Proposal to synthesize Equatable/Hashable for value types. (#706)

* Proposal to synthesize Equatable/Hashable for value types.

* Updates based on the WIP implementation

* Update to match the proposal template

* Remove the _original type declaration_ requirement

* Fix typos

* Forbid synthesis in extensions (see SR-4920).

Author: allevato

proposals/NNNN-synthesize-equatable-hashable.md

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+# Synthesizing `Equatable` and `Hashable` conformance
+
+* Proposal: [SE-NNNN](NNNN-synthesize-equatable-hashable.md)
+* Author: [Tony Allevato](https://github.com/allevato)
+* Review Manager: TBD
+* Status: **Awaiting review**
+
+## Introduction
+
+Developers have to write large amounts of boilerplate code to support
+equatability and hashability of complex types. This proposal offers a way for
+the compiler to automatically synthesize 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
+](https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160307/012099.html)
+
+## Motivation
+
+Building robust types in Swift can involve writing significant boilerplate
+code to support hashability and equatability. By eliminating the complexity for
+the users, we make `Equatable`/`Hashable` types much more appealing to users and
+allow them to use their own types in contexts that require equatability and
+hashability with no added effort on their part (beyond declaring the
+conformance).
+
+Equality is pervasive across many 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 basic `struct` is fairly uninteresting:
+
+```swift
+struct Person: Equatable {
+  static func == (lhs: Person, rhs: Person) -> Bool {
+    return lhs.firstName == rhs.firstName &&
+           lhs.lastName == rhs.lastName &&
+           lhs.birthDate == rhs.birthDate &&
+           ...
+  }
+}
+```
+
+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 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 for it to not only be inefficient, but incorrect as well.
+
+In particular, the code that must be written to implement equality for
+`enum`s is quite verbose:
+
+```swift
+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 `enum`s: those for which the cases have no associated values (which includes
+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:
+
+```swift
+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 `enum`s
+with a raw type, and the recently approved `Encodable`/`Decodable` protocols
+also support synthesis of their operations when possible. Since there is 
+precedent for synthesized conformances in Swift, we propose extending it to
+these fundamental protocols.
+
+## Proposed solution
+
+In general, we propose that a type synthesize conformance to
+`Equatable`/`Hashable` if all of its members are `Equatable`/`Hashable`. We
+describe the specific conditions under which these conformances are synthesized
+below, followed by the details of how the conformance requirements are
+implemented.
+
+### Requesting synthesis is opt-in
+
+Users must _opt-in_ to automatic synthesis by declaring their type as
+`Equatable` or `Hashable` without implementing any of their requirements. This
+conformance must be part of the _original type declaration_ and not on an
+extension (see [Synthesis in extensions](#synthesis-in-extensions) below for
+more on this).
+
+Any type that declares such conformance and satisfies the conditions below
+will cause the compiler to synthesize an implementation of `==`/`hashValue`
+for that type.
+
+Making the synthesis opt-in—as opposed to automatic derivation without
+an explicit declaration—provides a number of benefits:
+
+* The syntax for opting in is natural; there is no clear analogue in Swift
+  today for having a type opt out of a feature.
+
+* It requires users to make a conscious decision about the public API surfaced
+  by their types. Types cannot accidentally "fall into" conformances that the
+  user does not wish them to; a type that does not initially support `Equatable`
+  can be made to at a later date, but the reverse is a breaking change.
+
+* The conformances supported by a type can be clearly seen by examining
+  its source code; nothing is hidden from the user.
+
+* We reduce the work done by the compiler and the amount of code generated
+  by not synthesizing conformances that are not desired and not used.
+
+* As will be discussed later, explicit conformance significantly simplifies
+  the implementation for recursive types.
+
+There is one exception to this rule: the current behavior will be preserved that
+`enum` types with cases that have no associated values (including those with raw
+values) conform to `Equatable`/`Hashable` _without_ the user explicitly
+declaring those conformances. While this does add some inconsistency to `enum`s
+under this proposal, changing this existing behavior would be source-breaking.
+The question of whether such `enum`s should be required to opt-in as well can
+be revisited at a later date if so desired.
+
+### Overriding synthesized conformances
+
+Any user-provided implementations of `==` or `hashValue` will override the
+default implementations that would be provided by the compiler.
+
+### Conditions where synthesis is allowed
+
+For brevity, let `P` represent either the protocol `Equatable` or `Hashable` in
+the descriptions below.
+
+#### Synthesized requirements for `enum`s
+
+For an `enum`, synthesis of `P`'s requirements is based on the conformances of
+its cases' associated values. Computed properties are not considered.
+
+The following rules determine whether `P`'s requirements can be synthesized for
+an `enum`:
+
+* The compiler does **not** synthesize `P`'s requirements for an `enum` with no
+  cases because it is not possible to create instances of such types.
+
+* The compiler synthesizes `P`'s requirements for an `enum` with one or more
+  cases if and only if all of the associated values of all of its cases conform
+  to `P`.
+
+#### Synthesized requirements for `struct`s
+
+For a `struct`, synthesis of `P`'s requirements is based on the conformances of
+**only** its stored instance properties. Neither static properties nor computed
+instance properties (those with custom getters) are considered.
+
+The following rules determine whether `P`'s requirements can be synthesized for
+a `struct`:
+
+* The compiler trivially synthesizes `P`'s requirements for a `struct` with *no*
+  stored properties. (All instances of a `struct` with no stored properties can
+  be considered equal and hash to the same value if the user opts in to this.)
+
+* The compiler synthesizes `P`'s requirements for a `struct` with one or more
