Match

Syntax

decide(<Command>, <State>) {
    match <expression> {
        <pattern> -> [<Event>, ...]
        <pattern> -> [<Event>, ...]
        _ -> [<Event>, ...]
    }
}

Match blocks enable conditional event emission based on runtime values. They appear inside decide block bodies as an alternative to a direct emission statement.

Match Arms

Each arm has a pattern and an emission list. Arms are evaluated top-to-bottom.

Wildcard

The _ pattern matches any value. It typically appears as the last arm to ensure exhaustive coverage.

With Guards and Postconditions

Match blocks can be combined with require guards and ensure postconditions:

decide(UpdateStatus, Active) {
    require status != old.status
        else reject "Status unchanged"
    match status {
        "approved" -> [StatusApproved]
        "rejected" -> [StatusRejected]
        _ -> [StatusUpdated { newStatus: status }]
    }
    ensure currentStatus != old.currentStatus
}

Example

decide(SetPriority, Open) {
    match priority {
        1 -> [PrioritySetCritical]
        2 -> [PrioritySetHigh]
        _ -> [PrioritySetNormal { level: priority }]
    }
}