Question

data Film = Film String String Int [String] deriving (Eq, Ord, Show, Read)
--that's my custom data type, (Film *filmname, director, year of release, fans)

addFilm :: Film -> [Film] -> [Film]
addFilm newFilm filmList = filmList ++ [newFilm]

--this is for adding a new film to the list, but without the fans string array

该模块正在加载到WinGHCi中，没有任何错误，但我不知道应该用什么代替filmList。

---
<interactive>:63:30:
    Couldn't match type `Char' with `Film'
    Expected type: [Film]
      Actual type: [Char]
    In the second argument of `addFilm', namely `"Jordan Vogt-Roberts"'
    In the expression:
      addFilm "Kong: Skull Island" "Jordan Vogt-Roberts" 2017
    In an equation for `it':
        it = addFilm "Kong: Skull Island" "Jordan Vogt-Roberts" 2017
*Main> addFilm "Kong: Skull Island" "Jordan Vogt-Roberts" 2017 filmList

<interactive>:64:57: Not in scope: `filmList' --- that's what I input.

Answer 1

您需要使用数据构造函数，而不仅仅是参数。如果要创建Film类型的值，则需要编写例如：Film "Kong: Skull Island" "Jordan Vogt-Roberts" 2017 ["Fan One","Fan Two"]。

更具体地说，像data TypeName = DataConstructor String这样的数据声明会创建一个函数DataConstructor :: String -> TypeName。在您的情况下，即Film :: String -> String -> Int -> [String] -> Film。

示例：

skullIsland = Film "Kong: Skull Island" "Jordan Vogt-Roberts" 2017 ["Fan One","Fan Two"]
otherMovie = Film "Movie Name" "The Director" 2000 ["Lazersmoke","Saad"]

addFilm skullIsland [otherMovie,otherMovie]

但没有粉丝字符串数组

我不确定我明白你要做什么;粉丝列表是您数据类型的一部分，您不能忽略它。

我输入什么作为filmList参数的值？

1 个答案: