new "for" loop primitive in the Control category

HISTORY.md

@@ -26,6 +26,7 @@
     * 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 "for" loop primitive 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
     * added "width" and "height" as attribute selectors of the OF primitive for the stage
@@ -69,8 +70,9 @@
     * French
 
 ### 2019-04-24
-* Threads: new "combine" primitive in list category
+* Threads, Objects: new "combine" primitive in list category
 * Threads: added type-assertions for the new HOF prims
+* Threads, Objects: new "for" loop primitive in Control category
 
 ### 2019-04-23
 * Threads: fixed JS stack overflow issue for MAP primitive

src/objects.js

@@ -736,6 +736,12 @@ SpriteMorph.prototype.initBlocks = function () {
             category: 'control',
             spec: 'repeat until %b %loop'
         },
+        doFor: {
+            type: 'command',
+            category: 'control',
+            spec: 'for %upvar = %n to %n %cla',
+            defaults: ['i', 1, 10]
+        },
         doIf: {
             type: 'command',
             category: 'control',
@@ -2172,6 +2178,7 @@ SpriteMorph.prototype.blockTemplates = function (category) {
         blocks.push(block('doForever'));
         blocks.push(block('doRepeat'));
         blocks.push(block('doUntil'));
+        blocks.push(block('doFor'));
         blocks.push('-');
         blocks.push(block('doIf'));
         blocks.push(block('doIfElse'));
@@ -7747,6 +7754,7 @@ StageMorph.prototype.blockTemplates = function (category) {
         blocks.push(block('doForever'));
         blocks.push(block('doRepeat'));
         blocks.push(block('doUntil'));
+        blocks.push(block('doFor'));
         blocks.push('-');
         blocks.push(block('doIf'));
         blocks.push(block('doIfElse'));

src/threads.js

@@ -2098,6 +2098,37 @@ Process.prototype.doForEach = function (upvar, list, script) {
     this.evaluate(script, new List(), true);
 };
 
+Process.prototype.doFor = function (upvar, start, end, script) {
+    // perform a script for every integer step between start and stop,
+    // assigning the current iteration index to a variable with the
+    // name specified in the "upvar" parameter, so it can be referenced
+    // within the script.
+
+    var dta;
+    if (this.context.aggregation === null) {
+        this.context.aggregation = {
+            idx : Math.floor(start),
+            test : start < end ?
+                function () {return this.idx > end; }
+                    : function () {return this.idx < end; },
+            step : start < end ? 1 : -1,
+            parms : new List() // empty parameters, reusable to avoid GC
+        };
+    }
+
+    dta = this.context.aggregation;
+    this.context.outerContext.variables.addVar(upvar);
+    this.context.outerContext.variables.setVar(
+        upvar,
+        dta.idx
+    );
+    if (dta.test()) {return; }
+    dta.idx += dta.step;
+    this.pushContext('doYield');
+    this.pushContext();
+    this.evaluate(script, dta.parms, true);
+};
+
 // Process interpolated HOF primitives
 
 /*