new "is empty" predicate primitive in List category

HISTORY.md

@@ -25,7 +25,7 @@
     * new "get graphic effect" reporter
     * new "get pen attribute" reporter
     * new "write" command in pen category (used to be "label" in tools)
-    * new "map","keep", "combine" and "for each" primitives in list category
+    * new "is empty", "map","keep", "combine" and "for each" primitives in list category
     * new "for" loop and "if then else" reporter primitives in the Control category
     * added "neg", "lg" (log2) and "2^" selectors to monadic function reporter in Operators
     * added "^" reporter (power of) in the Operators category
@@ -71,6 +71,9 @@
     * German
     * French
 
+### 2019-04-26
+* Lists, Threads, Objects: new "is empty" predicate primitive in List category
+
 ### 2019-04-26
 * updated Catalan translation (for new HOF prims)
 * updated Spanish translation (for new HOF prims)

snap.html

@@ -7,11 +7,11 @@
         <script type="text/javascript" src="src/morphic.js?version=2019-02-07"></script>
         <script type="text/javascript" src="src/widgets.js?version=2019-04-05"></script>
         <script type="text/javascript" src="src/blocks.js?version=2019-04-11"></script>
-        <script type="text/javascript" src="src/threads.js?version=2019-04-26"></script>
-        <script type="text/javascript" src="src/objects.js?version=2019-04-26"></script>
+        <script type="text/javascript" src="src/threads.js?version=2019-04-27"></script>
+        <script type="text/javascript" src="src/objects.js?version=2019-04-27"></script>
         <script type="text/javascript" src="src/gui.js?version=2019-04-10"></script>
         <script type="text/javascript" src="src/paint.js?version=2019-02-22"></script>
-        <script type="text/javascript" src="src/lists.js?version=2019-02-07"></script>
+        <script type="text/javascript" src="src/lists.js?version=2019-04-27"></script>
         <script type="text/javascript" src="src/byob.js?version=2019-02-15"></script>
         <script type="text/javascript" src="src/tables.js?version=2019-02-07"></script>
         <script type="text/javascript" src="src/symbols.js?version=2019-03-07"></script>

src/lists.js

@@ -62,7 +62,7 @@ CellMorph, ArrowMorph, MenuMorph, snapEquals, Morph, isNil, localize, isString,
 MorphicPreferences, TableDialogMorph, SpriteBubbleMorph, SpeechBubbleMorph,
 TableFrameMorph, TableMorph, Variable, isSnapObject*/
 
-modules.lists = '2019-February-07';
+modules.lists = '2019-April-27';
 
 var List;
 var ListWatcherMorph;
@@ -95,6 +95,7 @@ var ListWatcherMorph;
     length()                - number of slots
     at(index)               - element present in specified slot
     contains(element)       - <bool>
+    isEmpty(element)        - <bool>
 
     conversion:
     -----------
@@ -245,6 +246,13 @@ List.prototype.contains = function (element) {
     });
 };
 
+List.prototype.isEmpty = function () {
+    if (this.isLinked) {
+        return isNil(this.first);
+    }
+    return !this.contents.length;
+};
+
 // List table (2D) accessing (for table morph widget):
 
 List.prototype.isTable = function () {

src/objects.js

@@ -84,7 +84,7 @@ BlockEditorMorph, BlockDialogMorph, PrototypeHatBlockMorph, localize,
 TableMorph, TableFrameMorph, normalizeCanvas, BooleanSlotMorph, HandleMorph,
 AlignmentMorph, Process, XML_Element, VectorPaintEditorMorph*/
 
-modules.objects = '2019-April-26';
+modules.objects = '2019-April-27';
 
 var SpriteMorph;
 var StageMorph;
@@ -1250,6 +1250,11 @@ SpriteMorph.prototype.initBlocks = function () {
             spec: '%l contains %s',
             defaults: [null, localize('thing')]
         },
+        reportListIsEmpty: {
+            type: 'predicate',
+            category: 'lists',
+            spec: 'is %l empty?'
+        },
         doAddToList: {
             type: 'command',
             category: 'lists',
@@ -2431,6 +2436,7 @@ SpriteMorph.prototype.blockTemplates = function (category) {
         blocks.push('-');
         blocks.push(block('reportListLength'));
         blocks.push(block('reportListContainsItem'));
+        blocks.push(block('reportListIsEmpty'));
         blocks.push('-');
         blocks.push(block('reportMap'));
         blocks.push(block('reportKeep'));
@@ -2616,6 +2622,7 @@ SpriteMorph.prototype.freshPalette = function (category) {
                         'reportCDR',
                         'reportListLength',
                         'reportListContainsItem',
+                        'reportListIsEmpty',
                         'doForEach',
                         'reportMap',
                         'reportKeep',
@@ -7985,6 +7992,7 @@ StageMorph.prototype.blockTemplates = function (category) {
         blocks.push('-');
         blocks.push(block('reportListLength'));
         blocks.push(block('reportListContainsItem'));
+        blocks.push(block('reportListIsEmpty'));
         blocks.push('-');
         blocks.push(block('reportMap'));
         blocks.push(block('reportKeep'));

src/threads.js

@@ -62,7 +62,7 @@ StageMorph, SpriteMorph, StagePrompterMorph, Note, modules, isString, copy,
 isNil, WatcherMorph, List, ListWatcherMorph, alert, console, TableMorph, Color,
 TableFrameMorph, ColorSlotMorph, isSnapObject, Map, newCanvas, Symbol*/
 
-modules.threads = '2019-April-26';
+modules.threads = '2019-April-27';
 
 var ThreadManager;
 var Process;
@@ -1791,6 +1791,11 @@ Process.prototype.reportListContainsItem = function (list, element) {
     return list.contains(element);
 };
 
+Process.prototype.reportListIsEmpty = function (list) {
+    this.assertType(list, 'list');
+    return list.isEmpty();
+};
+
 Process.prototype.doShowTable = function (list) {
     // experimental
     this.assertType(list, 'list');