#import "../../../components/index.typ": docs-chapter, docs-table, short-or-long

#show: docs-chapter.with(
  title: "Scripting",
  route: "/reference/scripting",
  description: "Automate your document with Typst's scripting capabilities.",
)

Typst embeds a powerful scripting language. You can automate your documents and create more sophisticated styles with code. Below is an overview over the scripting concepts.

= Expressions <expressions>
In Typst, markup and code are fused into one. All but the most common elements are created with _functions._ To make this as convenient as possible, Typst provides compact syntax to embed a code expression into markup: An expression is introduced with a hash (`#`) and normal markup parsing resumes after the expression is finished. If a character would continue the expression but should be interpreted as text, the expression can forcibly be ended with a semicolon (`;`). You can @reference:syntax:escapes[escape a literal `#` or `;` with a backslash].

```example
#emph[Hello] \
#emoji.face \
#"hello".len()
```

The example above shows a few of the available expressions, including @function[function calls], @reference:scripting:fields[field accesses], and @reference:scripting:methods[method calls]. More kinds of expressions are discussed in the remainder of this chapter. A few kinds of expressions are not compatible with the hash syntax (e.g. binary operator expressions). To embed these into markup, you can use parentheses, as in `[#(1 + 2)]`.

= Blocks <blocks>
To structure your code and embed markup into it, Typst provides two kinds of _blocks:_

- *Code block:* `{{ let x = 1; x + 2 }}` \
  When writing code, you'll probably want to split up your computation into multiple statements, create some intermediate variables and so on. Code blocks let you write multiple expressions where one is expected. The individual expressions in a code block should be separated by line breaks or semicolons. The output values of the individual expressions in a code block are joined to determine the block's value. Expressions without useful output, like `{let}` bindings yield `{none}`, which can be joined with any value without effect.

- *Content block:* `{[*Hey* there!]}` \
  With content blocks, you can handle markup/content as a programmatic value, store it in variables and pass it to @function[functions]. Content blocks are delimited by square brackets and can contain arbitrary markup. A content block results in a value of type @content[content]. An arbitrary number of content blocks can be passed as trailing arguments to functions. That is, `{list([A], [B])}` is equivalent to `{list[A][B]}`.

Content and code blocks can be nested arbitrarily. In the example below, `{[hello ]}` is joined with the output of  `{a + [ the ] + b}` yielding `{[hello from the *world*]}`.

```example
#{
  let a = [from]
  let b = [*world*]
  [hello ]
  a + [ the ] + b
}
```

= #short-or-long[Bindings][Bindings and Destructuring] <bindings>
As already demonstrated above, variables can be defined with `{let}` bindings. The variable is assigned the value of the expression that follows the `=` sign. A @reference:syntax:identifiers[valid variable name] may contain `-`, but cannot start with `-`. The assignment of a value is optional, if no value is assigned, the variable will be initialized as `{none}`. The `{let}` keyword can also be used to create a @function:defining-functions[custom named function]. Variables can be accessed for the rest of the containing block (or the rest of the file if there is no containing block).

```example
#let name = "Typst"
This is #name's documentation.
It explains #name.

#let my-add(x, y) = x + y
Sum is #my-add(2, 3).
```

Let bindings can also be used to destructure @array[arrays] and @dictionary[dictionaries]. In this case, the structure of the left-hand side of the assignment should mirror the array or dictionary: With bindings corresponding by position for arrays and by key name for dictionaries. The `..` operator can be used once in the pattern to collect the remainder of the array's or the dictionary's items.

```example
#let (x, y) = (1, 2)
The coordinates are #x, #y.

#let (a, .., b) = (1, 2, 3, 4)
The first element is #a.
The last element is #b.

#let books = (
  Shakespeare: "Hamlet",
  Homer: "The Odyssey",
  Austen: "Persuasion",
)

#let (Austen,) = books
Austen wrote #Austen.

#let (Homer: h) = books
Homer wrote #h.

#let (Homer, ..other) = books
#for (author, title) in other [
  #author wrote #title.
]
```

You can use the underscore to discard elements in a destructuring pattern:

```example
#let (_, y, _) = (1, 2, 3)
The y coordinate is #y.
```

