LYR-SEM0200

a type parameter that is never used

warning · spec 03-types · lint unused-generic, group unused, warn unless said

A function's type parameter is named nowhere in it — not in a parameter, the result or the body. A type's is not reported: a phantom parameter only the type's name uses is a type's own.

Explanation

A type parameter nothing names cannot be inferred from a call: every caller must write it, and it changes nothing.

Take it out, or name it where the declaration needs it. A name that begins with _ is not reported.

A type's parameter is not looked at: struct Id<T> { raw: int } makes Id<User> and Id<Order> two types though nothing in Id names T.

Wrong

fn first<T, U>(xs: T[]): T {
    return xs[0];
}

fn main(): void {
    let _ = first<int, string>([1, 2]);
}

Right

fn first<T>(xs: T[]): T {
    return xs[0];
}

fn main(): void {
    let _ = first([1, 2]);
}