Blogpost about value classes.

distance.scala

class Meter(val n: Int) extends AnyVal
  
def traveled(m: Meter) = 
  s"You traveled ${m.n} meters!"
  
val meters = new Meter(34)
traveled(meters) should equal ("You traveled 20 meters!")

distance_method.scala

class Meter(val n: Int) extends AnyVal { 
  def traveled = s"You traveled ${n} meters!"
}
  
val meters = new Meter(34)
meters.traveled should equal ("You traveled 20 meters!")

distance_method_javap.scala

// [meters.traveled] was compiled into:
22: invokevirtual	#37; //Method Meter$.traveled$extension:(I)Ljava/lang/String;
// so it's calling a method on the companion object, which takes an [int]
// still no Meter in sight here...

// :javap -v Meter
// does Meter even exist, if I'd want to use it explicitly?
public java.lang.String traveled();
  7:  invokevirtual #26; //Method Meter$.traveled$extension:(I)Ljava/lang/String;
// yes it exists, and is delegating all the work to the companion object
   
// :javap -v Meter$ // the [companion object]!
public final java.lang.String traveled$extension(int);
// which contains the actual implementation of traveled
// notice the $extension suffix marking that it's a generated method.

implicit_class.scala

implicit class AwesomeString(val s: String) {
  def awesome_! = s + "! Awesome!"
}
// an [implicit final def AwesomeString(String): AwesomeString] is generated for us automatically

"Scala".awesome_!  

// 3: ldc #21; //String Scala                                       vvvvvvvvvvvvvvvvvvvvvvvvvv
// 5: invokevirtual #25; //Method AwesomeString:(Ljava/lang/String;)LTesting$AwesomeString; << returns boxed!
// 8: invokevirtual #30; //Method AwesomeString.awesome_$bang:()Ljava/lang/String;
  
// Called: AwesomeString.awesome_$bang:()Ljava/lang/String;
// This impl. takes the `s` from the AwesomeString's instance!

implicit_value_class.scala

implicit class AwesomeString2(val s: String) extends AnyVal {
  def awesome_! = s + "! Awesome!"
}

"Scala".awesome_! 

// 18: ldc #35; //String Scala                                        vvvvvvvvvvvvvvvvvvvvv
// 20: invokevirtual #39; //Method AwesomeString2:(Ljava/lang/String;)Ljava/lang/String; << returns not boxed!
// 23: invokevirtual #42; //Method AwesomeString2$.awesome_$bang$extension:(Ljava/lang/String;)Ljava/lang/String;
                    
//                                       vvvvvvvvvvvvvvvvvvv
// Called: AwesomeString2.awesome_$bang:(Ljava/lang/String;)Ljava/lang/String;
// as you can see the method gets the string passed in right away

implicits_old.scala

trait AwesomeStringImplicits {
  
  // boilerplate...
  implicit def asAwesomeString(s: String) =
    new AwesomeString(s)
    
  class AwesomeString(s: String) {
    def awesome_! = s + "! Awesome!"
  }
}

object Main extends App 
  with AwesomeStringImplicits {
  
  "Scala".awesome_!
}

javap_distance.scala

// def traveled(m: Meter) was compiled as:
public java.lang.String traveled(int); // where's my Meter?!

// val meters = new Meter(34)
0:  bipush  34 // ok that's the int... 
2:  istore_1  // that's all, no Meter in sight!