KList updates

- exchange variance annotations on KList for a bounded existential
   in KList.toList
 - add foldr (reason for dropping variance annotations)
 - add functions stating equivalence between
   + KList[M,H :+: T] and KCons[H,T,M]
   + KList[M,HNil] and KNil

util/collection/KList.scala

@@ -9,7 +9,7 @@ import Types._
 * 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]
+sealed trait KList[+M[_], HL <: HList]
 {
 	type Raw = HL
 	/** Transform to the underlying HList type.*/
@@ -20,6 +20,13 @@ sealed trait KList[+M[_], +HL <: HList]
 	def toList: List[M[_]]
 	/** Convert to an HList. */
 	def combine[N[X] >: M[X]]: HL#Wrap[N]
+
+	def foldr[P[_ <: HList],N[X] >: M[X]](f: KFold[N,P]): P[HL]
+}
+trait KFold[M[_],P[_ <: HList]]
+{
+	def kcons[H,T <: HList](h: M[H], acc: P[T]): P[H :+: T]
+	def knil: P[HNil]
 }
 
 final case class KCons[H, T <: HList, +M[_]](head: M[H], tail: KList[M,T]) extends KList[M, H :+: T]
@@ -33,6 +40,8 @@ final case class KCons[H, T <: HList, +M[_]](head: M[H], tail: KList[M,T]) exten
 	def combine[N[X] >: M[X]]: (H :+: T)#Wrap[N] = HCons(head, tail.combine)
 
 	override def toString = head + " :^: " + tail.toString
+
+	def foldr[P[_ <: HList],N[X] >: M[X]](f: KFold[N,P]) = f.kcons(head, tail foldr f)
 }
 
 sealed class KNil extends KList[Nothing, HNil]
@@ -42,6 +51,7 @@ sealed class KNil extends KList[Nothing, HNil]
 	def :^: [M[_], H](h: M[H]) = KCons(h, this)
 	def toList = Nil
 	def combine[N[X]] = HNil
+	override def foldr[P[_ <: HList],N[_]](f: KFold[N,P]) = f.knil
 	override def toString = "KNil"
 }
 object KNil extends KNil
@@ -51,5 +61,12 @@ 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))
+	def fromList[M[_]](s: Seq[M[_]]): KList[M, _ <: HList] = if(s.isEmpty) KNil else KCons(s.head, fromList(s.tail))
+
+	// haven't found a way to convince scalac that KList[M, H :+: T] implies KCons[H,T,M]
+	// Therefore, this method exists to put the cast in one location.
+	implicit def kcons[H, T <: HList, M[_]](kl: KList[M, H :+: T]): KCons[H,T,M] =
+		kl.asInstanceOf[KCons[H,T,M]]
+	// haven't need this, but for symmetry with kcons:
+	implicit def knil[M[_]](kl: KList[M, HNil]): KNil = KNil
 }