Sample of dart fold method

dart_fold.dart

class PaymentLine {
  double price;
  String name;
  int qty;
  
  PaymentLine({ this.price, this.name, this.qty });
}

void main() {
  List<PaymentLine> payments = <PaymentLine>[];
  payments.add(PaymentLine()
               ..name = 'Food'
               ..price = 10.99
               ..qty = 1);
  
  payments.add(PaymentLine()
               ..name = 'Litre of Water'
               ..price = 2.49
               ..qty = 5);
  
  payments.add(PaymentLine()
              ..name = 'Autograph'
              ..price = 100.0
              ..qty = 1);
  
  double totalPrice = payments.fold(0, (double previousValue, PaymentLine nextLine) => previousValue += nextLine.price);
  int totalQty = payments.fold(0, (int previousValue, PaymentLine nextLine) => previousValue += nextLine.qty);
  
  print('Total Price: $totalPrice (should be 113.48)');
  print('Total Qty: $totalQty (should be 7)');
}