[swift-evolution] [swift-evolution-announce] [Review] SE-0185 - Synthesizing Equatable and Hashable conformance

Michael Ilseman milseman at apple.com
Wed Aug 9 18:58:34 CDT 2017

> On Aug 9, 2017, at 4:09 PM, Chris Lattner <clattner at nondot.org> wrote:
> Hello Swift community,
> The review of SE-0185 - "Synthesizing Equatable and Hashable conformance" begins now and runs through August 15, 2017. The proposal is available here:
> https://github.com/apple/swift-evolution/blob/master/proposals/0185-synthesize-equatable-hashable.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:
> 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?

+1, it addresses one of the biggest pain points that cause people to reach for metaprogramming facilities.

I have one clarifying point and tweak. The proposal states:

> A struct T: Hashable that satisfies the conditions above will receive a synthesized implementation of var hashValue: Int { get } that uses an unspecified hash function† to compute the hash value by incorporating the hash values of the fields as its terms, in definition order.

This means that if the hash-combine operation is not commutative, then hashValues are not stable modulo member reordering. Was this scenario explicitly thought through? I don’t think it’s likely to be an issue in practice, but it's an artifact of implicit and ordered member-wise synthesis. It’s not obvious that declaration order is clearly the best order, e.g. declaration order is not necessarily memory-layout order. It might be better to leave the precise order unspecified, so long as it's guaranteed consistent during the execution of a program.

> 	• 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?

It's a reasonably scoped chunk of functionality for the biggest pain points, so yes.

> 	• If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?

Haskell and Rust both have “deriving" constructs for generating these kinds of things, and that simplifies a lot of code.

> 	• How much effort did you put into your review? A glance, a quick reading, or an in-depth study?

Thought about it a bit; read it quickly.

> More information about the Swift evolution process is available at:
> https://github.com/apple/swift-evolution/blob/master/process.md
> Thank you,
> Chris Lattner
> Review Manager
> _______________________________________________
> swift-evolution-announce mailing list
> swift-evolution-announce at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution-announce

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