evalMoneyIn2.hs

evalMoneyIn :: (Conversion -> Maybe Rate) -> MoneyBag -> Currency -> Maybe Money
evalMoneyIn findRate (MoneyBag m) refCurrency = do
  let convert (curr, amount) = (* amount) <$> findRate (curr, refCurrency)
  amounts <- mapM convert (M.toList m)
  pure $ Money { amount = sum amounts, currency = refCurrency }