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51 lines
2.3 KiB
Plaintext
51 lines
2.3 KiB
Plaintext
created: 20150124182421000
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modified: 20181120125803533
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tags: [[Filter Syntax]]
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title: Filter Expression
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type: text/vnd.tiddlywiki
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<$railroad text="""
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[{:
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[: [[whitespace|"Filter Whitespace"]] ]
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("+"|"~"|:-|"-")
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[[run|"Filter Run"]]
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}]
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"""/>
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A <<.def "filter expression">> is the outermost level of the [[filter syntax|Filter Syntax]]. It consists of one or more [[runs|Filter Run]].
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If a run has:
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* no prefix, its output titles are [[dominantly appended|Dominant Append]] to the filter's output
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* the prefix `-`, output titles are <<.em removed>> from the filter's output (if such tiddlers exist)
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* 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
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* <<.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
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In technical / logical terms:
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|!Run |!Interpretation |!Output |
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|`run` |union of sets |... OR run |
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|`+run` |intersection of sets |... AND run |
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|`-run` |difference of sets |... AND NOT run |
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|`~run` |else |... ELSE run |
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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:
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|Prefix|Input|h
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|`-`, `~` or none| <$link to="all Operator">`[all[]]`</$link> tiddler titles, unless otherwise determined by the first [[filter operator|Filter Operators]]|
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|`+`|the filter output of all previous runs so far|
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Precisely because of varying inputs, be aware that both prefixes `-` and `+` do not behave inverse to one another!
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For example, in both of the following, `$:/baz` will only be removed if it actually exists:
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* <$link to="is Operator"> `foo bar $:/baz -[is[system]]`</$link>
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* <$link to="prefix Operator">`foo bar $:/baz -[prefix[$:/]]`</$link>
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To understand why, consider the input for both final runs with their `-` prefix.
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In order to remove `$:/baz` in any case, existing or not, simply use the `+` prefix with [[negated filter operators|Filter Operators]]:
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* <$link to="is Operator">`foo bar $:/baz +[!is[system]]`</$link>
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* <$link to="prefix Operator">`foo bar $:/baz +[!prefix[$:/]]`</$link>
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