lab3

data Product = Books String String|Video String|Disk String String Int deriving Show

getTitle :: Product -> String
getTitle (Books a b) = a
getTitle (Video c) = c
getTitle (Disk e f g) = e

getTitles :: [Product] -> [String]
getTitles [] = []
getTitles (x:xs) = getTitle x:getTitles xs

getAuthor :: Product -> String
getAuthor (Books a b) = b

predBooks :: Product -> Bool
predBooks (Books _ _) = True
predBooks _ = False

getBooks = filter predBooks

getSome :: [Product] -> [String]
getSome [] = []
getSome ((Disk _ _ _):xs) = getSome xs
getSome ((Video _):xs) = getSome xs
getSome ((Books _ x):xs) = x:getSome xs