> On May 4, 2017, at 3:01 PM, Xiaodi Wu via swift-evolution > <swift-evolution@swift.org> wrote: > > Hmm, I can see the appeal of automatically deriving Equatable and Hashable > conformance, but I'd like that to be opt-in. That is, types should declare > that they are Equatable or Hashable to begin with. It wouldn't have to take > extra syntax, as compiler magic could effectively synthesize default > implementations for == and/or hashValue when all members are themselves > Equatable or Hashable, respectively. With such a scheme, consideration can be > made to accommodating classes too.
Completely agreed. -Chris > On Thu, May 4, 2017 at 15:37 Tony Allevato via swift-evolution > <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote: > Hi all, > > A conversation on Twitter last night brought up some interest in this feature > and I was encouraged to revive this proposal. > > Jordan Rose mentioned > <https://twitter.com/UINT_MIN/status/859922619578986496> on Twitter that it > could possibly make it in by the Swift 4 deadline if others contributed—I > have a WIP branch (albeit not currently working because I rebased after a > couple months of it being idle) that does the work for enums but I got stuck > on the mutually recursive cases. If this got approved, I'd love to > collaborate with other interested folks to finish up the implementation. > > Link: https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad> > > > Deriving Equatable and Hashable for value types > > Proposal: SE-0000 > <https://github.com/apple/swift-evolution/blob/master/proposals/NNNN-name.md> > Author(s): Tony Allevato <https://github.com/allevato> > Status: Awaiting review > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#rationale> > Review manager: TBD > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#introduction>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 have to write 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 to Equatable and Hashable to > reduce this boilerplate, in a subset of scenarios where generating the > correct implementation is known to be possible. > > Swift-evolution thread: Universal Equatability, Hashability, and > Comparability <http://thread.gmane.org/gmane.comp.lang.swift.evolution/8919> > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#motivation>Motivation > > Building robust value types in Swift can involve writing significant > boilerplate code to support 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: > > struct Foo: Equatable { > static 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-valued Dictionary 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: > > enum Token: Equatable { > case string(String) > case number(Int) > case lparen > case rparen > > static func == (lhs: Token, rhs: Token) -> Bool { > switch (lhs, rhs) { > case (.string(let lhsString), .string(let rhsString)): > return lhsString == rhsString > case (.number(let lhsNumber), .number(let lhsNumber)): > return lhsNumber == rhsNumber > case (.lparen, .lparen), (.rparen, .rparen): > return true > default: > return false > } > } > } > Crafting a high-quality hash function for this enum would be similarly > inconvenient to write. > > Swift already derives Equatable and Hashable conformance for a small subset > of enums: those for which the cases have no associated values (including > enums with raw types). Two instances of such an enum are equal if they are > the same case, and an instance's hash value is its ordinal: > > enum Foo { > case zero, one, two > } > > let x = (Foo.one == Foo.two) // evaluates to false > let y = Foo.one.hashValue // evaluates to 1 > Likewise, conformance to RawRepresentable is automatically derived for enums > with a raw type. Since there is precedent for derived conformances in Swift, > we propose extending this support to more value types. > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#proposed-solution>Proposed > solution > > In general, we propose that value types derive conformance to > Equatable/Hashable if all of its members are Equatable/Hashable. We describe > the specific conditions under which these conformances are derived below, > followed by the details of how the conformance requirements are implemented. > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#protocol-derivability-conditions>Protocol > derivability conditions > > For brevity, let P represent either the protocol Equatable or Hashable in the > descriptions below. > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#derived-conformances-for-enums>Derived > conformances for enums > > For an enum, derivability of P is based on the conformances of its cases' > associated values. Computed properties are not considered. > > The following rules determine whether conformance to P can be derived for an > enum: > > An enum with no cases does not derive conformance to P, since it is not > possible to create instances of such types. > > An enum with one or more cases derives conformance to P if all of the > associated values of all of its cases conform to P (either explicitly or > derived). > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#derived-conformances-for-structs>Derived > conformances for structs > > For a struct, derivability of P is based on the conformances of its stored > instance properties only. Neither static properties nor computed instance > properties (those with custom getters) are considered. > > The following rules determine whether conformance to P can be derived for a > struct: > > A struct with no stored properties does not derive conformance to P. (Even > though it is vacuously true that all instances of a struct with no stored > properties could be considered equal and hash to the same value, the reality > is that such structs are more often used for grouping/nesting of other > entities and not for their singular value, and we don't consider it > worthwhile to generate extra code in this case.) > > A struct with one or more stored properties derives conformance to P if all > if the types of all of its stored properties conform to P (either explicitly > or derived). > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#considerations-for-recursive-types>Considerations > for recursive types > > For brevity in the discussion below, the term members refers only to those > members that are checked for deriving conformances: stored properties for > structs and associated values for enums. > > Recursive value types require a bit more care when determining whether a > conformance can be derived. Consider the following enum with an indirect case: > > enum TreeNode { > case empty > case leaf(value: Int) > case internal(left: TreeNode, right: TreeNode) > } > When examining the internal case, an application of the rules above implies > that "TreeNode derives P if TreeNode conforms to P"—a recursive condition. In > this situation, we note that any instance of this type—or of any recursive > type—forms a finite tree structure because the recursion must be terminated > eventually by using