typetag stuff.scala

import scalaz.{Tag, @@}

trait EmailAddress
implicit def EmailAddress(str: String): String @@ EmailAddress = {
  require(str.matches("[^@]+@.*"), "String is not an email") 
  //                   ^ Over simplified email regex, I know. 
  //                   Also it should be defined elsewhere so it's not compiled everytime. I know. This is just an example :)
  
  Tag[String,EmailAddress](str)
}
case class SomeClass(email: String @@ EmailAddress)

val foo = SomeClass("[email protected]")

foo.email + " is a string, so we treat it like a string."