What is the difference between "with" and "new" for context parameters in scala?-CodePudding

Is there a difference if I define a given value for a context parameter using the keyword with or using new?

for example is there a difference between human and cat givens?

 trait Talker:
  def talk:String

given human:Talker with
  def talk:String = "I am human"

given cat:Talker = new:
  def talk:String = "I am cat, meow"

def run(using t:Talker):Unit =
  println(t.talk)

CodePudding user response：

There is a clear difference on compiled class level when using new vs with.

When using with, there is a dedicated class created, we can see it in the de-compiled code. Here is how it looks in Java

public final class Talker$package {
   public static final class human$ implements Talker, Serializable {
      public static final Talker$package.human$ MODULE$ = new Talker$package.human$();

      private Object writeReplace() {
         return new ModuleSerializationProxy(Talker$package.human$.class);
      }

      public String talk() {
         return "I am human";
      }
   }
}

It has a dedicated class human$ that extends Talker.

And on the other hand when using new for cat, we have an anonymous class.

public final class Talker$package {
   public static Talker cat() {
      return Talker$package$.MODULE$.cat();
   }
}

where MODULE$.cat() eventually points to following

Talker var5 = new Talker() {
    public String talk() {
       return "I am cat, meow";
    }
};