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]($styling/#show-rules) provide context[^1] and numberings in the
outline, for instance, also provide the proper context to resolve counters.

## The context keyword
Style properties frequently change within a document, for example through set 
rules. To retrieve such properties in a consistent way, one must first specify 
the precise context where the property should be retrieved. This is achieved 
with the `context` keyword. Once the context has been fixed, the property 
information is available through standard field access syntax. For example, 
`text.lang` asks for the current language setting. In its simplest form, the 
`context` keyword refers to "right here":

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

Note that any attempt to access `#text.lang` directly, i.e. outside of a context,
will cause the compiler to issue an error message, since it cannot determine the 
precise location the query refers to. The field names supported 
by a given element function always correspond to the named parameters documented 
on each element's page.

Moreover, some functions, such as [`to-absolute`]($length.to-absolute) 
and [`counter.display`]($counter.display), 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:

```example
#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 property access 
changes accordingly: 

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

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

As you see, the result of a `[#context ..]` expression can 
be inserted into the document as [content]. Context blocks can 
contain arbitrary code beyond the field access. However,
and this is often surprising for newcomers, context-dependent 
property fields remain _constant_ throughout the context's scope. 
This has two important consequences: First, direct property 
assignments like `{text.lang = "de"}` are _not_ allowed – 
use `set` or `show` rules for this purpose. Second, changes to a 
property value within a context (e.g. by a `set` rule) are not 
observable by field access within that same context:

```example
#set text(lang: "en")
#context [
  Read 1: #text.lang

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

Both reads have the same output `{"en"}`, because `text.lang` is fixed
upon entry in the context and remains constant until the end of its scope 
(the closing `]`). Thus, the `text.lang` field is not affected by
`[#set text(lang: "fr")]`, although Read 2 occurs after it. Compare 
this to the previous example: There we got two different results because 
we created two different contexts.

However, immutability only applies to the property fields themselves. 
The appearance of content within a context _can_ be changed in the 
usual manner, e.g. by set rules. Consider the same example with font size:

```example
#set text(size: 40pt)
#context [
  Read 1: #text.size

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

Read 2 still outputs `{40pt}`, because `text.size` is a constant.
However, this output is printed in 25pt font, as specified by the set
rule before the read. 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_:

```example
#set text(lang: "en")
#context [
  Read 1: #text.lang

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

All of the above applies to `show` rules analogously. To demonstrate this, 
we define a function `{template}` (emulating what a document template 
might do) which is activated by an "everything" show rule in a context:

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

#set text(size: 40pt)
#context [
  Read 1: #text.size

  #show: template
  Read 2: #text.size \
  Read 3: #context text.size
]
```
Reads 1 and 2 print the original text size upon entry in the first 
context (since `text.size` remains constant there), but Read 3 is 
located in a nested context and reflects the new font size set by 
the `show` rule via the `template` function.

## Setting derived properties in a context
An important purpose of reading the current value of properties is
to use this information in the calculation of derived properties, 
instead of setting those properties manually. For example, 
you can double the font size like this:

```example
#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.
For the specific case of accessing `text.size`, context is usually 
not necessary as the `{1em}` unit is always equal to the current font 
size, so the above example is equivalent to 

```example
#[
  #set text(size: 2em)
  Large text \ 
]
Original size
```

but convenient alternatives like this are unavailable for most properties.
This makes contexts a powerful and versatile concept. For example, 
you can use a similar resizing technique to increase the spacing 
between the lines of a specific equation block (or any other content):

```example
#let spaced(spacing: 100%, body) = context {
  // Access current par.leading in a context.
  set par(leading: par.leading * spacing)
  body
}

Normal spacing:
$ x \ x $
Doubled spacing:
#spaced(spacing: 200%)[$ z \ z $]
```

The advantage of this technique is that the user does not have to know the 
original spacing in order to double it. To double the spacing of all 
equations, you can put the same calculations in a show rule. Note that 
it is not necessary to add the `context` keyword on the right-hand side 
of a `show` rule, because show rules establish a context automatically:

```example
Normal spacing:
$ x \ x $

#show math.equation.where(block: true): it => {
  // Access current par.leading in a context,
  // established automatically by the show rule.
  set par(leading: par.leading * 200%)
  it
}

Doubled spacing:
$ 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`]($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:

```example
#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.

```example
#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, Read 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, Read B accesses 
the counter in a nested context and will thus see the updated value.

```example
#let c = counter("mycounter")
#c.update(1)
#context [
  #c.update(2)
  Read A: #c.display() \
  Read 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.

```example
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/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]($state/#caution)). 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.

[^1]: Currently, all show rules provide styling context, but only show rules on
      [locatable]($location/#locatable) elements provide a location context.