Agents: Declaring an agent

type EmailError = NotSent | Denied(reason: String)

capability Email {
    fn send(to: String) ! EmailError
}

type Outbox: Email {
    fn send(self, to: String) ! EmailError {
        print("sent to {to}")
    }
}

agent Greeter {
    can Email.send

    limit Email.send {
        count <= 2 per day
    }

    on welcome(to: String) ! EmailError {
        try Email.send(to)
    }
}

fn main() {
    const bot = Greeter.start(Email: Outbox())
    bot.welcome("asha@example.com") catch e => print("refused: {e}")
    bot.welcome("omar@example.com") catch e => print("refused: {e}")
    bot.welcome("zoe@example.com") catch e => print("refused: {e}")
}