[swift-evolution] [Proposal] Foundation Swift Archival & Serialization

Colin Barrett colin at springsandstruts.com
Tue Mar 21 10:44:10 CDT 2017

Hi Itai,

Glad to see these proposal! I'm curious, have you or the other Swift folks
thought about how *users* of these new Codable protocols will interact with
resilience domains?

What I mean is that what appear to be private or internal identifiers, and
thus changeable at will, may actually be fragile in that changing them will
break the ability to decode archives encoded by previous versions.

Making this safer could mean:
- Encoding only public properties
- Adding some form of indirection (a la ObjC non-fragile ivars?)
- Compiler warning (or disallowing) changes to properties in certain

I imagine the specifics would need to follow the rest of the plans for

It's likely that this could be addressed by a future proposal, as for the
time being developers can simply "not hold it wrong" ;)


On Wed, Mar 15, 2017 at 6:52 PM Itai Ferber via swift-evolution <
swift-evolution at swift.org> wrote:

> Hi everyone,
> The following introduces a new Swift-focused archival and serialization
> API as part of the Foundation framework. We’re interested in improving the
> experience and safety of performing archival and serialization, and are
> happy to receive community feedback on this work.
> Because of the length of this proposal, the *Appendix* and *Alternatives
> Considered* sections have been omitted here, but are available in the full
> proposal <https://github.com/apple/swift-evolution/pull/639> on the
> swift-evolution repo. The full proposal also includes an *Unabridged API* for
> further consideration.
> Without further ado, inlined below.
> — Itai
> Swift Archival & Serialization
>    - Proposal: SE-NNNN <https://github.com/apple/swift-evolution/pull/639>
>    - Author(s): Itai Ferber <https://github.com/itaiferber>, Michael LeHew
>    <https://github.com/mlehew>, Tony Parker <https://github.com/parkera>
>    - Review Manager: TBD
>    - Status: *Awaiting review*
>    - Associated PRs:
>       - #8124 <https://github.com/apple/swift/pull/8124>
>       - #8125 <https://github.com/apple/swift/pull/8125>
> Introduction
> Foundation's current archival and serialization APIs (NSCoding,
> NSJSONSerialization, NSPropertyListSerialization, etc.), while fitting
> for the dynamism of Objective-C, do not always map optimally into Swift.
> This document lays out the design of an updated API that improves the
> developer experience of performing archival and serialization in Swift.
> Specifically:
>    - It aims to provide a solution for the archival of Swift struct and
>    enum types
>    - It aims to provide a more type-safe solution for serializing to
>    external formats, such as JSON and plist
> Motivation
> The primary motivation for this proposal is the inclusion of native Swift
> enum and struct types in archival and serialization. Currently,
> developers targeting Swift cannot participate in NSCoding without being
> willing to abandon enum and structtypes — NSCoding is an @objc protocol,
> conformance to which excludes non-class types. This is can be limiting in
> Swift because small enums and structs can be an idiomatic approach to
> model representation; developers who wish to perform archival have to
> either forgo the Swift niceties that constructs like enumsprovide, or
> provide an additional compatibility layer between their "real" types and
> their archivable types.
> Secondarily, we would like to refine Foundation's existing serialization
> APIs (NSJSONSerialization and NSPropertyListSerialization) to better
> match Swift's strong type safety. From experience, we find that the
> conversion from the unstructured, untyped data of these formats into
> strongly-typed data structures is a good fit for archival mechanisms,
> rather than taking the less safe approach that 3rd-party JSON conversion
> approaches have taken (described further in an appendix below).
> We would like to offer a solution to these problems without sacrificing
> ease of use or type safety.
> Agenda
> This proposal is the first stage of three that introduce different facets
> of a whole Swift archival and serialization API:
>    1. This proposal describes the basis for this API, focusing on the
>    protocols that users adopt and interface with
>    2. The next stage will propose specific API for new encoders
>    3. The final stage will discuss how this new API will interop with
>    NSCoding as it is today
> SE-NNNN provides stages 2 and 3.
> Proposed solution
> We will be introducing the following new types:
>    - protocol Codable: Adopted by types to opt into archival. Conformance
>    may be automatically derived in cases where all properties are also
>    Codable.
>    - protocol CodingKey: Adopted by types used as keys for keyed
>    containers, replacing String keys with semantic types. Conformance may
