TiddlyWiki5/editions/tw5.com/tiddlers/filters/syntax/Filter Expression.tid

created: 20150124182421000
modified: 20201027185638636
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 output 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
* <<.from-version "5.1.23">> named prefixes for filter runs are available.
* <<.from-version "5.1.23">> named prefix `:filter`, it receives the filter output so far as its input. The next run is evaluated for each title of the input, removing every input title for which the output is an empty list.

<<.tip "Compare named filter run prefix `:filter` with [[filter Operator]] which applies a subfilter to every input title, removing the titles that return an empty result from the subfilter">>

In technical / logical terms:

|!Run |!Equivalent named prefix |!Interpretation |!Output |
|`run` |`:or[run]` |de-duplicated union of sets |... OR run |
|`=run` |`:all[run]` |union of sets without de-duplication |... OR run |
|`+run` |`:and[run]` |intersection of sets |... AND run |
|`-run` |`:except[run]` |difference of sets |... AND NOT run |
|`~run` |`:else[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>