From 09330968cc7b1dab798a9774bdc8160ad12a787d Mon Sep 17 00:00:00 2001
From: Jeremy Ruston <jeremy@jermolene.com>
Date: Tue, 20 Nov 2018 13:29:44 +0000
Subject: [PATCH] New "else" prefix for filters (#3558)

* Experimental "else" prefix for filters

See #3557

* Docs updates for "else" filter runs
---
 core/modules/filters.js                                |  9 ++++++++-
 editions/test/tiddlers/tests/test-filters.js           |  7 +++++++
 .../tiddlers/filters/syntax/Filter Expression.tid      | 10 ++++++----
 3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/core/modules/filters.js b/core/modules/filters.js
index e9117bec1..fdad467d2 100644
--- a/core/modules/filters.js
+++ b/core/modules/filters.js
@@ -119,7 +119,7 @@ exports.parseFilter = function(filterString) {
 		p = 0, // Current position in the filter string
 		match;
 	var whitespaceRegExp = /(\s+)/mg,
-		operandRegExp = /((?:\+|\-)?)(?:(\[)|(?:"([^"]*)")|(?:'([^']*)')|([^\s\[\]]+))/mg;
+		operandRegExp = /((?:\+|\-|~)?)(?:(\[)|(?:"([^"]*)")|(?:'([^']*)')|([^\s\[\]]+))/mg;
 	while(p < filterString.length) {
 		// Skip any whitespace
 		whitespaceRegExp.lastIndex = p;
@@ -259,6 +259,13 @@ exports.compileFilter = function(filterString) {
 						results.splice(0,results.length);
 						$tw.utils.pushTop(results,operationSubFunction(source,widget));
 					};
+				case "~": // This operation is unioned into the result only if the main result so far is empty
+					return function(results,source,widget) {
+						if(results.length === 0) {
+							// Main result so far is empty
+							$tw.utils.pushTop(results,operationSubFunction(source,widget));
+						}
+					};
 			}
 		})());
 	});
diff --git a/editions/test/tiddlers/tests/test-filters.js b/editions/test/tiddlers/tests/test-filters.js
index 191168ae4..0ef8f551d 100644
--- a/editions/test/tiddlers/tests/test-filters.js
+++ b/editions/test/tiddlers/tests/test-filters.js
@@ -102,6 +102,13 @@ describe("Filter tests", function() {
 
 	// Our tests
 
+	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");
+		expect(wiki.filterTiddlers("~[[No such tiddler]]").join(",")).toBe("No such tiddler");
+		expect(wiki.filterTiddlers("[my-field[present]] ~[[No such tiddler]]").join(",")).toBe("No such tiddler");
+	});
+
 	it("should handle the lookup operator", function() {
 		expect(wiki.filterTiddlers("Six Seventh 8 +[lookup[Tiddler]]").join(",")).toBe("Missing inaction from TiddlerOne,,Tidd");
 		expect(wiki.filterTiddlers("Six Seventh 8 +[lookup:8[Tiddler]]").join(",")).toBe("Missing inaction from TiddlerOne,Tidd,Tidd");
diff --git a/editions/tw5.com/tiddlers/filters/syntax/Filter Expression.tid b/editions/tw5.com/tiddlers/filters/syntax/Filter Expression.tid
index 430a900cd..8550fa394 100644
--- a/editions/tw5.com/tiddlers/filters/syntax/Filter Expression.tid	
+++ b/editions/tw5.com/tiddlers/filters/syntax/Filter Expression.tid	
@@ -1,5 +1,5 @@
 created: 20150124182421000
-modified: 20171212073225103
+modified: 20181120125803533
 tags: [[Filter Syntax]]
 title: Filter Expression
 type: text/vnd.tiddlywiki
@@ -7,7 +7,7 @@ type: text/vnd.tiddlywiki
 <$railroad text="""
 [{:
   [: [[whitespace|"Filter Whitespace"]] ]
-  ("+"|:-|"-")
+  ("+"|"~"|:-|"-")
   [[run|"Filter Run"]]
 }]
 """/>
@@ -19,6 +19,7 @@ If a run has:
 * no prefix, its output titles are [[dominantly appended|Dominant Append]] to the filter's output
 * the prefix `-`, output titles are <<.em removed>> from the filter's output (if such tiddlers exist)
 * the prefix `+`, it receives the filter output so far as its input; its output then <<.em "replaces">> all filter ouput so far and forms the input for the next run
+* <<.from-version "5.1.18">> 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
 
 In technical / logical terms:
 
@@ -26,11 +27,12 @@ In technical / logical terms:
 |`run` |union of sets |... OR run |
 |`+run` |intersection of sets |... AND run |
 |`-run` |difference of sets |... AND NOT run |
+|`~run` |else |... ELSE run |
 
-A run's input is normally a list of all the non-[[shadow|ShadowTiddlers]] tiddler titles in the wiki (in no particular order). But the `+` prefix can change this:
+The input of a run is normally a list of all the non-[[shadow|ShadowTiddlers]] tiddler titles in the wiki (in no particular order). But the `+` prefix can change this:
 
 |Prefix|Input|h
-|`-` or none| <$link to="all Operator">`[all[]]`</$link> tiddler titles, unless otherwise determined by the first [[filter operator|Filter Operators]]|
+|`-`, `~` or none| <$link to="all Operator">`[all[]]`</$link> tiddler titles, unless otherwise determined by the first [[filter operator|Filter Operators]]|
 |`+`|the filter output of all previous runs so far|
 
 Precisely because of varying inputs, be aware that both prefixes `-` and `+` do not behave inverse to one another!