one of the other base cases. Therefore, without changing > the outcome, we can assume for the purposes of determining whether T derives > P that any members of type T already conform to P. If any members of > different types prohibit deriving P, then we know that the whole type cannot > derive it; likewise, if all of the other members permit deriving P, then we > know that T can derive it by recursively applying the derived operation. > > This property can be extended to mutually recursive types as well. Consider > this contrived example: > > enum A { > case value(Int) > case b(B) > } > > enum B { > case value(String) > case c(C) > } > > enum C { > case value(Double) > case a(A) > } > The rules state that—ignoring the trivial cases—"A derives P if B conforms to > P," and "B derives P if Cconforms to P," and "C derives P if A conforms to > P." The same observation about recursion and the finiteness of instances from > above holds here, so we can generalize the rule above as follows: > > A type T can derive P if all members of T conform to P or are of types found > in cycles that lead back to Tsuch that the members of those other types along > the cycle also all conform to P or are themselves along such a cycle. > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#other-considerations>Other > considerations > > When conditional conformances are supported in Swift, generic types should > conditionally derive Equatable and Hashable for type argument substitutions > where the rules above are satisfied. > > A notable side effect of this is that Optional<Wrapped> would derive > Equatable and Hashable conformance when Wrapped conforms to those protocols, > because it is an enum that would satisfy the rules described above. Its > implementation would not need to be in the standard library (but there is > also nothing preventing it from being there). > > Conditional conformances will also significantly improve derivability > coverage over other payload/member types. For example, consider a struct > containing a stored property that is an array of Equatable types: > > struct Foo { > var values: [String] > } > Today, Array<String> does not conform to Equatable, so its presence would > prohibit Foo from deriving Equatable. However, once Swift can express the > conformance Array<Element>: Equatable where Element: Equatable, Foo would > automatically derive Equatable as well. This makes derived conformances > significantly more powerful. > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#implementation-details>Implementation > details > > An enum T that derives Equatable will receive a compiler-generated > implementation of static == (lhs: T, rhs: T) -> Bool that returns true if and > only if lhs and rhs are the same case and have payloads that are > memberwise-equal. > > An enum T that derives Hashable will receive a compiler-generated > implementation of var hashValue: Int { get }that uses an unspecified hash > function† to compute the hash value by incorporating the case's ordinal > (i.e., definition order) followed by the hash values of its associated values > as its terms, also in definition order. > > A struct T that derives Equatable will receive a compiler-generated > implementation of static == (lhs: T, rhs: T) -> Bool that returns true if and > only if lhs.x == rhs.x for all stored properties in T. > > A struct T that derives Hashable will receive a compiler-generated > 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. > > † We intentionally leave the exact definition of the hash function > unspecified here. A multiplicative hash function with good distribution is > the likely candidate, but we do not rule out other possibilities. Users > should not depend on the nature of the generated implementation or rely on > particular outputs; we reserve the right to change it in the future. > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#overriding-derived-conformances>Overriding > derived conformances > > Any user-provided implementations of == or hashValue will override the > default implementations that would be provided by the compiler. This is > already the case 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. > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#open-questions>Open > questions > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#omission-of-fields-from-generated-computations>Omission > of fields from generated computations > > Some commenters have expressed a desire to tag certain properties of a struct > from being included in automatically generated equality tests or hash value > computations. 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. > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#explicit-or-implicit-derivation>Explicit > or implicit derivation > > As with raw-value enums today, should the derived conformance be completely > implicit, or should users have to explicitly list conformance with Equatable > and Hashable in order for the compiler to generate the derived implementation? > > If derived conformances were made explicit, it could be argued that this > should also be done for consistency across raw-value enums as well. This > would be a source-breaking change, which should be avoided at this stage. > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#impact-on-existing-code>Impact > on existing code > > This change would make types that satisfy the rules above Equatable and > Hashable when they previously were not. It is not expected that there would > be any behavioral changes because of this; since Equatable and Hashable have > associated type requirements, users cannot be dynamically testing for > conformance to them at runtime. > > Value types that already provide custom implementations of Equatable and > Hashable would keep the custom implementation because it would override the > compiler-provided default. > > This change would potentially increase binary size when it generates > conformances that did not exist before, at least for types where it is not > possible to know that the conformances are unused and thus cannot be > dead-stripped (i.e., public types). > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#alternatives-considered>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.) > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#acknowledgments>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. > > > <https://gist.github.com/allevato/2fd10290bfa84accfbe977d8ac07daad#rationale>Rationale > > On [Date], the core team decided to (TBD) this proposal. When the core team > makes a decision regarding this proposal, their rationale for the decision > will be written here. > > > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org <mailto:swift-evolution@swift.org> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution> > _______________________________________________ > swift-evolution mailing list > swift-evolution@swift.org > https://lists.swift.org/mailman/listinfo/swift-evolution
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