Pure F# で HList を実装しようとしたが…型推論に阻まれる

gistfile1.fs

[<AbstractClass>]
type HList<'A when 'A :> HList<'A>>() =
  abstract member Extend<'E> : 'E -> HCons<'E, 'A>
  abstract member Extender<'E> : unit -> Apply<unit, 'E * 'A, HCons<'E, 'A>>

and HList private () =
  static member Cons<'E, 'L when 'L :> HList<'L>>(e, l) = HCons<'E, 'L>(e, l)

and HCons<'E, 'L when 'L :> HList<'L>>(e: 'E, l: 'L) =
  inherit HList<HCons<'E, 'L>>()
  override __.Extender<'X>() = Apply.Cons<'X, HCons<'E, 'L>>()
  override this.Extend<'X>(e: 'X) = HList.Cons<'X, HCons<'E, 'L>>(e, this)
  member __.Head = e
  member __.tail = l

and [<AbstractClass>] Apply<'F, 'A, 'R>() =
  abstract member Apply: 'F * 'A -> 'R

and Apply private () =
  static member F() = { new Apply<'X -> 'Y, 'X, 'Y>() with
    member __.Apply(f, x) = f x }
  static member Id() = { new Apply<unit, _, _>() with
    member __.Apply((), x) = x }
  static member Cons<'E, 'L when 'L :> HList<'L>>() =
    { new Apply<unit, 'E * 'L, HCons<'E, 'L>>() with
      member __.Apply((), (p1, p2)) = HList.Cons<'E, 'L>(p1, p2) }

type HNil private () =
  inherit HList<HNil>()
  override this.Extend<'E>(e: 'E) = HList.Cons<'E, HNil>(e, this)
  override __.Extender<'E>() = Apply.Cons<'E, HNil>()