How to craft a String enum in Dart 3.x?

I'm used to design num enums in Dart 3.x, but there, I need a String enum.

I tried to do the same as before but I get compilers issues which I don't know how to solve:

> The invoked constructor isn't a 'const' constructor.
Try invoking a 'const' generative constructor.

This error points to each enum value.

And that one for the factory:

>The body might complete normally, causing 'null' to be returned, but the return type, 'MonetizationSubscriptionProductId<String>', is a potentially non-nullable type.
Try adding either a return or a throw statement at the end.

My code is as follows:

enum MonetizationSubscriptionProductId<String> {
  /// Premium certified individual subscription
  kSubscriptionIdPremiumCertifiedIndividuel('premium'),

  /// Premium certified individual professional subscription
  kSubscriptionIdPremiumCertifiedProfessional('premium_certified_professional');

  const MonetizationSubscriptionProductId(this.value);

  final String value;

  factory MonetizationSubscriptionProductId<String>.fromValue(String v) =>
      values.firstWhere((x) => x.value == v,
          orElse: () => throw Exception("Unknown value $v"));
}

UPDATE: Regarding the "You can't have factory constructors for enums.", see the working code I usually use for my enums.

// Smiley size
enum SmileySize {
  small(0),
  normal(1),
  large(2);

  const SmileySize(this.value);
  final int value;

  factory SmileySize.fromValue(int v) =>
      values.firstWhere((x) => x.value == v, orElse: () => normal);
}

The generic type for MonetizationSubscriptionProductId doesn't seem necessary. The enum fields aren't tied to their generic type (in fact, I couldn't come up with any use cases for generic enums; if you have any, it would be interesting to know).

If you want to have the fromValue function, I suggest creating a top-level method or a static function (not inside the enum itself, though; it won't work).

To put it all together, perhaps something like this is what you're looking for:

enum MonetizationSubscriptionProductId {
  /// Premium certified individual subscription
  kSubscriptionIdPremiumCertifiedIndividuel('premium'),

  /// Premium certified individual professional subscription
  kSubscriptionIdPremiumCertifiedProfessional('premium_certified_professional');

  const MonetizationSubscriptionProductId(this.value);

  final String value;
}

MonetizationSubscriptionProductId mapValueToMonetizationSubscriptionProductId(String v) =>
    MonetizationSubscriptionProductId.values.firstWhere(
      (x) => x.value == v,
      orElse: () => throw Exception("Unknown value $v"),
    );

After remark of @f-person and giving a second thought, there is no need to actually have the enum to be String-based.

Just do what I usually do and use the toString() to set the actual product ID I need, as follows:

enum MonetizationSubscriptionProductId {
  /// Premium certified individual subscription
  kSubscriptionIdPremiumCertifiedIndividuel(0),

  /// Premium certified individual professional subscription
  kSubscriptionIdPremiumCertifiedProfessional(1);

  const MonetizationSubscriptionProductId(this.value);

  final num value;

  factory MonetizationSubscriptionProductId.fromValue(num v) =>
      values.firstWhere((x) => x.value == v,
          orElse: () => throw Exception("Unknown value $v"));

  @override
  String toString() {
    switch (this) {
      case kSubscriptionIdPremiumCertifiedIndividuel:
        return 'premium';
      case kSubscriptionIdPremiumCertifiedProfessional:
        return 'premium_certified_professional';
    }
  }
}