From 6598455e830f35a32e499fc55fb3057e5cd63f61 Mon Sep 17 00:00:00 2001
From: Karl Knechtel <zahlman@gmail.com>
Date: Sat, 3 Oct 2015 08:32:47 -0400
Subject: [PATCH] Create Filter Operators.tid

---
 .../dev/tiddlers/new/Filter Operators.tid     | 94 +++++++++++++++++++
 1 file changed, 94 insertions(+)
 create mode 100644 editions/dev/tiddlers/new/Filter Operators.tid

diff --git a/editions/dev/tiddlers/new/Filter Operators.tid b/editions/dev/tiddlers/new/Filter Operators.tid
new file mode 100644
index 000000000..2bf019915
--- /dev/null
+++ b/editions/dev/tiddlers/new/Filter Operators.tid	
@@ -0,0 +1,94 @@
+tags: dev
+title: Filter Operators
+type: text/vnd.tiddlywiki
+
+! Overview
+
+Filter operators are modules (tiddlers of type `application/javascript`) with their `module-type` field set to `filteroperator`, exporting one or more functions implementing a filter.
+
+Each function will be called with three arguments:
+
+* A [[tiddler iterator|Tiddler Iterators]] representing the results of the previous filter step (or all tiddlers, if this filter appears first in an expression), conventionally named `source`.
+* An object, conventionally called `operator`, representing the arguments for this filter step, with the following keys:
+** //operator//: the name of the filter operator specified in the WikiText;
+** //operand//: the operand for the filter step (as a string; if the filter specified it in angle brackets or braces, the text reference or variable name will have already been resolved);
+** //prefix//: (optional) a string containing a single exclamation mark if the filter operator is to be negated;
+** //suffix//: (optional) a string containing an additional filter argument (typically a tiddler field name) following the filter name (separated by a colon in the filter syntax);
+** //regexp//: (optional, deprecated) used instead of //operand// if the filter operand is a regexp.
+* An object, conventionally called `options`, with the following keys:
+** //wiki//: The $tw.Wiki object;
+** //widget//: (optional) a widget node.
+
+The function should return either a new [[tiddler iterator|Tiddler Iterators]], or else an array of tiddler titles (as strings). The underlying filter mechanism will convert back and forth between iterators and arrays as needed.
+
+Normally
+
+! References
+
+There are several filter operators built into the core which can serve as a jumping off point for your own filter operators:
+
+https://github.com/Jermolene/TiddlyWiki5/tree/master/core/modules/filters
+
+! Example
+
+Suppose we want to make a filter operator that returns every other tiddler from the input list. A typical invocation might look like `[tags[interesting]everyother[]]`.
+
+We make a new tiddler, set its `type` and `module-type` appropriately, and begin writing the code:
+
+    (function(){
+    "use strict";
+    
+    exports.everyother = function(source, operator, options) {
+        // TODO
+    }
+    })();
+
+For the example filter syntax, our function will be called with
+
+* source: an iterator over all the tiddlers tagged as `interesting`
+* operator: an object {operator: "everyother", operand: ""}
+* options: an object with the current Wiki object and a widget object, neither of which we need
+
+As is usually the case, we don't care about `operator.operator` here (since that information has already been used to look up our function); we also don't care about `operator.operand`, since there is no meaningful operand for this operation.
+
+We could implement the operator by iterating over the input tiddlers and explicitly building a result array of titles:
+
+    (function(){
+    "use strict";
+    
+    exports.everyother = function(source, operator, options) {
+        var result = [];
+        var include = true;
+        source(function(tiddler, title) {
+            if (include) { result.push(title); }
+            include = !include;
+        });
+        return result;
+    }
+    })();
+
+That is, we supply a callback to `source` that negates `include` each time through (in order to grab every other result) and pushes the `title` of every other tiddler onto the result.
+
+Alternatively, we can return our own iterator, by returning a function that accepts a similar callback and only calls it on every other tiddler:
+
+    (function(){
+    "use strict";
+    
+    exports.everyother = function(source, operator, options) {
+        return function(callback) {
+            var include = true;
+            source(function(tiddler, title) {
+                if (include) { callback(tiddler, title); }
+                include = !include;
+            });
+        };
+    }
+    })();
+
+Either way, we could interpret the `!` flag on the filter, if present, to mean that we want the //other// half of the tiddlers, by using it to set the initial value of `include`:
+
+    var include = operator.prefix !== "!";
+
+! Filter Behaviour
+
+As with [JavaScript Macros], filter operators should not make modifications to tiddlers, but only return a list of tiddlers or a tiddler iterator.