[lptk]

As mentioned in a response to a sibling comment, we plan to support `or`, which should address the problem you mention. (If not, would you have an example of what you mean?)

> I worry that the semantics around exhaustiveness and mutable values may be confusing, though I guess OCaml already has that problem

Indeed, and it was until very recently a source of unsoundness: https://icfp24.sigplan.org/details/mlworkshop-2024-papers/8/...

[munificent]

Oh, wow, that's interesting.

We added pattern matching and exhaustiveness to Dart not that long ago, and dealing with exhaustiveness and mutability was a big concern since Dart (unlike more strictly functional languages) generally doesn't avoid mutability.

Our solution was that whenever a pattern accesses a property, the language implicitly caches that value. Any future accesses to the same property in that switch statement/expression use the previously cached value. That way, an entire set of switch cases is always operating on an immutable snapshot of data so that exhaustiveness can't be violated by side effects. Spec:

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