[IDEA] pragma to assign variables by evaluating a filter expression

[saqimtiaz]

Currently the only way to assign a variable using a filter that can be imported elsewhere is to use nested $set widgets and the syntax is a frequent pitfall, especially in terms of understanding the requirements for nesting.

It is recommended to introduce a new pragma shorthand so that user can use set widgets to assign variables using a familiar syntax like procedures and functions.

Possible syntax:

\set myvar() [tag[HelloThere]]

or

\function-now myvar() [tag[HelloThere]]
\function-now myvar(daysPerYear:365) [tag<daysPerYear>]

equivalent to:
<$set name="myvar" filter="[tag[HelloThere]]"></$set>

The latter has the advantage of emphasizing the similarity with functions, the difference being when the execution of the filter happens.

---

Update:

• Assign MVV as opposed to a title list
• consider alternative syntax \let myvar= [tag[HelloThere]] for consistency with variable assignment in filter expressions.

[pmario]

IMO \define already does the same thing - or do I miss something?

\define myvar() [tag[HelloThere]]
<$set name=myvar value="[tag[HelloThere]]">

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[saqimtiaz]

Define sets the filter as the variable, the proposal is for a pragma that evaluates a filter and sets the result as a variable.

[pmario]

Ah, OK, I did not read the text carefully enough. Sorry.

• Set-widget with filter creates a titleList output
• \set myvar() should also create a titleList
• \function x() a b c evaluates to "a" when used

<$set name="myvar" filter="[tag[HelloThere]]">

<$text text=<<myvar>>/>

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What about \function-set to avoid -eval with the same initial behaviour as a \function, but evaluated immediately.

1. \function-set myvar(daysPerYear:365) [tag<daysPerYear>] indicates, that it only outputs 1 element
2. \function-set myvar(daysPerYear:365) [tag<daysPerYear>filter:titlelist[]] would create a titleList and returns 1 element
3. \function-set myvar(daysPerYear:365) [tag<daysPerYear>filter:titlelist[]join[ ]] would return all elements. So the functionality is the same as a function

No 3) in short form

\set myvar(daysPerYear:365) [tag<daysPerYear>]

Just some thoughts.

[saqimtiaz]

• Set-widget with filter creates a titleList output

It would make sense to implement this keeping in mind multi valued variables. As such the pragma would not use titlelists and would assign a multivalued variable, and the user would have the ability to access just the first result via <varname> or all the values via (varname).

[pmario]

OK. If we have MVVs (multi valued variables), Jeremy did use :let at #8972 -- So we may be able to use a \let pragma.

Edit: Probably to similar to the <$let-widget. -- ... Not a good idea.

[pmario]

@saqimtiaz ... How do you intend to use <<myvar 10>> later on in the code? And how should it be evaluated?

\set myvar(daysPerYear:365) [tag<daysPerYear>]

[saqimtiaz]

How do you intend to use <<myvar 10>> later on in the code?

You wouldn't, as the filter would be evaluated and the result assigned already.

If we support parameters for this pragma they would just be for declaring variables from constants to be used within the function itself. It provides symmetry with how functions work with regards to accepting parameters, but I would not expect this to be a widely used feature.

[yaisog]

Within the tiddler where it is used, one must be careful to distinguish it from \function which uses variables within the scope of the widget that calls the function, where the new definition will use variables within the scope of the whole tiddler (or maybe dependent on the position relative to other \defines and such?) I would shy away from any pragma names that seem to relate it to functions and rather have something different; \set is not so bad, or maybe \filter?

I do like the general idea, though, as importing nested $sets like we would do now seems very hacky.