typst/docs/reference/language/context.md

description
How to deal with content that reacts to its location in the document.

Context

Sometimes, we want to create content that reacts to its location in the document. This could be a localized phrase that depends on the configured text language or something as simple as a heading number which prints the right value based on how many headings came before it. However, Typst code isn't directly aware of its location in the document. Some code at the beginning of the source text could yield content that ends up at the back of the document.

To produce content that is reactive to its surroundings, we must thus specifically instruct Typst: We do this with the {context} keyword, which precedes an expression and ensures that it is computed with knowledge of its environment. In return, the context expression itself ends up opaque. We cannot directly access whatever results from it in our code, precisely because it is contextual: There is no one correct result, there may be multiple results in different places of the document. For this reason, everything that depends on the contextual data must happen inside of the context expression.

Aside from explicit context expressions, context is also established implicitly in some places that are also aware of their location in the document: Show rules provide context¹ and numberings in the outline, for instance, also provide the proper context to resolve counters.

Behavior of the context keyword

Style properties frequently change within a document, for example by applying set rules. To retrieve such poperties in a consistent way, one must first specify the context where the query is to be executed. This is the purpose of the context keyword. Once the context has been fixed, the property information is available through a simple field access syntax. For example, text.lang asks for the current language setting. In its simplest form, the context keyword refers to "right here":

#set text(lang: "de")
// query the language setting "here"
#context text.lang

Note that calling #text.lang directly would be an error, because the request cannot be answered without knowledge of the context. The field names supported by a given element function always??|usually?? correspond to the names of the optional arguments in the element's constructor. Remark: it would be nice to have a .fields() function for all types, not just content.

Moreover, some functions, such as to-absolute (more examples) are only applicable in a context, because their results depend on the current settings of style properties. When another function foo() calls a context-dependent function, it becomes itself context-dependent:

#let foo() = 1em.to-absolute()
#context {
  // foo() cannot be called outside of a context
  foo() == text.size
}

When a property is changed, the response to the query changes accordingly:

#set text(lang: "en")
#context text.lang

#set text(lang: "de")
#context text.lang

The output of a #context ... call is read-only in the form of opaque [content]. Write access is prohibited, as it would often result in invalid code: If the context changes between read and write, overwriting a property would cause an inconsistent system state. In fact, context-dependent property fields are read-only even within the context itself:

#set text(lang: "en")
#context [
  call 1: #text.lang \

  #set text(lang: "fr")
  call 2: #text.lang
]

Both calls have the same output 'en', because text.lang is assigned upon entry in the context and remains constant until the end of its scope (the closing ]). Compare this to the previous example: there we got two different results because we created two different contexts.

However, the read-only restriction only applies to the property fields themselves. Content creation instructions do see the effect of the set rule. Consider the same example with font size:

#set text(size: 50pt)
#context [
  call 1: #text.size \

  #set text(size: 25pt)
  call 2: #text.size
]

The second call still outputs '50pt', because text.size is a constant. However, this output is printed in '25pt' font, as specified by the set rule before the call. This illustrates the importance of picking the right insertion point for a context to get access to precisely the right styles. If you need access to updated property fields after a set rule, you can use nested contexts:

#set text(lang: "en")
#context [
  call 1: #text.lang \

  #set text(lang: "fr")
  call 2: #context text.lang
]

All of the above applies to show rules analogously, for example:

#let template(body) = {
  set text(size: 25pt)
  body
}

#set text(size: 50pt)
#context [
  call 1: #text.size \

  #show: template
  call 2: #text.size \
  call 3: #context text.size
]

Controlling content creation within a context

The main purpose of retrieving the current values of properties is, of course, to use them in the calculation of derived properties, instead of setting those properties manually. For example, you can double the font size like this:

#context {
  // the context allows you to
  // retrieve the current text.size
  set text(size: text.size * 200%)
  [large text \ ] 
}
original size

Since set rules are only active until the end of the enclosing scope, 'original size' is printed with the original font size. The above example is equivalent to

