List; add swap(index0, index1)

doc/site/modules/core/list.markdown

@@ -127,6 +127,17 @@ System.print(list) //> [6, 7, 8, 9]
 
 It is a runtime error if `comparer` is not a function.
 
+### **swap**(index0, index1)
+
+Swaps values inside the list around. Puts the value from `index0` in `index1`,
+and the value from `index1` at `index0` in the list.
+
+<pre class="snippet">
+var list = [0, 1, 2, 3, 4]
+list.swap(0, 3)
+System.print(list) //> [3, 1, 2, 0, 4]
+</pre>
+
 ### **[**index**]** operator
 
 Gets the element at `index`. If `index` is negative, it counts backwards from

src/vm/wren_core.c

@@ -400,6 +400,21 @@ DEF_PRIMITIVE(list_indexOf)
   RETURN_NUM(wrenListIndexOf(vm, list, args[1]));
 }
 
+DEF_PRIMITIVE(list_swap)
+{
+  ObjList* list = AS_LIST(args[0]);
+  uint32_t indexA = validateIndex(vm, args[1], list->elements.count, "Index 0");
+  if (indexA == UINT32_MAX) return false;
+  uint32_t indexB = validateIndex(vm, args[2], list->elements.count, "Index 1");
+  if (indexB == UINT32_MAX) return false;
+
+  Value a = list->elements.data[indexA];
+  list->elements.data[indexA] = list->elements.data[indexB];
+  list->elements.data[indexB] = a;
+
+  RETURN_NULL;
+}
+
 DEF_PRIMITIVE(list_subscript)
 {
   ObjList* list = AS_LIST(args[0]);
@@ -1367,6 +1382,7 @@ void wrenInitializeCore(WrenVM* vm)
   PRIMITIVE(vm->listClass, "iteratorValue(_)", list_iteratorValue);
   PRIMITIVE(vm->listClass, "removeAt(_)", list_removeAt);
   PRIMITIVE(vm->listClass, "indexOf(_)", list_indexOf);
+  PRIMITIVE(vm->listClass, "swap(_,_)", list_swap);
 
   vm->mapClass = AS_CLASS(wrenFindVariable(vm, coreModule, "Map"));
   PRIMITIVE(vm->mapClass->obj.classObj, "new()", map_new);

test/core/list/swap.wren

@@ -0,0 +1,9 @@
+var list = [0, 1, 2, 3, 4]
+
+list.swap(0, 3) 
+System.print(list) // expect: [3, 1, 2, 0, 4]
+
+list.swap(-1, 2)
+System.print(list) // expect: [3, 1, 4, 0, 2]
+
+list.swap(8, 0) // expect runtime error: Index 0 out of bounds.