重写Data.Aeson只处理我的记录的一个字段 [英] Override how Data.Aeson handles only one field of my record

问题描述

  data Course = Course {
 id :: Maybe文本，
名称::文本，
删除:: Bool 
}派生（显示，通用）
 
实例FromJSON课程
实例ToJSON课程
  

我希望允许删除为可选项序列化的JSON结构，但不在我的应用程序中。如果解析时未指定，那么我想将删除设置为 False 。

我可以为 FromJSON 写一个手动实例，但我不想写出来所有的领域。我想声明如何处理删除，并让自动实例处理其他所有内容。

我会如何做到这一点？

解决方案

据我所知不是自定义泛型实例的方法，但是您可以以不同的方式构造您的类型：

 数据课程=课程
 {courseId :: Maybe Text  - 不要使用`id`，它已经是一个函数
，name :: Text 
}派生（Show，Generic）
 
 data可删除a =可删除
 {已删除:: Bool 
，item :: a 
}派生（显示）
 
实例FromJSON课程
实例ToJSON课程
 
实例FromJSON a => FromJSON（可删除的a）其中
 parseJSON（Object v）= do 
 i < -  parseJSON（Object v）
 d < -  v。：？ deleted。！= False 
 return $ Deletable di 
 parseJSON _ = mzero 
  

现在你可以做

 > let noDeleted ={\name \：\Math \，\courseId\：\12345\}:: Text 
> let withDeleted ={\name \：\Math \，\courseId\：\12345\，\deleted\：true}::文本
 
>解码noDeleted ::也许（可删除的课程）
 Just（Deletable {deleted = False，item = Course {courseId = Just12345，name =Math}}）
 
>解码noDeleted ::也许课程
只是（课程{courseId =只是12345，名称=数学}）
 
> decodeDeleted :: Maybe（Deletable Course）
 Just（Deletable {deleted = True，item = Course {courseId = Just12345，name =Math}}）
 
>解码withDeleted ::也许课程
 Just（Course {courseId = Just12345，name =Math}）
  

现在，您可以选择在需要时将课程标记为可删除，并且 FromJSON 实例处理所有事情。

I am making a REST API for university courses:

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

instance FromJSON Course
instance ToJSON Course

I would like to allow deleted to be optional in the serialized JSON structure, but not in my application. I want to set deleted to False if it isn't specified when parsing.

I could write a manual instance for FromJSON, but I don't want to have to write it out for all the fields. I want to declare how deleted is handled and let the automatic instance handle everything else.

How would I do this?

解决方案

To my knowledge there is not a way to customize the generic instance, but you could structure your type a bit differently:

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

Now you can do

> 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"})

And now you can just optionally tag a course as deletable when you need it, and the FromJSON instances take care of everything.

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