Tagger / Untagger

TagUntag.scala

/*
Based on @milessabin's https://gist.github.com/milessabin/89c9b47a91017973a35f
Note the swap: T for Tag and U for Untagged type
*/
  
type Tag[U, T] = { 
  type Tag = T 
  type Untagged = U
}

type @@[U, T] = U with Tag[U, T]

implicit class Tagger[U](untagged: U) {
  def tagged[T]: U @@ T = untagged.asInstanceOf[U @@ T]
}
implicit class Untagger[U, T](tagged: @@[U,T]) {
  def untag : U = tagged
}

trait New      
trait Persisted

case class Foo(name:String)
val foo = Foo("abc")

val taggedFoo = foo.tagged[New]
// taggedFoo: @@[Foo,New] = Foo(abc)
val untaggedFoo = taggedFoo.untag
// untaggedFoo: this.Untagged = Foo(abc)

val untaggedFooOneLiner = foo.tagged[Persisted].untag
// untaggedFooOneLiner: Foo = Foo(abc)

TagUntagWithoutUntaggedType.scala

/*
  When type Untagged is commented out, it's impossible to untag, although type U is known
*/
type Tag[U, T] = { 
  type Tag = T 
  // type Untagged = U
}

type @@[U, T] = U with Tag[U, T]

implicit class Tagger[U](untagged: U) {
  def tagged[T]: U @@ T = untagged.asInstanceOf[U @@ T]
}
implicit class Untagger[U, T](tagged: @@[U,T]) {
  def untag : U = tagged
}

trait New      
trait Persisted

case class Foo(name:String)
val foo = Foo("abc")

val taggedFoo = foo.tagged[New]
// taggedFoo: @@[Foo,New] = Foo(abc)
val untaggedFoo = taggedFoo.untag
//untaggedFoo: @@[Foo,New] = Foo(abc)

val untaggedFooOneLiner = foo.tagged[Persisted].untag
// untaggedFooOneLiner: Foo = Foo(abc)