[andreypopp]

Does it (Dart, C#) support exhaustivity checking for such matching over subclasses?

[munificent]

I don't know about C#, but, yes Dart has quite sophisticated exhaustiveness checking over sealed class hiearchies, including hierarchies that are DAGs and not just trees. It also handles record/tuple types and generics, nested arbitrarily deeply. And patterns that call arbitrary getters on objects.

For example, if you have this class hierarchy:

    sealed class A {}
    sealed class B1 implements A {}
    sealed class B2 implements A {}
    class C1 implements B1 {}
    class C2 implements B1, B2 {}
    class C3 implements B2 {}

    //       A
    //      / \
    //    B1   B2
    //   / \  / \
    // C1   C2   C3

Then it understands that this switch statement is exhaustive:

    void test(A a1, A a2) {
      switch ((a1, a2)) {
        case (A(), C1()): print(1);
        case (C2(), C2()): print(2);
        case (C3(), B2()): print(3);
        case (B1(), B2()): print(4);
      }
    }

Because they cover the Cartesian product of all possible combinations of subtypes like:

            a1
                   A
                  / \
                B1   B2
               / \  / \
             C1   C2   C3
    a2     +----+----+----+
         C1|      1       |
      B1<  +----+----+----+
    A<   C2|    |  2 |    |
      B2<  +    +----+ 3  +
         C3| 4       |    |
           +----+----+----+

This can extend to arbitrary dimensions based on how many record fields there are.

Figuring that all out was quite the challenge! The full design is here:

https://github.com/dart-lang/language/blob/main/accepted/3.0...

[tomcam]

thanks tons for that amazing comment. Helped me tremendously. Illustrations are incredibly helpful in cases like this.

[zengid]

Bravo! This is awesome.

[jjtheblunt]

https://dart.dev/language/class-modifiers-for-apis#the-seale...

that first paragraph, i believe, addresses your question, with excellent example from munificent in sibling comment.

[zengid]

Not sure about Dart, but for C# I don't think it can (due to classes being open by default).

If anyone is interested in a deeper dive on C# and adding Discriminated Unions, this interview by Nick Chapsas with Mads Torgersen is a great discussion: https://youtu.be/Nuw3afaXLUc?t=3939 (note, I'm pretty sure this time stamp is the right part, but the discussion might start before this. I'm at work and can't listen too closely to the video atm).

[zengid]

After re-watching, exhaustiveness is not a feature yet. If you look above to munificent's comment, you'll see that Dart has it! very cool!

[randyrand]

yes, you declare your class 'sealed' (introduced in Dart 3.0)