Add the haskell book

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diff --git a/Haskell-book/07/src/Ex.purs b/Haskell-book/07/src/Ex.purs
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+module Ex where
+
+import Data.Int (fromString)
+import Data.Maybe (Maybe(..))
+import Data.Tuple (Tuple(..))
+import Prelude
+import Unsafe.Coerce (unsafeCoerce)
+
+--
+-- Multiple choice
+--
+-- Exercise 2
+f :: Char -> String
+f = unsafeCoerce unit
+
+g :: String -> Array String
+g = unsafeCoerce unit
+
+e :: Char -> Array String
+e = g <<< f
+
+-- Exercise 5
+f' :: forall a. a -> a
+f' x = x
+
+g' :: Boolean
+g' = f' true
+
+--
+-- Let's write code
+--
+
+-- Exercise 1
+tensDigit :: Int -> Int
+tensDigit x = d
+    where xLast = x `div` 10
+          d     = xLast `mod` 10
+
+-- Exercise 2
+hunsD :: Int -> Int
+hunsD x = d
+    where xLast = x `div` 100
+          d     = xLast `mod` 10
+
+foldBool :: forall a. a -> a -> Boolean -> a
+foldBool x y z = case z of
+    false -> x
+    true  -> y
+
+findBool2 :: forall a. a -> a -> Boolean -> a
+findBool2 x y z
+    | z         = y
+    | otherwise = x
+
+foldBool3 :: forall a. a -> a -> Boolean -> a
+foldBool3 x _ false = x
+foldBool3 _ y true  = y
+
+-- Exercise 3
+g'' :: forall a b c. (a -> b) -> Tuple a c -> Tuple b c
+g'' f'' (Tuple a c) = Tuple (f'' a) c
+
+-- Exercise 5
+-- id :: a -> a
+-- id = x = x
+
+roundTrip :: Int -> Int
+roundTrip = fromMaybe <<< fromString <<< show
+    where fromMaybe (Just n) = n
+          fromMaybe Nothing = unsafeCoerce unit