Introduce runningFold operator, make scan alias to runningFold (Kotlin#2645)

* Otherwise Kotlin users with non-reactive background get confused by flow/stdlib inconsistency
    * Make it experimental to delay the final decision about the name

Fixes Kotlin#2641

Author: qwwdfsad

Diff for: kotlinx-coroutines-core/api/kotlinx-coroutines-core.api

@@ -994,6 +994,7 @@ public final class kotlinx/coroutines/flow/FlowKt {
 	public static synthetic fun retry$default (Lkotlinx/coroutines/flow/Flow;ILkotlin/jvm/functions/Function1;ILjava/lang/Object;)Lkotlinx/coroutines/flow/Flow;
 	public static synthetic fun retry$default (Lkotlinx/coroutines/flow/Flow;JLkotlin/jvm/functions/Function2;ILjava/lang/Object;)Lkotlinx/coroutines/flow/Flow;
 	public static final fun retryWhen (Lkotlinx/coroutines/flow/Flow;Lkotlin/jvm/functions/Function4;)Lkotlinx/coroutines/flow/Flow;
+	public static final fun runningFold (Lkotlinx/coroutines/flow/Flow;Ljava/lang/Object;Lkotlin/jvm/functions/Function3;)Lkotlinx/coroutines/flow/Flow;
 	public static final fun runningReduce (Lkotlinx/coroutines/flow/Flow;Lkotlin/jvm/functions/Function3;)Lkotlinx/coroutines/flow/Flow;
 	public static final fun sample (Lkotlinx/coroutines/flow/Flow;J)Lkotlinx/coroutines/flow/Flow;
 	public static final fun sample-HG0u8IE (Lkotlinx/coroutines/flow/Flow;D)Lkotlinx/coroutines/flow/Flow;

Diff for: kotlinx-coroutines-core/common/src/flow/operators/Transform.kt

@@ -74,6 +74,20 @@ public fun <T> Flow<T>.onEach(action: suspend (T) -> Unit): Flow<T> = transform
     return@transform emit(value)
 }
 
+/**
+ * Folds the given flow with [operation], emitting every intermediate result, including [initial] value.
+ * Note that initial value should be immutable (or should not be mutated) as it is shared between different collectors.
+ * For example:
+ * ```
+ * flowOf(1, 2, 3).scan(emptyList<Int>()) { acc, value -> acc + value }.toList()
+ * ```
+ * will produce `[], [1], [1, 2], [1, 2, 3]]`.
+ *
+ * This function is an alias to [runningFold] operator.
+ */
+@ExperimentalCoroutinesApi
+public fun <T, R> Flow<T>.scan(initial: R, @BuilderInference operation: suspend (accumulator: R, value: T) -> R): Flow<R> = runningFold(initial, operation)
+
 /**
  * Folds the given flow with [operation], emitting every intermediate result, including [initial] value.
  * Note that initial value should be immutable (or should not be mutated) as it is shared between different collectors.
@@ -84,7 +98,7 @@ public fun <T> Flow<T>.onEach(action: suspend (T) -> Unit): Flow<T> = transform
  * will produce `[], [1], [1, 2], [1, 2, 3]]`.
  */
 @ExperimentalCoroutinesApi
-public fun <T, R> Flow<T>.scan(initial: R, @BuilderInference operation: suspend (accumulator: R, value: T) -> R): Flow<R> = flow {
+public fun <T, R> Flow<T>.runningFold(initial: R, @BuilderInference operation: suspend (accumulator: R, value: T) -> R): Flow<R> = flow {
     var accumulator: R = initial
     emit(accumulator)
     collect { value ->
@@ -100,7 +114,7 @@ public fun <T, R> Flow<T>.scan(initial: R, @BuilderInference operation: suspend
  *
  * For example:
  * ```
- * flowOf(1, 2, 3, 4).runningReduce { (v1, v2) -> v1 + v2 }.toList()
+ * flowOf(1, 2, 3, 4).runningReduce { acc, value -> acc + value }.toList()
  * ```
  * will produce `[1, 3, 6, 10]`
  */

Diff for: kotlinx-coroutines-core/common/test/flow/operators/ScanTest.kt

@@ -23,6 +23,13 @@ class ScanTest : TestBase() {
         assertEquals(listOf(emptyList(), listOf(1), listOf(1, 2), listOf(1, 2, 3)), result)
     }
 
+    @Test
+    fun testFoldWithInitial() = runTest {
+        val flow = flowOf(1, 2, 3)
+        val result = flow.runningFold(emptyList<Int>()) { acc, value -> acc + value }.toList()
+        assertEquals(listOf(emptyList(), listOf(1), listOf(1, 2), listOf(1, 2, 3)), result)
+    }
+
     @Test
     fun testNulls() = runTest {
         val flow = flowOf(null, 2, null, null, null, 5)