[英]Aeson: how do I parse an object with an element that is a stringified object?
[英]How can I parse an Aeson Object into my own custom type?
我正在嘗試使用 Aeson 編寫 JSON 解析器。
我在代碼中調用 JSON 的方式:
testReq :: Request
testReq = parseRequest_ "https://api.openweathermap.org/data/2.5/onecall?lat=41.63526&lon=-70.92701&exclude=minutely&appid=93120a85abf28f8fb1cdae14ffd7435d&units=metric"
首先我定義我的自定義類型
type Celsius = Double
type HPA = Int --Hectopascal Pressure Unit
type Percent = Int
type Meter = Int
type MeterPerSec = Double
type CompassDegree = Int
data WeatherObj =
WeatherObj
{ time :: UTCTime
, temp :: Celsius
, feels_like :: Celsius
, pressure :: HPA
, humidity :: Percent
, visibility :: Meter
, wind_speed :: MeterPerSec
, wind_deg :: CompassDegree
}
deriving (Eq, Show, Generic)
接下來我編寫我的 FromJSON 實例,我知道它有效,因為如果我運行parseCurrentWeather testReq
我會返回WeatherObj {time = 2020-07-19 16:54:43 UTC, temp = 25.51, feels_like = 29.49, pressure = 1012, humidity = 83, visibility = 10000, wind_speed = 1.34, wind_deg = 247}
太完美了!
instance FromJSON WeatherObj where
parseJSON = withObject "weatherObj" $ \obj -> do
timeOffset <- obj .: "timezone_offset"
currentO <- obj .: "current"
dt <- currentO .: "dt"
temp <- currentO .: "temp"
feels_like <- currentO .: "feels_like"
pressure <- currentO .: "pressure"
humidity <- currentO .: "humidity"
visibility <- currentO .: "visibility"
wind_speed <- currentO .: "wind_speed"
wind_deg <- currentO .: "wind_deg"
pure $ WeatherObj (makeLocalTime dt timeOffset)
temp feels_like
pressure humidity
visibility wind_speed
wind_deg
parseCurrentWeather :: Request -> IO WeatherObj
parseCurrentWeather req = do
current <- fetchCurrentWeather req
pure $ getResponseBody current
現在我需要弄清楚如何解析每小時的天氣,這應該會給我 48 個對象。 當我運行parseHourly testReq
時,此代碼的工作原理是返回一長串 JSON ,沒有任何例外。 這個 JSON 絕對匹配鏈接中的 JSON。 到目前為止,我看起來很棒。
fetchHourly :: Request -> IO (Response HourlyWeathers) --Could also be IO (Response Object)
fetchHourly = httpJSON
data HourlyWeathers =
HourlyWeathers
{ getHours :: [Object] }
deriving (Eq, Show, Generic)
instance FromJSON HourlyWeathers where
parseJSON = withObject "hourlyWeather" $ \obj -> do
allHours <- obj .: "hourly"
pure $ HourlyWeathers allHours
parseHourly :: Request -> IO HourlyWeathers
parseHourly req = do
hours <- fetchHourly req
pure $ getResponseBody hours
現在我們遇到了有問題的代碼。 我想將 map objToWeatherObj
放到我使用parseHourly
生成的對象列表中。 我似乎無法克服的問題是,當我運行parseHourlyObjects
時,我得到了一個所有空的列表。
parseHourlyObjects :: Request -> IO [Maybe WeatherObj]
parseHourlyObjects req = do
hourly <- fetchHourly req
let x = getHours $ getResponseBody hourly
y = fmap objToWeatherObj x
pure y
objToWeatherObj :: Object -> Maybe WeatherObj
objToWeatherObj = (decode . encode)
我已經能夠為WeatherObj
編寫ToJSON
實例,但結果證明這無關緊要,因為我需要將通用Object
解析為WeatherObj
。 我相信我需要的 function 是decode
,盡管我可能是錯的。
鑒於:
data WeatherObj =
WeatherObj
{ time :: UTCTime
, temp :: Celsius
, feels_like :: Celsius
, pressure :: HPA
, humidity :: Percent
, visibility :: Meter
, wind_speed :: MeterPerSec
, wind_deg :: CompassDegree
}
deriving (Eq, Show, Generic, FromJSON)
請注意,它現在也派生自 FromJSON 。
你可以:
decode "{\"time\":\"...\",...}" :: Maybe WeatherObj
並獲得一個Maybe WeatherObj 。 通過編寫您自己的 FromJSON 實例,我認為您可能使您的生活比需要的更具挑戰性。
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