Derive Monoid for using Generics

GenericMonoid.hs

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE DeriveGeneric #-}

module GenericMonoid where

import Data.Monoid
import Data.Maybe
import Generics.OneLiner
import GHC.Generics

data MyThing = MyThing
  { fieldOne :: String
  , fieldTwo :: Sum Int
  } deriving Generic

gmempty :: forall a . (ADT a, Constraints a Monoid) => a
gmempty = head $ create @Monoid [mempty]

-- Note that this will fail if your type has multiple constructors 🤷‍♂️
gmappend :: forall a . (ADT a, Constraints a Semigroup) => a -> a -> a
gmappend a b =
  fromJust $ zipWithA @Semigroup @a @Maybe (\a b -> Just (a <> b)) a b

instance Semigroup MyThing where
  (<>) = gmappend

instance Monoid MyThing where
  mempty = gmempty