currency.hs

#!/usr/bin/env stack
-- stack --resolver lts-6.25 script

import Data.Ratio

type Currency = (String, Ratio Integer)
type Value = (Currency, Ratio Integer)

convert :: Value -> Currency -> Value
convert ((_, a), amount) (currency, b) = ((currency, b), (amount * a) * b)

main = do
  print (convert (("EUR", 1.2), 1000) ("USD", 1))
  print (convert (("USD", 1), 1000) ("EUR", 1.2))