> On Dec 28, 2015, at 12:59 PM, Joe Groff <jgr...@apple.com> wrote:
>
>>
>> On Dec 27, 2015, at 9:51 AM, Matthew Johnson via swift-evolution
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>
>>>
>>> On Dec 26, 2015, at 11:31 PM, Chris Lattner <clatt...@apple.com
>>> <mailto:clatt...@apple.com>> wrote:
>>>
>>> On Dec 25, 2015, at 12:04 PM, Matthew Johnson <matt...@anandabits.com
>>> <mailto:matt...@anandabits.com>> wrote:
>>>>>> Discussion on a couple of topics continues inline below as well.
>>>>>
>>>>> Great, comments below:
>>>>
>>>> Thanks for continuing the discussion. I have updated the proposal to
>>>> reflect the core functionality and moved everything else to the future
>>>> enhancements section. I think this draft is ready or nearly ready for a
>>>> PR.
>>>>
>>>> Here is a link to the current draft:
>>>> https://github.com/anandabits/swift-evolution/edit/flexible-memberwise-initialization/proposals/NNNN-flexible-memberwise-initialization.md
>>>>
>>>> <https://github.com/anandabits/swift-evolution/edit/flexible-memberwise-initialization/proposals/NNNN-flexible-memberwise-initialization.md>
>>>>
>>>> Here is a link to the commit diff:
>>>> https://github.com/anandabits/swift-evolution/commit/f15360d2e201709640f9137d86a8b705a07b5466?short_path=f5ec377#diff-f5ec377f4782587684c5732547456b70
>>>>
>>>> <https://github.com/anandabits/swift-evolution/commit/f15360d2e201709640f9137d86a8b705a07b5466?short_path=f5ec377#diff-f5ec377f4782587684c5732547456b70>
>>> Thanks again for pushing this forward, this is looking great.
>>
>> Sounds good. I just submitted a PR. I think it’s ready. Please let me
>> know if you feel any further changes are necessary.
>>
>>>
>>>>> It is also annoying that writing it as a static property would force you
>>>>> to write something like “X.a" instead of just “a”.
>>>>
>>>> I agree with this. I can accept an argument that this provides enough
>>>> value to avoid changing the language.
>>>>
>>>> That said, I do think default values for let properties are higher value.
>>>> If I had to pick one it would be default values and I would consider it to
>>>> be worthy of a breaking change in the language. But It would be great if
>>>> we can find a way to support both.
>>>
>>> I understand your desire, but this really is something we’ll have to
>>> discuss carefully. Changing Swift soft that “let x = 42” doesn’t
>>> necessarily mean that x is 42 is a pretty deep semantic change, which will
>>> be surprising for many people (as was noted by others on this thread). I
>>> agree that it would be great to get more flexible initialization for lets,
>>> but keep in mind that you can already do this long-hand if you want.
>>
>> I know changing the existing behavior would require very careful thinking.
>> I am absolutely open to any solution that allow a default value for let
>> properties to be specified whether it changes the existing behavior or not.
>> What is the best way I can help to move this discussion forward in a
>> productive manner? Would it be a good idea to start a new thread on the
>> topic? Or is this something you feel like the core team needs to mull over
>> for a while before we can have a productive conversation on the list?
>
> At least for structs, there's almost no reason for the memberwise-initialized
> properties to be `let`s, since preventing partial mutation of a value doesn't
> put any effective limits on users of the type. If you have:
>
> struct Foo {
> let x
> let y
>
> memberwise init(...)
> }
>
> then even though you can't say:
>
> var foo = Foo(x: 1, y: 1)
> foo.x = 2
>
> you can do the equivalent partial update by memberwise initialization:
>
> var foo = Foo(x: 1, y: 1)
> foo = Foo(x: 2, y: foo.y)
>
> and it's highly likely both forms will be optimized to the same thing.
Yes, understood. The use cases for `let` with a default I have in mind are
classes.
For example, UI widgets with some internal state but also appearance
properties. In my experience it is often not a requirement and worth the added
complexity to allow mutation of appearance attributes in custom UI widgets, but
they always have a default value that is often, but not always used. There are
definitely workarounds, but a `let` with a default value feels like the most
natural solution.
I hope we can continue discussing how it might be possible to support this
(whether by adding a feature or changing existing behavior).
>
> -Joe
>
>>
>>>
>>>>> This I still have a concern with, in two ways:
>>>>>
>>>>> 1) This still has tight coupling between the base and derived class.
>>>>> Properties in a based class are not knowable by a derived class in
>>>>> general (e.g. across module boundaries) and this directly runs afoul of
>>>>> our resilience plans. Specifically, across module boundaries, properties
>>>>> can change from stored to computed (or back) without breaking clients.
>>>>>
>>>>> 2) You’re introducing another unnecessary feature "super.init(…)” which
>>>>> will have to be independently justified.
>>>>>
>>>>
>>>> I will continue thinking about how this might be solved and also about
>>>> other cases where such a forwarding feature might be useful.
>>>
>>> Sounds good. This is definitely an interesting area to investigate, but I
>>> don't want the general goodness of your base memberwise init proposal to
>>> have to wait :-)
>>
>> I agree with you on not holding back the base proposal.
>>
>> I really appreciate you noticing that this should really be orthogonal to
>> the base proposal. I’ve already been giving this a lot of thought. It is
>> very clear to me now that a much more general parameter / argument
>> forwarding feature is the right approach. I am going to pursue a proposal
>> for that as well.
