LYR-SEM0030

a thrown, declared or caught type that is not an Error

error · spec 06-errors

A throw throws a value, a throws clause lists a type, or a catch names a type that does not conform to Error.

Explanation

What is thrown is an Error: a struct, class or enum that conforms to Error (with its fn message(): string), an interface value of one, or a type parameter constrained to it. A number or a string is not thrown.

Declare an error type with :: [Error] and throw, declare and catch that.

Wrong

fn f(): int throws {
    throw 5;
}

fn main(): void {
    let _ = try? f();
}

Right

enum Oops :: [Error] { Bad; fn message(): string { return "oops"; } }

fn f(): int throws Oops {
    throw Oops.Bad;
}

fn main(): void {
    let _ = try? f();
}