Access json data using path. Functional programming to the rescue :P

accessor.hs

{-# LANGUAGE OverloadedStrings #-}
import qualified Data.ByteString.Lazy as LBS
import qualified Data.Text as Text
import Data.Text (Text)
import Data.Aeson (encode, decode, Value, Value(..))
import qualified Data.HashMap.Strict as HashMap
import Data.Vector ((!))

  
path :: Text -> Value -> Value
path pathString = foldl1 (.) getters
  where
    getters :: [Value -> Value]
    getters = (map jsonGetter . reverse . Text.splitOn ".") pathString
    
    jsonGetter :: Text -> Value -> Value
    jsonGetter name = go 
      where go (Object jsonValue) = case HashMap.lookup name jsonValue of
              Just val -> val
              Nothing -> error $ "Error accessing " ++ show name
            go (Array jsonArray) = jsonArray ! (read . Text.unpack) name 
            go  _ = error $ "Error accessing " ++ show name


jsonPath :: Text -> LBS.ByteString -> LBS.ByteString
jsonPath pathString content =  
  let jsonValue = case decode content :: Maybe Value of
        Just v -> v
        Nothing -> error $ "Error parsing json " in
  encode . path pathString $ jsonValue

main :: IO ()
main = do
  content <- LBS.readFile "persons.json"
  LBS.putStrLn $ jsonPath "address.city" content
  LBS.putStrLn $ jsonPath "phoneNumbers.0.type" content

accessor.py

import json
from operator import itemgetter

compose = lambda f, g: lambda x: f(g(x))

jsongetter = lambda s: itemgetter(int(s)) if s.isdigit() else itemgetter(s)

def path(path):
    getters = map(jsongetter, reversed(path.split('.')))
    return reduce(compose, getters)

if __name__ == '__main__':
    with open('persons.json') as f:
        data = json.loads(f.read())
    print path('address.city')(data)
    print path('phoneNumbers.0.type')(data)

persons.json

{
    "firstName": "John",
    "lastName": "Smith",
    "age": 25,
    "address": {
        "streetAddress": "21 2nd Street",
        "city": "New York",
        "state": "NY",
        "postalCode": 10021
    },
    "phoneNumbers": [
        {
            "type": "home",
            "number": "212 555-1234"
        },
        {
            "type": "fax",
            "number": "646 555-4567"
        }
    ]
}

Here is a discussion about it :
http://www.reddit.com/r/haskell/comments/1ftrnq/accessing_json_data/