dart_iterable_extension.dart

void main() {
  final l = List.generate(5, (index) => index * 2);
  print(l);
  print(l.insertBetween(100));
  print(l.insertBetweenAndAround(100));
  print(l.fillUpTo(count: 10, value: 100));
  print(l.fillUpTo(count: 3, value: 100));
}

extension IterableExtension<T> on Iterable<T> {
  Iterable<T> insertBetween(T value) {
    return expand((v) => [v, value]).take(length * 2 - 1);
  }

  Iterable<T> insertBetweenAndAround(T value) {
    return [
      value,
      ...expand((v) => [v, value])
    ];
  }

  Iterable<T> fillUpTo({required int count, required T value}) {
    return [
      ...this,
      if (count > length) ...List.filled(count - length, value),
    ];
  }
}