created: 20230316151518640
modified: 20230327130626715
tags: [[Filter Run Prefix]]
title: Interchangeable Filter Run Prefixes
type: text/vnd.tiddlywiki

!! Interchangeable Filter Run Prefixes

In technical / logical terms:

|!Run |!Equivalent named prefix |!Interpretation |!Output |
|`[run]` |`:or[run]` |de-duplicated union of sets |... OR run |
|`+[run]` |`:and[run]` |accumulation of filter steps |... AND run |
|`-[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]` |<<.from-version "5.4.0">> 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:

|Prefix|Input|h
|`-`, `~`, `=`, `:intersection` or none| <$link to="all Operator">`[all[]]`</$link> tiddler titles, unless otherwise determined by the first [[filter operator|Filter Operators]]|
|`+`, `:filter`,  `:map`, `:reduce`,`:sort` |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!

For example, in both of the following, `$:/baz` will only be removed if it actually exists:

* <$link to="is Operator"> `foo bar $:/baz -[is[system]]`</$link>
* <$link to="prefix Operator">`foo bar $:/baz -[prefix[$:/]]`</$link>

To understand why, consider the input for both final runs with their `-` prefix.

In order to remove `$:/baz` in any case, existing or not, simply use the `+` prefix with [[negated filter operators|Filter Operators]]:

* <$link to="is Operator">`foo bar $:/baz +[!is[system]]`</$link>
* <$link to="prefix Operator">`foo bar $:/baz +[!prefix[$:/]]`</$link>

!! Difference between + and intersection

For the difference between `+` and `:intersection`, see [[Intersection Filter Run Prefix (Examples)]].

!! For Developers

To create a new filter run prefix, create a [[Javascript module|Modules]] with a [[module-type|ModuleType]] of `filterrunprefix`.
