more meaningful error messages for list operations

now asserting data types to list operations, due to popular request.
This is somewhat sad, because *always* type-asserting *everything* puts
a strain on performance and takes away some of the “liveliness”. But I
hear you, teachers :-/

history.txt

@@ -3511,6 +3511,10 @@ Fixes:
 * Objects, GUI: run “When I start as clone” scripts when manually cloning a sprite, only position at hand pointer if no such scripts exist 
 * Morphic, Objects: confine turtle direction readout to 0-360 degrees, thanks, Cynthia for the bug report!!
 
+170708
+------
+* Threads: Assert data types to list operations -> meaningful error messages
+
 
 Features:
 * polymorphic sprite-local custom blocks
@@ -3528,10 +3532,12 @@ Features:
 * clones share their original sprite’s scripts, not a shallow-copy of them
 * a highlight-colored balloon indicates the number of active processes per shared script
 * new musical “notes” symbol
+* Assert data types to list operations for more meaningful error messages
 
 Fixes:
 * changed keyboard shortcut indicator for “find blocks” to “^”
 * prevent Snap from “hanging” when encountering certain errors in visible stepping 
 * only mark implicit parameters if no formal ones exist
 * optimized thread-launch and script highlighting to a single frame instead of formerly two
+* changed direction attribute of sprites to automatically confine to 0-360 degrees
 * fixed some typos

threads.js

@@ -61,7 +61,7 @@ StageMorph, SpriteMorph, StagePrompterMorph, Note, modules, isString, copy,
 isNil, WatcherMorph, List, ListWatcherMorph, alert, console, TableMorph,
 TableFrameMorph, ColorSlotMorph, isSnapObject*/
 
-modules.threads = '2017-July-04';
+modules.threads = '2017-July-08';
 
 var ThreadManager;
 var Process;
@@ -1627,23 +1627,23 @@ Process.prototype.reportNewList = function (elements) {
 };
 
 Process.prototype.reportCONS = function (car, cdr) {
-    // this.assertType(cdr, 'list');
+    this.assertType(cdr, 'list');
     return new List().cons(car, cdr);
 };
 
 Process.prototype.reportCDR = function (list) {
-    // this.assertType(list, 'list');
+    this.assertType(list, 'list');
     return list.cdr();
 };
 
 Process.prototype.doAddToList = function (element, list) {
-    // this.assertType(list, 'list');
+    this.assertType(list, 'list');
     list.add(element);
 };
 
 Process.prototype.doDeleteFromList = function (index, list) {
     var idx = index;
-    // this.assertType(list, 'list');
+    this.assertType(list, 'list');
     if (this.inputOption(index) === 'all') {
         return list.clear();
     }
@@ -1660,7 +1660,7 @@ Process.prototype.doDeleteFromList = function (index, list) {
 
 Process.prototype.doInsertInList = function (element, index, list) {
     var idx = index;
-    // this.assertType(list, 'list');
+    this.assertType(list, 'list');
     if (index === '') {
         return null;
     }
@@ -1675,7 +1675,7 @@ Process.prototype.doInsertInList = function (element, index, list) {
 
 Process.prototype.doReplaceInList = function (index, list, element) {
     var idx = index;
-    // this.assertType(list, 'list');
+    this.assertType(list, 'list');
     if (index === '') {
         return null;
     }
@@ -1690,7 +1690,7 @@ Process.prototype.doReplaceInList = function (index, list, element) {
 
 Process.prototype.reportListItem = function (index, list) {
     var idx = index;
-    // this.assertType(list, 'list');
+    this.assertType(list, 'list');
     if (index === '') {
         return '';
     }
@@ -1704,12 +1704,12 @@ Process.prototype.reportListItem = function (index, list) {
 };
 
 Process.prototype.reportListLength = function (list) {
-    // this.assertType(list, 'list');
+    this.assertType(list, 'list');
     return list.length();
 };
 
 Process.prototype.reportListContainsItem = function (list, element) {
-    // this.assertType(list, 'list');
+    this.assertType(list, 'list');
     return list.contains(element);
 };