HList type sizeOf

gistfile1.scala

import shapeless._

trait Size[A] {
  def apply(): Int
}

implicit val shortSize = new Size[Short] {
  def apply() = 2
}

implicit val intSize = new Size[Int] {
  def apply() = 4
}

implicit val doubleSize = new Size[Double] {
  def apply() = 8
}

def sizeOf[A](implicit size: Size[A]) = size()

sizeOf[Int] == 4

trait HListTypeSize[L <: HList] {
  def apply(): List[Int]
}

implicit object HNilTypeSize extends HListTypeSize[HNil] {
  def apply() = Nil
}

implicit def hlistTypeSize[H, T <: HList](implicit size: Size[H], hlistTypeSize: HListTypeSize[T]) = 
  new HListTypeSize[H :: T] { 
    def apply() = size() :: hlistTypeSize() 
  }

def hsizeOf[L <: HList](implicit hlistTypeSize: HListTypeSize[L]): List[Int] = hlistTypeSize()

hsizeOf[Int :: Double :: Short :: Int :: HNil] == List(4, 8, 2, 4)

hsizeOf[Int :: String :: Short :: Int :: HNil] // doesn't compile. there's no implicit Size[String]