>    be automatically derived in most cases.
>    - protocol Encoder: Adopted by types which can take Codable values and
>    encode them into a native format.
>       - class KeyedEncodingContainer<Key : CodingKey>: Subclasses of this
>       type provide a concrete way to store encoded values by CodingKey.
>       Types adopting Encoder should provide subclasses of
>       KeyedEncodingContainer to vend.
>       - protocol SingleValueEncodingContainer: Adopted by types which
>       provide a concrete way to store a single encoded value. Types adopting
>       Encoder should provide types conforming to
>       SingleValueEncodingContainer to vend (but in many cases will be
>       able to conform to it themselves).
>    - protocol Decoder: Adopted by types which can take payloads in a
>    native format and decode Codable values out of them.
>       - class KeyedDecodingContainer<Key : CodingKey>: Subclasses of this
>       type provide a concrete way to retrieve encoded values from storage by
>       CodingKey. Types adopting Decoder should provide subclasses of
>       KeyedDecodingContainer to vend.
>       - protocol SingleValueDecodingContainer: Adopted by types which
>       provide a concrete way to retrieve a single encoded value from storage.
>       Types adopting Decoder should provide types conforming to
>       SingleValueDecodingContainer to vend (but in many cases will be
>       able to conform to it themselves).
> For end users of this API, adoption will primarily involve the Codable
>  and CodingKey protocols. In order to participate in this new archival
> system, developers must add Codable conformance to their types:
> // If all properties are Codable, implementation is automatically derived:public struct Location : Codable {
>     public let latitude: Double
>     public let longitude: Double}
> public enum Animal : Int, Codable {
>     case chicken = 1
>     case dog
>     case turkey
>     case cow}
> public struct Farm : Codable {
>     public let name: String
>     public let location: Location
>     public let animals: [Animal]}
> With developer participation, we will offer encoders and decoders
> (described in SE-NNNN, not here) that take advantage of this conformance to
> offer type-safe serialization of user models:
> let farm = Farm(name: "Old MacDonald's Farm",
>                 location: Location(latitude: 51.621648, longitude: 0.269273),
>                 animals: [.chicken, .dog, .cow, .turkey, .dog, .chicken, .cow, .turkey, .dog])let payload: Data = try JSONEncoder().encode(farm)
> do {
>     let farm = try JSONDecoder().decode(Farm.self, from: payload)
>     // Extracted as user types:
>     let coordinates = "\(farm.location.latitude, farm.location.longitude)"} catch {
>     // Encountered error during deserialization}
> This gives developers access to their data in a type-safe manner and a
> recognizable interface.
> Detailed design
> To support user types, we expose the Codable protocol:
> /// Conformance to `Codable` indicates that a type can marshal itself into and out of an external representation.public protocol Codable {
>     /// Initializes `self` by decoding from `decoder`.
>     ///
>     /// - parameter decoder: The decoder to read data from.
>     /// - throws: An error if reading from the decoder fails, or if read data is corrupted or otherwise invalid.
>     init(from decoder: Decoder) throws
>     /// Encodes `self` into the given encoder.
>     ///
>     /// If `self` fails to encode anything, `encoder` will encode an empty `.default` container in its place.
>     ///
>     /// - parameter encoder: The encoder to write data to.
>     /// - throws: An error if any values are invalid for `encoder`'s format.
>     func encode(to encoder: Encoder) throws}
> By adopting Codable, user types opt in to this archival system.
> Structured types (i.e. types which encode as a collection of properties)
> encode and decode their properties in a keyed manner. Keys may be String-convertible
> or Int-convertible (or both), and user types which have properties should
> declare semantic key enums which map keys to their properties. Keys must
> conform to the CodingKey protocol:
> /// Conformance to `CodingKey` indicates that a type can be used as a key for encoding and decoding.public protocol CodingKey {
>     /// The string to use in a named collection (e.g. a string-keyed dictionary).
>     var stringValue: String? { get }
>     /// Initializes `self` from a string.
>     ///
>     /// - parameter stringValue: The string value of the desired key.
>     /// - returns: An instance of `Self` from the given string, or `nil` if the given string does not correspond to any instance of `Self`.
>     init?(stringValue: String)
>     /// The int to use in an indexed collection (e.g. an int-keyed dictionary).
>     var intValue: Int? { get }
>     /// Initializes `self` from an integer.
