Haskell Aeson json encoding bytestrings - json

Question

I need to serialize a record in Haskell, and am trying to do it with Aeson. The problem is that some of the fields are ByteStrings, and I can't work out from the examples how to encode them. My idea is to first convert them to text via base64. Here is what I have so far (I put 'undefined' where I didn't know what to do):
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE OverloadedStrings #-}
module Main where
import qualified Data.Aeson as J
import qualified Data.ByteString as B
import qualified Data.ByteString.Base64 as B64
import qualified Data.Text as T
import qualified Data.Text.Encoding as E
import qualified GHC.Generics as G
data Data = Data
{ number :: Int
, bytestring :: B.ByteString
} deriving (G.Generic, Show)
instance J.ToJSON Data where
toEncoding = J.genericToEncoding J.defaultOptions
instance J.FromJSON Data
instance J.FromJSON B.ByteString where
parseJSON = undefined
instance J.ToJSON B.ByteString where
toJSON = undefined
byteStringToText :: B.ByteString -> T.Text
byteStringToText = E.decodeUtf8 . B64.encode
textToByteString :: T.Text -> B.ByteString
textToByteString txt =
case B64.decode . E.encodeUtf8 $ txt of
Left err -> error err
Right bs -> bs
encodeDecode :: Data -> Maybe Data
encodeDecode = J.decode . J.encode
main :: IO ()
main = print $ encodeDecode $ Data 1 "A bytestring"
It would be good if it was not necessary to manually define new instances of ToJSON and FromJSON for every record, because I have quite a few different records with bytestrings in them.

Answer 1

parseJson needs to return a value of type Parser B.ByteString, so you just need to call pure on the return value of B64.decode.
import Control.Monad
-- Generalized to any MonadPlus instance, not just Either String
textToByteString :: MonadPlus m => T.Text -> m B.ByteString
textToByteString = case B64.decode (E.encodeUtf8 x) of
Left _ -> mzero
Right bs -> pure bs
instance J.FromJSON B.ByteString where
parseJSON (J.String x) = textToByteString x
parseJSON _ = mzero
Here, I've chosen to return mzero both if you try to decode anything other than a JSON string and if there is a problem with the base-64 decoding.
Likewise, toJSON needs just needs to encode the Text value you create from the base64-encoded ByteString.
instance J.ToJSON B.ByteString where
toJSON = J.toJSON . byteStringToText
You might want to consider using a newtype wrapper instead of defining the ToJSON and FromJSON instances on B.ByteString directly.