An alternative approach to index-access in HLists

index.scala

// Context <https://gist.github.com/2346456>

val hlist = 1 :: "foo" :: 3.0f :: HNil
val nones = HStream.const(None)

val wrapSome = new (Id ~> Some) { def apply[T](t: T) = Some(t) }
val somelist = hlist.transform(wrapSome)

val stream = somelist +: nones

// `x` is the name of the value in `Some`

scala> stream(_0).x
res2: Int = 1

scala> stream(_1).x
res3: java.lang.String = foo

// this doesn't work as expected, ...

scala> stream(_2)
<console>:21: error: cyclic aliasing or subtyping involving type Unapplied

// ... but this does

scala> stream(_2).x
res4: Float = 3.0

scala> stream(_3).x
<console>:22: error: type mismatch;
 found   : scalaz.typelevel.Succ[scalaz.typelevel.Succ[scalaz.typelevel.Succ[scalaz.typelevel.Zero.type]]]#scala.Unapplied[Some[Int],[N(in type λ) <: scalaz.typelevel.Nat]N(in type λ)#Unapplied[Some[java.lang.String],[(some other)N(in type λ) <: scalaz.typelevel.Nat](some other)N(in type λ)#Unapplied[Some[Float],[B]None.type]]]
 required: ?{val x: ?}
Note that implicit conversions are not applicable because they are ambiguous:
 both method any2Ensuring in object Predef of type [A](x: A)Ensuring[A]
 and method any2ArrowAssoc in object Predef of type [A](x: A)ArrowAssoc[A]
 are possible conversion functions from scalaz.typelevel.Succ[scalaz.typelevel.Succ[scalaz.typelevel.Succ[scalaz.typelevel.Zero.type]]]#Unapplied[Some[Int],[N <: scalaz.typelevel.Nat]...              res0(_3).x
                  ^