Since an aeson JSON object is a Hashmap, you can use the Hasmap interface, in this case lookup
.
import qualified Data.HashMap.Strict as M
M.lookup "command" o == Just "get"
Question
I have a JSON request in the style of
{"command":"get","params":{"something":"something else"}}
and this code snippet from the Yesod book
{-# 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"
But I can't wrap my head around how I would destructure the generated Value data structure. How do I go about getting the values of keys etc. I realise I could parse to an explicitly defined data structure, but that would bring other kinds of problems to my use case.
In modValue I've put a comment where I can't figure out what to put. I tried treating it as a Map, since that's how it is implemented inside Aeson, but that obviously doesn't type check.
EDIT:
Adding Data.HashMap to imports and using the line
| M.lookup "command" o == Just "get" = return $ object [("result" .= (String "YAY"))]
gives the following error message.
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:
On a sudden hunch, I tracked down an error message I got earlier involving "unordered-containers". This is the package that Aeson uses. But I realised that I also had the package hashmap installed, which is imported as Data.HashMap. The hashmaps from unordered-containers are imported as Data.HashMap.Strict or Lazy!
Changing the line import qualified Data.HashMap as M
to import qualified Data.HashMap.Strict as M
fixed it anyway. Now the given answer works!
Solution
Since an aeson JSON object is a Hashmap, you can use the Hasmap interface, in this case lookup
.
import qualified Data.HashMap.Strict as M
M.lookup "command" o == Just "get"