Durable workflows,
as plain programs.

impl Flow<Void> for Trial {    fn run() {        welcome(self.user);        wait time.after(3.days());        if !hasCreatedAProject(self.user) {            sendTips(self.user);        }        wait time.after(10.days());   // a deploy meanwhile? it goes on        if !isSubscribed(self.user) {            offerDiscount(self.user, 20);            wait time.after(1.days());            endTrial(self.user);        }    }}

impl Flow<Result<String, Problem>> for Checkout {    fn run() -> Result<String, Problem> {        let hold = reserve(self.cart);         // undone if it fails        let payment = charge(self.cart)?;      // once, crash or not        let parcel = ship(self.cart, hold)?;   // fails? refund, release        receipt(self.cart, payment, parcel);        return Ok(parcel);    }}// Run again after a crash, with the same key: charged oncestep fn charge(cart: Cart) -> Result<String, Problem> {    let payment = bank.charge(cart.card, cart.cents, flow.key());    flow.undo(refund, payment);    return Ok(payment);}

impl Flow<String> for Agent {    fn run() -> String {        let messages = [Message.user(self.question)];        for _ in 0..20 {            let reply = complete(messages);   // kept: never asked twice            if reply.tool == null {                return reply.text;            }            if reply.tool == "refund" {       // money: a person says yes                askReviewer(self.reviewer, reply.text);                let approved = wait self.approvals.receive();                if !approved {                    return "refused by ${self.reviewer}";                }            }            messages.push(callTool(reply));   // each tool call, once        }        return "gave up after 20 turns";    }}

impl Flow<Decision> for Expense {    fn run() -> Decision {        let approver = managerOf(self.author);        for _ in 0..3 {            ask(approver, self.id);            // It may stop here for days, and pick up where it was            let answer = wait match {                case let a = self.answers.receive(): a;                case time.after(2.days()): null;            };            if answer != null {                if answer.approved {                    return Decision.Approved(approver);                }                return Decision.Refused(answer.why);            }            approver = managerOf(approver);   // no answer: one level up        }        return Decision.Unanswered;    }}