Set footer in template file using main file

[drowsy mist]

I made a template that has a footer I'd like to expose so I can set it in the main file. I have this code, but when I try to compile it, typst says "variables from outside the function are read-only and cannot be modified"

is there a way to do this?

// === template file
#let footer_text = ""

#let set_footer(txt) = {
  footer_text = txt
} 
  
#let template(doc) = [
  #set page(
    paper: "us-letter",
    footer: [#set text(10pt);#h(1fr)#footer_text],
  )
  #doc
]

// === main file
#import "tools/formatting.typ": *
  
#set_footer("Math | 461 | Quiz 1 | 30/9/2024")
#show: template

[digital night]

Use states or pass it to the set rule through an argument

[drowsy mist]

thanks for letting me know about states, never heard of them. I got it working. this is the code for reference:

// === template file
#let s = state("footer_text", "-1")

#let set_footer(expr) = [
  #s.update(footer_text => expr)
]

#let template(doc) = [
  #set page(
    paper: "us-letter",
    footer: [#set text(10pt);#h(1fr)#context s.get()],
  )
  #doc
]

// === main file
#import "tools/formatting.typ": *
#show: template
#set_footer("Math | 461 | Quiz 1 | 24/2/2024")

[digital night]

If you don't expect that footer text to change throughout the document, the following is a more typstian way:

#let template(footer: none) = (doc) => {
  set page(footer: align(right, text(size: 10pt, footer)))
  doc
}

// === main file
#show: template(footer: "Math | 461 | Quiz 1 | 24/2/2024")```