Merge datatypes with Aeson

MergeAeson.hs

{-# LANGUAGE TypeOperators, FlexibleInstances, DeriveGeneric, OverloadedStrings #-}
module JSON where

import GHC.Generics
import Data.Aeson

import qualified Data.HashMap.Lazy as M

data A = A {a :: String} deriving (Show, Generic)
data B = B {b :: Float} deriving (Show, Generic)

instance ToJSON A
instance ToJSON B

data Merge a b = Merge a b deriving Show

instance ToJSON (Merge A B) where
  toJSON (Merge a b) = mergeJSON [toJSON a, toJSON b]

mergeJSON :: [Value] -> Value
mergeJSON = Object . M.unions . map (\(Object x) -> x)

aVal :: A
aVal = A "str"

bVal :: B
bVal = B 12.34

cVal :: Merge A B
cVal = Merge aVal bVal

-- then do: encode cVal --> {"a": "str", "b": 12.34}

https://gist.github.com/cdparks/32217e570048dd2066549272b1640b5b