Deriving Example Eq instances from Haskell Book

eq-instances.hs

data TisAnInteger = TisAn Integer
instance Eq TisAnInteger where
  (==) (TisAn a) (TisAn b) = a == b
  
data TwoIntegers = Two Integer Integer
instance Eq TwoIntegers where
  (==) (Two a b) (Two c d) = (a == c) && (b == d)
  
data StringOrInt = TisAnInt Int | TisAString String
instance Eq StringOrInt where
  (==) (TisAnInt a) (TisAnInt b) = a == b
  (==) (TisAString a) (TisAString b) = a == b
  (==) _ _ = False
  
data Pair a = Pair a a
instance Eq a => Eq (Pair a) where
  (==) (Pair x y) (Pair x' y') = (x == x') && (y == y')
  
data Tuple a b = Tuple a b
instance (Eq a, Eq b) => Eq (Tuple a b) where
  (==) (Tuple x y) (Tuple x' y') = (x == x') && (y == y')
  
data Which a = ThisOne a | ThatOne a
instance Eq a => Eq (Which a) where
   (==) (ThisOne x) (ThisOne y) = x == y
   (==) (ThatOne x) (ThatOne y) = x == y
   (==) _ _ = False
   
data EitherOr a b = Hello a | Goodbye b
instance (Eq a, Eq b) => Eq (EitherOr a b) where
  (==) (Hello x) (Hello y)     = x == y
  (==) (Goodbye x) (Goodbye y) = x == y
  (==) _ _ = False