[swift-evolution] [Review] SE-0024 "Optional Value Setter `??=`"

James Campbell james at supmenow.com
Sat Feb 13 06:10:16 CST 2016 

One common case could be for a error catching, consider this:

let object = try? UnsupportedMessage(text: "James", type:.Text,
date:NSDate()) ?? FailedObject()

vs

var object = try? Message(text: "James", type:.Text, date:NSDate())
object ??= UnsupportedMessage()

Why would we need this? I have a chat application where if it gets a
message it doesn't understand it will throw an error. In the past I would
handle this errors by returning nil like so. However we want to communicate
to the user that they missing content by vending a UnsupportedMessage
object which shows the UI to prompt them to upgrade.

If the object being constructed has a lot of variables then it could be
lost off the side of the screen. I feel ??= is more concise in this case.

This could also be great for the null object pattern where you may not want
a variable to be nil, you may want some sort of null object that is vended
in cases of failure that could log this error.

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On Sat, Feb 13, 2016 at 11:20 AM, Taras Zakharko via swift-evolution <
swift-evolution at swift.org> wrote:

> I think the proposal should include some discussion on the common cases
> where this operator would be useful. As such, I don’t see much hard of
> including this, except that I am not sure that the problem is significant
> enough to warrant a reaction.
>
> — T
>
>
> On 13 Feb 2016, at 11:11, Radosław Pietruszewski via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> +0.5
>
> I am generally for this proposal, because I don’t see why not — there are
> cases where this is useful, and it’s symmetric with other X/X= operator
> pairs, so it’s not like it’s a completely new thing.
>
> However, I don’t think the operator is _as_ useful in Swift as it is in
> other languages.
>
> In Ruby, for example, it’s extremely common to use `||=` (an equivalent of
> ??=, more or less) to modify function arguments, e.g. assign default values
> if nil was passed. But you can’t do that in Swift since SE-0003 removed the
> ability to mark a parameter as `var`.
>
> So you can’t do that
>
> arg ??= default
>
> and you have to do
>
> let arg = arg ?? default
>
> Except you wouldn’t want the earlier version anyway, because `arg` would
> continue to be an optional. A notion not relevant in a dynamically typed
> language, but in Swift, it matters.
>
> And I see other cases like this. A proposal to add `??=` to Swift was one
> of the first Swift radars I filed, because it was just something I was
> really used to. But with time I realized there are actually relatively few
> cases where this is useful. And that’s all because of Swift’s
> characteristics: it’s statically typed, it pushes you to use constants and
> not variables where possible, optionality is explicit, and avoided when not
> necessary.
>
> Still, I searched through my code and found _a few_ instances of `??=` (my
> own implementation), almost all used for dealing with dictionaries (and a
> dictionary-like structure in a library called SwiftyUserDefaults) — and
> having this operator definitely improved the clarity of those places.
>
>
>    - What is your evaluation of the proposal?
>
> All in all, I’m for.
>
>
>    - Is the problem being addressed significant enough to warrant a
>    change to Swift?
>
> I’d say yes, because it’s simple to implement, and carries little risk
> AFAICT. Still, it’s not as significant an improvement as many of the other
> proposals.
>
>
>    - Does this proposal fit well with the feel and direction of Swift?
>
> Again, ??= is only useful in _some_ cases, and in many other cases you
> want to avoid optionality and mutability. So it’s not necessarily something
> we want to encourage a lot, but I don’t see real risk in people trying to
> damage their Swift code just so they can use ??=. And OTOH having nice
> tools for dealing with optionality when it’s necessary is a very Swifty
> thing to do, so overall, yes.
>
>
>    - If you have used other languages or libraries with a similar
>    feature, how do you feel that this proposal compares to those?
>
> Mostly dynamically typed languages with ||=, Ruby in particular. Also my
> own implementation of ??= in my projects and
> https://github.com/radex/swiftyuserdefaults
>
>
>    - How much effort did you put into your review? A glance, a quick
>    reading, or an in-depth study?
>
> A quick reading of the proposal.
>
> Best,
> — Radek
>
> On 13 Feb 2016, at 06:15, Douglas Gregor via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Hello Swift community,
>
> The review of SE-0024 "Optional Value Setter `??=`" begins now and runs
> through February 18, 2016. The proposal is available here:
>
>
> https://github.com/apple/swift-evolution/blob/master/proposals/0024-optional-value-setter.md
>
> Reviews are an important part of the Swift evolution process. All reviews
> should be sent to the swift-evolution mailing list at
>
> https://lists.swift.org/mailman/listinfo/swift-evolution
>
> or, if you would like to keep your feedback private, directly to the
> review manager. When replying, please try to keep the proposal link at the
> top of the message:
>
> Proposal link:
>
>
> https://github.com/apple/swift-evolution/blob/master/proposals/0024-optional-value-setter.md
>
> Reply text
>
> Other replies
>
> <https://github.com/apple/swift-evolution#what-goes-into-a-review-1>What
> goes into a review?
>
> The goal of the review process is to improve the proposal under review
> through constructive criticism and, eventually, determine the direction of
> Swift. When writing your review, here are some questions you might want to
> answer in your review:
>
>    - What is your evaluation of the proposal?
>    - Is the problem being addressed significant enough to warrant a
>    change to Swift?
>    - Does this proposal fit well with the feel and direction of Swift?
>    - If you have used other languages or libraries with a similar
>    feature, how do you feel that this proposal compares to those?
>    - How much effort did you put into your review? A glance, a quick
>    reading, or an in-depth study?
>
> More information about the Swift evolution process is available at
>
> https://github.com/apple/swift-evolution/blob/master/process.md
>
> Thank you,
>
> Doug Gregor
>
> Review Manager
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