You're probably right. It's very likely that you have worked on more C++
codebases than I have, and I haven't been working on code for
high-performance computing, so it's possible that I'm suffering from a
small sample size. But if you're working on a consumer app, I do think that
it's logical vtable dispatch is what you want most of the time. So in my
experience, functions need to be virtual more often than not, and the C++
code I've seen would be shorter if you had to explicitly mark methods as
nonvirtual rather than virtual.
When I said, "programmers want virtual functions 99% of the time," I was
mostly thinking of the legion of programmers who grew up learning languages
where virtual methods are the only kinds of methods. Objective-C,
JavaScript, Ruby, Python, Java, etc. I've worked with a few younger
programmers who are thrown to the C++ sharks by management, and once they
learn the difference between virtual and non-virtual methods, they tend to
mark all their methods virtual as a defensive measure.
You make a very good point about Swift being the second major language to
take value semantics correctly. My original point though, was once most iOS
developers move to Swift, I think it's possible that they'll just stick to
what they're comfortable with, using classes exclusively and writing
Massive View Controllers, because it's what they know, it's easy to do, and
it doesn't require learning sometimes conceptually difficult new concepts.
So my question is whether we want to make that more difficult for them. It
seems like there are benefits and disadvantages to both. I'm just trying to
raise the possibility that this may be the dominant programming paradigm in
Swift for some time, as unfortunate as that may be.
···
On Mon, Dec 21, 2015 at 10:04 AM, Dave Abrahams <dabrahams@apple.com> wrote:
On Dec 20, 2015, at 3:51 PM, Michael Buckley via swift-evolution < > swift-evolution@swift.org> wrote:
+0. This seems reasonable, and a lot of the arguments are compelling. The
argument put forth about library design especially so. But coming from C++,
where I have to prefix nearly every method in my classes with virtual, I'm
worried that we could end up with the same problem in Swift.We don't know what the dominant paradigm in swift will be ten years from
now. Inheritance has a raft of problems, but there's no guarantee that the
alternatives will be better in the long run. I suspect they will be, but I
also suspect we will find new and exciting problems in large codebases
using more functional patterns.While there's a lot of excitement in the Swift community right now about
final, value types, and other language features, but I fear that when the
rest of the world jumps on the Swift bandwagon, most are just going to use
classes exclusively over structs and continue their OOP practices, simply
because it's what they're used to.Making final the default may be a great way to discourage them. But it may
also get us right back to where we are in C++ today, where programmers want
virtual functions 99% of the time, but have to specify each function as
virtual.In my considerable experience with C++, that is not at all where we are
today. Increasingly, C++ is becoming seen as a language for
high-performance computing, and people working in that area learn that they
don't want to pay for virtual dispatch when they don't have to. It is true
that for some of them, reflexive use of OOP is hard to shake, but they do
learn eventually. Note also that Swift is really the second major language
to take value semantics seriously. The first was C++.On Sun, Dec 20, 2015 at 2:53 PM, Charles Srstka via swift-evolution < > swift-evolution@swift.org> wrote:
I agree with this. -1 to the proposal.
Charles
On Dec 17, 2015, at 8:00 PM, Rod Brown via swift-evolution < >> swift-evolution@swift.org> wrote:
To play devils advocate, take for example UINavigationController in UIKit
on iOS.I’ve seen multiple times in multiple projects legitimate reasons for
subclassing it, despite the fact that UIKit documentation says we “should
not need to subclass it”. So if we relied on Apple to “declare”, they most
probably wouldn’t, and these use cases (and some really impressive apps)
would become impossible.While I agree with all points made about “If it’s not declared
subclassable, they didn’t design it that way”, I think that ties everyone’s
hands too much. There is a balance between safety and functionality that
must be worked out. I think this errs way too far on the side of safety.Rod
On 18 Dec 2015, at 12:51 PM, Javier Soto <javier.api@gmail.com> wrote:
What if one framework provider thinks “you won’t need to subclass this
ever”If the framework author didn't design and implement that class with
subclassing in mind, chances are it's not necessarily safe to do so, or at
least not without knowledge of the implementation. That's why I think
deciding that a class can be subclassed is a decision that should be made
consciously, and not just "I forgot to make it final"
On Thu, Dec 17, 2015 at 5:41 PM Rod Brown <rodney.brown6@icloud.com> >> wrote:My opinion is -1 on this proposal. Classes seem by design to
intrinsically support subclassing.What if one framework provider thinks “you won’t need to subclass this
ever” but didn’t realise your use case for doing so, and didn’t add the
keyword? When multiple developers come at things from different angles, the
invariable situation ends with use cases each didn’t realise. Allowing
subclassing by default seems to mitigate this risk at least for the most
part.I think this definitely comes under the banner of “this would be nice”
without realising the fact you’d be shooting yourself in the foot when
someone doesn’t add the keyword in other frameworks and you’re annoyed you
can’t add it.On 18 Dec 2015, at 10:46 AM, Javier Soto via swift-evolution < >>> swift-evolution@swift.org> wrote:
Does it seem like there's enough interesest in this proposal? If so,
what would be the next steps? Should I go ahead and create a PR on the
evolution repo, describing the proposal version that Joe suggested, with
classes closed for inheritance by default outside of a module?Thanks!
