Parsing a nested array of objects with Aeson

Question

I want to parse a JSON object and create a JSONEvent with the given name and args

I'm using Aeson, and right now I'm stucked on converting "args":[{"a": "b"}] to a [(String, String)].

Thank's in advance!

{-# LANGUAGE OverloadedStrings #-}

import Control.Applicative
import Data.Aeson

data JSONEvent = JSONEvent [(String, String)] (Maybe String) deriving Show

instance FromJSON JSONEvent where
  parseJSON j = do
    o <- parseJSON j
    name <- o .:? "name"
    args <- o .:? "args" .!= []
    return $ JSONEvent args name

let decodedEvent = decode "{\"name\":\"edwald\",\"args\":[{\"a\": \"b\"}]}" :: Maybe JSONEvent

cheecheeo · Accepted Answer · 2012-01-06 22:28:44Z

Here's a bit more elaborate example based on ehird's example. Note that the explicit typing on calls to parseJSON is unnecessary but I find them useful for documentation and debugging purposes. Also I'm not sure what you intended, but with args with multiple values I simply concatenate all the args together like so:

*Main> decodedEvent2
Just (JSONEvent [("a","b"),("c","d")] (Just "edwald"))
*Main> decodedEvent3
Just (JSONEvent [("a","b"),("c","d")] (Just "edwald"))

Here's the code:

{-# LANGUAGE OverloadedStrings #-}

import Control.Applicative
import qualified Data.Text as T
import qualified Data.Foldable as F
import qualified Data.HashMap.Lazy as HM
import qualified Data.Vector as V
import Data.Aeson

import qualified Data.Attoparsec as P
import Data.Aeson.Types (Parser)
import qualified Data.Aeson.Types as DAT
import qualified Data.String as S

data JSONEvent = JSONEvent [(String, String)] (Maybe String) deriving Show

instance FromJSON JSONEvent where
  parseJSON = parseJSONEvent

decodedEvent = decode "{\"name\":\"edwald\",\"args\":[{\"a\": \"b\"}]}" :: Maybe JSONEvent
decodedEvent2 = decode "{\"name\":\"edwald\",\"args\":[{\"a\": \"b\"}, {\"c\": \"d\"}]}" :: Maybe JSONEvent
decodedEvent3 = decode "{\"name\":\"edwald\",\"args\":[{\"a\": \"b\", \"c\": \"d\"}]}" :: Maybe JSONEvent

emptyAesonArray :: Value
emptyAesonArray = Array $ V.fromList []

parseJSONEvent :: Value -> Parser JSONEvent
parseJSONEvent v =
  case v of
    Object o -> do
      name <- o .:? "name"
      argsJSON <- o .:? "args" .!= emptyAesonArray
      case argsJSON of
        Array m -> do
          parsedList <- V.toList <$> V.mapM (parseJSON :: Value -> Parser (HM.HashMap T.Text Value)) m
          let parsedCatList = concatMap HM.toList parsedList
          args <- mapM (\(key, value) -> (,) <$> (return (T.unpack key)) <*> (parseJSON :: Value -> Parser String) value) parsedCatList
          return $ JSONEvent args name
        _ -> fail ((show argsJSON) ++ " is not an Array.")
    _ -> fail ((show v) ++ " is not an Object.")

-- Useful for debugging aeson parsers
decodeWith :: (Value -> Parser b) -> String -> Either String b
decodeWith p s = do
  value <- P.eitherResult $ (P.parse json . S.fromString) s
  DAT.parseEither p value

ehird · Accepted Answer · 2012-01-06 16:51:24Z

3

I'm not an aeson expert, but if you have Object o, then o is simply a HashMap Text Value; you could use Data.HashMap.Lazy.toList to convert it into [(Text, Value)], and Data.Text.unpack to convert the Texts into Strings.

So, presumably you could write:

import Control.Arrow
import Control.Applicative
import qualified Data.Text as T
import qualified Data.Foldable as F
import qualified Data.HashMap.Lazy as HM
import Data.Aeson

instance FromJSON JSONEvent where
  parseJSON j = do
    o <- parseJSON j
    name <- o .:? "name"
    Object m <- o .:? "args" .!= []
    args <- map (first T.unpack) . HM.toList <$> F.mapM parseJSON m
    return $ JSONEvent args name

edited Jan 6, 2012 at 16:51

answered Jan 6, 2012 at 16:36

ehird

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2 Comments

masylum Over a year ago

I don' really get what's going on. I'm quite new at Haskell.. This is what I get:

ehird Over a year ago

@masylum: Ah, I misread your code slightly. Do you want the parsing to fail if, e.g. you have "args": [{"a":"b","c":d"}], i.e. an object with more than one key in the list? As for the "not in scope" error, you forgot the import Control.Arrow.

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