SkipLib

Swift standard library for Skip apps.

See what API is currently implemented here.

About

SkipLib vends the skip.lib Kotlin package. It serves two purposes:

1. SkipLib is a reimplementation of the Swift standard library for Kotlin on Android. Its goal is to mirror as much of the Swift standard library as possible, allowing Skip developers to use Swift standard library API with confidence.
2. SkipLib contains custom Kotlin API that the Skip transpiler takes advantage of when translating your Swift source to the equivalent Kotlin code. For example, the Kotlin language does not have tuples. Instead, SkipLib's Tuple.kt defines bespoke Kotlin Tuple classes. When the transpiler translates Swift code that references tuples, it uses these Tuple classes in the Kotlin it generates.

Dependencies

SkipLib depends on the skip transpiler plugin and has no additional library dependencies.

It is part of the core SkipStack and is not intended to be imported directly. The module is transparently adopted through the automatic addition of import skip.lib.* to transpiled files by the Skip transpiler.

Status

• SkipLib's Swift symbol files (see Implementation Strategy) are nominally complete. They should declare all Swift standard library API. This is difficult to validate, however, so if you find anything missing, please report it to us.
• Unimplemented API is appropriately marked with @available(*, unavailable) annotations. Skip will generate an error when you attempt to use an unimplemented API.
• In particular, a significant portion of the collections API is not yet implemented.
• Unit testing is not comprehensive.

See Swift Standard Library Support.

Contributing

We welcome contributions to SkipLib. The Skip product documentation includes helpful instructions and tips on local Skip library development.

The most pressing need is to reduce the amount of unimplemented API. To help fill in unimplemented API in SkipLib:

1. Find unimplemented API. Unimplemented API should be marked with @available(*, unavailable) in the Swift symbol files.
2. Write an appropriate Kotlin implementation. See Implementation Strategy below. For collections API, make sure your implementation is duplicated for String as well.
3. Write unit tests.
4. Submit a PR.

Other forms of contributions such as test cases, comments, and documentation are also welcome!

Implementation Strategy

Apart from the Skip transpiler itself, SkipLib implements the lowest levels of the Swift language. Its implementation strategy, therefore, differs from other Skip libraries.

Most Skip libraries call Kotlin API, but are written in Swift, relying on the Skip transpiler for translation to Kotlin. Most of SkipLib, however, is written in pure Kotlin. Consider SkipLib's implementation of Swift's Array. SkipLib divides its Array support into two files:

1. Sources/SkipLib/Array.swift acts as a Swift header file, declaring the Array type's Swift API but stubbing out the implementation. The // SKIP SYMBOLFILE comment at the top of the file marks it as such. Read more about special Skip comments in the Skip product documentation.
2. Sources/SkipLib/Skip/Array.kt contains the actual Array implementation in Kotlin.

This pattern is used for most Swift types throughout SkipLib. Meanwhile, SwiftLib implementations of constructs built directly into the Swift language - e.g. tuples or inout parameters - only have a Kotlin file, with no corresponding Swift symbol file.

Swift Standard Library Support

The following table summarizes SkipLib's Swift Standard Library API support on Android. Anything not listed here is likely not supported. Note that in your iOS-only code - i.e. code within #if !SKIP blocks - you can use any API you want.

Support levels:

