record names via Aeson a la dsal

Foo.hs

{-# LANGUAGE DeriveGeneric #-}                                                                                                                                                                                       
                                                                                                                                                                                                                     
module Foo where                                                                                                                                                                                                     
                                                                                                                                                                                                                     
import qualified GHC.Generics as G                                                                                                                                                                                   
import Data.Aeson                                                                                                                                                                                                    
import Data.Aeson.Lens                                                                                                                                                                                               
import Data.HashMap.Strict                                                                                                                                                                                           
import Control.Lens                                                                                                                                                                                                  
                                                                                                                                                                                                                     
data Color = Red | Yellow | Blue                                                                                                                                                                                     
  deriving (Show, G.Generic)                                                                                                                                                                                         
                                                                                                                                                                                                                     
data Foo = Foo {                                                                                                                                                                                                     
    name :: String                                                                                                                                                                                                   
  , id :: Int                                                                                                                                                                                                        
  , favoriteColor :: Color                                                                                                                                                                                           
  } deriving (Show, G.Generic)                                                                                                                                                                                       
                                                                                                                                                                                                                     
instance ToJSON Color where                                                                                                                                                                                          
    toEncoding = genericToEncoding defaultOptions                                                                                                                                                                    
                                                                                                                                                                                                                     
instance FromJSON Color                                                                                                                                                                                              
                                                                                                                                                                                                                     
instance ToJSON Foo where                                                                                                                                                                                            
    toEncoding = genericToEncoding defaultOptions                                                                                                                                                                    
                                                                                                                                                                                                                     
instance FromJSON Foo                                                                                                                                                                                                
                                                                                                                                                                                                                     
getRecFieldnames rec = (toJSON rec) ^.. _Object . to keys . traversed                                                                                                                                                
                                                                                                                                                                                                                     
--                                                                                                                                                                                                                   
-- λ> getRecFieldnames (Foo "Bob" 1 Red)                                                                                                                                                                             
-- ["name","id","favoriteColor"]                                                                                                                                                                                     
-- λ>                                                                                                                                                                                                                
--