versions.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE StandaloneDeriving #-}
{-# LANGUAGE TypeFamilies #-}

import Data.Functor.Const
import Data.Functor.Identity


data Nat = Z | S Nat

type V0 = 'Z
type V1 = 'S V0
type V2 = 'S V1
type V3 = 'S V2


type family Since (c :: Nat) (v :: Nat) :: * -> * where
    Since 'Z v = Identity
    Since c 'Z = Const ()
    Since ('S c) ('S v) = Since c v


data Person (v :: Nat) =
    Person {
      personName     :: Since V0 v String,
      personAge      :: Since V1 v Integer,
      personLocation :: Since V2 v String
    }

deriving instance Show (Person V0)
deriving instance Show (Person V1)
deriving instance Show (Person V2)