Add support for variable filter operands

Fixes #583

editions/test/tiddlers/tests/test-filters.js

@@ -270,6 +270,25 @@ describe("Filter tests", function() {
 		expect(wiki.filterTiddlers("[modifier{!!modifier}] +[sort[title]]",fakeWidget).join(",")).toBe("$:/TiddlerTwo,a fourth tiddler,one,Tiddler Three");
 	});
 
+	it("should handle variable operands", function() {
+
+		var widget = require("$:/core/modules/widgets/widget.js");
+	// Create a root widget for attaching event handlers. By using it as the parentWidget for another widget tree, one can reuse the event handlers
+		var rootWidget = new widget.widget({
+			type: "widget",
+			children: [{type: "widget", children: []}]
+		},{
+			wiki: $tw.wiki,
+			document: $tw.document
+		});
+		rootWidget.makeChildWidgets();
+		var anchorWidget = rootWidget.children[0];
+		rootWidget.setVariable("myVar","Tidd");
+		rootWidget.setVariable("myVar2","JoeBloggs");
+		expect(wiki.filterTiddlers("[prefix<myVar>] +[sort[title]]",anchorWidget).join(",")).toBe("Tiddler Three,TiddlerOne");
+		expect(wiki.filterTiddlers("[modifier<myVar2>] +[sort[title]]",anchorWidget).join(",")).toBe("TiddlerOne");
+	});
+
 	it("should handle the before and after operators", function() {
 		expect(wiki.filterTiddlers("[list[TiddlerSeventh]after[TiddlerOne]]").join(",")).toBe("Tiddler Three");
 		expect(wiki.filterTiddlers("[list[TiddlerSeventh]after[a fourth tiddler]]").join(",")).toBe("MissingTiddler");

editions/tw5.com/tiddlers/dev/TiddlerFilter Formal Grammar.tid

@@ -1,5 +1,5 @@
 created: 20140210141217955
-modified: 20140303091312363
+modified: 20140725091312363
 tags: dev
 title: TiddlerFilter Formal Grammar
 type: text/vnd.tiddlywiki
@@ -34,7 +34,7 @@ Whitespace is matched with javascript "\s+", which matches space, tab, carriage
 
 ''<opt-operator-suffix>'' ::= ''<string>'' | ""
 
-''<operand>'' ::= "[" ''<search-string>'' "]" | "{" ''<indirect-search-string>'' "}" | ''<regex>''
+''&lt;operand&gt;'' ::= "[" ''&lt;search-string&gt;'' "]" | "{" ''&lt;indirect-search-string&gt;'' "}" | "<" ''&lt;variable-name-string&gt;'' ">" | ''&lt;regex&gt;''
 
 ''&lt;regex&gt;'' ::= "/" ''&lt;string&gt;'' "/" ''&lt;opt-regex-args&gt;''
 

editions/tw5.com/tiddlers/filters/Introduction to Filters.tid

@@ -1,5 +1,5 @@
 created: 20140410101941871
-modified: 20140410103229640
+modified: 20140725103229640
 tags: introduction
 title: Introduction to Filters
 type: text/vnd.tiddlywiki
@@ -67,6 +67,16 @@ If a filter operator is written with curly brackets around the operand then it i
 [search{$:/temp/search}]
 ```
 
+! Variable Operands
+
+If a filter operator is written with angle brackets around the operand then it is taken to be the name of a variable containing the actual value. For example, this filter selects all tiddlers containing the title of the current tiddler:
+
+```
+[search<currentTiddler>]
+```
+
+(Note that the `currentTiddler` variable is used to track the current tiddler).
+
 ! Regular Expression Operands
 
 The "field" filter also accepts [[regular expressions|http://en.wikipedia.org/wiki/Regular_expression]] with the syntax `/regexp/(modifier)`. For example: