Introduce let filter run prefix for assigning filter run result to a variable

core/modules/filterrunprefixes/let.js

@@ -0,0 +1,37 @@
+/*\
+title: $:/core/modules/filterrunprefixes/let.js
+type: application/javascript
+module-type: filterrunprefix
+
+Assign a value to a variable
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter prefix function
+*/
+exports.let = function(operationSubFunction,options) {
+	// Save the variable name
+	var name = options.suffixes[0][0];
+	// Return the filter run prefix function
+	return function(results,source,widget) {
+		// Set the input source to the incoming results
+		var inputSource = widget.wiki.makeTiddlerIterator(results.toArray());
+		// Assign the result of the subfunction to the variable
+		var variables = {};
+		variables[name] = operationSubFunction(inputSource,widget)[0] || "";
+		// Clear the results
+		results.clear();
+		// Return the variables
+		return {
+			variables: variables
+		};
+	};
+};
+
+})();

core/modules/filters.js

@@ -348,7 +348,13 @@ exports.compileFilter = function(filterString) {
 		self.filterRecursionCount = (self.filterRecursionCount || 0) + 1;
 		if(self.filterRecursionCount < MAX_FILTER_DEPTH) {
 			$tw.utils.each(operationFunctions,function(operationFunction) {
-				operationFunction(results,source,widget);
+				var operationResult = operationFunction(results,source,widget);
+				if(operationResult) {
+					if(operationResult.variables) {
+						// If the filter run prefix has returned variables, create a new fake widget with those variables
+						widget = widget.makeFakeWidgetWithVariables(operationResult.variables);
+					}
+				}
 			});
 		} else {
 			results.push("/**-- Excessive filter recursion --**/");

editions/test/tiddlers/tests/data/let-filter-prefix/All.tid

@@ -0,0 +1,12 @@
+title: LetFilterRunPrefix/All
+description: Usage of "all[]" operator within "let" filter run prefix
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+<$text text={{{ [[colossus]] :let:another[all[]] [<another>] +[join[-]] }}}/>
++
+title: ExpectedResult
+
+<p>colossus</p>

editions/test/tiddlers/tests/data/let-filter-prefix/Simple.tid

@@ -0,0 +1,12 @@
+title: LetFilterRunPrefix/Simple
+description: Simple usage of "let" filter run prefix
+type: text/vnd.tiddlywiki-multiple
+tags: [[$:/tags/wiki-test-spec]]
+
+title: Output
+
+<$text text={{{ :let:varname[[magpie]] [<varname>] +[join[-]] }}}/>
++
+title: ExpectedResult
+
+<p>magpie</p>

editions/tw5.com/tiddlers/filters/syntax/Let Filter Run Prefix.tid

@@ -0,0 +1,23 @@
+created: 20250307212252946
+from-version: 5.3.7
+modified: 20250307212252946
+rp-input: all titles from previous filter runs
+rp-output: an empty title list is always returned from the "let" filter run prefix
+rp-purpose: assign the result of a filter run to a variable
+tags: [[Named Filter Run Prefix]]
+title: Let Filter Run Prefix
+type: text/vnd.tiddlywiki
+
+<$railroad text="""
+\start none
+\end none
+( ":let" ) 
+( ":" ) 
+( : "variable" )
+( ":" )
+[[run|"Filter Run"]]
+"""/>
+
+The `:let` filter run prefix assigns the first result of a filter run to a variable that is made available to the remaining [[filter runs|Filter Run]] in the [[filter expression|Filter Expression]]. If the filter run does not return any results then the variable is set to an empty string.
+
+Within the filter run the [[all Operator]] with an empty parameter retrieves all the titles from the previous filter runs, instead of the usual behaviour of retieving all the titles that were passed to the filter expression.

editions/tw5.com/tiddlers/filters/syntax/Named Filter Run Prefix.tid

@@ -23,6 +23,7 @@ A named filter run prefix can precede any [[run|Filter Run]] of a [[filter expre
 [[<":or"> |"Or Filter Run Prefix"]] |
 [[<":reduce"> |"Reduce Filter Run Prefix"]] |
 [[<":sort">  /"v5.2.0"/ |"Sort Filter Run Prefix"]] |
+[[<":let">  /"v5.3.7"/ |"Let Filter Run Prefix"]] |
 [[<":then">  /"v5.3.0"/ |"Then Filter Run Prefix"]]) [[run|"Filter Run"]]
 """/>