Tiddlywiki5: [Request] Else-if logic in list filter

[ibnishak]

Please consider introducing a param like "emptyFilter" for list widget which will run if the initial filter turns up empty. Like emptymessage, but a filter. So it would help creating logic like -
"List all tiddlers tagged fruits, and if there are no fruits, list all tiddlers tagged vegetables"

[Jermolene]

Hi @ibnishak I'd been thinking of tackling this via a new filter run prefix that only processes the run if the current result set is empty. For example, if we use ~ as the operator:

[enlist{MyTiddler}] ~[[Fallback]]

Then

• If "MyTiddler" contained "one two three", the list one two three would be returned
• If "MyTiddler" contained "[[one two]]", the list [[one two]] would be returned
• If "MyTiddler" were blank, a list containing the single entry Fallback would be returned

[ibnishak]

That would be better actually. That way it would permit piping more than one condition with filter steps. Sort of like switch-case. Any chance we could see this soon?

It would also enable modification of defaults without changing core. Like <$list filter="<<user-choices>> ~[[defaults]]"/>

[Jermolene]

Hi @ibnishak I've made a branch with a quick implementation. Much harder and more tedious is that we also need a bunch of documentation updates (including the railroad diagram in https://tiddlywiki.com/#Filter%20Expression)...

[AnthonyMuscio]

I was just looking at updating the documentation for this, will this else prefix be able to be used more than once in a set of filter runs? and even respond to a previous else prefix?

Eg
[all[tiddlers]has[fieldname]] ~[all[tiddlers]has[fieldname2]] ~[all[tiddlers]has[fieldname3]]

I have included an attempt at updating https://tiddlywiki.com/#Filter%20Expression

<$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
* 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
* the prefix `~`, A filter that is used if there is no titles selected so far, we could call this an else filter.

In technical / logical terms:

|!Run |!Interpretation |!Output |
|`run` |union of sets |... OR run |
|`+run` |intersection of sets |... AND run |
|`-run` |difference of sets |... AND NOT run |
|`~run` |if no result so far | 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:

|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|
|`~`|A new filter output if all previous runs so far resulted in no title|

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>

Regards
Tony

[Jermolene]

Hi @AnthonyMuscio thank you! I actually pushed some doc updates yesterday that you can see here:

https://github.com/Jermolene/TiddlyWiki5/pull/3558/files

I think we've made roughly equivalent changes, have I missed anything?

[AnthonyMuscio]

Looks good

[AnthonyMuscio]

Just tested from Pre-release !

The result of this does not look correct

<<currentTiddler>><br>
<$set name=display-title filter="[all[current]get[caption]] ~[all[current]get[title]]">
<<display-title>><br>
<$link to=<<currentTiddler>> tooltip="Open tiddler"><$text text=<<display-title>>/></$link><br>

</$set>

[pmario]

@ibnishak ... can this issue be closed? There is a new possibility in 5.1.18

[ibnishak]

Yes. Sorry for the delay.