Reflect library in Haskell (there will be probably better ways to do this)

reflect_haskell.hs

{-# LANGUAGE Rank2Types, FlexibleContexts, UndecidableInstances #-}
import Data.Reflection
import Data.Proxy
import Text.Printf
import Prelude

data Coin = Euro | USD

newtype Amount coin = Amount { _amount :: Int }

instance Reifies coin Coin => Show (Amount coin) where
  show a@(Amount amount) =
    case reflect a of
      Euro -> printf "%.2f euros" (fromIntegral amount * 1.0 :: Double)
      USD  -> printf "%.2f usd" (fromIntegral amount * 0.9 :: Double)

likeProxy :: Proxy coin -> Amount coin -> Amount coin
likeProxy _ = id

printDivisa :: Coin -> String
printDivisa coin = reify coin $ \proxy ->
  -- The lifted type can't scape this closure.
  show $ likeProxy proxy (Amount 100)