{-# LANGUAGE DerivingStrategies         #-}
{-# LANGUAGE DerivingVia                #-}
{-# LANGUAGE GeneralisedNewtypeDeriving #-}
{-# LANGUAGE StandaloneDeriving         #-}
{-# LANGUAGE FlexibleInstances          #-}
{-# LANGUAGE MultiParamTypeClasses      #-}

import Data.Foldable (fold)
import Data.Monoid   (Sum (..))

newtype Currency a = Currency a
  deriving (Show)
  deriving stock (Eq) -- Deriving strategies; Stock is the default strategy.
  deriving (Semigroup) via (Sum a) -- Deriving via

-- Standalone deriving
deriving instance Read a => Read (Currency a)
-- Standalone deriving and deriving strategies; Newtype is the newtype strategy.
deriving newtype instance Ord a => Ord (Currency a)
-- Standalone deriving, deriving strategies and deriving via; Num is a constraint for Sum.
deriving via (Sum a) instance Num a => Monoid (Currency a)

main :: IO ()
main = do
  print $ fold $ fmap Currency [2, 3, 5, 7, 11]