make函数返回数据结构的标题

Question

有几种数据类型.

data Book =BName| Author deriving (Eq,Show)
data Video = VName deriving (Eq,Show)
data CD =Cname | Auth | NOC deriving (Eq,Show)
data Product = Product Book Video CD deriving (Eq,Show)

make function getTitle返回结构名称(BName,CName或VName).例如getTitle(Book"name"noname") - >"name"getTitle(Vide"name") - >"name"等.

可能吗？

存在数据类型Book with field title and author,Videocassete with field author,CDdisk with fields title,composition of composition and author.1)创建数据类型可以引入此数据类型的产品2)使函数getTitle返回标题.3)使函数getTitles返回产品列表中的所有标题(使用函数getTtitle)4)使函数bookAuthors返回产品列表中的书籍作者5)使函数lookupTitle :: String - > [Product] - > Maybe返回带有输入名称的产品的产品6)使函数lookupTitles :: [String] - > [Product] - > [Product]输入params是名称列表和产品列表,并且每个名称从列表中获取产品产品.忽略第一个列表中的名称,而不是第二个列表中的产品.使用函数lookipTitle.

这都是一项任务.

---

data Book = Book String String deriving(Eq,Show)
data Video = Video String deriving(Eq,Show)
data CDisk = CDisk String String Int deriving(Eq,Show)
--titles
class Titleable a where
 getTitle :: a ->String
instance Titleable Book where
 getTitle (Book title _) = title
instance Titleable Video where
 getTitle (Video title) = title
instance Titleable CDisk where
 getTitle(CDisk title _ _) = title
--get titles
getTitles (x:xs) = [ getTitle x | x<-xs ]
lookupTitle _ [] =Nothing
lookupTitle::String->[Product]->Maybe Product
lookupTitle a (x:xs)  | getTitle x==a =Just x
                             | otherwise = lookupTitle a (x:xs)
lookupTitles::[String]->[Product]->[Product]
lookupTitles (x:xs) (y:ys) = [y|x<-xs,y<-ys,x==lookupTitle y]

但是1)我不知道如何制作函数bookAuthors(函数怎么能找到参数是book-type？)2)如何制作产品类型？我的意思是它是Book或Video或CDisk 3)lookupTitle和lookupTitle是正确的？

Answer 1

您确定要的是这样的数据类型吗？

data Book = BName | Author deriving (Eq,Show)

是指书籍是BName(书名？)还是作者？

我怀疑你想要类似的东西

data Book = Book BName Author

例如,一本书有书名和作者

如果你想捕捉"可标题"事物的本质,你可以使用类型类.

class Titleable a where
  getTitle :: a -> String

instance Titleable Book  where
  getTitle (Book title _) = title

并为您的其他类型编写实例.