reduce and :reduce handle empty input identically (#5255)

Fixes #5246. Now the reduce operator and :reduce filter run prefix will
both return empty output when their input is empty, so that both can be
chained together with the else operator or :else prefix.

core/modules/filters/reduce.js

@@ -48,7 +48,11 @@ exports.reduce = function(source,operator,options) {
 			accumulator = "" +  list[0];
 		}
 	}
-	return [accumulator];
+	if(results.length > 0) {
+		return [accumulator];
+	} else {
+		return [];
+	}
 };
 
 })();

editions/test/tiddlers/tests/test-prefixes-filter.js

@@ -77,6 +77,8 @@ describe("'reduce' and 'intersection' filter prefix tests", function() {
 		expect(wiki.filterTiddlers("[tag[shopping]] :reduce[get[quantity]add<accumulator>]").join(",")).toBe("22");
 		expect(wiki.filterTiddlers("[tag[shopping]] :reduce[get[price]multiply{!!quantity}add<accumulator>]").join(",")).toBe("27.75");
 		expect(wiki.filterTiddlers("[tag[shopping]] :reduce[<index>compare:number:gt[0]then<accumulator>addsuffix[, ]addsuffix<currentTiddler>else<currentTiddler>]").join(",")).toBe("Brownies, Chick Peas, Milk, Rice Pudding");
+		// Empty input should become empty output
+		expect(wiki.filterTiddlers("[tag[non-existent]] :reduce[get[price]multiply{!!quantity}add<accumulator>]").length).toBe(0);
 		expect(wiki.filterTiddlers("[tag[non-existent]] :reduce[get[price]multiply{!!quantity}add<accumulator>] :else[[0]]").join(",")).toBe("0");
 	});
 
@@ -93,7 +95,10 @@ describe("'reduce' and 'intersection' filter prefix tests", function() {
 		expect(wiki.filterTiddlers("[tag[shopping]reduce<num-items>]",anchorWidget).join(",")).toBe("22");
 		expect(wiki.filterTiddlers("[tag[shopping]reduce<add-price>]",anchorWidget).join(",")).toBe("27.75");
 		expect(wiki.filterTiddlers("[tag[shopping]reduce<join-with-commas>]",anchorWidget).join(",")).toBe("Brownies, Chick Peas, Milk, Rice Pudding");
-		expect(wiki.filterTiddlers("[tag[non-existent]reduce<add-price>,[0]]",anchorWidget).join(",")).toBe("0");
+		// Empty input should become empty output
+		expect(wiki.filterTiddlers("[tag[non-existent]reduce<add-price>,[0]]",anchorWidget).join(",")).not.toBe("0");
+		expect(wiki.filterTiddlers("[tag[non-existent]reduce<add-price>,[0]]",anchorWidget).length).toBe(0);
+		expect(wiki.filterTiddlers("[tag[non-existent]reduce<add-price>else[0]]",anchorWidget).join(",")).toBe("0");
 	});
 
 	it("should handle the :intersection prefix", function() {

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

@@ -7,18 +7,17 @@ type: text/vnd.tiddlywiki
 \define add-price() [get[price]multiply{!!quantity}add<accumulator>]
 \define num-items() [get[quantity]add<accumulator>]
 \define join-with-commas() [<index>compare:number:gt[0]then<accumulator>addsuffix[, ]addsuffix<currentTiddler>else<currentTiddler>]
+\define multiply-input() [multiply<accumulator>]
 \define display-variable(name)
 ''<$text text=<<__name__>>/>'': <code><$text text={{{ [<__name__>getvariable[]] }}}/></code>
 \end
-\define reduce-tip()
-Remember that <<.op reduce>> always produces output, so <<.op else>> will never trigger after <<.op reduce>>.
-\end
 
 These examples use the following predefined variables:
 
 * <<display-variable add-price>>
 * <<display-variable num-items>>
 * <<display-variable join-with-commas>>
+* <<display-variable multiply-input>>
 
 They also use the following data tiddlers:
 
