There are 3 syntactic modes in typst: markup, code and math. In markup, everything is literal, unless you put a `#` sigil like `#expr` in which case `expr` is parsed in code mode. In code mode everything is an identifier, as usual in programming. In math its a bit of an ugly tradeoff but its ok: single-letter things are parsed as literals but multi-letter tokens are parsed as identifiers. Finally, in code you can enclose in `[...]` to parse in markup mode. So typically, your document will be mostly in markup mode and you will encounter stuff like `#something[An argument]`, which is a function call to which you pass one content-typed argument.
So above, `y` is parsed as literal, while `dt` is parsed as an identifier, hence function call.
> In markup, everything is literal, unless you put a `#` sigil
Unless it's a "=", then it begins a new section. And unless it's a "-" or a "+" or a "/", then it's a list item. And unless it's a "<", then it defines a label, or a "@", then it's a reference to one. And unless it's a "_" or "*" or "`", then it changes font or style.
Right. Markup mode supports markup features - including all of that stuff for headings, lists, bold, italics and so on. It’s clearly inspired by markdown and similar languages. Personally I prefer * to \em{xxx} since it’s easier to type and it makes the text easier to read while I’m editing. (Or maybe just more familiar since give written so much markdown at this point).
I think I would have preferred a consistent sigil, but I guess with fast enough feedback you'd get used to the quirks.
I do wonder if this has over-optimised on short equations—to me heavy maths use implies multi-page equations with very specific formatting requirements (something amsmath has no issues with, which isn't surprising given who the authors are).
So above, `y` is parsed as literal, while `dt` is parsed as an identifier, hence function call.