[swift-evolution] Pitch: Automatically deriving Equatable/Hashable for more value types
Chris Lattner
clattner at nondot.org
Sat May 6 18:07:06 CDT 2017
> On May 4, 2017, at 3:01 PM, Xiaodi Wu via swift-evolution <swift-evolution at 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 at swift.org <mailto:swift-evolution at 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.
>
>
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