> Extensibility

> [Data] The schema gives us a fixed set of variants, over which you can of course write any function you want.

> [Objects] We have a fixed set of exposed operations, but different variants can be constructed (including an improved ability to evolve a variant without impacting clients).

No mention of the expression problem? The TL;DR is, sometimes[1] we want both. And sometimes[2] it’s an exceptionally good idea for a lot of slightly different sets of variants to coexist withn the same program, which there also isn’t really a satisfactory solution for.

[1] https://www.craftinginterpreters.stuffwithstuff.com/represen...

[2] https://nanopass.org/

Why is the most relevant comment at the bottom of the page?

The expression problem absolutely captures the nature of the issue and a lot has been written about it.