created: 20150124182421000
modified: 20190610165329062
tags: [[Filter Syntax]]
title: Filter Expression
type: text/vnd.tiddlywiki

<$railroad text="""
[{:
  [: [[whitespace|"Filter Whitespace"]] ]
  ("+"|"~"|:-|"-"|"=")
  [[run|"Filter Run"]]
}]
"""/>

A <<.def "filter expression">> is the outermost level of the [[filter syntax|Filter Syntax]]. It consists of one or more [[runs|Filter Run]].

If a run has:

* no prefix, its output titles are [[dominantly appended|Dominant Append]] to the filter's output
* <<.from-version "5.1.20">> the prefix `=`, output titles are appended to the filter's output without de-duplication
* 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:

|!Run |!Interpretation |!Output |
|`run` |de-duplicated union of sets |... OR run |
|`=run` |union of sets without de-duplication |... OR run |
|`+run` |intersection of sets |... AND run |
|`-run` |difference of sets |... AND NOT run |
|`~run` |else |... ELSE 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). 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]]|
|`+`|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>