-- book.hs: code to process a list of books and print the year

data BookInfo = Book Int String [String]
                deriving (Show)

year (Book yr title authors) = yr

allYears [] = []
allYears (book:rest) = (year book) : (allYears rest)

-- aveYear: average of years of all books in library
--          Not defined if library is empty
aveYear library = 
   let years = allYears library in
      (fromIntegral (sum years)) / (fromIntegral (length years))


-- testData
lib = [Book 1851 "Moby Dick" ["Herman Melville"],
       Book 1851 "The House of the Seven Gables" ["Nathaniel Hawthorne"],
       Book 1937 "The Hobbit" ["JRR Tolkien"],
       Book 1990 "Good Omens" ["Terry Pratchett", "Niel Gaiman"]
       ]

-- aveYear lib should return 1907.25