a.dart

extension ObjectExt<T> on T {
  R let<R>(R Function(T) x) => x(this);
}

class ProgramCardDataFamily {
  const ProgramCardDataFamily();

  ProgramCardDataProvider call( int data) {
    return ProgramCardDataProvider( data);
  }
}

abstract class Provider<T> {
  T read();
}

class ProgramCardDataProvider implements Provider<int> {
  ProgramCardDataProvider( this.data);

  final int data;
  @override
  int read() {
    return data;
  }
}

T watch<T>(Provider<T> provider) {
  return provider.read();
}

void main() async {
  int? data = 3;
  print('a');
  data.let(ProgramCardDataFamily()).let(watch);
  print('b');
}