json - Aeson：解析枚举数据类型

Question

如何声明以下数据类型的 FromJSON 实例：

data Privacy = Everyone | 
           AllFriends | 
           FriendsOfFriends | 
           Self

以便遵循枚举数据类型的以下字符串：

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

这里暗示了一个可能的解决方案，但我无法编译该代码。

谢谢！

Answer 1

这种方式涉及较少的样板，但由于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

Answer 2

FromJSON 定义应为：

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

完整的工作示例：

{-# 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

Answer 3

我找到的解决方案是使用purefrom 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"