/* sbt -- Simple Build Tool
 * Copyright 2010 Mark Harrah
 */
package sbt

import Types._

/**
 * A minimal heterogeneous list type.  For background, see
 * http://apocalisp.wordpress.com/2010/07/06/type-level-programming-in-scala-part-6a-heterogeneous-listĀ basics/
 */
sealed trait HList {
  type Wrap[M[_]] <: HList
}
sealed trait HNil extends HList {
  type Wrap[M[_]] = HNil
  def :+:[G](g: G): G :+: HNil = HCons(g, this)

  override def toString = "HNil"
}
object HNil extends HNil
final case class HCons[H, T <: HList](head: H, tail: T) extends HList {
  type Wrap[M[_]] = M[H] :+: T#Wrap[M]
  def :+:[G](g: G): G :+: H :+: T = HCons(g, this)

  override def toString = head + " :+: " + tail.toString
}

object HList {
  // contains no type information: not even A
  implicit def fromList[A](list: Traversable[A]): HList = ((HNil: HList) /: list)((hl, v) => HCons(v, hl))
}