Ah I see, so not exactly the kind of thing you'd notice making a document but you would if trying to extend the actual language. I mean TeX is basically an overpowered lambda calculus with a focus on text markup and generation.
I suppose the fact that LaTeX works at all is a testament to TeX's power, but yes I can imagine a better base language could be relevant for something like Typst.
> so not exactly the kind of thing you'd notice making a document
By "user interface" I mean (like Martin, in the linked video) the surface language, the way you interact with TeX, its syntax and how it presents itself to the user, the programming model. So definitely the kind of thing you'd notice making a document. Leaky abstractions and footguns are basically everywhere when you write a big LaTeX document.
> I mean TeX is basically an overpowered lambda calculus with a focus on text markup and generation.
No, lambda calculus has capture-avoiding substitution, aka hygienic macro expansion if you will. TeX has naive substitution. By the way, macro expansion is typically CBN, which is very much a rare and weird evaluation order (yes, Haskell, i know). TeX is much closer to some kind of assembly language for a virtual machine.
I suppose the fact that LaTeX works at all is a testament to TeX's power, but yes I can imagine a better base language could be relevant for something like Typst.