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type

The type keyword introduces a type alias: a new name for an existing (possibly complex) type. Aliases have zero runtime cost and are structurally transparent to the underlying type.

Syntax & Example​

type UserID = u64;
type Coordinates = (f64, f64);
type Cache<K, V> = HashMap<K, (V, u64)>;
type Port = u16;
type Milliseconds = u64;

fn connect(host: string, port: Port, timeout: Milliseconds): bool {
    return true;
}

Use Cases​

  • Giving domain-meaningful names to primitive types (Port, Milliseconds).
  • Simplifying long generic signatures into a single alias.
  • Defining function-signature aliases for callbacks and handlers.
  • Building union/sum aliases (type Json = string | i64 | bool | null).

See also: Type Aliases & Custom Types.