Extend set variable widget to work with filters

core/modules/widgets/setvariable.js

@@ -39,9 +39,21 @@ Compute the internal state of the widget
 SetWidget.prototype.execute = function() {
 	// Get our parameters
 	this.setName = this.getAttribute("name","currentTiddler");
+	this.setFilter = this.getAttribute("filter");
 	this.setValue = this.getAttribute("value");
+	this.setEmptyValue = this.getAttribute("emptyValue");
 	// Set context variable
-	this.setVariable(this.setName,this.setValue,this.parseTreeNode.params);
+	var value = this.setValue;
+	if(this.setFilter) {
+		var results = this.wiki.filterTiddlers(this.setFilter,this);
+		if(!this.setValue) {
+			value = $tw.utils.stringifyList(results);
+		}
+		if(results.length === 0 && this.setEmptyValue !== undefined) {
+			value = this.setEmptyValue;
+		}
+	}
+	this.setVariable(this.setName,value,this.parseTreeNode.params);
 	// Construct the child widgets
 	this.makeChildWidgets();
 };
@@ -51,7 +63,7 @@ Selectively refreshes the widget if needed. Returns true if the widget or any of
 */
 SetWidget.prototype.refresh = function(changedTiddlers) {
 	var changedAttributes = this.computeAttributes();
-	if(changedAttributes.name || changedAttributes.value) {
+	if(changedAttributes.name || changedAttributes.filter || changedAttributes.value || changedAttributes.emptyValue) {
 		this.refreshSelf();
 		return true;
 	} else {

editions/prerelease/tiddlers/SetWidget.tid

@@ -0,0 +1,57 @@
+title: SetWidget
+created: 201311151827
+modified: 20141113212520422
+tags: Widgets
+caption: set
+
+! Introduction
+
+The set variable widget assigns a value to a specified [[variable|WidgetVariables]]. The new value of the variable is availale to the content within the set variable widget.
+
+! Content and Attributes
+
+The content of the `<$set>` widget is the scope for the value assigned to the variable.
+
+|!Attribute |!Description |
+|name |The name of the variable to assign (defaults to "currentTiddler") |
+|value |The value to assign to the variable if the filter is missing or not empty |
+|filter |An optional filter to be evaluated and assigned to the variable (see below) |
+|emptyValue |The value to assign to the variable if the filter is present and evaluates to an empty list (see below) |
+
+!! Simple Variable Assignment
+
+The simplest way of using set variable widget assigns a string to a variable. The following example assigns a literal string
+
+```
+<$set name="myVariable" value="Some text">
+<<myVariable>>
+</$set>
+```
+
+Both the name and value attributes can be transcluded. For example:
+
+```
+<$set name=<<anotherVariable>> value={{template!!text}}>
+<<myVariable>>
+</$set>
+```
+
+!! Conditional Variable Assignment
+
+This form of the set variable widget chooses one of two specified values according to whether a filter evaluates to an empty list. Here's an example that sets a variable according to whether the current tiddler is called "myMagicTitle":
+
+```
+<$set name="myVariable" filter="[all[current]field:title[myMagicTitle]]" value="It's magic" emptyValue="It's not magic">
+<<myVariable>>
+</$set>
+```
+
+!! Filtered List Variable Assignment
+
+This form of the set variable widget evaluates the filter and assigns the result to the variable as a space-separated list (using double square brackets for titles containing spaces).
+
+```
+<$set name="myVariable" filter="[tag[Introduction]]">
+<<myVariable>>
+</$set>
+```