LYR-SEM0046

a lambda that can end without its value

error · spec 10-concurrency

A block lambda whose result is not void — a generator lambda with a result, or a lambda an inline function sets in place — has a path that ends its body without returning the value.

Explanation

A lambda that gives a result gives it on every path, like a function. A generator lambda that returns on one path returns on all of them; a lambda an inline function sets in place gives its value as its last expression, since a return there leaves the enclosing function.

Return on every path, or end the body with the value as its tail expression.

Wrong

fn main(): void {
    let g = (early: bool) => {
        yield 1;
        if (early) { return "early"; }
    };
    let _ = g(true);
}

Right

fn main(): void {
    let g = (early: bool) => {
        yield 1;
        if (early) { return "early"; }
        return "late";
    };
    let _ = g(true);
}