calculus-use-case-with-operation.dart

class CalculatorUseCase {
  num memory;
  Operation operation; // an instance of Operation class
  num operandA;
  num operandB;

  CalculatorUseCase(
      {Operation operation, this.operandA = 0, this.operandB = 0}) {
    this.operation = operation ?? Add;
  }

  num get value => operation.oper(operandB)(operandA);
  String get format => operation.format(operandB)(operandA);
  ...
}