HList in Elm

HList.elm

module HList exposing (HList, cons, nil first rest)

type HList x xs
  = Nil
  | Cons x xs
  
 
nil = Nil

cons : x1 -> HList x2 xs -> HList x1 (HList x2 xs)
cons head tail =
  Cons head tail
  

first : HList head tail -> Maybe head
first hlist =
  case hlist of
    Nil ->
      Nothing
    Cons head tail ->
      Just head
      
rest : HList head tail -> Maybe tail
rest hlist =
  case hlist of
    Nil ->
      Nothing
    Cons head tail ->
      Just tail