Question

我正在为大学课程制作REST API：

data Course = Course {
    id :: Maybe Text,
    name :: Text,
    deleted :: Bool
} deriving(Show, Generic)

instance FromJSON Course
instance ToJSON Course

我想允许deleted在序列化的JSON结构中是可选的，但不是在我的应用程序中。如果在解析时未指定deleted，我想将False设置为FromJSON。

我可以为{{1}}编写一个手动实例，但我不想为所有字段写出来。我想声明如何处理删除并让自动实例处理其他所有内容。

我该怎么做？

Answer 1

据我所知，没有办法自定义通用实例，但你可以稍微改变一下你的类型：

data Course = Course
    { courseId :: Maybe Text    -- Don't use `id`, it's already a function
    , name :: Text
    } deriving (Show, Generic)

data Deletable a = Deletable
    { deleted :: Bool
    , item :: a
    } deriving (Show)

instance FromJSON Course
instance ToJSON Course

instance FromJSON a => FromJSON (Deletable a) where
    parseJSON (Object v) = do
        i <- parseJSON (Object v)
        d <- v .:? "deleted" .!= False
        return $ Deletable d i
    parseJSON _ = mzero

现在你可以做到

> let noDeleted = "{\"name\":\"Math\",\"courseId\":\"12345\"}" :: Text
> let withDeleted = "{\"name\":\"Math\",\"courseId\":\"12345\",\"deleted\":true}" :: Text

> decode noDeleted :: Maybe (Deletable Course)
Just (Deletable {deleted = False, item = Course {courseId = Just "12345", name = "Math"}})

> decode noDeleted :: Maybe Course
Just (Course {courseId = Just "12345", name = "Math"})

> decode withDeleted :: Maybe (Deletable Course)
Just (Deletable {deleted = True, item = Course {courseId = Just "12345", name = "Math"}})

> decode withDeleted :: Maybe Course
Just (Course {courseId = Just "12345", name = "Math"})

现在，您可以选择在需要时将课程标记为可删除，FromJSON实例可以处理所有内容。

覆盖Data.Aeson如何仅处理记录的一个字段

1 个答案: