Reference · Records and errors

err

err(any=) -> error(any)

What it does

An error value carrying a payload: text, a number, a map, anything. An error value is a value, not a stop: it can be stored, printed, returned, compared, checked with is_err, read with .error, replaced with ?? or passed on from a function with ?. Builtins such as int("x") and json.parse return error values made this way. Without an argument the payload is null.

Examples

let e = err("no such user")print(e, type_of(e), e.error)fn find(id: int) -> map ! str = if id == 1 { { id: 1, name: "Ann" } } else { err(`no user ${id}`) }print(find(1), find(2))print(find(2) ?? { name: "anon" })

err("no such user") error no such user{ id: 1, name: "Ann" } err("no user 2"){ name: "anon" }

Edge cases

• The payload can be a map, so an error can carry a code and a message.
• err() is err(null).
• Two errors are equal when their payloads are.
• ?? sees an error value as it sees null; 0, false and "" are not replaced.
• An error wrapped in err again is an error whose payload is an error.

let e = err({ code: 404, msg: "gone" })print(e.error.code, e.error.msg)print(err(), err("x") == err("x"), err("x") == err("y"))print(err("x") ?? "fallback", 0 ?? "fallback")print(err(err("inner")).error)

404 goneerr(null) true falsefallback 0err("inner")

Errors

err itself never fails. Using an error value as if it were its payload is a runtime error: adding to one, indexing it, calling it. A function whose signature declares ! str and returns an error with another payload is a runtime error too, and quillo check reports it before the program runs.

print(err("x") + 1)

line 1: cannot add error and int

Chaining

With ? the error of a call ends the function with that error; with ?? it becomes a default. match can take an error apart:

fn parse_age(s: str) -> int ! str = match int(s) { int as n if n >= 0 => n, _ => err(`bad age: ${s}`) }fn next_year(s: str) -> int ! str = parse_age(s)? + 1print(next_year("41"), next_year("x"))print(match int("x") { error as e => `failed: ${e.error}`, _ => "ok" })

42 err("bad age: x")failed: not an int: "x"

See also

is_err, unwrap, int, json.parse

Edit this page on GitHub · every example's output was made by running it (tools/refcheck.mjs) · questions on Discord or r/quillo