write-you-a-haskell/chapter10/generics.hs

{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE DefaultSignatures #-}

import GHC.Generics

-- Auxiliary class
class GEq' f where
  geq' :: f a -> f a -> Bool

instance GEq' U1 where
  geq' _ _ = True

instance (GEq c) => GEq' (K1 i c) where
  geq' (K1 a) (K1 b) = geq a b

instance (GEq' a) => GEq' (M1 i c a) where
  geq' (M1 a) (M1 b) = geq' a b

instance (GEq' a, GEq' b) => GEq' (a :+: b) where
  geq' (L1 a) (L1 b) = geq' a b
  geq' (R1 a) (R1 b) = geq' a b
  geq' _      _      = False

instance (GEq' a, GEq' b) => GEq' (a :*: b) where
  geq' (a1 :*: b1) (a2 :*: b2) = geq' a1 a2 && geq' b1 b2

--
class GEq a where
  geq :: a -> a -> Bool
  default geq :: (Generic a, GEq' (Rep a)) => a -> a -> Bool
  geq x y = geq' (from x) (from y)

-- Base equalities
instance GEq Char where geq = (==)
instance GEq Int where geq = (==)
instance GEq Float where geq = (==)

-- Equalities derived from structure of (:+:) and (:*:)
instance GEq a => GEq (Maybe a)
instance (GEq a, GEq b) => GEq (a,b)

main :: IO ()
main = do
  print $ geq 2 (3 :: Int)
  print $ geq 'a' 'b'
  print $ geq (Just 'a') (Just 'a')
  print $ geq ('a','b') ('a', 'b')