[swift-evolution] Optional Setting

[Javier Soto]

I think it would be worth mentioning in the proposal the threading
implications of this operator. The trickyness comes from the fact that
suddenly that expression is performing a call to both the getter *and* the
setter, so even if those 2 were to be protected via locks, there's still
the possibility of race-conditions. Someone else mentioned this is of
course also the case with operators such as ++ (which I believe Lattner was
in favor of stripping from the language)

Not saying that this operator shouldn't exist for this reason, but I do
think it's important to realize the complexity it brings to add an operator
that does more than is immediately obvious, since that's often times a
recipe for any developer making this sort of mistake.
I would lean towards -1, but I do acknowledge that in some circumstances it
can simplify some code.

On Wed, Dec 16, 2015 at 8:50 AM James Campbell via swift-evolution <
swift-evolution at swift.org> wrote:

> I've started a formal proposal here:
>
> https://github.com/apple/swift-evolution/pull/63
>
> On Wed, Dec 16, 2015 at 4:48 PM, Dave Abrahams via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>>
>> On Dec 16, 2015, at 6:22 AM, Kevin Wooten via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>
>> On Dec 16, 2015, at 4:12 AM, Al Skipp via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> On 16 Dec 2015, at 00:58, Marc Knaup via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>> I tend towards -1 for multiple reasons:
>>
>>    - It has little value for local variables. In most cases you want to
>>    use the value you assign to a local variable and assigning it to an
>>    optional variable would require a subsequent unwrapping. In most cases
>>    where local variables are involved "var x = y ?? z" is satisfying as it
>>    creates a non-optional value iff z is non-optional.
>>
>>    - It seems to be a rare use case that you set a value of an optional
>>    property which is currently nil and without also using that value directly
>>    within the same context. Quickly checking my Swift apps reveals only very
>>    little such use cases.
>>
>>    - The remaining cases could expressed like "object.property =
>>    object.property ?? …" or using "if object.property == nil { … }".
>>    While it is true that variable and property name could be very long,
>>    this is an unlikely case of an already rare case which decreases the value
>>    of the proposed assignment operator even further.
>>
>>    - Most important though is that such an optional assignment operator
>>    would work differently from all other assignment operators. The right
>>    operand would never be executed if the variable being assigned is already
>>    non-nil. This will likely be unexpected for a lot of developers who expect
>>    similar behavior like in all other assignments.
>>
>> I think these are all very good points. Seems like the only really
>> practical use would be restricted to:
>> object.property ??= val
>>
>> Instead of:
>> object.property = object.property ?? val
>>
>> Is it worth it for that one scenario? As Marc pointed out, the ??
>> operator is much more versatile as it can also be used to return a
>> non-optional value.
>>
>>
>> After perusing our Swift code it turns out that we use the long form (a =
>> a ?? def) quite a bit.  As it was previously mentioned it, when the
>> variables is named “a” it’s clearly not an issue, but this is…
>>
>>     messagesViewController.chatTitleName =
>>  messagesViewController.chatTitleName ?? “Default”
>>
>> (Those are effectively real world variable names).
>>
>> I think quite a bit of the clarity of this statement is lost by the
>> duplication and the proposed form..
>>
>>     messagesViewController.chatTitleName ??= “Default”
>>
>> clears it up fairly well.
>>
>>
>> A few points:
>>
>> 1. I've always thought we needed something like this; glad to see it
>> discussed
>>
>> 2. This is also applicable to dictionaries:
>>
>>   messagesViewController.titleNames["chat"] ??= "Default"
>>
>> 3. I think it may be time for a formal proposal :-)
>>
>> 4. One way the community can help us to evaluate it would be to create
>> the API in an extension in your own code, actually apply it in your
>> project, and evaluate what it does for readability.
>>
>> Thanks,
>>
>> -Dave
>>
>>
>>
>>
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>>
>>
>
>
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-- 
Javier Soto
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