埃宋：解析枚舉數據類型

我如何可以聲明以下數據類型的FromJSON的實例：埃宋：解析枚舉數據類型

data Privacy = Everyone | 
      AllFriends | 
      FriendsOfFriends | 
      Self

所以，下面的字符串來枚舉數據類型很榮幸：

"EVERYONE" -> Everyone 
"ALL_FRIENDS" -> AllFriends 
"FRIENDS_OF_FRIENDS" -> FriendsOfFriends 
"SELF" -> Self 
_ -> Parsing error

一可能的解決方案是暗示here，但我不能讓代碼編譯。

謝謝！

來源

2014-11-22 Damian Nadales

的FromJSON定義應爲：

instance FromJSON Privacy where 
    parseJSON (Object v) = createPrivacy <$> (v .: "value")

完整的工作例如：

{-# LANGUAGE OverloadedStrings #-} 

import Data.Text 
import Data.Aeson 
import Control.Applicative 
import Control.Monad 

data Privacy = Everyone | 
       AllFriends | 
       FriendsOfFriends | 
       Self 
    deriving (Show) 

instance FromJSON Privacy where 
    parseJSON (Object v) = createPrivacy <$> (v .: "value") 
    parseJSON _   = mzero 

createPrivacy :: String -> Privacy 
createPrivacy "EVERYONE" = Everyone 
createPrivacy "ALL_FRIENDS" = AllFriends 
createPrivacy "FRIENDS_OF_FRIENDS" = FriendsOfFriends 
createPrivacy "SELF" = Self 
createPrivacy _ = error "Invalid privacy setting!" 

main = do 
    let a = decode "{\"value\":\"ALL_FRIENDS\",\"foo\":12}" :: Maybe Privacy 
    print a

來源

2014-11-22 12:21:08 ErikR

感謝這個答案。因爲我有一個枚舉的數據類型作爲JSON對象的一個屬性，所以使用我在我的回答中描述的模式結束了。使用「值」需要我有額外的不必要的嵌套層次。然而你的代碼回答了我的問題。 – 2014-11-22 12:31:57

我找到的解決方案是使用pure從Control.Applicative。

import Control.Applicative ((<$>), (<*>), pure) 

data Privacy = Everyone | 
     AllFriends | 
     FriendsOfFriends | 
     Self 

instance FromJSON Privacy where 
    parseJSON (String s) = pure $ mkPrivacy s 
    parseJSON _ = fail "Failed to parse Privacy object" 

instance ToJSON Privacy where 
    toJSON Everyone = "EVERYONE" 
    toJSON AllFriends = "ALL_FRIENDS" 
    toJSON FriendsOfFriends = "FRIENDS_OF_FRIENDS" 
    toJSON Self = "SELF" 

mkPrivacy :: Text -> Privacy 
mkPrivacy "EVERYONE" = Everyone 
mkPrivacy "ALL_FRIENDS" = AllFriends 
mkPrivacy "FRIENDS_OF_FRIENDS" = FriendsOfFriends 
mkPrivacy "SELF" = Self 
mkPrivacy _ = error "Invalid privacy token"

來源

2014-11-22 12:19:32

這樣涉及較少的樣板，但它可能無法做到高效由於T.unpack

data Privacy = Everyone | AllFriends | FriendsOfFriends | Self deriving (Read, Show) 

instance FromJSON Privacy where 
    parseJSON (String s) = fmap read (pure $ T.unpack s) 
    parseJSON _ = mzero

來源

2015-06-07 12:04:24 sportanova

埃宋：解析枚舉數據類型

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