Can we just copy&paste the solution from Haskell instead of creating our own?
It's just better in every aspect. Deriving `Equatable` and `Hashable` would
become
struct Polygon deriving Equatable, Hashable {
...
}
This has several advantages:
- you don't have to guess wether `Equatable` or `Hashable` should be
automatically derived or not.
- Deriving becomes an explicit choice.
- If you need a custom `Equatable` implementation (for whatever reason), you
can still do it.
- It doesn't break any code that is unaware of the change
- It can be extended in future versions of Swift, without introducing any new
incompatibilities. For example, `CustomStringConvertible` could be derived just
as easily.
- It is compatible with generics. E.g. `struct Shape<T> deriving Equatable`
will make every `Shape<X>` equatable if `X` is equatable. But if `X` is not
equatable, `Shape<X>` can be used as well. (Unless `X` is not used, in which
case every `Shape<T>` would be equatable. Unless something in the definition of
`Shape` makes deriving `Equatable` impossible => this produces an error.)
- It is proven to work in production.
-Michael
> Am 26.05.2016 um 03:48 schrieb Mark Sands via swift-evolution
> <[email protected]>:
>
> Thanks so much for putting this together, Tony! Glad I was able to be some
> inspiration. :^)
>
>
> On Wed, May 25, 2016 at 1:28 PM, Tony Allevato via swift-evolution
> <[email protected]> wrote:
> I was inspired to put together a draft proposal based on an older discussion
> in the Universal Equality, Hashability, and Comparability thread
> <http://thread.gmane.org/gmane.comp.lang.swift.evolution/8919/> that recently
> got necromanced (thanks Mark Sands!).
>
> I'm guessing that this would be a significant enough change that it's not
> possible for the Swift 3 timeline, but it's something that would benefit
> enough people that I want to make sure the discussion stays alive. If there
> are enough good feelings about it, I'll move it from my gist into an actual
> proposal PR.
>
> Automatically deriving Equatable andHashable for value types
>
> • Proposal: SE-0000
> • Author(s): Tony Allevato
> • Status: Awaiting review
> • Review manager: TBD
> Introduction
>
> Value types are prevalent throughout the Swift language, and we encourage
> developers to think in those terms when writing their own types. Frequently,
> developers find themselves writing large amounts of boilerplate code to
> support equatability and hashability of value types. This proposal offers a
> way for the compiler to automatically derive conformance toEquatable and
> Hashable to reduce this boilerplate, in a subset of scenarios where
> generating the correct implementation is likely to be possible.
>
> Swift-evolution thread: Universal Equatability, Hashability, and Comparability
>
> Motivation
>
> Building robust value types in Swift can involve writing significant
> boilerplate code to support concepts of hashability and equatability.
> Equality is pervasive across many value types, and for each one users must
> implement the == operator such that it performs a fairly rote memberwise
> equality test. As an example, an equality test for a struct looks fairly
> uninteresting:
>
> func ==(lhs: Foo, rhs: Foo) -> Bool
> {
>
> return lhs.property1 == rhs.property1 &&
>
> lhs
> .property2 == rhs.property2 &&
>
> lhs
> .property3 == rhs.property3 &&
>
>
> ...
>
> }
>
> What's worse is that this operator must be updated if any properties are
> added, removed, or changed, and since it must be manually written, it's
> possible to get it wrong, either by omission or typographical error.
>
> Likewise, hashability is necessary when one wishes to store a value type in a
> Set or use one as a multi-valuedDictionary key. Writing high-quality,
> well-distributed hash functions is not trivial so developers may not put a
> great deal of thought into them – especially as the number of properties
> increases – not realizing that their performance could potentially suffer as
> a result. And as with equality, writing it manually means there is the
> potential to get it wrong.
>
> In particular, the code that must be written to implement equality for enums
> is quite verbose. One such real-world example (source):
>
> func ==(lhs: HandRank, rhs: HandRank) -> Bool
> {
>
> switch
> (lhs, rhs) {
>
> case (.straightFlush(let lRank, let lSuit), .straightFlush(let rRank , let
> rSuit)):
>
> return lRank == rRank && lSuit ==
> rSuit
>
> case (.fourOfAKind(four: let lFour), .fourOfAKind(four: let
> rFour)):
>
> return lFour ==
> rFour
>
> case (.fullHouse(three: let lThree), .fullHouse(three: let
> rThree)):
>
> return lThree ==
> rThree
>
> case (.flush(let lRank, let lSuit), .flush(let rRank, let
> rSuit)):
>
> return lSuit == rSuit && lRank ==
> rRank
>
> case (.straight(high: let lRank), .straight(high: let
> rRank)):
>
> return lRank ==
> rRank
>
> case (.threeOfAKind(three: let lRank), .threeOfAKind(three: let
> rRank)):
>
> return lRank ==
> rRank
>
> case (.twoPair(high: let lHigh, low: let lLow, highCard: let
> lCard),
>
> .twoPair(high: let rHigh, low: let rLow, highCard: let
> rCard)):
>
> return lHigh == rHigh && lLow == rLow && lCard ==
> rCard
>
> case (.onePair(let lPairRank, card1: let lCard1, card2: let lCard2, card3: let
> lCard3),
>
> .onePair(let rPairRank, card1: let rCard1, card2: let rCard2, card3: let
> rCard3)):
>
> return lPairRank == rPairRank && lCard1 == rCard1 && lCard2 == rCard2 &&
> lCard3 ==
> rCard3
>
> case (.highCard(let lCard), .highCard(let
> rCard)):
>
> return lCard ==
> rCard
>
> default
> :
>
> return false
>
> }
> }
>
> Crafting a high-quality hash function for this enum would be similarly
> inconvenient to write, involving another large switchstatement.
