Haskell 错误：预期类型为“* -> *”，但“电影”具有类型“*”

Question

我创建了一个“电影”代数数据类型（根据任务的要求）：

data Movie = Movie { title :: String, director :: String, releaseYear :: Int} 

然后添加一个函子：

instance Functor Movie where
    fmap _ Nothing = Nothing 
    fmap f (Movie title director releaseYear) = Movie (f title) (f director) (f releaseYear)

之后我写了几个函数来测试程序的运行：

appendYear :: String -> String
appendYear title = title ++ " (2023)"

sampleMovie :: Movie
sampleMovie = Movie "Cool Film" "Amazing Director" 2000

modifiedMovie :: Movie
modifiedMovie = fmap appendYear sampleMovie

在我尝试调用函数之后...

main :: IO ()
main = do
    print sampleMovie
    print modifiedMovie

...我有以下内容：

[1 of 1] Compiling Main             ( main.hs, main.o )
main.hs:3:18: error:
    • Expected kind ‘* -> *’, but ‘Movie’ has kind ‘*’
    • In the first argument of ‘Functor’, namely ‘Movie’
      In the instance declaration for ‘Functor Movie’
  |
3 | instance Functor Movie where
  |                  ^^^^^

请帮我解决问题。我最近才开始学习Haskell，所以我使用在线编译器

尝试在其他任务上做类似的事情：

data Possibly a = NotThere
                | There a 
                deriving (Show, Functor)

instance Functor Possibly where
    fmap _ NotThere = NotThere
    fmap f (There a) = There (f a)

...但这并没有让我得到任何结果。

Answer 1

这里有几个问题：首先：

fmap _ Nothing = Nothing

没有意义：

Nothing

是

Maybe

类型的数据构造函数，而不是

Movie

类型。

但最重要的是

Functor

适用于参数化类型：这允许更改类型。因此，这意味着

Movie

应该定义为：

data Movie a = Movie { title :: a, director :: String, releaseYear :: Int}

instance Functor Movie where
    fmap f m(Movie {title=t}) = m { … }

我将实施

…

部分作为练习。