Add then and else operators

Fixes #4147

core/modules/filters/else.js

@@ -0,0 +1,30 @@
+/*\
+title: $:/core/modules/filters/else.js
+type: application/javascript
+module-type: filteroperator
+
+Filter operator for replacing an empty input list with a constant, passing a non-empty input list straight through
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports.else = function(source,operator,options) {
+	var results = [];
+	source(function(tiddler,title) {
+		results.push(title);
+	});
+	if(results.length === 0) {
+		return [operator.operand];
+	} else {
+		return results;
+	}
+};
+
+})();

core/modules/filters/then.js

@@ -0,0 +1,26 @@
+/*\
+title: $:/core/modules/filters/then.js
+type: application/javascript
+module-type: filteroperator
+
+Filter operator for replacing any titles with a constant
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports.then = function(source,operator,options) {
+	var results = [];
+	source(function(tiddler,title) {
+		results.push(operator.operand);
+	});
+	return results;
+};
+
+})();

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

@@ -128,6 +128,15 @@ function setupWiki(wikiOptions) {
 // Our tests
 function runTests(wiki) {
 
+	it("should handle the then and else operators", function() {
+		expect(wiki.filterTiddlers("[modifier[JoeBloggs]then[JaneBloggs]]").join(",")).toBe("JaneBloggs");
+		expect(wiki.filterTiddlers("[!modifier[JoeBloggs]then[JaneBloggs]]").join(",")).toBe("JaneBloggs,JaneBloggs,JaneBloggs,JaneBloggs,JaneBloggs");
+		expect(wiki.filterTiddlers("[modifier[DaveBloggs]then[JaneBloggs]]").join(",")).toBe("");
+		expect(wiki.filterTiddlers("[modifier[JoeBloggs]else[JaneBloggs]]").join(",")).toBe("TiddlerOne");
+		expect(wiki.filterTiddlers("[!modifier[JoeBloggs]else[JaneBloggs]]").join(",")).toBe("$:/ShadowPlugin,$:/TiddlerTwo,Tiddler Three,a fourth tiddler,one");
+		expect(wiki.filterTiddlers("[modifier[DaveBloggs]else[JaneBloggs]]").join(",")).toBe("JaneBloggs");
+	});
+
 	it("should handle the ~ prefix", function() {
 		expect(wiki.filterTiddlers("[modifier[JoeBloggs]] ~[[No such tiddler]]").join(",")).toBe("TiddlerOne");
 		expect(wiki.filterTiddlers("[modifier[JaneBloggs]] ~[[No such tiddler]]").join(",")).toBe("No such tiddler");

editions/tw5.com/tiddlers/filters/Conditional Operators.tid

@@ -0,0 +1,24 @@
+created: 20190802113703788
+modified: 20190802132727925
+tags: Filters
+title: Conditional Operators
+type: text/vnd.tiddlywiki
+
+<<.from-version "5.1.20">>The conditional filter operators allow ''if-then-else'' logic to be expressed within filters.
+
+The foundation is the convention that an empty list can be used to represent the boolean value ''false'' and a list with at one (or more) entries to represent ''true''.
+
+The conditional operators are:
+
+* [[then Operator]] replaces any input values with a constant string. For example:
+** <<.inline-operator-example "[[HelloThere]is[missing]then[FOO]]">>
+** <<.inline-operator-example "[[Missing Tiddler]is[missing]then[FOO]]">>
+* [[else Operator]] if the title list is empty then returns a constant string, otherwise returns the original title list
+** <<.inline-operator-example "[[HelloThere]is[tiddler]else[BAR]]">>
+** <<.inline-operator-example "[[Missing Tiddler]is[tiddler]else[BAR]]">>
+
+These operators can be combined. For example:
+
+* <<.inline-operator-example "[[New Tiddler]is[missing]then[I am missing]else[No I am not missing]]">>
+
+<<list-links "[tag[Conditional Operators]]">>

editions/tw5.com/tiddlers/filters/else Operator.tid

@@ -0,0 +1,15 @@
+caption: else
+created: 20190802113024942
+modified: 20190802113919945
+op-input: a [[selection of titles|Title Selection]]
+op-output: the original input titles unless empty, in which case return a list with the single entry  <<.place E>>
+op-parameter: a string
+op-parameter-name: E
+op-purpose: if the list of input titles is empty then return a list consisting of a single constant string, otherwise return the original titles
+tags: [[Conditional Operators]] [[Filter Operators]]
+title: else Operator
+type: text/vnd.tiddlywiki
+
+<<.from-version "5.1.20">> See [[Conditional Operators]] for an overview.
+
+<<.operator-examples "else">>

editions/tw5.com/tiddlers/filters/examples/else Operator (Examples).tid

@@ -0,0 +1,9 @@
+created: 20190802113334259
+modified: 20190802113551566
+tags: [[else Operator]] [[Operator Examples]]
+title: else Operator (Examples)
+type: text/vnd.tiddlywiki
+
+<<.operator-example 1 "[[HelloThereMissing]is[missing]else[yes]]">>
+<<.operator-example 2 "[[HelloThere]is[missing]else[yes]]">>
+

editions/tw5.com/tiddlers/filters/examples/then Operator (Examples).tid

@@ -0,0 +1,9 @@
+created: 20190802113310992
+modified: 20190802113555129
+tags: [[then Operator]] [[Operator Examples]]
+title: then Operator (Examples)
+type: text/vnd.tiddlywiki
+
+<<.operator-example 1 "[[HelloThereMissing]is[missing]then[yes]]">>
+<<.operator-example 2 "[[HelloThere]is[missing]then[yes]]">>
+

editions/tw5.com/tiddlers/filters/then Operator.tid

@@ -0,0 +1,15 @@
+caption: then
+created: 20190802112756430
+modified: 20190802113849579
+op-input: a [[selection of titles|Title Selection]]
+op-output: the input titles with each one replaced by the string <<.place T>>
+op-parameter: a string
+op-parameter-name: T
+op-purpose: replace input titles by a constant string
+tags: [[Conditional Operators]] [[Filter Operators]]
+title: then Operator
+type: text/vnd.tiddlywiki
+
+<<.from-version "5.1.20">> See [[Conditional Operators]] for an overview.
+
+<<.operator-examples "then">>