For building newtypes over types that have an invariant type parameter (higher-kinded types).

Example:

// Only needed for type-class derivation
import cats._
import cats.implicits._

type Nel[A] = Nel.Type[A]

object Nel extends NewtypeK[List] {
  def apply[A](head: A, tail: A*): Nel[A] =
    unsafeCoerce(head :: tail.toList)

  def unapply[F[_], A](list: F[A])(
    implicit ev: F[A] =:= Nel[A]
  ): Some[(A, List[A])] = {
    val l = value(list)
    Some((l.head, l.tail))
  }

  implicit def eq[A: Eq]: Eq[Nel[A]] =
    derive
  implicit val traverse: Traverse[Nel] =
    deriveK
  implicit val monad: Monad[Nel] =
    deriveK
}

NOTE: the type-parameter is invariant. See NewtypeCovariantK for working with covariant type parameters.