Get current page margins in typst

Question

In typst I want to write code that does something if the current location is at (or close to) the left-hand margin.

How can I get the value of the current page margins as numbers?

For example, the code below does what I want with the default 25mm margins, but I want it to work with arbitrary page margins. So I want to change the hard-coded 25 to a function which returns the current left margin value, in mm, as a number.

#let lr() = locate(l=>if l.position().x.mm()<=25 [*left* ] else  [_right_ ])

#let n = 0
#while n < 100 {
  n = n+1
  [#lr()]
}

Answer 1

With Typst 0.11 and context you can access the values set by set rules. Hence, you could use this snippet:

#set page(margin: (left: 10%))
#context {
  // In Typst 0.11, `auto` is not yet resolved in context blocks so we
  // need to check for it and apply the default
  let left = if page.margin == auto or page.margin.left == auto {
    // https://typst.app/docs/reference/layout/page/#parameters-margin
    // The margins are set automatically to 2.5/21 times the smaller dimension of the page
    calc.min(page.width, page.height)*2.5/21
  } else {
    // l.position returns a length but page.margin.left returns a relative
    // length. We resolve it here manually.
    page.margin.left.length + page.margin.left.ratio * page.width
  }

  let lr() = locate(l=> if l.position().x <= left [*left* ] else  [_right_ ])

  let n = 0
  while n < 100 {
    n = n+1
    [#lr()]
  }
}

As you see above, there are still some peculiarities with regards to the resolution of auto and relative lengths, but we expect to fix that in the future.