On Tue, Dec 8, 2015 at 7:40 AM Matthew Johnson via swift-evolution < >>> swift-evolution@swift.org> wrote:
I understand the rationale, I just disagree with it.
IMO adding a keyword to state your intention for inheritance is not a
significant obstacle to prototyping and is not artificial bookkeeping. I
really don't understand how this would conflict with "consequence-free"
rapid development. It is a good thing to require people to stop and think
before using inheritance. Often there is a more appropriate alternative.The assumption that it is straightforward to fix problems within a
module if you later decide you made a mistake is true in some respects but
not in others. It is not uncommon for apps to be monolithic rather than
being well factored into separate modules, with many developers
contributing and the team changing over time. While this is not ideal it
is reality.When you have the full source it is certainly *possible* to solve any
problem but it is often not straightforward at all. Here is an example of
a real-work scenario app developers might walk into:1) A class is developed without subclassing in mind by one developer.
2) After the original developer is gone another developer adds some
subclasses without stopping to think about whether the original developer
designed for subclassing, thereby introducing subtle bugs into the app.
3) After the second developer is gone the bugs are discovered, but by
this time there are nontrivial dependencies on the subclasses.
4) A third developer who probably has little or no context for the
decisions made by previous developers is tasked with fixing the bugs.This can be quite a knot to untangle, especially if there are problems
modifying the superclass to properly support the subclasses (maybe this
breaks the contract the superclass has with its original clients).It may have been possible to avoid the whole mess if the second
developer was required to add 'inheritable' and 'overrideable' keywords or
similar. They are already required to revisit the source of it while
adding the keywords which may lead to consideration of whether the
implementation is sufficient to support inheritance in their currently
intended manner.Implementation inheritance is a blunt tool that often leads to
unanticipated problems. IMO a modern language should steer developers away
from it and strive to reduce the cases where it is necessary or more
convenient. Making final the default would help to do this.Supporting sealed classes and methods that can only be subclassed or
overridden within the same module is not in conflict with final by
default. Both are good ideas IMO and I would like to see both in Swift.I hope the core team is willing to revisit this decision with community
input. If not I will let it go, although I doubt I will ever agree with
the current decision.Matthew
Sent from my iPad
On Dec 7, 2015, at 10:30 PM, John McCall <rjmccall@apple.com> wrote:
>>> On Dec 7, 2015, at 7:18 PM, Matthew Johnson via swift-evolution < >>>> swift-evolution@swift.org> wrote:
>>> Defaults of public sealed/final classes and final methods on a
class by default are a tougher call. Either way you may have design issues
go unnoticed until someone needs to subclass to get the behavior they want.
So when you reach that point, should the system error on the side of rigid
safety or dangerous flexibility?
>>
>> This is a nice summary of the tradeoff. I strongly prefer safety
myself and I believe the preference for safety fits well with the overall
direction of Swift. If a library author discovers a design oversight and
later decides they should have allowed for additional flexibility it is
straightforward to allow for this without breaking existing client code.
>>
>> Many of the examples cited in argument against final by default have
to do with working around library or framework bugs. I understand the
motivation to preserve this flexibility bur don't believe bug workarounds
are a good way to make language design decisions. I also believe use of
subclasses and overrides in ways the library author may not have intended
to is a fragile technique that is likely to eventually cause as many
problems as it solves. I have been programming a long time and have never
run into a case where this technique was the only way or even the best way
to accomplish the task at hand.
>>
>> One additional motivation for making final the default that has not
been discussed yet is the drive towards making Swift a protocol oriented
language. IMO protocols should be the first tool considered when dynamic
polymorphism is necessary. Inheritance should be reserved for cases where
other approaches won't work (and we should seek to reduce the number of
problems where that is the case). Making final the default for classes and
methods would provide a subtle (or maybe not so subtle) hint in this
direction.
>>
>> I know the Swift team at Apple put a lot of thought into the
defaults in Swift. I agree with most of them. Enabling subclassing and
overriding by default is the one case where I think a significant mistake
was made.
>
> Our current intent is that public subclassing and overriding will be
locked down by default, but internal subclassing and overriding will not
be. I believe that this strikes the right balance, and moreover that it is
consistent with the general language approach to code evolution, which is
to promote “consequence-free” rapid development by:
>
> (1) avoiding artificial bookkeeping obstacles while you’re hacking
up the initial implementation of a module, but
>
> (2) not letting that initial implementation make implicit source and
binary compatibility promises to code outside of the module and
>
> (3) providing good language tools for incrementally building those
initial prototype interfaces into stronger internal abstractions.
>
> All the hard limitations in the defaults are tied to the module
boundary because we assume that it’s straightforward to fix any problems
within the module if/when you decided you made a mistake earlier.
>
> So, okay, a class is subclassable by default, and it wasn’t really
designed for that, and now there are subclasses in the module which are
causing problems. As long as nobody's changed the default (which they
could have done carelessly in either case, but are much less likely to do
if it’s only necessary to make an external subclass), all of those
subclasses will still be within the module, and you still have free rein to
correct that initial design mistake.
>
> John.
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