created: 20150124182421000
modified: 20150124184900000
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]].

A run's input 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.

* If a run has no `+` or `-` prefix, its output titles are [[dominantly appended|Dominant Append]] to the filter's output.
* If a run has a `-` prefix, its output titles are <<.em removed>> from the filter's output (if they were present).
* If a run has a `+` prefix, it receives the filter's output so far as its input. The filter's output is then completely <<.em replaced>> by the run's output. Any subsequent run reverts to receiving all tiddler titles as its input, unless it too has a `+` prefix.

In concise technical terms:

|!Run |!Interpretation |!Output |
|`run` |union of sets |... OR run |
|`+run` |intersection of sets |... AND run |
|`-run` |difference of sets |... AND NOT run |