tail))
-    if (bf.isInstanceOf[Stream.StreamBuilderFactory[_]]) loop(this).asInstanceOf[That]
-    else super.++(that)
+    loop(this).asInstanceOf[That]
+// was:    if (bf.isInstanceOf[Stream.StreamBuilderFactory[_]]) loop(this).asInstanceOf[That]
+//    else super.++(that)
   }
 
   /** Create a new stream which contains all elements of this stream
@@ -137,8 +138,9 @@ self =>
   override def map[B, That](f: A => B)(implicit bf: BuilderFactory[B, That, Stream[A]]): That = {
     def loop(these: Stream[A]): Stream[B] =
       if (these.isEmpty) Stream.Empty else new Stream.Cons(f(these.head), loop(these.tail))
-    if (bf.isInstanceOf[Stream.StreamBuilderFactory[_]]) loop(this).asInstanceOf[That]
-    else super.map(f)
+    loop(this).asInstanceOf[That]
+// was:    if (bf.isInstanceOf[Stream.StreamBuilderFactory[_]]) loop(this).asInstanceOf[That]
+//         else super.map(f)
   }
 
   /** Applies the given function <code>f</code> to each element of
@@ -154,10 +156,11 @@ self =>
       else {
         val seg = f(these.head)
         if (seg.isEmpty) loop(these.tail)
-        else seg.toStream ++ loop(these.tail)
+        else seg.toStream append loop(these.tail)
       }
-    if (bf.isInstanceOf[Stream.StreamBuilderFactory[_]]) loop(this).asInstanceOf[That]
-    else super.flatMap(f)
+    loop(this).asInstanceOf[That]
+// was:    if (bf.isInstanceOf[Stream.StreamBuilderFactory[_]]) loop(this).asInstanceOf[That]
+//    else super.flatMap(f)
   }
 
   /** Returns all the elements of this stream that satisfy the
@@ -200,8 +203,9 @@ self =>
     def loop(these: Stream[A], elems2: Iterator[B]): Stream[(A1, B)] =
       if (these.isEmpty || !elems2.hasNext) Stream.Empty
       else new Stream.Cons((these.head, elems2.next), loop(these.tail, elems2))
-    if (bf.isInstanceOf[Stream.StreamBuilderFactory[_]]) loop(this, that.iterator).asInstanceOf[That]
-    else super.zip[A1, B, That](that)
+    loop(this, that.iterator).asInstanceOf[That]
+// was:    if (bf.isInstanceOf[Stream.StreamBuilderFactory[_]]) loop(this, that.iterator).asInstanceOf[That]
+//    else super.zip[A1, B, That](that)
   }
 
   /** Zips this iterable with its indices. `s.zipWithIndex` is equivalent to
@@ -315,8 +319,9 @@ self =>
     def loop(len: Int, these: Stream[A]): Stream[B] =
       if (these.isEmpty) Stream.fill(len)(elem)
       else new Stream.Cons(these.head, loop(len - 1, these.tail))
-    if (bf.isInstanceOf[Stream.StreamBuilderFactory[_]]) loop(len, this).asInstanceOf[That]
-    else super.padTo(len, elem)
+    loop(len, this).asInstanceOf[That]
+// was:    if (bf.isInstanceOf[Stream.StreamBuilderFactory[_]]) loop(len, this).asInstanceOf[That]
+//    else super.padTo(len, elem)
   }
 
   /** A list consisting of all elements of this list in reverse order.
@@ -347,6 +352,14 @@ self =>
  */
 object Stream extends SequenceFactory[Stream] {
 
+  /** The factory for streams.
+   *  @note Methods such as map/flatMap will not invoke the Builder factory,
+   *        but will return a new stream directly, to preserve laziness.
+   *        The new stream is then cast to the factory's result type.
+   *        This means that every BuilderFactory that takes a
+   *        Stream as its From type parameter must yield a stream as its result parameter.
+   *        If that assumption is broken, cast errors might result.
+   */
   class StreamBuilderFactory[A] extends VirtualBuilderFactory[A]
 
   implicit def builderFactory[A]: BuilderFactory[A, Stream[A], Coll] = new StreamBuilderFactory[A]