+  stored properties if and only if all of the types of all of its stored
+  properties conform to `P`.
+
+### Considerations for recursive types
+
+By making the synthesized conformances opt-in, recursive types have their
+requirements fall into place with no extra effort. In any cycle belonging to a
+recursive type, every type in that cycle must declare its conformance
+explicitly. If a type does so but cannot have its conformance synthesized
+because it does not satisfy the conditions above, then it is simply an error for
+_that_ type and not something that must be detected earlier by the compiler in
+order to reason about _all_ the other types involved in the cycle. (On the other
+hand, if conformance were implicit, the compiler would have to fully traverse
+the entire cycle to determine eligibility, which would make implementation much
+more complex).
+
+### Implementation details
+
+An `enum T: Equatable` that satisfies the conditions above will receive a
+synthesized implementation of `static func == (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: Hashable` that satisfies the conditions above will receive a
+synthesized implementation of `var hashValue: Int { get }` that uses an
+unspecified hash function<sup>†</sup> 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: Equatable` that satisfies the conditions above will receive a
+synthesized implementation of `static func == (lhs: T, rhs: T) -> Bool` that
+returns `true` if and only if `lhs.x == rhs.x` for all stored properties `x` in
+`T`. If the `struct` has no stored properties, this operator simply returns
+`true`.
+
+A `struct T: Hashable` that satisfies the conditions above will receive a
+synthesized implementation of `var hashValue: Int { get }` that uses an
+unspecified hash function<sup>†</sup> to compute the hash value by incorporating
+the hash values of the fields as its terms, in definition order. If the `struct`
+has no stored properties, this property evaluates to a fixed value not specified
+here.
+
+<sup>†</sup> The choice of hash function is left as an implementation detail,
+not a fixed part of the design; as such, users should not depend on specific
+characteristics of its behavior. The most likely implementation would call the
+standard library's `_mixInt` function on each member's hash value and then
+combine them with exclusive-or (`^`), which mirrors the way `Collection` types
+are hashed today.
+
+## Source compatibility
+
+By making the conformance opt-in, this is a purely additive change that does
+not affect existing code. We also avoid source-breaking changes by not changing
+the behavior for `enum`s with no associated values, which will continue to
+implicitly conform to `Equatable` and `Hashable` even without explicitly
+declaring the conformance.
+
+## Effect on ABI stability
+
+This feature is purely additive and does not change ABI.
+
+## Effect on API resilience
+
+N/A.
+
+## Alternatives considered
+
+In order to realistically scope this proposal, we considered but ultimately
+deferred the following items, some of which could be proposed additively in the
+future.
+
+### Synthesis in extensions
+
+Requirements will be synthesized only for protocol conformances that are
+_part of the type declaration itself;_ conformances added in extensions will
+not be synthesized.
+
+For `struct`s, synthesizing a requirement would not be safe in an extension
+in a different module or in a different file in the same module because any
+`private` or `fileprivate` members of the `struct` would not be accessible
+there. Extensions within the same file would be safe now that `private` members
+are also accessible from extensions of the containing type in the same file.
+
+However, to align with `Codable` in the context of
+[SR-4920](https://bugs.swift.org/browse/SR-4920), we will also currently
+forbid synthesized requirements in extensions in the same file; this specific
+case can be revisited later for all derived conformances.
+
+We note that conformances to `enum` types would be safe to synthesize anywhere
+because the cases and their associated values are always as accessible as the
+`enum` type itself, but we apply the same rule above for consistency; users do
+not have to memorize an intricate table of what is derivable and where.
+
+### Synthesis for `class` types and tuples
+
+We do not synthesize conformances for `class` types. The conditions above become
+more complicated in inheritance hierarchies, and equality requires that
+`static func ==` be implemented in terms of an overridable instance method for
+it to be dispatched dynamically. Even for `final` classes, the conditions are
+not as clear-cut as they are for value types because we have to take superclass
+behavior into consideration. Finally, since objects have reference identity,
+memberwise equality may not necessarily imply that two instances are equal.
+
+We do not synthesize conformances for tuples at this time. While this would
+nicely round out the capabilities of value types, allow the standard library to
+remove the hand-crafted implementations of `==` for up-to-arity-6 tuples, and
+allow those types to be used in generic contexts where `Equatable` conformance
+is required, adding conformances to non-nominal types would require additional
+work.
+
+### Omitting fields from synthesized conformances
+
+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.
+
+Such a feature, which could be implemented with an attribute such as
+`@transient`, would likely also play a role in other protocols like
+`Encodable`/`Decodable`. This could be done as a purely additive change on top
+of this proposal, so we propose not doing this at this time.
+
+### Implicit derivation
+
+An earlier draft of this proposal made derived conformances implicit (without
+declaring `Equatable`/`Hashable` explicitly). This has been changed
+because&mdash;in addition to the reasons mentioned earlier in the
+proposal&mdash;`Encodable`/`Decodable` provide a precedent for having the
+conformance be explicit. More importantly, however, determining derivability for
+recursive types is _significantly more difficult_ if conformance is implicit,
+because it requires examining the entire dependency graph for a particular type
+and to properly handle cycles in order to decide if the conditions are
+satisfied.
+
+### Support for `Comparable`
+
+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, Mark Sands for necromancing the
+swift-evolution thread that convinced me to write this up, and everyone on
+swift-evolution since then for giving me feedback on earlier drafts.