src/shop/shop.oxl

@error pub data Declined { pub why: str }

pub data Receipt { pub id: i64 }

/// An external payment provider (in production a gRPC service; here a contract with a default implementation).
pub service Payments {
  @public rpc charge(cents: i64) -> Receipt | Declined
}

impl Payments {
  rpc charge(cents) {
    if cents > 100000 { fail Declined{ why: "limit" } }
    Receipt{ id: cents }
  }
}

/// Charges `cents`; a timeout is retried once, because charging is idempotent in this toy provider.
pub fn pay(cents: i64) -> Receipt | Declined | NetError {
  match Payments.charge(cents) {
    Receipt r => r
    Timeout => Payments.charge(cents)?
    other => other?
  }
}