$i < 2 ? null : $i + 5 * ) * => Dict( * 7 => [2], 8 => [3], 9 => [4], 10 => [5], 11 => [6], 12 => [7], 13 => [8], 14 => [9], 15 => [10] * ) * * Dict\group_by( * [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10], * fn($i) => $i < 2 ? null : ($i >= 7 ? 12 : $i +5) * ) * => Dict(7 => [2], 8 => [3], 9 => [4], 10 => [5], 11 => [6], 12 => [7, 8, 9, 10]) * * @template Tk of array-key * @template Tv * * @param iterable $values * @param (Closure(Tv): ?Tk) $key_func * * @return array> */ function group_by(iterable $values, Closure $key_func): array { $result = []; foreach ($values as $value) { $key = $key_func($value); if (null === $key) { continue; } Psl\invariant( Type\array_key()->matches($key), 'Expected $key_func to return a value of type array-key, value of type (%s) returned.', gettype($key), ); /** @var Tk $key */ $result[$key] = $result[$key] ?? []; $result[$key][] = $value; } return $result; }