Type level programming in Scala: Heterogeneous list

HList.scala

package com.dragisak.hlist

object HList {

  sealed trait HList {
    type prepend[A] <: HList
    def ::[A](a: A) :prepend[A]
  }

  case class HCons[H, Tail <: HList](head: H, tail: Tail) extends HList  {
    override type prepend[A] = HCons[A, HCons[H, Tail]]
    override def ::[A](a: A): prepend[A] = HCons(a, this)
  }

  case object HNil extends HList {
    override type prepend[A] = HCons[A, HNil.type ]
    override def ::[A](a: A): prepend[A] = HCons(a, this)
  }
}

Example:

import com.dragisak.hlist.HList._

val x = 2 :: false :: 2 :: Some(4) :: "foo" :: HNil


val a = x.head // Int
val b = x.tail.head // Boolean

Clear and neat!

Nice! Thank you!