Is it acceptable to return null from a factory constructor in Dart?

Question

I've been writing some code in Dart. I really love the factory constructor, but I'm afraid that I'm abusing it's usefulness. In particular, when I write a value object class, I sometimes return null if the validation fails.

class EmailAddress {
  static final RegExp _regex = new RegExp(...);
  final String _value;

  factory EmailAddress(String input) {
    return _regex.hasMatch(input) ? new EmailAddress._internal(input) : null;
  }

  const EmailAddress._internal(this._value);

  toString() => _value;
}

At first, this doesn't seem all that bad. However, when you actually use it, this is what you see.

methodThatCreatesAnEmailAddress() {
  var emailAddress = new EmailAddress("definitely not an email address");
  ...
}

The argument why this is bad is that a developer coming from another statically typed language, such as Java or C++, would expect emailAddress to always be initialized to a non-null value. The argument why this is perfectly acceptable is that the constructor is factory and, as such, is allowed to return a null value.

So is this bad practice or taking advantage of a useful feature?

Answer 1

With null-safe Dart, factory constructors are no longer permitted to return null.

Existing factory constructors that return null are expected to be replaced with static methods when migrated.