How to check for boolean state?

[huz]

I want to check whether a state is true or not:

#state("bla").update(true)

#let b = context ( state("bla").get() )

#b // shows true

#if b {
  [bla is true]
}

gives the error

Expected boolean, found content
#if b {

Checking for b==true, b==[true], b==“true” etc. doesn’t work.
Also it doesn’t work when using a string “true”.

[SillyFreak]

See here:

Why is the value I receive from context always content?

When working with context, you need to put everything that depends on the contextual information into the context block/expression itself. The explicit context expression is a trade-off: You get to know where in the document you are (accessible via here()) and which set rules are active there (accessible via things like text.lang). In return, the context value itself becomes opaque. You cannot peek into it, so everything that depends on the contextual information must happen within it. Let’s lo…

[huz]

… and I cannot compare content with strings or boolean? Or what is the answer?
b is not contextual, or is it?

[SillyFreak]

Content is a type that is not meant to be inspected, but there’s different kinds of content. The content returned by context is special, because what it represents is… contextual. Not only is it not meant to be inspected, it can’t be inspected (it’s “opaque”), and that includes comparisons. b == [true] is simply not true, because b is contextual. [true] == [true] or [true].text == "true" work, althought that’s not the common usage of these content values.

So what can you do? The linked post has this example:

@laurmaedje

#let val() = text.lang + "!"

#context {
  if val().starts-with("d") {
    ..
  }
}

Adapted to your situation, that would be

#state("bla").update(true)

#context if state("bla").get() {
  [bla is true]
}

Now only the result of the if, not its condition, is an opaque context value.

[huz]

Why can #b then be typed out if I cannot inspect it?

[SillyFreak]

It is a value. It can be displayed, as you did, so it’s useful to type out. But you can’t inspect it. Consider this:

#state("bla").update(true)

#let b = context ( state("bla").get() )

#b

#state("bla").update(false)

#b

This displays true first, then false. Imagine instead of #b, we did some inspection:

#state("bla").update(true)

#let b = context ( state("bla").get() )

// get a value depending on b
#col = if b == true { "red" } else { "green" }

#state("bla").update(false)

If b is true, we want red. Otherwise, we want green.

Since b is #b, the color is #col

If this worked, we’d get

If b is true, we want red. Otherwise, we want green.

Since b is false, the color is red

This is why contextual values can’t be inspected: it would lead to outdated inspection results being used by accident. (Apart from that I’m pretty sure the way Typst is built, allowing this at all would actually not be simple. But I hope this shows that it’s also not desirable.)

[huz]

Yes I want to set an inspection result and save it for the rest of the document. And state is the only way I can set a value in an including file and use in the included file. So this contextuality makes it impossible again.

[SillyFreak]

ok so can you maybe give two things that should be influenced by your boolean? I’m pretty sure that it will not be impossible to do them, even with contextuality.

For example, the snippet that I showed can be made to work, like this:

#state("bla").update(true)

// make this a function and don't use context here
// b() can only be called inside context
#let b() = state("bla").get()

// get a value depending on b()
#col = context if b() { "red" } else { "green" }

#state("bla").update(false)

If b is true, we want red. Otherwise, we want green.

Since b is #context b(), the color is #col

And it will say “Since b is false, the color is green”.

[huz]

The page layout would depend on it. Which I think is not possible even in an #if clause, let alone #context

#context { set page ("a5") if b() }

[SillyFreak]

so the including (main) file sets b, and the included (part) file should set its page size according to it? You could probably do that like this:

#show: it => context {
  set page ("a5") if b()
  it
}

Strangely, it seems that the set page only takes effect if there is some content on the page before. This is probably related to the workaround I had to use here: Why is the page counter in the background of my page wrong after using counter.update()? - #11 by SillyFreak – if I have the time, I’ll see if the situation has changed since the 0.11.1 release. (EDIT: I just tested this using a nightly Tinymist based on Typst 8c813cb (2024-09-22) and it seems to work as intended)

That said, context that goes over large ranges, e.g. entire sections, is indeed not ideal (as far as I know). If the boolean is set once and then just exists (i.e. you don’t update it, it’s just configuration), I would actually use import: Why can't I use a function in a chapter even though I imported it at the start of my main file? - #2 by SillyFreak

It should be possible to import "main.typ": b if you don’t want to put the variables in a separate preamble.typ (or similar) file.

[laurmaedje]

Context over large ranges shouldn’t be a problem anymore with Typst 0.12. In 0.11.1 there are a few bugs which let it cause problems. I used to discourage it, partly because of these bugs and partly because I wasn’t sure how it would affect things, but going forward I think it is unproblematic.

[huz]

@huz

The page layout would depend on it. Which I think is not possible even in an #if clause, let alone #context

#context { set page ("a5") ) if b() }

@SillyFreak

so the including (main) file sets b, and the included (part) file should set its page size according to it? You could probably do that like this:

#show: it => context {
  set page ("a5") if b()
  it
}

Yes this seems to work.