Feature request: PlantUML `?=` syntax support

[HeinrichAD]

PlantUML supports to assign a value to a variable, only if it is not already defined, with the syntax !$a ?= "foo".

!$a ?= "foo" could be considered the short form of the following:

!if %not(%variable_exists("$a"))
  !$a = "foo"
!endif

Since there is a workaround, this is prio low, but once you start working with a lot of customizable parameters, it's pretty annoying and the code gets messy quickly.

References

Documentation: https://plantuml.com/preprocessing#788fa5fb2276ed17

Example below in action: PlantUML Online Example

Example

= Test

[plantuml]
----
@startuml
Alice -> Bob : 1. **$name** should be empty

!$name ?= "Charlie"
Alice -> Bob : 2. **$name** should be Charlie

!$name = "David"
Alice -> Bob : 3. **$name** should be David

!$name ?= "Ethan"
Alice -> Bob : 4. **$name** should be David
@enduml
----

Expected

Actual

Syntax Error

``` > asciidoctor -r asciidoctor-diagram --failure-level ERROR -b xhtml5 -a data-uri -D generated test.adoc asciidoctor: ERROR: test.adoc: line 5: Failed to generate image: PlantUML preprocessing failed: [From (line 4) ] @startuml Alice -> Bob : 1. **$name** should be empty !$name ?= "Charlie" ^^^^^ Syntax Error? asciidoctor: FAILED: /documents/test.adoc: Failed to load AsciiDoc document - PlantUML preprocessing failed: [From (line 4) ] @startuml Alice -> Bob : 1. **$name** should be empty !$name ?= "Charlie" ^^^^^ Syntax Error? (RuntimeError) Use --trace for backtrace ```

[pepijnve]

This seems to have been added in a recent version of PlantUML. The currently bundled version is 1.2021.2. I've updated the gem containing the PlantUML jar to 1.2021.5. Could you update the gem on your end and retest?

[HeinrichAD]

Thank you. I confirm the functionality. It's working without issues.

Example of above with asciidoctor-diagram.