Add remaining haskell book exercises

Haskell-book/13/Cipher.hs

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+module Cipher where
+
+import Data.Char
+import Data.List
+
+vigenere :: String -> String
+vigenere = f (cycle [0, 11, 11, 24])
+    where f _      []       = []
+          f ys     (' ':xs) = ' ' : (f ys xs)
+          f (y:ys) (x:xs)   = (decode (x, y)) : (f ys xs)
+              where decode (x, y) = chr ((mod ((ord x) - 65 + y) 26) + 65)
+
+main :: IO ()
+main = do
+    i <- getLine
+    putStrLn $ vigenere i

Haskell-book/13/Palindrome.hs

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+module Palindrome where
+
+import Control.Monad
+import Data.Char
+import System.Exit
+
+format :: String -> String
+format s = filter (\x -> x >= 'a' && x <= 'z') (map toLower s)
+
+palindrome :: IO ()
+palindrome = forever $ do
+    line1 <- getLine
+    case ((format line1) == (format $ reverse line1)) of
+      True -> putStrLn "It's a palindrome!"
+      False -> do
+          putStrLn "Nope!"
+          exitSuccess
+
+main :: IO ()
+main = palindrome

Haskell-book/13/Person.hs

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+module Person where
+
+type Name = String
+type Age = Integer
+
+data Person = Person Name Age deriving Show
+
+data PersonInvalid = NameEmpty
+                   | AgeTooLow
+                   | PersonInvalidUnknown String
+                   deriving (Eq, Show)
+
+mkPerson :: Name -> Age -> Either PersonInvalid Person
+mkPerson name age
+  | name /= "" && age > 0 = Right $ Person name age
+  | name == "" = Left NameEmpty
+  | not (age > 0) = Left AgeTooLow
+  | otherwise =
+      Left $ PersonInvalidUnknown $
+          "Name was: " ++ show name ++ " Age was: " ++ show age
+
+showPerson :: Either PersonInvalid Person -> String
+showPerson (Left NameEmpty) = "Name is empty"
+showPerson (Left AgeTooLow) = "Age is too low"
+showPerson (Left (PersonInvalidUnknown e)) = e
+showPerson (Right p) = "Yay! Successfully got a person: " ++ (show p)
+
+gimmePerson :: IO ()
+gimmePerson = do
+    putStr "Enter your name: "
+    name <- getLine
+    putStr "Enter your age: "
+    age <- getLine
+    putStrLn $ showPerson $ mkPerson name (read age)