main.dart

void main() {
  var d = StubDoer(() {
    print("Also doing...");
  });

  d = wrapWithTimes(100)(d);
  d = wrapWithMetrics()(d);
  d = wrapWithTracing()(d);
  
  doStuff(d);
}

typedef WrapFunc<T> = T Function(T);

abstract class Doer {
  void Do();
}

typedef DoFn = Function();

class StubDoer extends Doer {
  DoFn doFn;

  StubDoer(this.doFn);

  @override
  void Do() {
    doFn();
  }
}

WrapFunc<Doer> wrapWithTimes(int n) {
  return (Doer d) {
    return StubDoer(() {
      for (int i = 0; i < n; i++) {
        d.Do(); 
      }
    });
  };
}

WrapFunc<Doer> wrapWithMetrics() {
  return (Doer d) {
    return StubDoer(() {
      var startTime = DateTime.now();      
      d.Do();
      var durMs = DateTime.now().difference(startTime).inMicroseconds;
      print('Took $durMs us');
    });
  };
}

WrapFunc<Doer> wrapWithTracing() {
  return (Doer d) {
    return StubDoer(() {
      d.Do();
    });
  };
}

void doStuff(Doer d) {
  d.Do();
}