Introduce letlist widget for assigning lists to variables

Terrible name. Annoyingly, we can't overload the existing let or set widgets.

core/modules/widgets/letlist.js

@@ -0,0 +1,91 @@
+/*\
+title: $:/core/modules/widgets/letlist.js
+type: application/javascript
+module-type: widget
+
+This widget allows defining multiple variables as lists, allowing
+the later variables to depend upon the earlier ones
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var Widget = require("$:/core/modules/widgets/widget.js").widget;
+
+var LetListWidget = function(parseTreeNode,options) {
+	// Initialise
+	this.initialise(parseTreeNode,options);
+};
+
+/*
+Inherit from the base widget class
+*/
+LetListWidget.prototype = new Widget();
+
+/*
+Render this widget into the DOM
+*/
+LetListWidget.prototype.render = function(parent,nextSibling) {
+	this.parentDomNode = parent;
+	this.computeAttributes();
+	this.execute();
+	this.renderChildren(parent,nextSibling);
+};
+
+LetListWidget.prototype.computeAttributes = function() {
+	// Before computing attributes, we must make clear that none of the
+	// existing attributes are staged for lookup, even on a refresh
+	var changedAttributes = {},
+		self = this;
+	this.currentValueFor = Object.create(null);
+	$tw.utils.each($tw.utils.getOrderedAttributesFromParseTreeNode(this.parseTreeNode),function(attribute) {
+		var value = self.computeAttribute(attribute),
+			name = attribute.name;
+		// Now that it's prepped, we're allowed to look this variable up
+		// when defining later variables
+		if(value !== undefined) {
+			self.currentValueFor[name] = self.wiki.filterTiddlers(value,self);
+		}
+	});
+	// Run through again, setting variables and looking for differences
+	$tw.utils.each(this.currentValueFor,function(value,name) {
+		if (!$tw.utils.isArrayEqual(self.attributes[name],value)) {
+			self.attributes[name] = value;
+			self.setVariable(name,value);
+			changedAttributes[name] = true;
+		}
+	});
+	return changedAttributes;
+};
+
+LetListWidget.prototype.getVariableInfo = function(name,options) {
+	// Special handling: If this variable exists in this very $let, we can
+	// use it, but only if it's been staged.
+	if ($tw.utils.hop(this.currentValueFor,name)) {
+		var value = this.currentValueFor[name];
+		return {
+			text: value[0] || "",
+			resultList: value
+		};
+	}
+	return Widget.prototype.getVariableInfo.call(this,name,options);
+};
+
+/*
+Refresh the widget by ensuring our attributes are up to date
+*/
+LetListWidget.prototype.refresh = function(changedTiddlers) {
+	var changedAttributes = this.computeAttributes();
+	if($tw.utils.count(changedAttributes) > 0) {
+		this.refreshSelf();
+		return true;
+	}
+	return this.refreshChildren(changedTiddlers);
+};
+
+exports["letlist"] = LetListWidget;
+
+})();

editions/test/tiddlers/tests/data/letlist-widget/SelfReference.tid

@@ -0,0 +1,19 @@
+title: LetListWidget/SelfReference
+description: Using self references with the "letlist" widget
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+<$letlist
+	original="[all[tiddlers]sort[]]"
+	varname="[varlist[original]]"
+>
+<$text text={{{ [varlist[varname]] +[join[-]] }}}/>
+</$letlist>
++
+title: ExpectedResult
+
+<p>
+$:/core-ExpectedResult-Output
+</p>

editions/test/tiddlers/tests/data/letlist-widget/Simple.tid

@@ -0,0 +1,16 @@
+title: LetListWidget/Simple
+description: Simple usage of the "letlist" widget
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+<$letlist varname="[all[tiddlers]sort[]]">
+<$text text={{{ [varlist[varname]] +[join[-]] }}}/>
+</$letlist>
++
+title: ExpectedResult
+
+<p>
+$:/core-ExpectedResult-Output
+</p>

editions/tw5.com/tiddlers/filters/syntax/varlist Operator.tid

@@ -9,6 +9,6 @@ tags: [[Filter Operators]] [[Selection Constructors]]
 title: varlist Operator
 type: text/vnd.tiddlywiki
 
-The parameter specifies the name of a variable that has been assigned a list of items by a [[let filter run prefix|Let Filter Run Prefix]]. The operator retrieves all the list items.
+The parameter specifies the name of a variable that has been assigned a list of items by a [[let filter run prefix|Let Filter Run Prefix]]. The operator retrieves all the list items stored in the variable.
 
 <<.operator-examples "varlist">>