NewtypeK

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.