door.hs

{-# LANGUAGE GADTs, DataKinds, TypeFamilies, FlexibleInstances #-}

-- from my stackoverflow question: https://stackoverflow.com/questions/40939508/translate-a-scala-type-example-to-haskell

data Status = Open | Closed deriving(Show)

data Door (status :: Status) = Door
  --OpenDoor :: Door Open
  --ClosedDoor :: Door Closed

instance Show (Door Open) where
  show _ = "Door Open"

instance Show (Door Closed) where
  show _ = "Door Closed"

open :: Door Closed -> Door Open
open _ = Door

close :: Door Open -> Door Closed
close _ = Door

type family Toggle (s :: Status) where
  Toggle Open = Closed
  Toggle Closed = Open

toggle :: Door s -> Door (Toggle s)
toggle _ = Door
--toggle OpenDoor = ClosedDoor
--toggle ClosedDoor = OpenDoor

main :: IO ()
main = print openedAgainDoor
  where
    closedDoor = Door :: Door Closed
    openDoor = open closedDoor
    closedAgainDoor = close openDoor
    openedAgainDoor = toggle closedAgainDoor
    -- closeClosedDoor = close closedDoor
    -- openOpenedDoor = open openDoor