Just some ideas from briefly mucking about with zippers with nkpart and newmana

JsonZipper.hs

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}

import qualified Data.Map as M
import Control.Applicative as A

newtype JObject = JObject (M.Map String Json)
newtype JList = JList [Json]

data Json
  = JLs JList
  | JObj JObject
  | JPrim String
  | JNull

data ListContext = ListContext JList JList JContext
data ObjectContext = ObjectContext String JObject JContext

data JContext
  = Top
  | ListCtx JList JList JContext
  | ObjectCtx String JObject JContext

data JZipper focus context = JZipper focus context

class Focus t where
  focus :: JZipper Json context -> Maybe (JZipper t context)
  blur  :: JZipper t context -> JZipper Json context

instance Focus JObject where
  focus (JZipper (JObj object) context) = Just (JZipper object context)
  focus _ = Nothing
  blur (JZipper object context) = JZipper (JObj object) context

class Up t c where
  up :: JZipper (Maybe Json) c -> JZipper t JContext

instance Up JList ListContext where
  up (JZipper focus (ListContext (JList left) (JList right) context))
    = JZipper (JList (reverse left ++ maybe id (:) focus right)) context

instance Up JObject ObjectContext where
  up (JZipper focus (ObjectContext field (JObject object) context))
    = JZipper (JObject (M.update (const focus) field object)) context

class Down i t c where
  down :: i -> JZipper t JContext -> Maybe (JZipper Json c)

instance Down String JObject ObjectContext where
  down field (JZipper (JObject object) context) = refocus <$> M.lookup field object
    where
      refocus on = JZipper on (ObjectContext field (JObject (M.delete field object)) context)

-- TODO: get rid of the partial functions!
instance Down Int JList ListContext where
  down index (JZipper (JList list) context)
    | index < length list = Just $ JZipper (list !! index) (ListContext left right context)
    | otherwise = Nothing
    where
      left = JList $ reverse $ take index list
      right = JList $ drop (index+1) list

relabel :: (String -> String) -> JZipper focus ObjectContext -> JZipper focus ObjectContext
relabel f (JZipper focus (ObjectContext field others context))
  = JZipper focus (ObjectContext (f field) others context)