[zenhack]

Related, capnproto has a corresponding feature (which it calls unions), but being a dsl for defining _protocols_, you obviously have to deal with the possibility of introducing new variants as the protocol evolves.

I wrote the Haskell implementation. It maps unions to variants (what Rust calls enums), BUT it always generates an extra 'unkown' variant, which gets used whenever the variant is one not recognized by the generated code.

In this case the value proposition of variants/enums, including exhaustiveness checking, is still really useful -- it can not only deal with the possibility of more variants added in the future, it forces you to handle that possibility, which imo is the best of both worlds.

But yes, in general, when modeling data that comes from the outside world, you need to about overfitting.

Vaguely related: https://lexi-lambda.github.io/blog/2020/01/19/no-dynamic-typ...