main.dart

extension Nullable<T extends Object> on T? {
  T unwrap([String message = '']) {
    if (this == null){
      throw Exception('Null $T: $message');
    } else {
      return this!;
    }
  }
  
  T orDefault(T defaultValue) {
    return this ?? defaultValue;
  }  
  
  
  R when<R extends Object>(R Function(T) nonNull,{required  R Function() orElse}){
    if (this == null){
      return orElse();
    } else {
      return nonNull(this!);
    }
  }
}

void main() {

  final String? hi = 'Hi';
  final String? none = null;
  
  Greet(hi);
  Greet(none);
  final String name = none.orDefault('No name');
  print(name);
  none.unwrap('No name');
}

void Greet(String? greeting){
  print(greeting.when((s) => s + ' You!', orElse: () => 'Goodbye'));
}