Consider adding support for `` (backticks) as an alternate string (expression) delimiter?

[njreid]

I find myself combining Common Expression Language (https://cel.dev/) into KDL to provide limited filtering/matching behavior.

It would be really convenient to have backticks supported as an alternative set of string delimiters, which could be used for writing expressions.

CEL is a nice fit here because it's explicitly not Turing complete, evaluates fast and has a pretty obvious/intuitive syntax for those operations it does support.

route "/" {
       allow `Authorization.username=="me@thing.com"`
       documentRoot "/var/www"
} 

[tabatkins]

Could you elaborate on what the backtick is helping with here? A quick look at CEL doesn't suggest that it has meaning there - is this just about being able to easily write strings that themselves contain quotes?

If so, then raw strings are probably what you want:

route "/" {
       allow #"Authorization.username=="me@thing.com""#
       documentRoot "/var/www"
} 

[njreid]

The backticks are useful in that they separate expressions in a given syntax** from all other string values. This helps a lot with tooling, because you can selectively syntax highlight and auto-complete within those expression blocks. LSPes can ensure that expressions only reference nodes/props/args which actually exist in the doc (or imported ancestors).
tl'dr - expressions are different from strings, and it helps to indicate that in the doc.

** I'll use CEL, but this could be configurable with a top-of-file frontmatter-style comment, like what is canonical for indicating KDL version.

[tabatkins]

I'm not sure I understand. Are you implying that backtick-strings are semantically distinct from quoted strings?

For syntax highlighting, there's various existing ways to address that. A given highlighter can make assumptions about a particular file type, such as Sublime Text's default python highlighter assuming that raw strings are regexes. Or you can tag the strings to indicate the language, like (cel)"foo bar".

[njreid]

In a word, yes - in the same way as double quote strings and backticks strings are semantically distinct in Javascript and Golang (and many others).

The (cel)"...." type hint is ok, but verbose. It breaks the DSLish feel which KDL does so well in all other respects.

I wouldn't want to give up the ability to have distinct raw strings by using #"..." for expressions exclusively.

The idea is that expressions evaluate into whatever type the property or argument is expecting (via the schema). So,

pipeline name=`$env` {}

Might turn into:

pipeline name="prod" {} 

after the file is parsed by some processing entity.