Destructuring also works in argument lists of functions ...

```example
#let left = (2, 4, 5)
#let right = (3, 2, 6)
#left.zip(right).map(
  ((a,b)) => a + b
)
```

... and on the left-hand side of normal assignments. This can be useful to swap variables among other things.

```example
#{
  let a = 1
  let b = 2
  (a, b) = (b, a)
  [a = #a, b = #b]
}
```

= Conditionals <conditionals>
With a conditional, you can display or compute different things depending on whether some condition is fulfilled. Typst supports `{if}`, `{else if}` and `{else}` expressions. When the condition evaluates to `{true}`, the conditional yields the value resulting from the if's body. Otherwise, it yields the value resulting from the else's body.

```example
#if 1 < 2 [
  This is shown
] else [
  This is not.
]
```

Each branch can have a code or content block as its body.

- `{if condition {..}}`
- `{if condition [..]}`
- `{if condition [..] else {..}}`
- `{if condition [..] else if condition {..} else [..]}`

= Loops <loops>
With loops, you can repeat content or compute something iteratively. Typst supports two types of loops: `{for}` and `{while}` loops. The former iterate over a specified collection whereas the latter iterate as long as a condition stays fulfilled. Just like blocks, loops _join_ the results from each iteration into one value.

In the example below, the three sentences created by the for loop join together into a single content value and the length-1 arrays in the while loop join together into one larger array.

```example
#for c in "ABC" [
  #c is a letter.
]

#let n = 2
#while n < 10 {
  n = (n * 2) - 1
  (n,)
}
```

For loops can iterate over a variety of collections:

- `{for value in array {..}}` \
  Iterates over the items in the @array[array]. The destructuring syntax described in @reference:scripting:bindings[Let binding] can also be used here.

- `{for pair in dict {..}}` \
  Iterates over the key-value pairs of the @dictionary[dictionary]. The pairs can also be destructured by using `{for (key, value) in dict {..}}`. It is more efficient than `{for pair in dict.pairs() {..}}` because it doesn't create a temporary array of all key-value pairs.

- `{for letter in "abc" {..}}` \
  Iterates over the characters of the @str[string]. Technically, it iterates over the grapheme clusters of the string. Most of the time, a grapheme cluster is just a single codepoint. However, a grapheme cluster could contain multiple codepoints, like a flag emoji.

- `{for byte in bytes("😀") {..}}` \
  Iterates over the @bytes[bytes], which can be converted from a @str[string] or @read[read] from a file without encoding. Each byte value is an @int[integer] between `{0}` and `{255}`.

To control the execution of the loop, Typst provides the `{break}` and `{continue}` statements. The former performs an early exit from the loop while the latter skips ahead to the next iteration of the loop.

```example
#for letter in "abc nope" {
  if letter == " " {
    break
  }

  letter
}
```

The body of a loop can be a code or content block:

- `{for .. in collection {..}}`
- `{for .. in collection [..]}`
- `{while condition {..}}`
- `{while condition [..]}`

= Fields <fields>
You can use _dot notation_ to access fields on a value. For values of type @content, you can also use the @content.fields[`fields`] function to list the fields.

The value in question can be either:
- a @dictionary[dictionary] that has the specified key,
- a @symbol[symbol] that has the specified modifier,
- a @module[module] containing the specified definition,
- @content[content] consisting of an element that has the specified field. The available fields match the arguments of the @function:element-functions[element function] that were given when the element was constructed.

```example
#let it = [= Heading]
#it.body \
#it.depth \
#it.fields()

#let dict = (greet: "Hello")
#dict.greet \
#emoji.face

```

= Methods <methods>
A _method call_ is a convenient way to call a function that is scoped to a value's @type[type]. For example, we can call the @str.len function in the following two equivalent ways:

```example
#str.len("abc") is the same as
#"abc".len()
```

The structure of a method call is `{value.method(..args)}` and its equivalent full function call is `{type(value).method(value, ..args)}`. The documentation of each type lists its scoped functions. You cannot currently define your own methods.

```example
#let values = (1, 2, 3, 4)
#values.pop() \
#values.len() \

#("a, b, c"
    .split(", ")
    .join[ --- ])

#"abc".len() is the same as
#str.len("abc")
```

There are a few special functions that modify the value they are called on (e.g. @array.push). These functions _must_ be called in method form. In some cases, when the method is only called for its side effect, its return value should be ignored (and not participate in joining). The canonical way to discard a value is with a let binding: `{let _ = array.remove(1)}`.

