simple array path traversal

list_path.dart

/// Traverse the list by the given path.
/// ex: listPath<int>([1, 2, [3, 4]], '2.1') => 4
T? listPath<T>(Object list, String path) {
  if (path.isEmpty) return null;
  var result = list;

  for (final part in path.split('.')) {
    if (result is! List) return null;
    final idx = int.tryParse(part);
    if (idx == null || idx < 0 || result.length <= idx) return null;
    result = result[idx] as Object;
  }

  if (result is T) return result as T;
  return null;
}

void main() {
  assert(listPath<int>([], '') == null, 'error');
  assert(listPath<int>([1], '0') == 1, '0 error');
  assert(listPath<int>([1], '1') == null, '1 error');
  assert(listPath<int>([1], '-1') == null, '-1 error');
  assert(listPath<int>([], '1.1') == null, '1.1 error');
  assert(listPath<int>([1, 2, [3, 4]], '0') == 1, '0 error');
  assert(listPath<int>([1, 2, [3, 4]], '2.1') == 4, '2.1 error');
  assert(listPath<int>([1, 2, [3, 4]], '2.2') == null, '2.2 error'); 
  assert(listPath<int>([1, 2, [3, 4]], '2.1.1') == null, '2.1.1 error');
  assert(listPath<int>([], 'a') == null, 'a error');
  assert(listPath<int>([], 'a.b') == null, 'a.a error');
}