Introduce => as a shortcut syntax for the let filter run prefix

Also relax the requirement for a filter run prefix to be followed by an opening square bracket

core/modules/filters.js

@@ -148,7 +148,7 @@ exports.parseFilter = function(filterString) {
 		p = 0, // Current position in the filter string
 		match;
 	var whitespaceRegExp = /(\s+)/mg,
-		operandRegExp = /((?:\+|\-|~|=|\:(\w+)(?:\:([\w\:, ]*))?)?)(?:(\[)|(?:"([^"]*)")|(?:'([^']*)')|([^\s\[\]]+))/mg;
+		operandRegExp = /((?:\+|\-|~|(?:=>?)|\:(\w+)(?:\:([\w\:, ]*))?)?)(?:(\[)|(?:"([^"]*)")|(?:'([^']*)')|([^\s\[\]]+))/mg;
 	while(p < filterString.length) {
 		// Skip any whitespace
 		whitespaceRegExp.lastIndex = p;
@@ -159,37 +159,38 @@ exports.parseFilter = function(filterString) {
 		// Match the start of the operation
 		if(p < filterString.length) {
 			operandRegExp.lastIndex = p;
-			match = operandRegExp.exec(filterString);
-			if(!match || match.index !== p) {
-				throw $tw.language.getString("Error/FilterSyntax");
-			}
 			var operation = {
 				prefix: "",
 				operators: []
 			};
-			if(match[1]) {
-				operation.prefix = match[1];
-				p = p + operation.prefix.length;
-				if(match[2]) {
-					operation.namedPrefix = match[2];
-				}
-				if(match[3]) {
-					operation.suffixes = [];
-					 $tw.utils.each(match[3].split(":"),function(subsuffix) {
-						operation.suffixes.push([]);
-						$tw.utils.each(subsuffix.split(","),function(entry) {
-							entry = $tw.utils.trim(entry);
-							if(entry) {
-								operation.suffixes[operation.suffixes.length -1].push(entry);
-							}
+			match = operandRegExp.exec(filterString);
+			if(match && match.index === p) {
+				if(match[1]) {
+					operation.prefix = match[1];
+					p = p + operation.prefix.length;
+					if(match[2]) {
+						operation.namedPrefix = match[2];
+					}
+					if(match[3]) {
+						operation.suffixes = [];
+						$tw.utils.each(match[3].split(":"),function(subsuffix) {
+							operation.suffixes.push([]);
+							$tw.utils.each(subsuffix.split(","),function(entry) {
+								entry = $tw.utils.trim(entry);
+								if(entry) {
+									operation.suffixes[operation.suffixes.length -1].push(entry);
+								}
+							});
 						});
-					 });
+					}
+				}
+				if(match[4]) { // Opening square bracket
+					p = parseFilterOperation(operation.operators,filterString,p);
+				} else {
+					p = match.index + match[0].length;
 				}
-			}
-			if(match[4]) { // Opening square bracket
-				p = parseFilterOperation(operation.operators,filterString,p);
 			} else {
-				p = match.index + match[0].length;
+				p = parseFilterOperation(operation.operators,filterString,p);
 			}
 			if(match[5] || match[6] || match[7]) { // Double quoted string, single quoted string or unquoted title
 				operation.operators.push(
@@ -346,6 +347,8 @@ exports.compileFilter = function(filterString) {
 					return filterRunPrefixes["and"](operationSubFunction, options);
 				case "~": // This operation is unioned into the result only if the main result so far is empty
 					return filterRunPrefixes["else"](operationSubFunction, options);
+				case "=>": // This operation is applied to the main results so far, and the results are assigned to a variable
+					return filterRunPrefixes["let"](operationSubFunction, options);
 				default: 
 					if(operation.namedPrefix && filterRunPrefixes[operation.namedPrefix]) {
 						return filterRunPrefixes[operation.namedPrefix](operationSubFunction, options);

editions/test/tiddlers/tests/data/let-filter-prefix/ShortcutSyntax.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={{{ [[magpie]] =>varname [<varname>] +[join[-]] }}}/>
++
+title: ExpectedResult
+
+<p>magpie</p>

editions/tw5.com/tiddlers/filters/syntax/Interchangeable Filter Run Prefixes.tid

@@ -14,6 +14,7 @@ In technical / logical terms:
 |`-[run]` |`:except[run]` |difference of sets |... AND NOT run |
 |`~[run]` |`:else[run]` |else |... ELSE run |
 |`=[run]` |`:all[run]` |union of sets without de-duplication |... OR run |
+|`=>[run]` |`:let[run]` |assign results to a variable |... LET run |
 
 The input of a run is normally a list of all the non-[[shadow|ShadowTiddlers]] tiddler titles in the wiki (in no particular order).<br>But the `+` prefix can change this:
 

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

@@ -19,4 +19,10 @@ The `:let` filter run prefix assigns the title list resulting from previous filt
 
 The variable is made available to the remaining [[filter runs|Filter Run]] in the [[filter expression|Filter Expression]]. Only the first item in the result list is returned when the variable is accessed in the usual way (or an empty string if the result list is empty). Using round brackets instead of angle brackets around a variable name as an operand retrieves the complete list of items in the result list.
 
+This prefix has an optional [[shortcut syntax|Shortcut Filter Run Prefix]] symbol `=>run`. For example:
+
+```
+=[<myfun1>] =[<myfun2>] =>myvar
+```
+
 The `:let` filter run prefix always clears the current result list.

editions/tw5.com/tiddlers/filters/syntax/Shortcut Filter Run Prefixes.tid

@@ -9,7 +9,7 @@ Shortcut prefixes are commonly used by advanced users because they are fast to t
 <$railroad text="""
 \start none
 \end none
-(-|:"+"|"-"|"~"|"=")
+(-|:"+"|"-"|"~"|"="|"=>")
 [[run|"Filter Run"]]
 """/>
 
@@ -23,7 +23,8 @@ If a run has:
 
 * the prefix `~`, if the filter output so far is an empty list then the output titles of the run are [[dominantly appended|Dominant Append]] to the filter's output. If the filter output so far is not an empty list then the run is ignored. <<.from-version "5.1.18">>
 
-* the prefix `=`, output titles are appended to the filter's output without de-duplication. <<.from-version "5.1.20">> 
+* the prefix `=`, output titles are appended to the filter's output without de-duplication. <<.from-version "5.1.20">>
 
+* the prefix `=>`, the input is assigned to the variable named with the output title. <<.from-version "5.3.7">>
 
 {{Interchangeable Filter Run Prefixes}}