#{
  set text(size: 2em)
  [large text \ ]
}
original size

but convenient alternatives like this do not exist for most properties. For example, to double the spacing between the lines of an equation block, you can use the same technique in a show rule. In this case, explicitly adding the context keyword is not necessary, because a show rule establishes a context automatically:

#let spaced-eq(spacing: 100%, body) = {
  show math.equation.where(block: true): it => {
    // access current par.leading in the
    // context of the show rule
    set par(leading: par.leading * spacing)
    it
  }
  body
}

normal spacing:
$
x \
x
$
doubled spacing:
#spaced-eq(spacing: 200%)[$
z \
z
$]

Location context

We've already seen that context gives us access to set rule values. But it can do more: It also lets us know where in the document we currently are, relative to other elements, and absolutely on the pages. We can use this information to create very flexible interactions between different document parts. This underpins features like heading numbering, the table of contents, or page headers dependent on section headings.

Some functions like counter.get implicitly access the current location. In the example below, we want to retrieve the value of the heading counter. Since it changes throughout the document, we need to first enter a context expression. Then, we use get to retrieve the counter's current value. This function accesses the current location from the context to resolve the counter value. Counters have multiple levels and get returns an array with the resolved numbers. Thus, we get the following result:

#set heading(numbering: "1.")

= Introduction
#lorem(5)

#context counter(heading).get()

= Background
#lorem(5)

#context counter(heading).get()

For more flexibility, we can also use the [here] function to directly extract the current [location] from the context. The example below demonstrates this:

• We first have {counter(heading).get()}, which resolves to {(2,)} as before.
• We then use the more powerful [counter.at] with [here], which in combination is equivalent to get, and thus get {(2,)}.
• Finally, we use at with a [label] to retrieve the value of the counter at a different location in the document, in our case that of the introduction heading. This yields {(1,)}. Typst's context system gives us time travel abilities and lets us retrieve the values of any counters and states at any location in the document.

#set heading(numbering: "1.")

= Introduction <intro>
#lorem(5)

= Background <back>
#lorem(5)

#context [
  #counter(heading).get() \
  #counter(heading).at(here()) \
  #counter(heading).at(<intro>)
]

The rule that context-dependent variables and functions remain constant within a given context also applies to location context. The function counter.display() is an example for this behavior. Below, call A will access the counter's value upon entry into the context, i.e. '1' - it cannot see the effect of {c.update(2)}. In contrast, call B accesses the counter in a nested context and will thus see the updated value.

#let c = counter("mycounter")
#c.update(1)
#context [
  #c.update(2)
  call A: #c.display() \
  call B: #context c.display()
]

As mentioned before, we can also use context to get the physical position of elements on the pages. We do this with the [locate] function, which works similarly to counter.at: It takes a location or other [selector] that resolves to a unique element (could also be a label) and returns the position on the pages for that element.

Background is at: \
#context locate(<back>).position()

= Introduction <intro>
#lorem(5)
#pagebreak()

= Background <back>
#lorem(5)

There are other functions that make use of the location context, most prominently [query]. Take a look at the introspection category for more details on those.

Compiler iterations

To resolve contextual interactions, the Typst compiler processes your document multiple times. For instance, to resolve a locate call, Typst first provides a placeholder position, layouts your document and then recompiles with the known position from the finished layout. The same approach is taken to resolve counters, states, and queries. In certain cases, Typst may even need more than two iterations to resolve everything. While that's sometimes a necessity, it may also be a sign of misuse of contextual functions (e.g. of state). If Typst cannot resolve everything within five attempts, it will stop and output the warning "layout did not converge within 5 attempts."

A very careful reader might have noticed that not all of the functions presented above actually make use of the current location. While {counter(heading).get()} definitely depends on it, {counter(heading).at(<intro>)}, for instance, does not. However, it still requires context. While its value is always the same within one compilation iteration, it may change over the course of multiple compiler iterations. If one could call it directly at the top level of a module, the whole module and its exports could change over the course of multiple compiler iterations, which would not be desirable.

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1. Currently, all show rules provide styling context, but only show rules on locatable elements provide a location context. ↩︎