state

Manages stateful parts of your document.

Let's say you have some computations in your document and want to remember the result of your last computation to use it in the next one. You might try something similar to the code below and expect it to output 10, 13, 26, and 21. However this does not work in Typst. If you test this code, you will see that Typst complains with the following error message: Variables from outside the function are read-only and cannot be modified.

#let x = 0
#let compute(expr) = {
  x = eval(
    expr.replace("x", str(x))
  )
  [New value is #x. ]
}

#compute("10") \
#compute("x + 3") \
#compute("x * 2") \
#compute("x - 5")

State and document markup

Why does it do that? Because, in general, this kind of computation with side effects is problematic in document markup and Typst is upfront about that. For the results to make sense, the computation must proceed in the same order in which the results will be laid out in the document. In our simple example, that's the case, but in general it might not be.

Let's look at a slightly different, but similar kind of state: The heading numbering. We want to increase the heading counter at each heading. Easy enough, right? Just add one. Well, it's not that simple. Consider the following example:

#set heading(numbering: "1.")
#let template(body) = [
  = Outline
  ...
  #body
]

#show: template

= Introduction
...

Here, Typst first processes the body of the document after the show rule, sees the Introduction heading, then passes the resulting content to the template function and only then sees the Outline. Just counting up would number the Introduction with 1 and the Outline with 2.

Managing state in Typst

So what do we do instead? We use Typst's state management system. Calling the state function with an identifying string key and an optional initial value gives you a state value which exposes a few methods. The two most important ones are display and update:

• The display method shows the current value of the state. You can optionally give it a function that receives the value and formats it in some way.

• The update method modifies the state. You can give it any value. If given a non-function value, it sets the state to that value. If given a function, that function receives the previous state and has to return the new state.

Our initial example would now look like this:

#let s = state("x", 0)
#let compute(expr) = [
  #s.update(x =>
    eval(expr.replace("x", str(x)))
  )
  New value is #s.display().
]

#compute("10") \
#compute("x + 3") \
#compute("x * 2") \
#compute("x - 5")

State managed by Typst is always updated in layout order, not in evaluation order. The update method returns content and its effect occurs at the position where the returned content is inserted into the document.

As a result, we can now also store some of the computations in variables, but they still show the correct results:

...

#let more = [
  #compute("x * 2") \
  #compute("x - 5")
]

#compute("10") \
#compute("x + 3") \
#more

This example is of course a bit silly, but in practice this is often exactly what you want! A good example are heading counters, which is why Typst's counting system is very similar to its state system.

Time Travel

By using Typst's state management system you also get time travel capabilities! By combining the state system with locate and query, we can find out what the value of the state will be at any position in the document from anywhere else. In particular, the at method gives us the value of the state at any location and the final methods gives us the value of the state at the end of the document.

...

Value at `<here>` is
#locate(loc => s.at(
  query(<here>, loc)
    .first()
    .location()
))

#compute("10") \
#compute("x + 3") \
*Here.* <here> \
#compute("x * 2") \
#compute("x - 5")

A word of caution

To resolve the values of all states, Typst evaluates parts of your code multiple times. However, there is no guarantee that your state manipulation can actually be completely resolved.

For instance, if you generate state updates depending on the final value of a state, the results might never converge. The example below illustrates this. We initialize our state with 1 and then update it to its own final value plus 1. So it should be 2, but then its final value is 2, so it should be 3, and so on. This example display 4 because Typst simply gives up after a few attempts.

#let s = state("x", 1)
#locate(loc => {
  s.update(s.final(loc) + 1)
})
#s.display()

In general, you should typically not generate state updates from within locate calls or display calls of state or counters. Instead, pass a function to update that determines the value of the state based on its previous value.

Create a new state identified by a key.