AESON：解析动态结构

Question

我有一个这样的JSON结构

{
  "tag1": 1,
  "tag2": 7,
  ...
}

我有这样的类型

data TagResult { name :: String, numberOfDevicesTagged :: Int } deriving (Show, Eq)
newtype TagResultList = TagResultList { tags :: [TagResult] }

标签名称当然是完全动态的，我在编译时不知道它们。我想创建一个实例 FromJSON 来解析 JSON 数据，但我无法编译它。

如何定义 parseJSON 来实现这一点？

Answer 1

您可以利用 Object 是 HasMap 的事实并在运行时提取密钥。然后，您可以按如下方式编写 FromJSON 实例 -

{-# LANGUAGE OverloadedStrings #-}
module Main where

import Data.Aeson
import qualified Data.Text as T
import qualified Data.HashMap.Lazy as HashMap

data TagResult = TagResult { name :: String
                           , numberOfDevicesTagged :: Int
                           } deriving (Show, Eq)


newtype TagResultList = TagResultList { tags :: [TagResult] } deriving Show


instance ToJSON TagResult where

  toJSON (TagResult tag ntag) =
    object [ T.pack tag .= ntag ]

instance ToJSON TagResultList where

  toJSON (TagResultList tags) =
    object [ "tagresults" .= toJSON tags ]


instance FromJSON TagResult where

  parseJSON (Object v) =
    let (k, _) = head (HashMap.toList v)
    in TagResult (T.unpack k) <$> v .: k

  parseJSON _ = fail "Invalid JSON type"

instance FromJSON TagResultList where

  parseJSON (Object v) =
    TagResultList <$> v .: "tagresults"


main :: IO ()
main = do

  let tag1 = TagResult "tag1" 1
      tag2 = TagResult "tag2" 7
      taglist = TagResultList [tag1, tag2]

  let encoded = encode taglist
      decoded = decode encoded :: Maybe TagResultList

  print decoded

上面的程序应该打印标签结果列表。

Just (TagResultList {tags = [TagResult {name = "tag1", numberOfDevicesTagged = 1},TagResult {name = "tag2", numberOfDevicesTagged = 7}]})

AESON：解析动态结构

AESON: Parse dynamic structure

json

haskell

aeson