Haskell Aeson解构泛型解析

Question

我有一个JSON请求的风格

{"command":"get","params":{"something":"something else"}}

和Yesod书中的这段代码片段

{-# LANGUAGE OverloadedStrings #-}
import Network.Wai (Response, responseLBS, Application, requestBody)
import Network.HTTP.Types (status200, status400)
import Network.Wai.Handler.Warp (run)
import Data.Aeson.Parser (json)
import Data.Conduit.Attoparsec (sinkParser)
import Control.Monad.IO.Class (liftIO)
import Data.Aeson (Value(..), encode, object, (.=))
import Control.Exception (SomeException)
import Data.ByteString (ByteString)
import Data.Conduit (ResourceT, ($$))
import Control.Exception.Lifted (handle)

main :: IO ()
main = run 3000 app

app :: Application
app req = handle invalidJson $ do
    value <- requestBody req $$ sinkParser json
    newValue <- liftIO $ modValue value
    return $ responseLBS
        status200
        [("Content-Type", "application/json")]
        $ encode newValue

invalidJson :: SomeException -> ResourceT IO Response
invalidJson ex = return $ responseLBS
    status400
    [("Content-Type", "application/json")]
    $ encode $ object
        [ ("message" .= show ex)
        ]

-- Application-specific logic would go here.
modValue :: Value -> IO Value
modValue (Object o) 
    | -- key "command" corresponds to value "get"
    | otherwise = fail "Invalid command"

但我无法理解如何解构生成的Value数据结构.我如何获取密钥等的值.我意识到我可以解析为明确定义的数据结构,但这会给我的用例带来其他类型的问题.

在modValue中,我发表评论,我无法弄清楚要放什么.我尝试将它视为一个Map,因为它是在Aeson中实现的,但显然不会进行类型检查.

编辑:

将Data.HashMap添加到导入并使用该行

    | M.lookup "command" o == Just "get" = return $ object [("result" .= (String "YAY"))]

给出以下错误消息.

main.hs:39:26:
Couldn't match expected type `M.Map k0 a0'
            with actual type `aeson-0.6.0.2:Data.Aeson.Types.Internal.Object'
In the second argument of `M.lookup', namely `o'
In the first argument of `(==)', namely `M.lookup "command" o'
In the expression: M.lookup "command" o == Just "get"

EDIT2:

在突然的预感中,我追踪了我之前收到的涉及"无序容器"的错误消息.这是Aeson使用的包.但我意识到我还安装了包hashmap,它作为Data.HashMap导入.来自无序容器的哈希映射被导入为Data.HashMap.Strict或Lazy!

无论如何,更改线路import qualified Data.HashMap as M以import qualified Data.HashMap.Strict as M固定它.现在给出的答案有效!

Answer 1

由于aeson JSON对象是Hashmap,因此在这种情况下可以使用Hasmap接口lookup.

import qualified Data.HashMap.Strict as M

M.lookup "command" o == Just "get"