Actor A sends a message to Actor B but it is intercepted by D. This can be generalized to have decorators that can catch messages.

Decorator.pony

actor A
  let _b: B
  let _d: D
  let e: Env

  new create(env: Env)=>
    _b = B(env)
    _d = D(env)
    e = env

  be messageB()=>
    e.out.print("Calling B")
    let b = _b
    let f: {()} val = recover lambda()(b)=>  b.foo() end end
    _d(consume f)

actor B
  let env': Env

  new create(env: Env)=>
    env' = env
  be foo()=>
    env'.out.print("In B")

actor D
  let e: Env
  new create( env:Env)=>
    e = env

  be apply(f: {()} val)=>
    e.out.print("In D")
    f()

actor Main
  new create(env: Env) =>
    let a = A(env)
    a.messageB()