Can we assign / change traits to the scala class during runtime? How - any sample code? Like Strategy Pattern (of Gang of four Design Pattern)

Question

To explain my question:

Class : Toy

Trait1: Speak like Male

Trait2: Speak like Female

Can I change the behavior (traits) of Toy during runtime so sometimes the same object speaks like male and sometimes the same object speaks like female?

I want to change the speaking behavior at runtime.

Answer 1

Scala really doesn't do that. There's Kevin Wright's autoproxy plugin which can do it, and you can instantiate and object with either trait, without that trait being part of the base class.

I personally think that trying to accomplish things that way is to go against the grain of Scala: hard and prone to getting stuck. It is better to design a solution that doesn't require such things -- in fact, Scala grain tends much more to the functional, which put focus on everything being immutable, and replacing one object with a new one as a result of computation.

Answer 2

sealed trait Speaker
case object Male extends Speaker
case object Female extends Speaker

class Toy(name: String, speaks: Speaker = Male) { 
  def speak = speaks match {
    case Male   => "ugh"
    case Female => "What time do you call this?"
  }
}

Then

barbie = ken.copy(speaks = Female)

You cannot change the traits which an object extends at runtime, because a trait is mixed in to create a class (in a .class file). A given object has exactly one class and this can never be changed.