eqInstance.hs

module EqExercise where

data TisAnInteger = TisAn Integer

instance Eq TisAnInteger where
  (==) (TisAn a) (TisAn a') = a == a'

data TwoIntegers =
  Two Integer Integer

instance Eq TwoIntegers where
  (==) (Two a a')
       (Two a'' a'') =
    a == a'' && a' == a'''

data StringOrInt =
    TisAnInt  Int
  | TisAString String

instance Eq StringOrInt where
  (==) (TisAnInt a) (TisAnInt a') = a == a'
  (==) (TisAString a) (TisAString a') = a == a'
  (==) _ _ = False

data Pair a =
  Pair a a

instance Eq a => Eq (Pair a) where
  (==) (Pair a a') (Pair a'' a''') =
    a == a'' && a' == a'''

data Tuple a b =
  Tuple a b

instance (Eq a, Eq b) => Eq (Tuple a b) where
  (==) (Tuple a b) (Tuple a' b') =
    a == a' && b == b'

data Which a =
  ThisOne a | ThatOne a

instance Eq a => Eq a where
  (==) (ThisOne a) (ThisOne a') = a == a'
  (==) (ThatOne a) (ThatOne a') = a == a'
  (==) _ _ = False

data EitherOr a b =
    Hello a
  | Goodbye b

instance (Eq a, Eq b) => Eq (EitherOr a b) where
  (==) (Hello a) (Hello a') = a == a'
  (==) (Goodbye b) (Goodbye b') = b == b'
  (==) _ _ = False