akkadotnet/akka.net

enum DelegationType { Handled, Inner }
struct Delegation<T> {
  public readonly DelegationType Type;
  public readonly T Value;
  public Delegation(DelegationType type, T value) {
    Type = type;
    Value = value;
  }
}
using Interceptor<T1, T2> = Func<T1, Delegation<T2>>;
static class Delegation {
  static Delegation<T> Inner<T>(T value) => new Delegation<T>(DelegationType.Inner, value);
  static Delegation<T> Handled<T>(T value) => new Delegation<T>(DelegationType.Handled, value);
  // extension methods
  static Interceptor<T1, T3> Then<T1, T2, T3>(this Interceptor<T1, T2> first, Interceptor<T2, T3> second) =>
    (input) => {
      var delegation = first(input);
      return delegation.Type == Handled ? delegation : second(delegation.Value);
    }    
}

// example 
class SomeActor : UntypedActor {
  Interceptor<int, int> _increment = i => Delegation.Inner(i + 1);
  Interceptor<int, int> _double = i => Delegation.Inner(i * 2);
  Interceptor<int, int> _pipeline = _increment.Then(_double);
  void OnReceive(object msg) => Console.WriteLine(_pipeline((int)msg).Value);
}