Learning Dart - 4 Create a factory via top-level function

main.dart

import 'dart:math' as math;

abstract class Shape {
  num get area;
}

class Circle implements Shape {
  final num radius;
  num get area => math.pi * math.pow(radius, 2);

  Circle(this.radius);
}

class Square implements Shape {
  final num side;
  num get area => math.pow(side, 2);

  Square(this.side);
}

Shape shapeFactory(String type) {
  if (type == 'circle') return Circle(2);
  if (type == 'square') return Square(2);
  throw 'Can\'t create $type';
}

main() {
  final circle = shapeFactory('circle');
  final square = shapeFactory('square');
  
  print(circle.area);
  print(square.area);
}