• ✅ – Full
• 🟢 – High
• 🟡 - Medium
• 🟠 – Low

Support	API
🟢	Actor Non-private mutable properties are not supported
✅	Any
✅	AnyActor
✅	AnyHashable
✅	AnyObject
🟢	Array init() init(repeating: Element, count: Int) init(_ sequence: any Sequence<Element>) See Collection for collection API support
✅	assert
✅	assertionFailure
✅	AsyncSequence
✅	AsyncStream When invoking the init(unfolding:) constructor, use a labeled argument rather than a trailing closure
🟢	Bool static func random() -> Bool static func random(using gen: inout RandomNumberGenerator) -> Bool
✅	CaseIterable
✅	CGAffineTransform
✅	CGFloat
✅	CGPoint
✅	CGRect
✅	CGSize
🟢	Character init(_: Character) init(_: String) var isNewline: Bool var isWhitespace: Bool var isUppercase: Bool var isLowercase: Bool func lowercased() -> String func uppercased() -> String
🟢	Codable See Codable
🟡	Collection Note: This list represents the combined supported API of Swift's many collection types: Sequence, Collection, BidirectionalCollection, etc func allSatisfy(_ predicate: (Element) throws -> Bool) rethrows -> Bool mutating func append(_ newElement: Element) mutating func append(contentsOf newElements: any Sequence<Element>) func contains(_ element: Element) -> Bool func contains(where predicate: (Element) throws -> Bool) rethrows -> Bool func distance(from start: Int, to end: Int) -> Int func drop(while predicate: (Element) throws -> Bool) rethrows -> [Element] func dropFirst(_ k: Int = 1) -> [Element] func dropLast(_ k: Int = 1) -> [Element] func elementsEqual(_ other: any Sequence<Element>) -> Bool func elementsEqual(_ other: any Sequence<Element>, by areEquivalent: (Element, Element) throws -> Bool) rethrows -> Bool func enumerated() -> any Sequence<(offset: Int, element: Element)> var endIndex: Int func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> [Element] func first(where predicate: (Element) throws -> Bool) rethrows -> Element? func firstIndex(of element: Element) -> Int? func firstIndex(where predicate: (Element) throws -> Bool) rethrows -> Int? var first: Element? func flatMap<RE>(_ transform: (Element) throws -> any Sequence<RE>) rethrows -> [RE] func formIndex(_ i: inout Int, offsetBy distance: Int) func formIndex(after i: inout Int) func index(_ i: Int, offsetBy distance: Int) -> Int func index(after i: Int) -> Int var indices: any Sequence<Int> var isEmpty: Bool func joined<RE>() -> [RE] where Element: Sequence<RE> func joined<RE>(separator: any Sequence<RE>) -> [RE] where Element: Sequence<RE> func joined(separator: String) -> String func makeIterator() -> any IteratorProtocol<Element> func map<RE>(_ transform: (Element) throws -> RE) rethrows -> [RE] func max() -> Element? func max(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> Element? func min() -> Element? func min(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> Element? var underestimatedCount: Int func prefix(_ maxLength: Int) -> [Element] func prefix(through end: Int) -> [Element] func prefix(upTo end: Int) -> [Element] mutating func popFirst() -> Element? mutating func popLast() -> Element? func randomElement() -> Element? func randomElement(using generator: inout any RandomNumberGenerator) -> Element? func reduce<R>(_ initialResult: R, _ nextPartialResult: (_ partialResult: R, Element) throws -> R) rethrows -> R func reduce<R>(into initialResult: R, _ updateAccumulatingResult: (_ partialResult: inout R, Element) throws -> Void) rethrows -> R func remove(at i: Int) -> Element mutating func removeAll(keepingCapacity keepCapacity: Bool = false) mutating func removeAll(where shouldBeRemoved: (Element) throws -> Bool) rethrows mutating func removeFirst() -> Element mutating func removeFirst(_ k: Int) mutating func removeLast() -> Element mutating func removeLast(_ k: Int) mutating func reverse() func reversed() -> [Element] mutating func shuffle() mutating func shuffle<T: RandomNumberGenerator>(using generator: inout T) mutating func sort() mutating func sort(by areIncreasingOrder: (Element, Element) throws -> Bool) rethrows mutating func swapAt(_ i: Int, _ j: Int) subscript(bounds: Range<Int>) -> any Collection<Element> subscript(position: Int) -> Element func starts(with possiblePrefix: Any) -> Bool func starts(with possiblePrefix: Any, by areEquivalent: (Element, Element) throws -> Bool) rethrows -> Bool func suffix(from start: Int) -> [Element] func suffix(_ maxLength: Int) -> [Element] func sorted() -> [Element] func sorted(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> [Element] var startIndex: Int var count: Int func count<E>(where: (E) throws -> Bool) rethrows -> Int func withContiguousStorageIfAvailable<R>(_ body: (Any) throws -> R) rethrows -> R? func forEach(_ body: (Element) throws -> Void) rethrows func drop(while predicate: (Element) throws -> Bool) rethrows -> [Element] func dropFirst(_ k: Int = 1) -> [Element] func dropLast(_ k: Int = 1) -> [Element] func enumerated() -> any Sequence<(offset: Int, element: Element)> func filter(_ isIncluded: (Element) throws -> Bool) rethrows -> [Element] func first(where predicate: (Element) throws -> Bool) rethrows -> Element? func map<RE>(_ transform: (Element) throws -> RE) rethrows -> [RE] func max(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> Element? func min(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> Element? func reduce<R>(_ initialResult: R, _ nextPartialResult: (_ partialResult: R, Element) throws -> R) rethrows -> R func reduce<R>(into initialResult: R, _ updateAccumulatingResult: (_ partialResult: inout R, Element) throws -> Void) rethrows -> R func reversed() -> [Element] func shuffled() -> [Element] func shuffled<T: RandomNumberGenerator>(using generator: inout T) -> [Element] func flatMap<RE>(_ transform: (Element) throws -> any Sequence<RE>) rethrows -> [RE] func compactMap<RE>(_ transform: (Element) throws -> RE?) rethrows -> [RE] func sorted(by areInIncreasingOrder: (Element, Element) throws -> Bool) rethrows -> [Element] func joined<RE>() -> [RE] where Element: Sequence<RE> func joined<RE>(separator: any Sequence<RE>) -> [RE] where Element: Sequence<RE> func joined(separator: String) -> String func starts(with possiblePrefix: Any) -> Bool func contains(_ element: Element) -> Bool func min() -> Element? func max() -> Element? func sorted() -> [Element] var startIndex: Int var endIndex: Int var indices: any Sequence<Int> func index(_ i: Int, offsetBy distance: Int) -> Int func distance(from start: Int, to end: Int) -> Int func index(after i: Int) -> Int func formIndex(after i: inout Int) func formIndex(_ i: inout Int, offsetBy distance: Int) func randomElement() -> Element? func randomElement(using generator: inout any RandomNumberGenerator) -> Element? mutating func popFirst() -> Element? var first: Element? func prefix(upTo end: Int) -> [Element] func suffix(from start: Int) -> [Element] func prefix(through end: Int) -> [Element] mutating func removeFirst() -> Element mutating func removeFirst(_ k: Int) func firstIndex(of element: Element) -> Int? func firstIndex(where predicate: (Element) throws -> Bool) rethrows -> Int? mutating func shuffle() mutating func shuffle<T: RandomNumberGenerator>(using generator: inout T) mutating func sort() mutating func sort(by areIncreasingOrder: (Element, Element) throws -> Bool) rethrows mutating func reverse() mutating func swapAt(_ i: Int, _ j: Int) mutating func append(_ newElement: Element) mutating func append(contentsOf newElements: any Sequence<Element>) mutating func insert(_ newElement: Element, at i: Int) mutating func insert(contentsOf newElements: any Sequence<Element>, at i: Int) mutating func remove(at i: Int) -> Element mutating func removeAll(keepingCapacity keepCapacity: Bool = false) mutating func removeAll(where shouldBeRemoved: (Element) throws -> Bool) rethrows mutating func popLast() -> Element? mutating func removeLast() -> Element mutating func removeLast(_ k: Int) subscript(bounds: Range<Int>) -> any Collection<Element> subscript(position: Int) -> Element
✅	Comparable
✅	CustomDebugStringConvertible
✅	CustomStringConvertible
🟢	Decodable See Codable
🟢	Dictionary init() init(minimumCapacity: Int) init(uniqueKeysWithValues keysAndValues: any Sequence<(Key, Value)>) func filter(_ isIncluded: ((Key, Value)) throws -> Bool) rethrows -> Dictionary<Key, Value> subscript(key: Key) -> Value? subscript(key: Key, default defaultValue: Value) -> Value func mapValues<T>(_ transform: (Value) throws -> T) rethrows -> Dictionary<Key, T> func compactMapValues<T>(_ transform: (Value) throws -> T?) rethrows -> Dictionary<Key, T> mutating func updateValue(_ value: Value, forKey key: Key) -> Value? mutating func removeValue(forKey key: Key) -> Value? var keys: any Collection<Key>) var values: any Collection<Value> mutating func removeAll(keepingCapacity keepCapacity: Bool = false) See Collection for collection API support