>>
>>>
>>>>> Other comments:
>>>>>
>>>>> In "Impact on existing code”, given your proposal, I think that we should
>>>>> keep the implicit memberwise initializer on classes, start generating it
>>>>> for root classes, and generate it for derived classes whose parent has a
>>>>> DI with no arguments (e.g. subclasses of NSObject). We should keep the
>>>>> current behavior where it is generated with internal behavior, and it is
>>>>> surpressed if *any* initializers are defined inside of the type.
>>>>
>>>> I’ll update that section to reflect these comments.
>>>>
>>>> One question I have is what the implicit memberwise initializer should do
>>>> in the case of private members. If we make it follow the rules of this
>>>> proposal we would break existing structs with private members that are
>>>> currently receiving the implicit memberwise initializer.
>>>>
>>>> I think this would be a good breaking change for both consistency with
>>>> this proposal and because implicitly exposing private members via the
>>>> initializer was a questionable choice. A mechanical migration could
>>>> generate code to add an explicit implementation of the previously implicit
>>>> initializer that doesn’t qualify under the rules of the new proposal. How
>>>> do you feel about this?
>>>
>>> I don’t have a strong opinion about this, and I can see reasonable
>>> arguments on either side. Breaking source compatibility in this case isn’t
>>> a show-stopper, since this will roll out in Swift 3.
>>
>> Glad to hear breaking compatibility is ok in this case if it is required for
>> consistency.
>>
>>>
>>> Here are the pros and cons as I see them with disallow-ing more-private
>>> fields to be published through less-private memberwise inits:
>>>
>>> Neutral: Either approach is fine for “CGRect” like types that are really
>>> just public bags of public fields.
>>> Pro: Makes logical sense at first blush. Memberwise inits publishing
>>> private state would be odd/surprising.
>>> Pro: Safer default, in that you don’t accidentally publish stuff you don’t
>>> want through a memberwise init.
>>> Con: This puts tension between access control for stored properties and
>>> memberwise inits. You have to choose between narrower access control or
>>> getting the benefit of a memberwise init. Another way to say it: this
>>> design means that narrow access control leads to boilerplate.
>>>
>>> I’m sure there are others as well.
>>>
>>> Again, I don’t have a strong opinion, but I’d lean slightly towards
>>> publishing all stored properties through the memberwise init. If you don’t
>>> have a strong opinion either, it would be fine to add a section pointing
>>> out the tradeoffs, and we can all discuss it during the review period. I
>>> suspect some of the other core team folks will have an opinion on this as
>>> well.
>>
>> I briefly addressed this in the alternatives considered section. I’ll fill
>> that out with additional details including the points you raise.
>>
>> I feel pretty strongly that we should enforce the access control rules
>> stated in the proposal. In addition to the Pros you note:
>>
>> 1. I think it is usually the right thing to do. If the caller can’t see a
>> member it probably doesn’t make sense to allow them to initialize it.
>>
>> 2. If we expose more private-members by default then memberwise
>> initialization is useless under the current proposal in many cases. There
>> would be no way to prevent synthesis of parameters for more-private members.
>> We have to choose between allowing callers to initialize our internal state
>> or forgoing the benefit of memberwise initialization.
>>
>> 3. If a proposal for `@nomemberwise` is put forward and adopted that would
>> allow us to prevent synthesis of parameters for members as desired.
>> Unfortunately `@nomemberwise` would need to be used much more heavily than
>> it otherwise would (i.e. to prevent synthesis of memberwise parameters for
>> more-private members). It would be better if `@nomemberwise` was not
>> necessary most of the time.
>>
>> 4. If callers must be able to provide memberwise arguments for more-private
>> members directly it is still possible to allow that while taking advantage
>> of memberwise initialization for same-or-less-private members. You just
>> need to declare a `memberwise init` with explicitly declared parameters for
>> the more-private members and initialize them manually in the body.
>> Requiring the programmer to manually write any code that exposes
>> more-private members is a arguably a very good thing.
>>
>> I think #4 above addresses the con you mention pretty well and #2 above is a
>> significant drawback to not enforcing the access control rule (#3 is also
>> pretty significant IMO).
>>
>> I’m sure this will be a point of discussion during review. I’m prepared to
>> defend the decision I made but will also keep my mind open to opposing
>> arguments.
>>
>>>
>>> I sent you a PR (my first! :-) with some nit-picky details on the latest
>>> writeup, to fix typos, clarify a few things, and reduce redundancy.
>>
>> Thanks!
>>
>>> One point that I left:
>>>
>>>> The *implicitly* synthesized initializer will be identical to an
>>>> initializer declared *explicitly* as follows:
>>>>
>>>> 1. For structs and root classes: `memberwise init(...) {}`
>>>> 2. For derived classes: `memberwise init(...) { super.init() }`
>>>
>>> Note that these are both equivalent, since derived class initializers
>>> default to super.init() at the bottom of their body today. This is why you
>>> don’t have to call super.init() when deriving from NSObject, for example.
>>
>> I’ll add a note to make this clear.
>>
>>>
>>>
>>>>> Thanks again for pushing this forward, you can also put me down as the
>>>>> review manager if you’d like.
>>>>>
>>>>
>>>> You’re very welcome. It’s a privilege to be able to contribute ideas,
>>>> have them taken seriously and hopefully see them lead to progress in the
>>>> language. I’ve really enjoyed the process and discussions with the core
>>>> team as well as the broader community.
>>>>
>>>> It’s really incredible to see the Swift team embrace the community so
>>>> openly and so graciously!
>>>>
>>>> Merry Christmas!
>>>
>>> You too Matthew, thanks again,
>>>
>>> -Chris
>>>
>>
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