Add the haskell book

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+module EqInstances where
+    -- Exercise: Eq Instances
+    -- p. 178
+
+import Prelude
+
+-- Ex. 1
+data TisAnInteger =
+    TisAn Int
+
+instance eqTisAn :: Eq TisAnInteger where
+    eq x y = x == y
+
+-- Ex. 2
+data TwoIntegers =
+    Two Int Int
+
+instance ewTwoIntegers :: Eq TwoIntegers where
+    eq (Two x y) (Two x' y') = x == x' && y == y'
+
+-- Ex. 3
+data StringOrInt =
+    TisAnInt Int
+  | TisAString String
+
+instance eqStringOrInt :: Eq StringOrInt where
+    eq (TisAnInt x) (TisAnInt y) = x == y
+    eq (TisAString x) (TisAString y) = x == y
+    eq _ _ = false
+
+-- Ex. 4
+data Pair a =
+    Pair a a
+
+instance eqPair :: Eq a => Eq (Pair a) where
+    eq (Pair a b) (Pair a' b') = a == a' && b == b'
+
+-- Ex. 5
+data Tuple a b =
+    Tuple a b
+
+instance eqTuple :: (Eq a, Eq b) => Eq (Tuple a b) where
+    eq (Tuple a b) (Tuple a' b') = a == a' && b == b'
+
+-- Ex. 6
+data Which a =
+    ThisOne a
+  | ThatOne a
+
+instance eqWhich :: Eq a => Eq (Which a) where
+    eq (ThisOne x) (ThisOne y) = x == y
+    eq (ThatOne x) (ThatOne y) = x == y
+    eq _ _ = false
+
+-- Ex. 7
+data EitherOr a b =
+    Hello a
+  | Goodbye b
+
+instance eqEitherOr :: (Eq a, Eq b) => Eq (EitherOr a b) where
+    eq (Hello _) (Goodbye _) = false
+    eq (Goodbye _) (Hello _) = false
+    eq _ _ = true