fn fill_order(&mut self, market_order: &mut Order) {
        for limit_order in self.orders.iter_mut() {
            if limit_order.size >= market_order.size {
                limit_order.size -= market_order.size;
                market_order.size = 0.0;
                break;
            } else {
                market_order.size -= limit_order.size;
                limit_order.size = 0.0;
            }
        }
    }

I want to rewrite this function with functional style