Tagging example for Folex

01 tag package.scala

import scala.language.higherKinds

package object tag {
  private def cast[T, U](v: U): T = v.asInstanceOf[T]

  type Tag[T, +U] = {type Raw = T ; type Gag <: U }
  type @@[T, +U] = T with Tag[T, U]
  type Tagged[T, +U] = T with Tag[T, U]

  def tag[T, U](value:T):T @@ U = cast(value)

  trait TaggedType[T] {

    sealed trait Tag

    type Raw = T
    type Type = T @@ Tag

    def apply(raw: T): T @@ Tag = cast(raw)
  }

  //For adding tag at toplevel
  implicit class TopLevelTagger[T](val value: T) {
    def @@(typ: TaggedType[T]): T @@ typ.Tag = cast(value)
  }

  //For changing tag
  implicit class TopLevelChangeTagger[T,U](val value: Tag[T,U]) {
    def @@(typ: TaggedType[T]): T @@ typ.Tag = cast(value)
  }

  /**
  * Using `@@@` instead of `@@` because of Intellij Idea losts autocomplete at toplevel tagging
   * and marks with `red` (compiles, but annoying and not usable)
*/

  //For adding tag at `type bound` level
  implicit class TypeBoundTagger[F[_], T](val value: F[T]) {
    def @@@(typ: TaggedType[T]): F[T @@ typ.Tag] = cast(value)
  }

  //For changing tag at `type bound` level
  implicit class TypeBoundChangeTagger[F[_], T, U](val value: F[T @@ U]) {
    def @@@(typ: TaggedType[T]): F[T @@ typ.Tag] = cast(value)
  }
}

02 Folex.scala

import cats.data.Ior
import tag._

object UserRelations {
  
  object IsFollower extends TaggedType[Boolean]
  type IsFollower = IsFollower.Type

  object IsFollowing extends TaggedType[Boolean]
  type IsFollowing = IsFollowing.Type

  type Rel = Ior[IsFollowing, IsFollower]

  implicit class OptionTagOps[U, I](opt: Option[U with Tag[U,I]]) {
    def getOrElseT(default: U): U @@ I = {

      opt.getOrElse(tag[U, I](default))
    }
  }

  val example1 = Ior.left(IsFollower(true)).left.getOrElseT(false) //works fine

  val casted1: IsFollower = example1 //works fine

  val example2: IsFollower = Ior.left(IsFollower(true)).left.getOrElseT(false) //now works fine
  
}