content: -- file: ch03/BookStore.hs
  data BookInfo = Book Int String [String]
                  deriving (Show)
  
  -- file: ch03/BookStore.hs
  data MagzineInfo = Magzine Int String [String]
                     deriving (Show)
  
  -- file: ch03/BookStore.hs
  myInfo = Book 9780135072455 "Algebra of Programming"
                ["Richard Bird", "Oege de Moor"]
  
  -- file: ch03/BookStore.hs
  -- 稍后就会介绍 CustomerID 的定义
  
  data BookReview = BookReview BookInfo CustomerID String
  
  -- file: ch03/BookStore.hs
  type CustomerID = Int
  type ReviewBody = String
  
  data BetterReview = BetterReview BookInfo CustomerID ReviewBody
  
  -- file: ch03/BookStore.hs
  type BookRecord = (BookInfo, BookReview)
  
  -- file: ch03/BookStore.hs
  type CardHolder = String
  type CardNumber = String
  type Address = [String]
  data BillingInfo = CreditCard CardNumber CardHolder Address
                   | CashOnDelivery
                   | Invoice CustomerID
                     deriving (Show)
  
  -- file: ch03/BookStore.hs
  bookID      (Book id title authors) = id
  bookTitle   (Book id title authors) = title
  bookAuthors (Book id title authors) = authors
  