evalMoneyIn.hs

evalMoneyIn :: (Conversion -> Maybe Rate) -> MoneyExpr -> Currency -> Maybe Money
evalMoneyIn findRate expr refCurrency = visit expr
  where
    visit (KnownAmount (Money amt curr)) = do
      rate <- findRate (curr, refCurrency)
      pure $ Money (rate * amt) refCurrency

    visit (MoneyAdd subs) = do
      ms <- mapM visit subs
      pure $ Money (sum (map amount ms)) refCurrency

    visit (MoneyMul expr factor) = do
      m <- visit expr
      pure $ Money (factor * amount m) refCurrency