Deriving typeclasses for zio-prelude newtypes outside of the object scope (scala2 only)

newtypes.amm

import $ivy.`dev.zio::zio-prelude:1.0.0-RC15`

import zio.prelude.Newtype

object newtypes {
  def derive[A, STA <: Newtype[A], F[_]](implicit ev: STA <:< Newtype[A], typeClass: F[A]): F[STA#Type] =
    typeClass.asInstanceOf[F[STA#Type]]

  trait Auto {
    implicit def auto[A, STA <: Newtype[A], F[_]](implicit ev: STA <:< Newtype[A], typeClass: F[A]): F[STA#Type] =
      derive
  }

  object auto extends Auto
}

// Usage

object elsewhere {
  object Foo extends Newtype[String]
  type Foo = Foo.Type

  trait Show[A] {
    def show(a: A): String
  }

  implicit val stringShow: Show[String] = new Show[String]{ def show(a: String): String = a }

  // import newtypes.auto._ // auto derive all the things
  implicit val fooShow: Show[Foo] = newtypes.derive // semiauto derivation

  implicitly[Show[Foo]]
}