Optionals

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 don't 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.

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

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