Question

我正在尝试解决F＃中的练习。我必须编写一个可以区分书籍和电影的代码，并将其作为书籍或电影放在列表中。由于书籍没有文件大小，因此可以通过文件大小来区分。例如，如果我放入一本书，则代码必须将它添加为列表，就像电影一样。我将样本结果和输入链接起来。预先谢谢你。

type Movie =
    { movieName: string
      duration: Nat
      fileSize: Nat }

type Book =
    { bookName: string
      pages: Nat }

type Activity =
    | Watch of Movie
    | Read of Book


let rec createActivities(hl: (string * Nat * Nat) list): Activity list = 
    match hl with
      | [] -> []
      | x::xs -> ....

以下是输入内容：

createActivities([
                "The Hobbit"                , 304N, 0N
                "The Fellowship of the Ring", 228N, 50N
                "The Name of the Wind"      , 662N, 0N
                "The Emoji Movie"           , 86N , 1024N
                "The Hobbit"                , 164N, 9001N
                "The Fellowship of the Ring", 700N, 0N

结果：

[
                Read { bookName = "The Hobbit"; pages = 304N }
                Watch { movieName = "The Fellowship of the Ring"; duration = 228N; fileSize = 50N }
                Read { bookName = "The Name of the Wind"; pages = 662N }
                Watch { movieName = "The Emoji Movie"; duration = 86N; fileSize = 1024N }
                Watch { movieName = "The Hobbit"; duration = 164N; fileSize = 9001N }
                Read { bookName = "The Fellowship of the Ring"; pages = 700N }
            ]

Answer 1

F＃中的match表达式可以非常高级，在match表达式的各个部分中都有子匹配。例如，您的匹配表达式中的x::xs大小写可以变成(name, duration, filesize) :: xs。并且，如果您为其中之一指定一个值，则只有当元组的那一部分具有该值时，它才会匹配。考虑到这一点，我将您的匹配表达式写成这样：

let rec createActivities(hl: (string * Nat * Nat) list): Activity list = 
    match hl with
      | [] -> []
      | (name, pages, 0N) :: xs -> Read { bookName = name; pages = pages } :: createActivities xs
      | (name, duration, fileSize) :: xs -> Watch { movieName = name; duration = duration; fileSize = fileSize } :: createActivities xs

此方法的工作原理是match个案例将按从上到下的顺序进行处理，并使用第一个匹配的案例。因此，如果将元组作为0N作为其第三元素，则将使用第二个匹配情况，否则将使用第三个匹配情况。因此match表达式可以保持非常简单和简洁。

Answer 2

我可以这样解决问题。感谢您的帮助。

let rec createActivities(hl: (string * Nat * Nat) list): Activity list =    
  match hl with 
  | [] -> []
  | (name, pagesorduration, Size) :: xs -> 
    if Size = 0N then Read { bookName = name; pages = pagesorduration } :: createActivities(xs) 
    else Watch { movieName = name; duration = pagesorduration; fileSize = Size } :: createActivities xs

F＃列表拆分，划分

2 个答案: