HList experiment

HList.purs

module Data.HList 
  ( (.:.)
  , head
  , tail
  ) where

newtype HList a = HList a

infixr 5 .:.

(.:.) :: forall h t. h -> HList { | t } -> HList { head :: h, tail :: HList { | t } }
(.:.) h t = HList { head: h, tail: t }

head :: forall h t. HList { head :: h, tail :: HList { | t } } -> h
head (HList r) = r.head

tail :: forall h t. HList { head :: h, tail :: HList { | t } } -> HList { | t }
tail (HList r) = r.tail

type HNil = HList {}

foreign import hnil "var hnil = null;" :: HNil

instance showHList :: Show (HList h) where
  show a = unsafeShowHList a

foreign import unsafeShowHList
  """
    function unsafeShowHList(hlist) {
      var i = "";

      while (hlist) {
        i = i + JSON.stringify(hlist.head) + " .:. ";
        hlist = hlist.tail;
      }

      return i + "hnil";
    }
  """ :: forall a. HList a -> String