$iterable * @param (Closure(Tv): Ts) $scalar_func * * @return list */ function unique_by(iterable $iterable, Closure $scalar_func): array { /** @var list $unique */ $unique = []; /** @var list $original_values */ $original_values = []; foreach ($iterable as $v) { $scalar = $scalar_func($v); if (!Iter\contains($unique, $scalar)) { $unique[] = $scalar; $original_values[] = $v; } } return $original_values; }