[swift-evolution] [Review] SE-0018 Flexible Memberwise Initialization

Alex Johnson ajohnson at quickleft.com
Wed Jan 6 17:26:27 CST 2016

(this is mostly a repost of a message I sent to the "[draft]" thread for
this proposal, with some light editing to better match terminology in the

*What is your evaluation of the proposal?*

I like this proposal. I think it will bring some much-needed ease-of-use.

I have reservations about the "..." placeholder for the memberwise
initialization parameters, though. I know this was suggested by
Chris Lattner, so I'm inclined to defer to his judgement. But, here are my

First, it's very close to the varags syntax (e.g. "Int...") which can also
appear in initializer parameter lists.

Second, and I think more important, I'm not sure that it's all that *useful*.
It's presence isn't necessary for triggering memberwise initialization
synthesis; that is already done by the "memberwise" keyword.

The primary example given in the proposal is:

memberwise init(anInt: Int, anotherInt: Int, ...) {

  /* code using anInt and anotherInt */


That is, it's used to indicate where the synthesized parameters appear in
the parameter list if there are also custom (non-memberwise) parameters.

My question is, *could the memberwise initialization parameters always be
last?* That would eliminate the need for the placeholder.

I don't think I've seen a compelling case for embedding the "..." *within*
a list of custom arguments, like:

memberwise init(anInt: Int, ..., anotherInt: Int) {
  /* code using anInt and anotherInt */

It's been mentioned several times in the discussion of this proposal that
this behavior is purely optional. If it turns out that there are rare cases
where placing the memberwise params in the middle is useful, authors can
use manual initialization.

On Wed, Jan 6, 2016 at 2:47 PM, Chris Lattner via swift-evolution <
swift-evolution at swift.org> wrote:

> Hello Swift community,
> The review of "Flexible Memberwise Initialization" begins now and runs
> through January 10th. The proposal is available here:
> https://github.com/apple/swift-evolution/blob/master/proposals/0018-flexible-memberwise-initialization.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.
> 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 you 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,
> -Chris
> Review Manager
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution


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