[justinpombrio]

> I think my statement is still correct in that "a symbol could be a data structure," right? Like you could take a list of dicts and emit a tree of dicts.

Yeah I guess. Text combinator libraries like Haskell's Parsec and Rust's Nom are typically parametric over the type of "characters". Realistically I don't think I've ever seen anyone use one of those libraries for an input that wasn't text-like, though; do you have a use case in mind?

> But wait, a list is a kind of tree, or rather, there is a parse tree of recursive head/tail branches.

Yes, so you can run into parsing problems when working with trees, if you work really hard at it. But if you do the correct action is "reconsider your life choices" and not "use parsing theory".