Filter operators: THEN ELSE ~(ELSE)

[Mohammad]

I am wondering how to use filter operator THEN, ELSE and ~(else)

Why the below fails

<$list filter="[<a>then<b>else<c>]>


</$list>

What is the difference between else and ~?

<$list filter="[<a>else<b>]">


</$list>

and 

<$list filter="[[<a>] ~[<b>]]">


</$list>

--Mohammad

[Eric Shulman]

On Wednesday, December 4, 2019 at 6:51:20 AM UTC-8, Mohammad wrote:

I am wondering how to use filter operator THEN, ELSE and ~(else)

Why the below fails

<$list filter="[<a>then<b>else<c>]>

not sure.  gotta think about this a bit... 

What is the difference between else and ~?

<$list filter="[<a>else<b>]">

and 

<$list filter="[[<a>] ~[<b>]]">

 

The big difference here is that else[...] can only substitute a single value, while ~[...] can be a complex filter expression

-e

[PMario]

Hi,

then and else are used in a filter run. ~ starts a new run.

See the docs list.

-m

[TiddlyTweeter]

Could you give a concrete example? Just asking for a friend :-)

[Mohammad]

Yes, I hope to see more example to see how they work!

--Mohammad

[TonyM]

I think the best way for me is to think of then and else operators is they return a string, or title if the list is not empty then or else if list is empty.

Generate/list titles > then[hastitles]else[notitles]

So the above coded as a filter will only ever return hastitles or notitles.

The ~ on the other hand does test for empty list as well and can return a single title or new list.

Along with match these new operators will allow us to write filters more logically and in more natural language, I would say plain English like but the titles can be any language.

By being able to generate a string/title output on ones choosing from a filter, the wiki text can have further tests to respond to the generated result.

Happy to post examples as I collect them for myself.

Tony

[TonyM]

You examples look wrong to me

[<a>then<b>else<c>]

Basicaly you are using three variables as titles and you are relying on an empty variable to be the same. In your example you are only testing for the existence of a and returning either b or c.

This may be what you want?

tony

[Xavier Cazin]

Hi Mohammad,

A good concrete example could be a followup to your recent template question. Here, we used the ~[...] construct to compute a tiddler title (or a list thereof):

<$list variable=template filter="[tag[a]] ~[tag[b]] ~[[tid3]]">
<$transclude mode=block tiddler=<<template>> />
</$list>

The computation of tid2 could have been of course arbitrarily more complex than [tag[b]].

If we wanted to use the else operator on the other hand, the only thing we could have done would have been to directly write tiddler titles that were previously known, like in:

<$list variable=template filter="[tag[a]else[tid2]else[tid3]]">
<$transclude mode=block tiddler=<<template>> />
</$list>

Or at most, got from a variable or a  field:

<$list variable=template filter="[tag[a]else<variable-holding-tid2>else{field-holding-tid3}]">
<$transclude mode=block tiddler=<<template>> />
</$list>

By the way, your test below is indeed intriguing. It looks like the operator title (hidden in the <a> expression) always constructs something that won't be considered empty by the rest of the filter, whatever the value of the variable <a> is. In other words, it always returns b. It sounds like a bug to me :-)

<$list filter="[<a>then<b>else<c>]">

</$list>

-- Xavier

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[TonyM]

Mohammad et al,

Also consider the use of 

getvariable Operator

New in: 5.1.20 The usual way to retrieve a variable value within a filter is with the angle brackets notation. For example, [<currentTiddler>] will retrieve the value of the variable called currentTiddler.  

eg Untested by me just now

[<a>] or [<a>match[yes]] or [<a>else[novalue]]

Also

is Operator

New in: 5.1.20 is[variable]] test exists as a variable (whether or not that variable has a non-blank value)
blank

New in: 5.1.20 is[blank] (i.e. is a zero length string)

Tony