package net

import shapeless.{::, HList, HNil}

trait SizeOf[A] {
  def value: Int
}
object SizeOf {

  def apply[A](implicit ev: SizeOf[A]): SizeOf[A] = ev

  implicit def hlistSizeOf[H, L <: HList](implicit ev: SizeOf[L]) =
    new SizeOf[H :: L] {
      def value: Int = 1 + ev.value
  }

  implicit val hnilSizeOf = new SizeOf[HNil] {
    def value: Int = 0
  }

}

import net.SizeOf
import shapeless.{$colon$colon, HList, HNil}

scala> SizeOf[String :: HNil]
res0: net.SizeOf[shapeless.::[String,shapeless.HNil]] = net.SizeOf$$anon$1@439fd7ab

scala> res0.value
res1: Int = 1

scala> SizeOf[String :: Int :: Int :: HNil]
res2: net.SizeOf[shapeless.::[String,shapeless.::[Int,shapeless.::[Int,shapeless.HNil]]]] = net.SizeOf$$anon$1@7f9d9cf8

scala> res2.value
res4: Int = 3