Immutable

:::info[Note] This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information. :::

Returns an immutable (ImmSet), deep copy of the current Set

public function immutable(): ImmSet<Tv>;

This method is interchangeable with toImmSet().

Returns

• ImmSet<Tv> - an ImmSet that is a deep copy of the current Set.

Examples

function expects_immutable(ImmSet<string> $is): void {
  \var_dump($is);
}

<<__EntryPoint>>
function basic_usage_main(): void {
  $s = Set {'red', 'green', 'blue', 'yellow'};

  // Get a deep, immutable copy of $s
  $immutable_s = $s->immutable();

  expects_immutable($immutable_s);
}