Implicit existence based Either

exists.scala

//Either based on the existence of a typeclass.
sealed trait Exists[TC[_], N, Y]{
  def apply(no: N, yes: Y): Either[N, Y]
}

object Exists extends LowPriorityExists{
  def apply[TC[_]] = new{
    def apply[N, Y](n: N, y: Y)(implicit exists: Exists[TC, N, Y]) = exists(n, y)
  }
  
  implicit def yes[TC[_], N, Y](implicit tc: TC[Y]) = new Exists[TC, N, Y]{
    def apply(no: N, yes: Y): Either[N, Y] = Right(yes)
  }
}

trait LowPriorityExists{
  implicit def no[TC[_], N, Y] = new Exists[TC, N, Y]{
    def apply(no: No, yes: Y): Either[N, Y] = Left(no)
  }
}

usecase.scala

Exists[Monoid](that :: Nil, that).fold(
  other :: _,
  implicitly[Monoid[That]].append(_, other)
)