Tour · 7 of 25

Errors are values in the return type, and `?` passes them up

A function that can fail returns Result[T, E]: .Ok(value) or .Err(error). The failure is in the signature, so no caller is surprised by a hidden exception. Postfix ? unwraps an .Ok, and on an .Err it returns that error from the current function, so the happy path reads top to bottom. Forget the ? and you are holding a Result, not a number, and the compiler says so. Real code uses an error enum instead of string; a later page shows how.

Run: Ctrl+Enter · Pages: Alt+← Alt+→

fn checkout(cents: i32, coupon: string, card: string) -> Result[string, string]: percent := discount(coupon)? # ← on .Err, return it to the caller receipt := charge(card, cents, percent)? .Ok(receipt)pub fn main() -> void $ Console: for coupon in ["SPRING10", "FREESTUFF"]: match checkout(5000, coupon, "4242"): .Ok(receipt) => println(receipt) .Err(reason) => println("checkout failed: $reason")# Delete the `?` after `discount(coupon)` and Run:# type-mismatch: in argument: expected i32, found Result[i32, string]# ── plumbing ──fn discount(coupon: string) -> Result[i32, string]: if coupon == "SPRING10": .Ok(10) else: .Err("unknown coupon $coupon")fn charge(card: string, cents: i32, percent: i32) -> Result[string, string]: due := cents * (100 - percent) / 100 if card == "0000": .Err("card declined") else: .Ok("charged $due cents to card $card")

Errors are values in the return type, and ? passes them up

Errors are values in the return type, and `?` passes them up