>     ///
>     /// - parameter intValue: The integer value of the desired key.
>     /// - returns: An instance of `Self` from the given integer, or `nil` if the given integer does not correspond to any instance of `Self`.
>     init?(intValue: Int)}
> For most types, String-convertible keys are a reasonable default; for
> performance, however, Int-convertible keys are preferred, and Encoders may
> choose to make use of Ints over Strings. Framework types should provide
> keys which have both for flexibility and performance across different types
> of Encoders. It is generally an error to provide a key which has neither
> a stringValue nor an intValue.
> By default, CodingKey conformance can be derived for enums which have
> either String or Int backing:
> enum Keys1 : CodingKey {
>     case a // (stringValue: "a", intValue: nil)
>     case b // (stringValue: "b", intValue: nil)}
> enum Keys2 : String, CodingKey {
>     case c = "foo" // (stringValue: "foo", intValue: nil)
>     case d         // (stringValue: "d", intValue: nil)}
> enum Keys3 : Int, CodingKey {
>     case e = 4 // (stringValue: "e", intValue: 4)
>     case f     // (stringValue: "f", intValue: 5)
>     case g = 9 // (stringValue: "g", intValue: 9)}
> Coding keys which are not enums, have associated values, or have other
> raw representations must implement these methods manually.
> In addition to automatic CodingKey conformance derivation for enums,
> Codableconformance can be automatically derived for certain types as well:
>    1. Types whose properties are all either Codable or primitive get an
>    automatically derived String-backed CodingKeys enum mapping properties
>    to case names
>    2. Types falling into (1) and types which provide a CodingKeys enum (directly
>    or via a typealias) whose case names map to properties which are all
>    Codableget automatic derivation of init(from:) and encode(to:) using
>    those properties and keys. Types may choose to provide a custom
>    init(from:) or encode(to:) (or both); whichever they do not provide
>    will be automatically derived
>    3. Types which fall into neither (1) nor (2) will have to provide a
>    custom key type and provide their own init(from:) and encode(to:)
> Many types will either allow for automatic derivation of all codability
> (1), or provide a custom key subset and take advantage of automatic method
> derivation (2).
> Encoding and Decoding
> Types which are encodable encode their data into a container provided by
> their Encoder:
> /// An `Encoder` is a type which can encode values into a native format for external representation.public protocol Encoder {
>     /// Populates `self` with an encoding container (of `.default` type) and returns it, keyed by the given key type.
>     ///
>     /// - parameter type: The key type to use for the container.
>     /// - returns: A new keyed encoding container.
>     /// - precondition: May not be called after a previous `self.container(keyedBy:)` call of a different `EncodingContainerType`.
>     /// - precondition: May not be called after a value has been encoded through a prior `self.singleValueContainer()` call.
>     func container<Key : CodingKey>(keyedBy type: Key.Type) -> KeyedEncodingContainer<Key>
>     /// Returns an encoding container appropriate for holding a single primitive value.
>     ///
>     /// - returns: A new empty single value container.
>     /// - precondition: May not be called after a prior `self.container(keyedBy:)` call.
>     /// - precondition: May not be called after a value has been encoded through a previous `self.singleValueContainer()` call.
>     func singleValueContainer() -> SingleValueEncodingContainer
>     /// The path of coding keys taken to get to this point in encoding.
>     var codingKeyContext: [CodingKey] { get }}
> // Continuing examples from before; below is automatically generated by the compiler if no customization is needed.public struct Location : Codable {
>     private enum CodingKeys : CodingKey {
>         case latitutude
>         case longitude
>     }
>     public func encode(to encoder: Encoder) throws {
>         // Generic keyed encoder gives type-safe key access: cannot encode with keys of the wrong type.
>         let container = encoder.container(keyedBy: CodingKeys.self)
>         // The encoder is generic on the key -- free key autocompletion here.
>         try container.encode(latitude, forKey: .latitude)
>         try container.encode(longitude, forKey: .longitude)
>     }}
> public struct Farm : Codable {
>     private enum CodingKeys : CodingKey {
>         case name
>         case location
>         case animals
>     }
>     public func encode(to encoder: Encoder) throws {
>         let container = encoder.container(keyedBy: CodingKeys.self)
>         try container.encode(name
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