enum SubscriptionType {
    free,
    monthly,
    annal,
  }

  extension SubscriptionTypeExtension on SubscriptionType {
    bool get isFree => this == SubscriptionType.free;
    bool get isMonthly => this == SubscriptionType.monthly;
    bool get isAnnual => this == SubscriptionType.annual;
  }

  final subscriptionType = SubscriptionType.monthly;

  void main() {
    print(subscriptionType.isMonthly); // true
    print(subscriptionType.isAnnual); // false
  }