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

[Tony Allevato]

So the concern is that you have a Set of values with derived hashes, it
gets encoded, and then a later run of the program decodes that payload, the
hash values are different and performance suffers because collisions occur
where they previously didn't? (Correctness would never be an issue, since
collisions would still be resolved by testing equality.)

*Technically* that's possible, I suppose, if (1) the developer reordered
the fields between runs, (2) the implementation of the hash function
changed between language versions, or (3) it involved some sort of runtime
nondeterminism or randomness. If we ignore the cases where the code or the
runtime has to be recompiled, then only (3) applies, and I don't foresee
the derived hashes being implemented non-deterministically.

For what it's worth, this doesn't seem like a problem that's unique to
synthesized requirements—it could happen with any type that changed its
hash or introduced nondeterminism.

On Wed, Aug 16, 2017 at 8:01 PM Xiaodi Wu <xiaodi.wu at gmail.com> wrote:

> If I understand the question correctly, Robert's asking whether the
> possibility that hashes might differ on every execution would causes issues
> if, upon deserializing, one stumbles upon a hash collision which did not
> occur with the original hashes. And I suppose the answer to that is...yes?
>
>
> On Wed, Aug 16, 2017 at 6:16 PM, Tony Allevato via swift-evolution <
> swift-evolution at swift.org> wrote:
>
>> From what I can tell the hash values of elements in a Set don't come into
>> play during encoding/decoding: they're merely serialized as the sequence of
>> their elements.
>> https://github.com/apple/swift/blob/2e5817ebe15b8c2fc2459e08c1d462053cbb9a99/stdlib/public/core/Codable.swift#L4073-L4097
>>
>>
>> On Wed, Aug 16, 2017 at 4:06 PM Robert Bennett via swift-evolution <
>> swift-evolution at swift.org> wrote:
>>
>>> How do unstable hash values play with Codable? If you encode and save a
>>> Set, might you have problems interacting with it in the future? (This is
>>> more a Codable question than Hashable, but I figure I might as well ask
>>> here)
>>>
>>> On Aug 16, 2017, at 7:02 PM, Matthew Johnson via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>>
>>> On Aug 16, 2017, at 5:59 PM, Rudolf Adamkovic via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> That's great. Thanks!
>>>
>>>
>>> Yes, excellent news!  I am *really* looking forward to seeing this
>>> proposal make it into an Xcode release!
>>>
>>>
>>> R+
>>>
>>> On 17 Aug 2017, at 00:46, John McCall <rjmccall at apple.com> wrote:
>>>
>>> On Aug 16, 2017, at 6:35 PM, Rudolf Adamkovič via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> This is fantastic news! Any chance of this landing in Swift 4.x instead
>>> of 5?
>>>
>>>
>>> It it likely to be available in 4.1, but not 4.0.
>>>
>>> John.
>>>
>>>
>>> R+
>>>
>>> On 17 Aug 2017, at 00:29, 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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