Fixes #3091. Review by community.

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@21179 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

src/library/scala/collection/TraversableLike.scala

@@ -612,6 +612,46 @@ self =>
   def reduceRightOption[B >: A](op: (A, B) => B): Option[B] =
     if (isEmpty) None else Some(reduceRight(op))
   
+  /**
+   * Produces a collection containing cummulative results of applying the operator going left to right.
+   * $willNotTerminateInf
+   * $orderDependent
+   * 
+   * @tparam B      the type of the elements in the resulting collection
+   * @tparam That   the actual type of the resulting collection
+   * @param z       the initial value
+   * @param op      the binary operator applied to the intermediate result and the element
+   * @param bf      $bfinfo
+   * @return        collection with intermediate results
+   */
+  def scanLeft[B, That](z: B)(op: (B, A) => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
+    val (_, b) = foldLeft(z, bf(repr) += z) { (acc, x) =>
+      val next = op(acc._1, x)
+      (next, acc._2 += next)
+    }
+    b.result
+  }
+  
+  /**
+   * Produces a collection containing cummulative results of applying the operator going right to left.
+   * $willNotTerminateInf
+   * $orderDependent
+   * 
+   * @tparam B      the type of the elements in the resulting collection
+   * @tparam That   the actual type of the resulting collection
+   * @param z       the initial value
+   * @param op      the binary operator applied to the intermediate result and the element
+   * @param bf      $bfinfo
+   * @return        collection with intermediate results
+   */
+  def scanRight[B, That](z: B)(op: (A, B) => B)(implicit bf: CanBuildFrom[Repr, B, That]): That = {
+    val (_, b) = foldRight(z, bf(repr) += z) { (x, acc) =>
+      val next = op(x, acc._1)
+      (next, acc._2 += next)
+    }
+    b.result
+  }
+  
   /** Sums up the elements of this collection.
    * 
    *   @param   num  an implicit parameter defining a set of numeric operations

test/files/pos/scan.scala

@@ -0,0 +1,18 @@
+
+
+
+
+object Test {
+
+  def main(args: Array[String]) {
+    val lst = List(1, 2, 3, 4, 5)
+    
+    assert(lst.scanLeft(0)(_ + _) == List(0, 1, 3, 6, 10, 15))
+    assert(lst.scanRight(0)(_ + _) == List(0, 5, 9, 12, 14, 15))
+    
+    val emp = List[Int]()
+    assert(emp.scanLeft(0)(_ + _) == List(0))
+    assert(emp.scanRight(0)(_ + _) == List(0))
+  }
+
+}

test/files/scalacheck/scan.scala

@@ -0,0 +1,17 @@
+import org.scalacheck._
+import Prop._
+import Gen._
+
+
+object Test extends Properties("TraversableLike.scanLeft") {
+  property("scanLeft") = forAll { (xs: List[Int], z: Int) => {
+    val sums = xs.scanLeft(z)(_ + _)
+    (xs.size == 0) || sums.zip(sums.tail).map(x => x._2 - x._1) == xs
+  }}
+}
+
+
+
+
+
+