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+{-# LANGUAGE TypeOperators, DefaultSignatures, FlexibleInstances, DeriveGeneric #-}
+{-# LANGUAGE FlexibleContexts #-}
+
+module Main where
+
+import GHC.Generics hiding (C, D)
+
+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
+
+-- No instances for P or Rec because geq is only applicable to types of kind *
+
+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 types instances (ad-hoc)
+instance GEq Char where geq = (==)
+instance GEq Int where geq = (==)
+instance GEq Float where geq = (==)
+{-
+-- Generic instances
+instance (GEq a) => GEq (Maybe a)
+instance (GEq a) => GEq [a]
+-}
+
+data C = C0 | C1
+ deriving Generic
+
+data D a = D0 | D1 { d11 :: a, d12 :: (D a) }
+ deriving Generic
+
+data (:**:) a b = a :**: b
+ deriving Generic
+
+-- Example values
+c0 = C0
+c1 = C1
+
+d0 :: D Char
+d0 = D0
+d1 = D1 'p' D0
+
+p1 :: Int :**: Char
+p1 = 3 :**: 'p'
+
+-- Generic instances
+instance GEq C
+instance (GEq a) => GEq (D a)
+instance (GEq a, GEq b) => GEq (a :**: b)
+
+-- Tests
+teq0 = geq c0 c1
+teq1 = geq d0 d1
+teq2 = geq d0 d0
+teq3 = geq p1 p1
+
+main = mapM_ print [teq0, teq1, teq2, teq3]