Merge pull request #48 from scala/flatten

Flatten and interaction between flatMap and Options

src/main/scala/strawman/collection/Iterable.scala

@@ -285,6 +285,9 @@ trait IterableOps[+A, +CC[X], +C] extends Any {
   /** Flatmap */
   def flatMap[B](f: A => IterableOnce[B]): CC[B] = fromIterable(View.FlatMap(coll, f))
 
+  def flatten[B](implicit ev: A => IterableOnce[B]): CC[B] =
+    fromIterable(View.FlatMap(coll, ev))
+
   /** Returns a new $coll containing the elements from the left hand operand followed by the elements from the
     *  right hand operand. The element type of the $coll is the most specific superclass encompassing
     *  the element types of the two operands.

src/main/scala/strawman/collection/package.scala

@@ -1,6 +1,6 @@
 package strawman
 
-import scala.{Any, AnyVal, Array, Char, IllegalArgumentException, IndexOutOfBoundsException, Int, NoSuchElementException, Unit, UnsupportedOperationException}
+import scala.{Any, AnyVal, Array, Boolean, Char, IllegalArgumentException, IndexOutOfBoundsException, Int, NoSuchElementException, Unit, UnsupportedOperationException}
 import scala.Predef.String
 import scala.reflect.ClassTag
 
@@ -71,6 +71,13 @@ package object collection extends LowPriority {
     }
   }
 
+  implicit def optionToIterableOnce[A](maybeA: scala.Option[A]): IterableOnce[A] =
+     new Iterator[A] {
+       private var _hasNext = maybeA.nonEmpty
+       def next(): A = if (_hasNext) { _hasNext = false; maybeA.get } else Iterator.empty.next()
+       def hasNext: Boolean = _hasNext
+     }
+
 }
 
 class LowPriority {

src/test/scala/strawman/collection/test/FlattenTest.scala

@@ -0,0 +1,23 @@
+package strawman
+package collection.test
+
+import org.junit.Test
+import strawman.collection.Seq
+import strawman.collection.immutable.{ImmutableArray, List}
+
+import scala.{Int, Some, Unit}
+import scala.Predef.{$conforms, assert}
+
+class FlattenTest {
+
+  def f(xs: Seq[Seq[Int]], ys: Seq[Int]): Unit = {
+    assert(xs.flatten == ys)
+    assert(ys.flatMap(y => Some(y)) == ys.map(y => Some(y)).flatten)
+  }
+
+  @Test
+  def flattenTest: Unit = {
+    f(List(ImmutableArray(1, 2, 3)), List(1, 2, 3))
+  }
+
+}