Thanks for Cap'n Proto. It's better than ProtoBuf, but I prefer FlatBuffers even more. I think the article is clearly indicating the issues that a wider community of conventional type systems in their mainstream languages is not fully aware of. And I disagree with your comments. Firstly, I don't like that you are labelling the author of the article as a "PL design theorist who doesn't have a clue" (my interpretation applied):

> his article appears to be written by a programming language design theorist who, unfortunately, does not understand (or, perhaps, does not value) practical software engineering.

I'm not the author, but they mention their prior industrial experience with protobufs at Google, among other unnamed places.

I'm not a PL theorist either, and I see that you don't fully understand the problems of composability, compatibility, and versioning and are too eager to dismiss them based on your prior experience with inferior type systems. And here's why I think it is the case:

> > This is especially true when it comes to protocols, because in a distributed system, you cannot update both sides of a protocol simultaneously. I have found that type theorists tend to promote "version negotiation" schemes where the two sides agree on one rigid protocol to follow, but this is extremely painful in practice: you end up needing to maintain parallel code paths, leading to ugly and hard-to-test code. Inevitably, developers are pushed towards hacks in order to avoid protocol changes, which makes things worse.

You are conflating your experience with particular conventional tooling with a general availability of superior type systems and toolings out there. There's a high demand in utilising their properties in protocol designs today, where most of the currently popular protocols are hampering type systems for no good reason (no productivity gain, no performance gain, no resource utilisation gain).

Version negotiation is not the only option available to a protocol designer. It is possible to use implicit-for-client and explicit-for-developer strategies to schema migration. It is also possible to semi-automate inference of those strategies. Example [1]

> This seems to miss the point of optional fields. Optional fields are not primarily about nullability but about compatibility. Protobuf's single most important feature is the ability to add new fields over time while maintaining compatibility.

There are at least two ways to achieve compatibility, and the optional fields that expand a domain type to the least common denominator of all encompassing possibilities is the wrong solution to this. Schema evolution via unions, versioning, and migrations is the proper approach that allows for strict resolution of compatibility issues with a level of granularity (distinct code paths) you like.

> Real-world practice has also shown that quite often, fields that originally seemed to be "required" turn out to be optional over time, hence the "required considered harmful" manifesto. In practice, you want to declare all fields optional to give yourself maximum flexibility for change.

This is false. In practice I want a schema versioning and deprecation policies, and not ever-growing domain expansion to the blob of all-optional data.

> It's that way because the "oneof" pattern long-predates the "oneof" language construct. A "oneof" is actually syntax sugar for a bunch of "optional" fields where exactly one is expected to be filled in.

this is not true either, and it doesn't matter what pattern predates which other pattern. Tagged unions are neither a language construct nor a syntax sugar, it's a property of Type Algebra where you have union- and product-compositions. Languages that implement Type Algebra don't do it to just add another fancy construct, they do it to benefit from mathematical foundations of these concepts.

> How do you make this change without breaking compatibility?

you version it, and migrate over time at your own pace without bothering your clients too often [1]

[1] https://github.com/typeable/schematic#migrations