For example, I want a function to force its argument to be text, even though it is used in math mode. These don’t work:
#let fun(arg) = $"#arg"_n$
#let fun(ft) = $#text[#arg]_n$
Is there a way so that fun, which is called in math content, shows arg as text?
The upright text you’re describing is the one in non-maths fonts.
Text in math mode should use text font is a pending issue.
#show math.equation: set text(font: "Fira Math")
// Using quotes -> upright Fira Math
$"Upright text" x + y = z$
// Using function `math.op` -> upright Fira Math with slightly less spacing
$op("Upright text") x + y = z$
// Using function `math.upright` -> upright Fira Math
$upright("Upright text") x + y = z$
// Using function `text` -> upright Fira Math
$text("Upright text") x + y = z$
// Using function `text` with own font -> own upright font
$text(font: "Libertinus Serif", "Upright text") x + y = z$
So you would want to use the last option, where Libertinus Serif is known to be the default outside font:
#let fun(arg) = $text(font: "Libertinus Serif", arg)_n$
// In code, use as any of the following
$fun("xyz") 1 + 1 = 2$
$#fun[xyz] 1 + 1 = 2$
$#fun([xyz]) 1 + 1 = 2$
Further automation might be possible throught context. That way, the outside font would be automatically applied.
Quick searches bring up these relevant posts:
I am not sure this is what you are looking for, but this may be it:
#let fun(arg) = $#arg _n$
$x_n$
#fun($x$)
$fun(x)$
$fun("x")$
If not, could you please also describe what you are expecting or provide a more comprehensive example?
In principle it’s impossible, because the function itself can’t change the type of argument produced on the side calling the function. In general, when a function is called in math mode, the argument will be some kind of content, as demonstrated:
#let fun(arg) = str(type(arg))
$fun(x) + fun(1) + fun(1 / 2) + fun(integral d x)$
