[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.

[munificent]

You're welcome.

The real challenge is that pictures like this only work for simple two-element tuples. Once you have more fields in the tuple, or nested destructuring, the manifold of the value space the patterns have to cover quickly gets into higher dimensions that are basically impossible to visualize.

[tomcam]

Yeah but you helped me over the initial hump. Much easier to get to the more involved cases now.

[zengid]

Bravo! This is awesome.