JSON with default value using Nullable

DefaultJson.hs

{-# LANGUAGE DataKinds        #-}
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE PolyKinds        #-}
{-# LANGUAGE TypeOperators    #-}

module DefaultJson where

import           Data.Aeson
import           Data.Extensible
import           Data.Maybe      (fromMaybe)

type Foo = Record FooFields
type FooFields =
  '[ "a" >: Int
   , "b" >: Int
   ]

type Foo' = Nullable (Field Identity) :* FooFields

defaultFoo :: Foo
defaultFoo
    = #a @= 10
   <: #b @= 50
   <: nil

fromNullable :: RecordOf h xs -> Nullable (Field h) :* xs -> RecordOf h xs
fromNullable def =
  hmapWithIndex $ \m x -> fromMaybe (hlookup m def) (getNullable x)