[SOLVED] Trait method in type class returns the trait itself question

Question
1

Suppose I have a trait called MyTrait

trait MyTrait[T] {
    def method[A](value: A): MyTrait[T]
}

In the companion object I want to implement method, but I do not know how to return the trait itself because if the code new MyTrait[TypeA] { ... } then it will become an infinite cycle.

object MyTrait {
    val instance = new MyTrait[TypeA] {
        override def method[A](value: A) = ???
    }
}

What I can think of is to return this in the end. But that’s seemingly not functional because it shares the instance.

So my question. Is there a way to achieve this (i.e. method return a new MyTrait instance)? Or generally it’s not doing so in type class? Any suggestions?

Thanks

2

What should method return in your case? Itself? Another instance, if so which one?

3

The trait MyTrait’s method will return MyTrait as specified in the trait

trait MyTrait[T] {
    def method[A](value: A): MyTrait[T]
}
4

I am asking in your implementation, what should it do.

5

Maybe a concrete example would be easier to explain the effect I want to achieve. Suppose I have a Setting trait.

trait Setting[T] {

    def set[A](key: String, value: A): Setting[T]

}

The implementation I would try is (but I do not know how to achieve the effect that it can return a new instance of Setting type without repeating the code)

object Setting {
    implicit val setting = new Setting[typesafe.Config] {
        override def set[A](key: String, default: A): Setting[Config] = new Setting[typesafe.Config] {
            override def set[A](key: String, default: A): Setting[Config] = new Setting[typesafe.Config] { 
                 .... // this becomes infinite new Setting and override set method 
            }
        }
    }
}
6

Why not just having a class?

sealed trait Setting[T] {
  def set[A](key: String, value: A): Setting[T]
}

object Setting {
  private final class SettingImpl extends Setting[typesafe.Config] {
    override def set[A](key: String, default: A): Setting[typesafe.Config] =
      new SettingImpl() // I guess some logic here.
  }

  implicit final val setting: Setting[typesafe.Config] = new SettingImpl()
}
1 Like
7

That passed on my head but I didn’t pick it up. Thanks for the advice. It does help and simpler!