= Modules <modules>
You can split up your Typst projects into multiple files called _modules._ A module can refer to the content and definitions of another module in multiple ways:

- *Including:* `{include "bar.typ"}` \
  Evaluates the file at the @path[path] `bar.typ` and returns the resulting @content[content].

- *Import:* `{import "bar.typ"}` \
  Evaluates the file at the @path[path] `bar.typ` and inserts the resulting @module[module] into the current scope as `bar` (filename without extension). You can use the `as` keyword to rename the imported module: `{import "bar.typ" as baz}`. You can import nested items using dot notation: `{import "bar.typ": baz.a}`.

- *Import items:* `{import "bar.typ": a, b}` \
  Evaluates the file at the @path[path] `bar.typ`, extracts the values of the variables `a` and `b` (that need to be defined in `bar.typ`, e.g. through `{let}` bindings) and defines them in the current file. Replacing `a, b` with `*` loads all variables defined in a module. You can use the `as` keyword to rename the individual items: `{import "bar.typ": a as one, b as two}`

Instead of a string or @path[path], you can also use a @module[module value], as shown in the following example:

```example
#import emoji: face
#face.grin
```

= Packages <packages>
To reuse building blocks across projects, you can also create and import Typst _packages._ A package import is specified as a triple of a namespace, a name, and a version.

```example
>>> #let add(x, y) = x + y
<<< #import "@preview/example:0.1.0": add
#add(2, 7)
```

The `preview` namespace contains packages shared by the community. You can find all available community packages on #link("https://typst.app/universe")[Typst Universe].

If you are using Typst locally, you can also create your own system-local packages. For more details on this, see the #link("https://github.com/typst/packages")[package repository].

= Operators <operators>
The following table lists all available unary and binary operators with effect, arity (unary, binary) and precedence level (higher binds stronger). Some operations, such as @calc.rem-euclid[modulus], do not have a special syntax and can be achieved using functions from the @calc module.

#docs-table(
  table.header[Operator][Effect][Arity][Precedence],

  [`{-}`],
  [Negation],
  [Unary],
  [7],

  [`{+}`],
  [No effect (exists for symmetry)],
  [Unary],
  [7],

  [`{*}`],
  [Multiplication],
  [Binary],
  [6],

  [`{/}`],
  [Division],
  [Binary],
  [6],

  [`{+}`],
  [Addition],
  [Binary],
  [5],

  [`{-}`],
  [Subtraction],
  [Binary],
  [5],

  [`{==}`],
  [Check equality],
  [Binary],
  [4],

  [`{!=}`],
  [Check inequality],
  [Binary],
  [4],

  [`{<}`],
  [Check less-than],
  [Binary],
  [4],

  [`{<=}`],
  [Check less-than or equal],
  [Binary],
  [4],

  [`{>}`],
  [Check greater-than],
  [Binary],
  [4],

  [`{>=}`],
  [Check greater-than or equal],
  [Binary],
  [4],

  [`{in}`],
  [Check if in collection],
  [Binary],
  [4],

  [`{not in}`],
  [Check if not in collection],
  [Binary],
  [4],

  [`{not}`],
  [Logical "not"],
  [Unary],
  [3],

  [`{and}`],
  [Short-circuiting logical "and"],
  [Binary],
  [3],

  [`{or}`],
  [Short-circuiting logical "or"],
  [Binary],
  [2],

  [`{=}`],
  [Assignment],
  [Binary],
  [1],

  [`{+=}`],
  [Add-Assignment],
  [Binary],
  [1],

  [`{-=}`],
  [Subtraction-Assignment],
  [Binary],
  [1],

  [`{*=}`],
  [Multiplication-Assignment],
  [Binary],
  [1],

  [`{/=}`],
  [Division-Assignment],
  [Binary],
  [1],
)
