Tagged unions

An enum variant may carry a value:

enum Token {
    End,
    Number(Int),
    Name(String),
}

var token = Token.Number(42);

Such an enum stores a tag together with the active variant’s payload.

Different payloads

record Position {
    line: Int,
    column: Int,
}

enum ParseEvent {
    Finished,
    Text(String),
    Location(Position),
}

Each value holds exactly one variant at a time.

Inspect the tag

The current language exposes the active tag through .tag:

var event = ParseEvent.Text("name");
println(toString(event.tag));

Pattern matching and payload narrowing are not implemented yet. Until they are, tagged unions are most useful when code can pass or store the complete value without extracting every payload.

Ownership

Payloads follow the ownership rules of their types:

class Node {
    pub next: Node;
}

enum Link {
    Empty,
    Some(Node),
}

A class reference stored in a tagged union is traced by ARC and the cycle collector. Strings and aggregates are copied and destroyed using their normal generated operations.