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如何用Aeson解析这个JSON?

aeson haskell parsing json

Varun Madiath · 技术社区 · 8 年前

我有以下JSON片段:

{
  "weather": [
    {
      "id": 803,
      "main": "Clouds",
      "description": "broken clouds",
      "icon": "04n"
    }
  ],
  "main": {
    "temp": 271.979,
    "pressure": 1024.8,
    "humidity": 100,
    "temp_min": 271.979,
    "temp_max": 271.979,
    "sea_level": 1028.51,
    "grnd_level": 1024.8
  },
  "id": 6332485,
  "name": "Queensbridge Houses",
  "cod": 200
}

我想从中解析以下类型:

data WeatherResponse = WeatherResponse
  { temp :: Double
  , humidity :: Double
  , weatherMain :: T.Text
  } deriving Show

我一直在尝试使用下面的代码来实现,但我不断遇到错误。我终于找到了所有要匹配的类型,但它解析错误,而且不知道它在哪里失败。

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE ScopedTypeVariables #-}

import Data.Aeson
import Data.Aeson.Types (Parser, Array)
import Data.Time (defaultTimeLocale, formatTime, getZonedTime)

import qualified Data.ByteString.Lazy as BL
import qualified Data.Vector as V
import qualified Data.Text as T

data WeatherResponse = WeatherResponse
  { temp :: Double
  , humidity :: Double
  , weatherMain :: T.Text
  } deriving Show

lambda3 :: Value -> Parser T.Text
lambda3 o = do
  withText "main" (\t -> do
                      return t
                  ) o

parseInner :: Value -> Parser T.Text
parseInner a = withArray "Inner Array" (lambda3 . (V.head)) a

instance FromJSON WeatherResponse where
  parseJSON =
    withObject "Root Object" $ \o -> do
    mainO <- o .: "main"
    temp <- mainO .: "temp"
    humidity <- mainO .: "humidity"
    weatherO <- o .: "weather"
    weatherMain <- parseInner weatherO
    return $ WeatherResponse temp humidity weatherMain

getSampleData = BL.readFile "/home/vmadiath/.xmonad/weather.json"

main = do
  text <- getSampleData
  let (result :: Either String WeatherResponse) = eitherDecode text
  putStrLn . show  $ result

我只是得到了下面的输出,但这并不能让我知道我错在哪里。

$ runhaskell lib/code.hs
Left "Error in $: expected main, encountered Object"

我把整件事都放在一个可以看的要点上 here

我想知道代码有什么问题,以及如何修复它。如果你有关于如何以更可读的方式写这篇文章的建议,我也很想知道。目前,我主要对这两个独立的功能感到恼火 lambda3 和 parseInner 令人讨厌)

1 回复 | 直到 8 年前

Sibi 8 年前

在我看来,你把这件事弄得太复杂了。这样的方法应该有效:

instance FromJSON WeatherResponse where
  parseJSON (Object v) = do
      weatherValue <- head <$> v .: "weather"
      WeatherResponse <$> ((v .: "main") >>= (.: "temp"))
                          <*> ((v .: "main") >>= (.: "humidity"))
                          <*> weatherValue .: "main"

它的输出:

[nix-shell:~/haskell-sample]$ ./weather
Right (WeatherResponse {temp = 271.979, humidity = 100.0, weatherMain = "Clouds"})