@@ -30,32 +29,10 @@ They also use the following data tiddlers:
 </$list>
 </ul>
 
-Number of items: 
-
 <<.operator-example 1 "[tag[shopping]reduce<num-items>]">>
-
-Total price:
-
 <<.operator-example 2 "[tag[shopping]reduce<add-price>]">>
-
-Using `<index>` to act differently on the first item than the rest:
-
-<<.operator-example 3 "[tag[shopping]reduce<join-with-commas>]">>
-
-Empty input, no second parameter:
-
-<<.operator-example 4 "[tag[non-existent]reduce<add-price>]">>
-
-Note how the output contains a single item with no text. This is not "empty output" for the purposes of the <<.op else>> operator.
-
-<$macrocall $name=".tip" _=<<reduce-tip>> />
-
-Empty input, no second parameter, followed by <<.op else>>:
-
-<<.operator-example 5 "[tag[non-existent]reduce<add-price>else[0]]">>
-
-Note how the output still contains a single item with no text: <<.op else>> did not trigger. If you want the value to be 0 when <<.op reduce>> has no items to process, you need to specify 0 as the initial value by passing it as a second parameter to <<.op reduce>>.
-
-Empty input, second parameter provided:
-
-<<.operator-example 6 "[tag[non-existent]reduce<add-price>,[0]]">>
+<<.operator-example 3 "[tag[shopping]reduce<join-with-commas>]" "Uses `<index>` to act differently on the first item than the rest">>
+<<.operator-example 4 "[tag[non-existent]reduce<add-price>]" "Empty input produces empty output">>
+<<.operator-example 5 "[tag[non-existent]reduce<add-price>else[0]]" "Use `else` to ensure output if input was empty">>
+<<.operator-example 6 "=1 =2 =3 +[reduce<multiply-input>]" "Empty initial value is treated as 0 by mathematical operators">>
+<<.operator-example 7 "=1 =2 =3 +[reduce<multiply-input>,[1]]" "Setting initial value is sometimes necessary for correct results">>

editions/tw5.com/tiddlers/filters/reduce.tid

@@ -10,10 +10,6 @@ tags: [[Filter Operators]]
 title: reduce Operator
 type: text/vnd.tiddlywiki
 
-\define reduce-tip()
-The <<.op reduce>> operator will always produce output, even if its input was empty. If its input is empty, the output of <<.op reduce>> will be the initial value of the accumulator, i.e. the value of the second parameter. One result of this fact is that the <<.op else>> operator will never be triggered if it follows a <<.op reduce>>. The "Empty input" examples show what happens when <<.op reduce>> receives no input.
-\end
-
 <<.from-version "5.1.23">> The <<.op reduce>> operator runs a subfilter for each input title, passing the result of the previous subfilter run as a variable. The initial value of the accumulator can optionally be specified. It returns the result of the final subfilter run.
 
 The <<.op reduce>> operator is used to flatten a list of items down to a single item by repeatedly applying a formula. A typical use is to add up the values in a given field of a list of tiddlers.
@@ -26,10 +22,17 @@ The following variables are available within the subfilter:
 * ''revIndex'' - the reverse numeric index of the current list item (with zero being the last item in the list)
 * ''length'' - the total length of the input list
 
-<$macrocall $name=".tip" _=<<reduce-tip>> />
+If the <<.op reduce>> operator receives no input, its output will be empty. The [[else Operator]] can be useful in such cases.
 
+<<.tip "Literal filter operands cannot contain square brackets but you can work around the issue by using a variable:">>
 
-<<.tip "Compare with the analagous named filter run prefix `:reduce`">>
+```
+<$set name="sum-input" value="[add<accumulator>]">
+{{{ =1 =2 =3 +[reduce<sum-input>] }}}
+</$set>
+```
+
+<<.tip "Compare with the analagous named [[filter run prefix|Filter Expression]] `:reduce`">>
 
 ```
 \define num-items() [get[quantity]add<accumulator>]
@@ -43,4 +46,6 @@ is equivalent to:
 [tag[shopping]] :reduce[get[quantity]add<accumulator>]
 ```
 
+<$macrocall $name=".tip" _="""If the optional second parameter is not given, the initial accumulator value will be empty. Numerical operators treat empty input as if it was the number 0. See the multiply-input examples for how this can affect the result of <<.op reduce>> in some cases."""/>
+
 <<.operator-examples "reduce">>

editions/tw5.com/tiddlers/filters/syntax/Filter Run Prefix (Examples).tid

@@ -39,6 +39,8 @@ is equivalent to:
 [tag[shopping]reduce<num-items>]
 ```
 
-Specifying a default value:
+Specifying a default value when input is empty:
 
 `[tag[non-existent]] :reduce[get[price]multiply{!!quantity}add<accumulator>] :else[[0]]`
+
+<$macrocall $name=".tip" _="""Unlike the [[reduce Operator]], the `:reduce` prefix cannot specify an initial value for the accumulator, so its initial value will always be empty (which is treated as 0 by mathematical operators). So `=1 =2 =3 :reduce[multiply<accumulator>]` will produce 0, not 6. If you need to specify an initial accumulator value, use the [[reduce Operator]]."""/>