>
> Swift already provides implicit protocol conformance in some cases; notably,
> enums with raw values conform toRawRepresentable, Equatable, and Hashable
> without the user explicitly declaring them:
>
> enum Foo: Int
> {
>
> case one = 1
>
>
> case two = 2
>
> }
>
>
> let x = (Foo.one == Foo.two) // works
> let y = Foo.one.hashValue // also works
> let z = Foo.one.rawValue // also also works
> Since there is precedent for this in Swift, we propose extending this support
> to more value types.
>
> Proposed solution
>
> We propose that a value type be Equatable/Hashable if all of its members are
> Equatable/Hashable, with the result for the outer type being composed from
> its members.
>
> Specifically, we propose the following rules for deriving Equatable:
>
> • A struct implicitly conforms to Equatable if all of its fields are of
> types that conform to Equatable – either explicitly, or implicitly by the
> application of these rules. The compiler will generate an implementation of
> ==(lhs: T, rhs: T)that returns true if and only if lhs.x == rhs.x for all
> fields x in T.
>
> • An enum implicitly conforms to Equatable if all of its associated
> values across all of its cases are of types that conform to Equatable –
> either explicitly, or implicitly by the application of these rules. The
> compiler will generate an implementation of ==(lhs: T, rhs: T) that returns
> true if and only if lhs and rhs are the same case and have payloads that are
> memberwise-equal.
>
> Likewise, we propose the following rules for deriving Hashable:
>
> • A struct implicitly conforms to Hashable if all of its fields are of
> types that conform to Hashable – either explicitly, or implicitly by the
> application of these rules. The compiler will generate an implementation of
> hashValue that uses a pre-defined hash function† to compute the hash value of
> the struct from the hash values of its members.
>
> Since order of the terms affects the hash value computation, we recommend
> ordering the terms in member definition order.
>
> • An enum implicitly conforms to Hashable if all of its associated
> values across all of its cases are of types that conform to Hashable – either
> explicitly, or implicitly by the application of these rules. The compiler
> will generate an implementation of hashValue that uses a pre-defined hash
> function† to compute the hash value of an enum value by using the case's
> ordinal (i.e., definition order) followed by the hash values of its
> associated values as its terms, also in definition order.
>
> † We leave the exact definition of the hash function unspecified here; a
> multiplicative hash function such as Kernighan and Ritchie or Bernstein is
> easy to implement, but we do not rule out other possibilities.
>
> Overriding defaults
>
> Any user-provided implementations of == or hashValue should override the
> default implementations that would be provided by the compiler. This is
> already possible today with raw-value enums so the same behavior should be
> extended to other value types that are made to implicitly conform to these
> protocols.
>
> Open questions
>
> Omission of fields from generated computations
>
> Should it be possible to easily omit certain properties from automatically
> generated equality tests or hash value computation? This could be valuable,
> for example, if a property is merely used as an internal cache and does not
> actually contribute to the "value" of the instance. Under the rules above, if
> this cached value was equatable, a user would have to override == and
> hashValue and provide their own implementations to ignore it. If there is
> significant evidence that this pattern is common and useful, we could
> consider adding a custom attribute, such as @transient, that would omit the
> property from the generated computations.
>
> Explicit or implicit derivation
>
> As with raw-value enums today, should the derived conformance be completely
> explicit, or should users have to explicitly list conformance with Equatable
> and Hashable in order for the compiler to generate the derived implementation?
>
> Impact on existing code
>
> This change will have no impact on existing code because it is purely
> additive. Value types that already provide custom implementations of == or
> hashValue but satisfy the rules above would keep the custom implementation
> because it would override the compiler-provided default.
>
> Alternatives considered
>
> The original discussion thread also included Comparable as a candidate for
> automatic generation. Unlike equatability and hashability, however,
> comparability requires an ordering among the members being compared.
> Automatically using the definition order here might be too surprising for
> users, but worse, it also means that reordering properties in the source code
> changes the code's behavior at runtime. (This is true for hashability as well
> if a multiplicative hash function is used, but hash values are not intended
> to be persistent and reordering the terms does not produce a significant
> behavioral change.)
>
> Acknowledgments
>
> Thanks to Joe Groff for spinning off the original discussion thread, Jose
> Cheyo Jimenez for providing great real-world examples of boilerplate needed
> to support equatability for some value types, and to Mark Sands for
> necromancing the swift-evolution thread that convinced me to write this up.
>
>
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>
>
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