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

Nevin Brackett-Rozinsky nevin.brackettrozinsky at gmail.com
Thu Aug 17 15:04:33 CDT 2017

I am really glad this is happening, it will make implementing basic data
types much nicer, and it is exactly the sort of feature that saves
developers from having to waste time thinking about trivial rote
boilerplate both when writing and when reading code—an excellent and
welcome addition to the language.

As for Haravikk’s scenario of marking a type as Equatable to trigger an
error so you remember to implement the protocol later, you can achieve the
same goal by tagging it “MakeMeEquatable” instead. The compiler will give
an error (because there is no such protocol) and then you can come back to
fix it another time.


On Wed, Aug 16, 2017 at 6:29 PM, Chris Lattner via swift-evolution <
swift-evolution at swift.org> wrote:

> Proposal Link: https://github.com/apple/swift-evolution/blob/master/
> proposals/0185-synthesize-equatable-hashable.md
> The review of SE-0185 “Synthesizing Equatable and Hashable conformance”
> ran from August 9…15, 2017. Feedback for the feature was glowingly
> positive, and the proposal is accepted.  The core team discussed the
> concerns raised in the feedback thread for the proposal.  Here are some
> rough notes (not intended to be exhaustive), but it is important to
> recognize that the proposal follows the design of the auto-synthesized
> Codable proposal, and that many of these same points were discussed when it
> came up:
> - The core team feels that adding compiler magic for this case is
> reasonable because it solves an important user need, and doesn’t preclude
> the introduction of a more general feature (e.g. like a macro system, or
> Rust's ‘deriving’ feature) in the future.  When/if that feature is designed
> and built, the compiler magic can be replaced with standard library magic.
> - The hash value of a type is not intended to be stable across rebuilds or
> other changes to the code.  It is ok to change if fields are reordered, the
> standard library changes the hash function, etc.  Tony pointed this out
> on-thread, saying:  The stdlib documentation for hashValue states "Hash
> values are not guaranteed to be equal across different executions of your
> program. Do not save hash values to use during a future execution.”
> - The code synthesized is meant to feel like a default implementation that
> you’re getting for free from a (constrained) extension on the protocol.
> This is why conformance to the protocol itself is all that is required, not
> something like “AutoEquatable”.
> Many thanks to Tony Allevato for driving forward this proposal.  The patch
> just needs final code review now - I think we’re all looking forward to
> this landing, congrats!
> Chris Lattner
> Review Manager
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> swift-evolution at swift.org
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