Complete redo of Eq instances exercise

06/06.05-eq-instances.hs

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+-- 1.
+data TisAnInteger = TisAn Integer
+  deriving Show
+
+instance Eq TisAnInteger where
+  (==) (TisAn x) (TisAn y) = x == y
+
+-----------------------------------------------------------------------------
+-- 2.
+data TwoIntegers = Two Integer Integer
+  deriving Show
+
+instance Eq TwoIntegers where
+  (==) (Two x y) (Two x' y') = x == x' && y == y'
+
+-----------------------------------------------------------------------------
+-- 3.
+data StringOrInt = TisAnInt   Int
+                 | TisAString String
+                 deriving Show
+
+instance Eq StringOrInt where
+  (==) (TisAnInt x) (TisAnInt y)     = x == y
+  (==) (TisAString x) (TisAString y) = x == y
+  (==) _ _                           = False
+
+-----------------------------------------------------------------------------
+-- 4.
+data Pair a = Pair a a
+  deriving Show
+
+instance Eq a => Eq (Pair a) where
+  (==) (Pair x y) (Pair x' y') = x == x' && y == y'
+
+-----------------------------------------------------------------------------
+-- 5.
+data Tuple a b = Tuple a b
+  deriving Show
+
+instance (Eq a, Eq b) => Eq (Tuple a b) where
+  (==) (Tuple x y) (Tuple x' y') = x == x' && y == y'
+
+-----------------------------------------------------------------------------
+-- 6.
+data Which a = ThisOne a
+             | ThatOne a
+             deriving Show
+
+instance Eq a => Eq (Which a) where
+  (==) (ThisOne x) (ThisOne x') = x == x'
+  (==) (ThatOne x) (ThatOne x') = x == x'
+  (==) _ _                      = False
+
+-----------------------------------------------------------------------------
+-- 7.
+data EitherOr a b = Hello a
+                  | Goodbye b
+                  deriving Show
+
+instance (Eq a, Eq b) => Eq (EitherOr a b) where
+  (==) (Hello x) (Hello x')     = x == x'
+  (==) (Goodbye x) (Goodbye x') = x == x'
+  (==) _ _                      = False
+
+