如何用aeson解析嵌套的JSON [英] How to parse nested JSON with aeson

问题描述

我试图用aeson解析以下表单的JSON

  {field：{name： ...}} 
 
或
 
 {tag：{name：...}} 
 
或
 
 {line：{number：...}} 
  

构造以下数据类型：

  data规则=行整数
 |字段L.ByteString 
 |标签L.ByteString 
  

不幸的是，我遇到了两个我没有找到解决方案的问题，即：

1. 如何解析嵌套的JSON？查看（。：） a>，它使用查找来提取特定键的值。我很犹豫要做这样的事情，因为它似乎太依赖于aeson如何实现的具体细节。我错误地认为这是一个问题吗？

如何根据哪个键在JSON中使用正确的数据构造函数？我所有的< |>所做的努力都让我无处可寻。

我会发布我写的代码很远，但我甚至没有到达有什么值得发布的地步。

解决方案

/ b>

  { - ＃LANGUAGE OverloadedStrings＃ - } 
 
导入Control.Applicative 
导入数据。 Aeson 
导入Data.Aeson.Types 
将限定的Data.ByteString导入为B 
将限定的Data.ByteString.Lazy导入为L 
将限定的Data.Map导入为M 
 
数据Rule =行整数
 |字段L.ByteString 
 |标签L.ByteString 
派生显示
 
实例FromJSON规则其中
 parseJSON j = do 
o < -  parseJSON j  - 负责JSON类型检查
案例M.toList（o :: Object）
 [（field，Object o'）]  - >字段< $> o'。：name
 [（tag，Object o'）]  - >标签< $> o'。：name
 [（line，Object o'）]  - >行< $> o'。：number
 _  - >失败规则：意外格式
  

I am trying to parse JSON of the following form using aeson

{"field":{"name":"..."}}

or

{"tag":{"name":"..."}}

or

{"line":{"number":"..."}}

to construct the following data type

data Rule = Line Integer
          | Field L.ByteString
          | Tag L.ByteString

Unfortunately, I face two problems that I've not found solutions to, namely:

1. How do I parse nested JSON? Looking at the implementation of (.:), it uses lookup to extract a specific key's value. I'm hesitant to do something like this as it seems to be relying too much on the specifics of how aeson implements things. Am I wrong in thinking this is an issue?

2. How do I use the correct data constructor based on which key is present in the JSON? All my efforts with <|> have led me nowhere.

I would post the code I've written thus far, but I haven't even gotten to the point where I have anything worth posting.

解决方案

How about the following?

{-# LANGUAGE OverloadedStrings #-}

import Control.Applicative
import           Data.Aeson
import           Data.Aeson.Types
import qualified Data.ByteString      as B
import qualified Data.ByteString.Lazy as L
import qualified Data.Map             as M

data Rule = Line Integer
          | Field L.ByteString
          | Tag L.ByteString
          deriving Show

instance FromJSON Rule where
  parseJSON j = do
    o <- parseJSON j -- takes care of JSON type check
    case M.toList (o :: Object) of
      [("field", Object o')] -> Field <$> o' .: "name"
      [("tag",   Object o')] -> Tag   <$> o' .: "name"
      [("line",  Object o')] -> Line  <$> o' .: "number"
      _                      -> fail "Rule: unexpected format"

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