extension on List<num> {
  bool get isTriangle {
    sort();
    return length == 3 && this[0] + this[1] > this[2];
  }
}

class Triangle {
  bool equilateral(double a, double b, double c) =>
      [a, b, c].isTriangle && {a, b, c}.length == 1;

  bool isosceles(double a, double b, double c) =>
      [a, b, c].isTriangle && {a, b, c}.length <= 2;

  bool scalene(double a, double b, double c) =>
      [a, b, c].isTriangle && {a, b, c}.length == 3;
}