我有一个
风格的JSON请求{"command":"get","params":{"something":"something else"}}
以及来自Yesod book
的此代码段{-# 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。我试过把它当作一张地图,因为它是在Aeson里面实现的,但显然不会打字。
编辑:
将Data.HashMap添加到导入并使用行
| 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,它被导入为Data.HashMap。来自无序容器的哈希映射将导入为Data.HashMap.Strict或Lazy!
将行import qualified Data.HashMap as M
更改为import qualified Data.HashMap.Strict as M
无论如何都要修复它。现在给出的答案有效!
答案 0 :(得分:5)
自aeson JSON object is a Hashmap起,您可以使用Hasmap interface,在这种情况下lookup
。
import qualified Data.HashMap.Strict as M
M.lookup "command" o == Just "get"