new "index of (element) in (list)" primitive

src/lists.js

@@ -62,7 +62,7 @@ CellMorph, ArrowMorph, MenuMorph, snapEquals, localize, isString,
 MorphicPreferences, TableDialogMorph, SpriteBubbleMorph, SpeechBubbleMorph,
 TableFrameMorph, TableMorph, Variable, isSnapObject, Costume, contains*/
 
-modules.lists = '2020-April-24';
+modules.lists = '2020-April-25';
 
 var List;
 var ListWatcherMorph;
@@ -95,7 +95,8 @@ var ListWatcherMorph;
     length()                - number of slots
     at(index)               - element present in specified slot
     contains(element)       - <bool>
-    isEmpty(element)        - <bool>
+    isEmpty()               - <bool>
+    indexOf(element)        - index of element's first occurrence, 0 if none
 
     conversion:
     -----------
@@ -253,6 +254,27 @@ List.prototype.isEmpty = function () {
     return !this.contents.length;
 };
 
+List.prototype.indexOf = function (element) {
+    var pair = this,
+        idx = 1,
+        i, len;
+    while (pair.isLinked) {
+        if (snapEquals(pair.first, element)) {
+            return idx;
+        }
+        pair = pair.rest;
+        idx += 1;
+    }
+    // in case I'm arrayed
+    len = pair.contents.length;
+    for (i = 0; i < len; i += 1) {
+        if (snapEquals(pair.contents[i], element)) {
+            return idx + i;
+        }
+    }
+    return 0;
+};
+
 // List table (2D) accessing (for table morph widget):
 
 List.prototype.isTable = function () {

src/objects.js

@@ -84,7 +84,7 @@ BlockEditorMorph, BlockDialogMorph, PrototypeHatBlockMorph,  BooleanSlotMorph,
 localize, TableMorph, TableFrameMorph, normalizeCanvas, VectorPaintEditorMorph,
 HandleMorph, AlignmentMorph, Process, XML_Element, WorldMap, copyCanvas*/
 
-modules.objects = '2020-April-20';
+modules.objects = '2020-April-25';
 
 var SpriteMorph;
 var StageMorph;
@@ -1282,6 +1282,12 @@ SpriteMorph.prototype.initBlocks = function () {
             category: 'lists',
             spec: 'is %l empty?'
         },
+        reportListIndex: {
+            type: 'reporter',
+            category: 'lists',
+            spec: 'index of %s in %l',
+            defaults: [localize('thing')]
+        },
         doAddToList: {
             type: 'command',
             category: 'lists',
@@ -2627,6 +2633,7 @@ SpriteMorph.prototype.blockTemplates = function (category) {
         blocks.push(block('reportCDR'));
         blocks.push('-');
         blocks.push(block('reportListLength'));
+        blocks.push(block('reportListIndex'));
         blocks.push(block('reportListContainsItem'));
         blocks.push(block('reportListIsEmpty'));
         blocks.push('-');
@@ -2814,6 +2821,7 @@ SpriteMorph.prototype.freshPalette = function (category) {
                         'reportListItem',
                         'reportCDR',
                         'reportListLength',
+                        'reportListIndex',
                         'reportListContainsItem',
                         'reportListIsEmpty',
                         'doForEach',
@@ -8677,6 +8685,7 @@ StageMorph.prototype.blockTemplates = function (category) {
         blocks.push(block('reportCDR'));
         blocks.push('-');
         blocks.push(block('reportListLength'));
+        blocks.push(block('reportListIndex'));
         blocks.push(block('reportListContainsItem'));
         blocks.push(block('reportListIsEmpty'));
         blocks.push('-');

src/threads.js

@@ -61,7 +61,7 @@ StageMorph, SpriteMorph, StagePrompterMorph, Note, modules, isString, copy, Map,
 isNil, WatcherMorph, List, ListWatcherMorph, alert, console, TableMorph, Color,
 TableFrameMorph, ColorSlotMorph, isSnapObject, newCanvas, Symbol, SVG_Costume*/
 
-modules.threads = '2020-April-24';
+modules.threads = '2020-April-25';
 
 var ThreadManager;
 var Process;
@@ -1834,6 +1834,11 @@ Process.prototype.reportListLength = function (list) {
     return list.length();
 };
 
+Process.prototype.reportListIndex = function(element, list) {
+    this.assertType(list, 'list');
+    return list.indexOf(element);
+};
+
 Process.prototype.reportListContainsItem = function (list, element) {
     this.assertType(list, 'list');
     return list.contains(element);