sbt/util/collection/KList.scala

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

import Types._

/** A higher-order heterogeneous list.  It has a type constructor M[_] and
* type parameters HL.  The underlying data is M applied to each type parameter.
* Explicitly tracking M[_] allows performing natural transformations or ensuring
* all data conforms to some common type. */
sealed trait KList[+M[_], +HL <: HList]
{
	type Raw = HL
	/** Transform to the underlying HList type.*/
	def down(implicit ev: M ~> Id): HL
	/** Apply a natural transformation. */
	def transform[N[_]](f: M ~> N): KList[N, HL]
	/** Convert to a List. */
	def toList: List[M[_]]
	/** Convert to an HList. */
	def combine[N[X] >: M[X]]: HL#Wrap[N]
}

final case class KCons[H, T <: HList, +M[_]](head: M[H], tail: KList[M,T]) extends KList[M, H :+: T]
{
	def down(implicit f: M ~> Id) = HCons(f(head), tail down f)
	def transform[N[_]](f: M ~> N) = KCons( f(head), tail transform f )
	// prepend
	def :^: [N[X] >: M[X], G](g: N[G]) = KCons(g, this)
	def toList = head :: tail.toList
	
	def combine[N[X] >: M[X]]: (H :+: T)#Wrap[N] = HCons(head, tail.combine)

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

sealed class KNil extends KList[Nothing, HNil]
{
	def down(implicit f: Nothing ~> Id) = HNil
	def transform[N[_]](f: Nothing ~> N) = KNil
	def :^: [M[_], H](h: M[H]) = KCons(h, this)
	def toList = Nil
	def combine[N[X]] = HNil
	override def toString = "KNil"
}
object KNil extends KNil

object KList
{
	// nicer alias for pattern matching
	val :^: = KCons
	
	def fromList[M[_]](s: Seq[M[_]]): KList[M, HList] = if(s.isEmpty) KNil else KCons(s.head, fromList(s.tail))
}