Enums

An enum is a closed list of named alternatives. Variants are always written with their enum's name.

type TicketStatus = Enum {    open    waiting    closed}function reopen() -> TicketStatus {    return TicketStatus.open}

A variant can carry its own fields, which makes an enum a tagged union: each value is exactly one variant and has only that variant's fields.

type PaymentOutcome = Enum {    pending    paid: {        receipt_id: ReceiptId        paid_at: Instant    }    declined: {        reason: DeclineReason    }}function declined_payment(reason: DeclineReason) -> PaymentOutcome {    return PaymentOutcome.declined {        reason: reason    }}function label(outcome: PaymentOutcome) -> OutcomeLabel {    return match outcome {        PaymentOutcome.pending -> "pending"        PaymentOutcome.paid -> "paid"        PaymentOutcome.declined -> "declined"    }}

The only way to read a variant's fields is to match on it. Payload fields have field types too, written PaymentOutcome.declined.reason.

An enum can't be created from text: TicketStatus("open") is an error. Text from outside, such as a request body, becomes a variant only as it is read and checked on the way in. In JSON a plain variant is its name as a string ("open"), and a variant with fields is an object with a tag naming the variant, plus its fields.

Enums have no methods or display labels. A variant is a name, or a name with a block of its fields; one given a label, open: "Open", is type.enum_label. Wording for people comes from translations: a domain's language files can give each value of an enum a wording, which a template then writes out (Enums with wording).