Your Own Capabilities

capability Name {
    fn method(param: Type) -> Result ! ErrorWithDenied
}

type PayError = Declined | Denied(reason: String)

capability Payments {
    fn refund(order: Int, amount: Int) ! PayError
    fn charge(order: Int, amount: Int) ! PayError
}

type TestBank: Payments {
    fn refund(self, order: Int, amount: Int) ! PayError {
        print("refunded {amount} on order {order}")
    }

    fn charge(self, order: Int, amount: Int) ! PayError {
        if amount > 1_000 {
            fail PayError.Declined
        }
        print("charged {amount} on order {order}")
    }
}

agent Shop {
    can Payments

    on buy(order: Int, amount: Int) ! PayError {
        try Payments.charge(order, amount)
    }

    on cancel(order: Int, amount: Int) ! PayError {
        try Payments.refund(order, amount)
    }
}

fn main() {
    const shop = Shop.start(Payments: TestBank())
    shop.buy(1, 300) catch e => print("failed: {e}")
    shop.buy(2, 5_000) catch e => print("failed: {e}")
    shop.cancel(1, 300) catch e => print("failed: {e}")
}