The problem is that I'm not talking about markup. Markup - unless it's so lightweight as to be near unnoticeable is not a solution. Unless every application you're transferring it via supports CSTML, the representation needs to be as compact and readable as a regular programming language.

To take your example, pretty much anything longer than [1, true, "3"] is a non-starter if someone is pasting it into Slack, or sending an e-mail. The CSTML representation isn't readable to them, and would take additional steps on both sides vs. just writing the source representation. I'm not going to tell people how to do something by writing it into some other tool and pasting some large blob into Slack or my e-mail client and expect the recipient to reverse the process.

That is the problem space. How you represent that as an AST isn't the problem - that's easy. How you represent it in a way that everyone can read and write and that "passes seamlessly" via existing tools is the problem.

(I must also admit that I think the choice of serialization format for CSTML is utterly baffling and feels like it adds a lot of NIH)