type Predicate<T> = Closure<(T), bool>;

function filter<T>(vec<T> $vec, Predicate<T> $predicate): vec<T> {
    $result = vec[];
    foreach $vec as $item {
        if $predicate($item) {
            $result[] = $item;
        }
    }

    return $result;
}

type Mapper<K, V, U> = Closure<(K, V), U>|SomeOtherType<K, V, U>;

function map<K, V, U>(dict<K, V> $dict, Mapper<K, V, U> $mapper): dict<K, U> {
    $result = dict[];
    if $mapper is SomeOtherType<_, _, _> {
        foreach $dict as $key => $value {
            $result[$key] = $mapper->doMap($key, $value);
        }
    } else {
        foreach $dict as $key => $value {
            $result[$key] = $mapper($key, $value);
        }
    }

    return $result;
}