Parsing RoseTree JSON in Haskell

Question

I am trying to parse JSON representation of a RoseTree. Here is a snapshot I have:

module RoseTree2 where

import Data.Tree
import Data.Aeson
import qualified Data.Text as T
import Control.Applicative

data RoseTree2 = RoseNode Int [RoseTree2] deriving (Show)

instance ToJSON RoseTree2 where
toJSON (RoseNode n cs) =
    object [T.pack "value" .= show n
    , T.pack "children".= show cs]

instance FromJSON RoseTree2 where
    parseJSON (Object o) =
        RoseNode <$> o.: T.pack "value"
        <*> o.: T.pack "children"

But I am getting following error on fileload:

RoseTree2.hs:10:10:
    No instance for (GToJSON (GHC.Generics.Rep RoseTree2))
      arising from a use of `aeson-0.7.0.6:Data.Aeson.Types.Class.$gdmtoJSON'
    Possible fix:
      add an instance declaration for
      (GToJSON (GHC.Generics.Rep RoseTree2))
    In the expression:
      (aeson-0.7.0.6:Data.Aeson.Types.Class.$gdmtoJSON)
    In an equation for `toJSON':
        toJSON = (aeson-0.7.0.6:Data.Aeson.Types.Class.$gdmtoJSON)
    In the instance declaration for `ToJSON RoseTree2'
Failed, modules loaded: none.

Could please someone tell me what's wrong with my definition of a JSON parser and how could I fix it? Thanks!

Answer 1

You need to indent the definition of toJSON

instance ToJSON RoseTree2 where
  toJSON (RoseNode n cs) =
    object [T.pack "value" .= show n
    , T.pack "children".= show cs]

Answer 2

You forgot to indent the line after instance ToJSON RoseTree2 so the instance block is closed, and it defaults to

default toJSON :: (Generic a, GToJSON (Rep a)) => a -> Value
toJSON = genericToJSON defaultOptions