Type Families

{-# LANGUAGE TypeFamilies #-}
module Lib
(
) where


import Data.Char (ord, chr)


--

type family Rep a
type instance Rep Int = Char
type instance Rep Char = Int

class Convertible1 a where
  convert1 :: a -> Rep a

instance Convertible1 Int where
  convert1 = chr

instance Convertible1 Char where
  convert1 = ord

-- (2) Associated form
class Convertible2 a where
  type Rep2 a
  convert2 :: a -> Rep2 a

instance Convertible2 Int where
  type Rep2 Int = Char
  convert2 = chr

instance Convertible2 Char where
  type Rep2 Char = Int
  convert2 = ord

--

newtype Age = MkAge { unAge :: Int }

type family Inspect x
type instance Inspect Age = Int
type instance Inspect Int = Bool

class Boom a where
  boom :: a -> Inspect a

instance Boom Int where
  boom = (== 0)

instance Boom Age where
  boom = unAge

main = do
  print $ convert1 'c'
  print $ convert1 (42::Int)
  print $ boom (MkAge 42)
  print $ boom (42::Int)