# Pastebin Ty5NHw5K
data X a = X {someVal :: Int, otherVal :: a}
  deriving Generic

data SomeX = IntX (X Int) | DoubleX (X Double)
  deriving Generic

class GHasSomeVal (f :: Type -> Type) where
  gsomeVal :: Lens' (f x) Int

instance GHasSomeVal (K1 R (X a)) where
  gsomeVal = _K1 . #someVal

instance GHasSomeVal f => GHasSomeVal (M1 i c f) where
  gsomeVal = _M1 . gsomeVal

instance (GHasSomeVal f, GHasSomeVal g) => GHasSomeVal (f :+: g) where
  gsomeVal f = \case
    L1 x -> L1 <$> gsomeVal f x
    R1 x -> R1 <$> gsomeVal f x

theSomeVal :: (Generic s, GHasSomeVal (Rep s)) => Lens' s Int
theSomeVal = generic . gsomeVal