Add remaining haskell book exercises

Haskell-book/13/Person.hs

@@ -0,0 +1,34 @@
+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)