LYR-PAR0033

a malformed pattern

error · spec 09-patterns

Where a pattern stands, the token begins none; a - in a pattern is not followed by a number; a range pattern's bound is no literal; or an array pattern holds two rests ...

Explanation

A pattern is a literal or a range of literals, a name that binds, _, a variant, a tuple, an array, or a type test. Only a number is negated in a pattern, and the bounds of a range are literals, so no pattern runs code. An array pattern takes at most one ..: with two, the positions between them could not be placed.

Write a pattern of one of these forms.

Wrong

fn main(): void {
    let xs = [1, 2, 3];
    let n = match (xs) {
        [a, .., ..] => a,
        _ => 0,
    };
}

Right

fn main(): void {
    let xs = [1, 2, 3];
    let n = match (xs) {
        [a, ..] => a,
        _ => 0,
    };
    let _ = n;
}