LYR-SEM0098

a refutable pattern without a path for a miss

error · spec 09-patterns

A let pattern that can fail has no else, the else of a let … else does not leave on every path, a for head or lambda parameter has a refutable pattern, or a let condition stands outside an if/while head.

Explanation

A pattern that can fail needs somewhere to go when it does. let .Num(n) = s else { return 0; }; names it, and the else must leave (return, throw, break, continue, a panic), because the names are not bound when it runs. A loop head or a lambda parameter has no such path, and a let condition belongs directly in if (…) or while (…).

Add an else that leaves, use match, or bind a name and take it apart in the body.

Wrong

enum Shape {
    Num(int),
    Empty,
}

fn number(s: Shape): int {
    let .Num(n) = s;
    return n;
}

fn main(): void {
    let _ = number(Shape.Empty);
}

Right

enum Shape {
    Num(int),
    Empty,
}

fn number(s: Shape): int {
    let .Num(n) = s else { return 0; };
    return n;
}

fn main(): void {
    let _ = number(Shape.Empty);
}