cid4.scala

class SetCorrelationIdBackend(delegate: SttpBackend[Task, Nothing]) 
  extends SttpBackend[Task, Nothing] {
    
  override def send[T](request: sttp.Request[T, Nothing]): Task[Response[T]] = {
    // suspending the calculation of the correlation id until the request 
    // send is evaluated
    Task {
      CorrelationId() match {
        case Some(cid) => request.header(CorrelationIdHeader, cid)
        case None => request
      }
    }.flatMap(delegate.send)
  }

  override def close(): Unit = delegate.close()

  override def responseMonad: MonadError[Task] = delegate.responseMonad
}