[swift-evolution] Optional Setting

Wed Dec 16 16:04:18 CST 2015

The threading issue affects all operators like +=, -=, *= etc. and they are
not going to be removed.
I don't think ??= would be any different in this case.

On Wed, Dec 16, 2015 at 10:58 PM, Javier Soto via swift-evolution <
swift-evolution at swift.org> wrote:

> 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
>>>
>>>
>>>
>>>
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>
>>>
>>
>>
>> --
>>  Wizard
>> james at supmenow.com
>> +44 7523 279 698
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>>
> --
> Javier Soto
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> https://lists.swift.org/mailman/listinfo/swift-evolution
>
>
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