type num = int|float;

final readonly class Collection<T> {
    public function __construct(
        private vec<T> $items = vec[],
    ) {}

    public function filter(Closure<(T), bool> $func): Collection<T> {
        $result = vec[];
        foreach $this->items as $item {
            if $func($item) {
                $result[] = $item;
            }
        }

        return new Collection::<T>($result);
    }

    public function map<Tout>(Closure<(T), Tout> $func): Collection<Tout> {
        $result = vec[];
        foreach $this->items as $item {
            $result[] = $func($item);
        }

        return new Collection::<Tout>($result);
    }

    public function sum(): int where T is num {
        $result = 0;
        foreach $this->items as $item {
            $result += $item;
        }

        $result
    }
}

function example(): int {
    $collection = new Collection::<num>(vec[
        1, 2, 3.0, 4.0, 5, 6, 7.0, 9.0, 10
    ]);

    $collection
      ->filter(fn(num $n): bool => $n < 8)
      ->map::<float>(
        fn(num $n): float => $n is float ? $n : $n + 0.0
      )
      ->sum()
}