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!

Answer 1

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"