Optionals

Values that may be present or absent — the none literal and the ? type suffix.

An optional is a value that may be present or absent. The literal none represents absence; a ? suffix on a type makes its values optional. Together they let a field be omitted, a function return no answer, or a slot be cleared explicitly.

Optional types

Suffix any type with ? to make it optional. A T? field accepts either a T value or the literal none. A field without ? is required and rejects none.

type Profile {
  name:  utf8       // required
  bio:   utf8?      // optional — may be none
  age:   u32?       // optional
}

complete = Profile { name: "Alice", bio: "Author.", age: 34u32 }
partial  = Profile { name: "Bob",   bio: none,      age: none }

The none literal

none is the value that fills an absent optional. It is also the result of builtins that do not return a useful value (error, panic, assert) — those have type none and cannot meaningfully be used in a value position.

note: utf8? = none

Working with optionals

Pattern-match an optional to handle the two cases. The if let shorthand is convenient when you only care about the present case. See Control Flow.

display = match maybe_name {
  none => "anonymous",
  n    => n,
}

shout = if let n = maybe_name {
  to_upper(n)
} else {
  "ANONYMOUS"
}

Defaults with ??

The ?? operator picks the left value unless it is none, in which case it returns the right side — the concise way to give an optional a default. It chains left-to-right and binds looser than every other operator, and the right side only evaluates when needed.

width  = box.width ?? 480.0
theme  = page.theme ?? site.theme ?? :nord
label  = trim(raw_label) ?? "untitled"   // (trim(raw_label)) ?? "untitled"

Optionals vs unions

Use T? when the only states are present and absent. When absence carries information (a reason, a fallback, multiple shapes), reach for a union instead.

Examples

Optional fields and the ?? default

A ? suffix makes a field optional; ?? supplies a fallback when it is none.

type Profile {
  name: utf8       // required
  bio:  utf8?      // optional — may be none
}

partial = Profile { name: "Bob", bio: none }
theme   = page.theme ?? site.theme ?? :nord

Expected: partial.bio is none; theme falls back to site.theme, then :nord.

- Unions

- Control Flow