[swift-evolution] [swift-evolution-announce] [Review] SE-0117: Default classes to be non-subclassable publicly

Tino Heth 2th at gmx.de
Tue Jul 12 15:44:13 CDT 2016

Thanks for the detailed answer — I didn't expect it.

I'll re-order the original message, since you had a genuine question (to bad for me if it was just a rhetorical one :) whose answer might be more interesting than the pointless remarks afterwards ;-)

> If you were writing a library, what would make you decide between “experimental” and not?

I guess I would mark everything immature/experimental/whatever on release (as long as it's not to hard to do so…), and then decide step by step wether a method should be public, overridable, private — or removed completely ;-)
Of course, there wouldn't be an universal scale, but most likely each developer/team would keep a reliable standard (some developers are very careful, some are bolder…)
If we are honest, a "sealed" that is only applied because it is the default is actually "experimental" — and in the age of open source, I don't think this should be equivalent to "you can't use this" (some conservative developers consider Swift itself to be immature ;-)
The whole concept is just a spontaneous idea, and maybe I'd come to the conclusion I actually don't like it; but as we can mark stuff as deprecated to notify users that they should stop using a certain method, it might as well be useful to annotate something as unstable to prevent certain developers start using a method (yet).

Now for the remarks — they are pointless indeed, because I'm not expecting to change the mind of anyone with a well-grounded opinion
> The binary compatibility concerns are less important, but the ability of a library author to reason about behavior is still critical. Here’s a scenario I really don’t want to see happen:
> 1. Client X adds a dependency on library A, via the package manager. They override a public method in library A to get the behavior they want.
> 2. Library A ships an update which fixes many bugs and security issues, and happens to eliminate the internal calls to the public method A was using. That is, overriding Athat method no longer has any effect within library A; only clients see the difference.
> 3. The developer for client X goes to their manager and asks for time to fix the issue. The manager tells them not to update right now, but maybe after this next release.
> 4. Client X never gets a new version of Library A ever again.

Imho this is remarkable in two aspects:
It is the first real example I'm aware of where I see a true problem, and it "overthrows" (there might be a less dramatic word I'm not aware of) the position that "sealed" primarily solves a problem for library-authors.

I agree that this problem is much less likely with sealed as default, but there's another one in this scenario:
Client X depends on the later-removed feature in Library A — but as this feature isn't accessible for him, he won't ever start using Library A at all…
None the less, I have to admit that I actually prefer the problem in the sealed-scenario, as it doesn't directly effect developer T, but rather pointy-haired boss Y, who just lost a potential customer ;-)

I can't resist to add another culinary comparison to Jonathan Hull's excellent knife-analogy:
As the owner of a restaurant, would you fill your menu with food that's easy to cook to please your chef, or would you rather include food that tastes fantastic to please your customers?

> I guess another way of saying this is that library authors avoid these miscommunications by "under-promising”, and if that “under-promising” is something that’s actually in the language instead of maybe/maybe-not being documented, then both sides are much more likely to be on the same page.
Damn, maybe I should have put this in front… why make false promises at all, when you just can tell the truth?

Best regards,

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