Merge pull request #10624 from shardulc/2.13.x

Specify behavior of folds and reduces when operator is not associative or collection is unordered

src/library/scala/collection/ArrayOps.scala

@@ -761,20 +761,19 @@ final class ArrayOps[A](private val xs: Array[A]) extends AnyVal {
     true
   }
 
-  /** Applies a binary operator to a start value and all elements of this array,
-    * going left to right.
-    *
-    *  @param   z    the start value.
-    *  @param   op   the binary operator.
-    *  @tparam  B    the result type of the binary operator.
-    *  @return  the result of inserting `op` between consecutive elements of this array,
-    *           going left to right with the start value `z` on the left:
-    *           {{{
-    *             op(...op(z, x_1), x_2, ..., x_n)
-    *           }}}
-    *           where `x,,1,,, ..., x,,n,,` are the elements of this array.
-    *           Returns `z` if this array is empty.
-    */
+  /** Applies the given binary operator `op` to the given initial value `z` and
+   *  all elements of this array, going left to right. Returns the initial value
+   *  if this array is empty.
+   *
+   *  If `x,,1,,`, `x,,2,,`, ..., `x,,n,,` are the elements of this array, the
+   *  result is `op( op( ... op( op(z, x,,1,,), x,,2,,) ... ), x,,n,,)`.
+   *
+   *   @param    z       An initial value.
+   *   @param    op      A binary operator.
+   *   @tparam   B       The result type of the binary operator.
+   *   @return           The result of applying `op` to `z` and all elements of this array,
+   *                     going left to right. Returns `z` if this array is empty.
+   */
   def foldLeft[B](z: B)(op: (B, A) => B): B = {
     def f[@specialized(Specializable.Everything) T](xs: Array[T], op: (Any, Any) => Any, z: Any): Any = {
       val length = xs.length
@@ -867,20 +866,20 @@ final class ArrayOps[A](private val xs: Array[A]) extends AnyVal {
     res
   }
 
-  /** Applies a binary operator to all elements of this array and a start value,
-    * going right to left.
-    *
-    *  @param   z    the start value.
-    *  @param   op   the binary operator.
-    *  @tparam  B    the result type of the binary operator.
-    *  @return  the result of inserting `op` between consecutive elements of this array,
-    *           going right to left with the start value `z` on the right:
-    *           {{{
-    *             op(x_1, op(x_2, ... op(x_n, z)...))
-    *           }}}
-    *           where `x,,1,,, ..., x,,n,,` are the elements of this array.
-    *           Returns `z` if this array is empty.
-    */
+  /** Applies the given binary operator `op` to all elements of this array and
+   *  the given initial value `z`, going right to left. Returns the initial
+   *  value if this array is empty.
+   *
+   *  If `x,,1,,`, `x,,2,,`, ..., `x,,n,,` are the elements of this array, the
+   *  result is `op(x,,1,,, op(x,,2,,, op( ... op(x,,n,,, z) ... )))`.
+   *
+   *   @param    z       An initial value.
+   *   @param    op      A binary operator.
+   *   @tparam   B       The result type of the binary operator.
+   *   @return           The result of applying `op` to all elements of this array
+   *                     and `z`, going right to left. Returns `z` if this array
+   *                     is empty.
+   */
   def foldRight[B](z: B)(op: (A, B) => B): B = {
     def f[@specialized(Specializable.Everything) T](xs: Array[T], op: (Any, Any) => Any, z: Any): Any = {
       var v = z
@@ -907,15 +906,17 @@ final class ArrayOps[A](private val xs: Array[A]) extends AnyVal {
 
   }
 
-  /** Folds the elements of this array using the specified associative binary operator.
-    *
-    *  @tparam A1     a type parameter for the binary operator, a supertype of `A`.
-    *  @param z       a neutral element for the fold operation; may be added to the result
-    *                 an arbitrary number of times, and must not change the result (e.g., `Nil` for list concatenation,
-    *                 0 for addition, or 1 for multiplication).
-    *  @param op      a binary operator that must be associative.
-    *  @return        the result of applying the fold operator `op` between all the elements, or `z` if this array is empty.
-    */
+  /** Alias for [[foldLeft]].
+   *
+   *  The type parameter is more restrictive than for `foldLeft` to be
+   *  consistent with [[IterableOnceOps.fold]].
+   *
+   *   @tparam A1     The type parameter for the binary operator, a supertype of `A`.
+   *   @param z       An initial value.
+   *   @param op      A binary operator.
+   *   @return        The result of applying `op` to `z` and all elements of this array,
+   *                  going left to right. Returns `z` if this string is empty.
+   */
   def fold[A1 >: A](z: A1)(op: (A1, A1) => A1): A1 = foldLeft(z)(op)
 
   /** Builds a new array by applying a function to all elements of this array.