class Range<T extends num> {
  final T from;
  final T to;

  num get magnitude => to - from;

  Range(this.from, this.to);

  factory Range.fromList(List<T> list) {
    assert(list.length == 2);
    return Range(list.first, list.last);
  }
}

// Extension on Iterable<Range<T>> is not allowed. Error given:
// The name 'T' isn't a type, so it can't be used as a type argument.
extension RangeOfRanges on Iterable<Range<num>> {
  Range<num> range() {
    final min = (map((range) => range.from).toList()..sort()).first;
    final max = (map((range) => range.to).toList()..sort()).last;

    return Range<num>(min, max);
  }
}

void main() {
  final intRange = Range(-3, 6);
  print(intRange.magnitude);
  print(intRange.magnitude.runtimeType); // int, all good so far

  final rangeList = <Range<double>>[
    Range(3.5, 5.3),
    Range(6.3, 9.2),
    Range(-3.0, 13.1),
  ];
  final rangeOfRanges = rangeList.range();
  // I want to call a function that accepts Range<double>.
  print(rangeOfRanges.runtimeType); // Range<num> :(

  final nastyWorkaround = Range(
    rangeOfRanges.from.toDouble(),
    rangeOfRanges.to.toDouble()
  );
  print(nastyWorkaround.runtimeType); // Finally: Range<double>
}