returning one or the other JSON instance

decodeOneOf.hs

import Data.Aeson (decode, FromJSON)
import Data.ByteString.Lazy (ByteString)
import Data.Monoid (First(..), (<>))

decodeOneOf :: (FromJSON x, FromJSON y) => ByteString -> Maybe (Either x y)
decodeOneOf bs = getFirst (First (fmap Left (decode bs)) <> First (fmap Right (decode bs)))

-- or more straightforwardly

decodeOneOf' :: (FromJSON x, FromJSON y) => ByteString -> Maybe (Either x y)
decodeOneOf' bs | Just x <- decode bs = Just (Left x)
                | Just y <- decode bs = Just (Right y)
                | otherwise           = Nothing