✅	DiscardingTaskGroup
🟢	Double static var nan: Double static var infinity: Double static var pi: Double var isNan: Bool var isFinite: Bool var isInfinite: Bool static func random(in range: Range<Double>) -> Double func rounded() -> Double func rounded(_ rule: FloatingPointRoundingRule) -> Double
🟢	Encodable See [Codable](#codable)
✅	Equatable
✅	Error
✅	fatalError
🟢	Float static var nan: Float static var infinity: Float static var pi: Float var isNan: Bool var isFinite: Bool var isInfinite: Bool static func random(in range: Range<Float>) -> Float func rounded() -> Float func rounded(_ rule: FloatingPointRoundingRule) -> Float
✅	Hashable
✅	Hasher
✅	Identifiable
🟢	Int8 static var min: Int8 static var max: Int8 static func random(in range: Range<Int8>) -> Int8 static func random(in range: Range<Int8>, using gen: inout RandomNumberGenerator) -> Int8
🟢	Int16 static var min: Int16 static var max: Int16 static func random(in range: Range<Int16>) -> Int16 static func random(in range: Range<Int16>, using gen: inout RandomNumberGenerator) -> Int16
🟢	Int32 static var min: Int32 static var max: Int32 static func random(in range: Range<Int32>) -> Int32 static func random(in range: Range<Int32>, using gen: inout RandomNumberGenerator) -> Int32
🟢	Int Kotlin Ints are 32 bit static var min: Int static var max: Int static func random(in range: Range<Int>) -> Int static func random(in range: Range<Int>, using gen: inout RandomNumberGenerator) -> Int
🟢	Int64 static var min: Int64 static var max: Int64 static func random(in range: Range<Int64>) -> Int64 static func random(in range: Range<Int64>, using gen: inout RandomNumberGenerator) -> Int64
🟢	@MainActor
🟢	MainActor static func run<T>(body: () throws -> T) async -> T
🟢	math.h var M_E: Double var M_LOG2E: Double var M_LOG10E: Double var M_LN2: Double var M_LN10: Double var M_PI: Double func acosf(_ x: Float) -> Float func acos(_ x: Double) -> Double func acosl(_ x: Double) -> Double func asinf(_ x: Float) -> Float func asin(_ x: Double) -> Double func asinl(_ x: Double) -> Double func atanf(_ x: Float) -> Float func atan(_ x: Double) -> Double func atanl(_ x: Double) -> Double func atan2f(_ x: Float, _ y: Float) -> Float func atan2(_ x: Double, _ y: Double) -> Double func atan2l(_ x: Double, _ y: Double) -> Double func cosf(_ x: Float) -> Float func cos(_ x: Double) -> Double func cosl(_ x: Double) -> Double func sinf(_ x: Float) -> Float func sin(_ x: Double) -> Double func sinl(_ x: Double) -> Double func tanf(_ x: Float) -> Float func tan(_ x: Double) -> Double func tanl(_ x: Double) -> Double func acoshf(_ x: Float) -> Float func acosh(_ x: Double) -> Double func acoshl(_ x: Double) -> Double func asinhf(_ x: Float) -> Float func asinh(_ x: Double) -> Double func asinhl(_ x: Double) -> Double func atanhf(_ x: Float) -> Float func atanh(_ x: Double) -> Double func atanhl(_ x: Double) -> Double func coshf(_ x: Float) -> Float func cosh(_ x: Double) -> Double func coshl(_ x: Double) -> Double func sinhf(_ x: Float) -> Float func sinh(_ x: Double) -> Double func sinhl(_ x: Double) -> Double func tanhf(_ x: Float) -> Float func tanh(_ x: Double) -> Double func tanhl(_ x: Double) -> Double func expf(_ x: Float) -> Float func exp(_ x: Double) -> Double func expl(_ x: Double) -> Double func exp2f(_ x: Float) -> Float func exp2(_ x: Double) -> Double func exp2l(_ x: Double) -> Double func expm1f(_ x: Float) -> Float func expm1(_ x: Double) -> Double func expm1l(_ x: Double) -> Double func logf(_ x: Float) -> Float func log(_ x: Double) -> Double func logl(_ x: Double) -> Double func log10f(_ x: Float) -> Float func log10(_ x: Double) -> Double func log10l(_ x: Double) -> Double func log2f(_ x: Float) -> Float func log2(_ x: Double) -> Double func log2l(_ x: Double) -> Double func log1pf(_ x: Float) -> Float func log1p(_ x: Double) -> Double func log1pl(_ x: Double) -> Double func logbf(_ x: Float) -> Float func logb(_ x: Double) -> Double func logbl(_ x: Double) -> Double func abs(_ x: Double) -> Double func abs(_ x: Int) -> Int func abs(_ x: Int64) -> Int64 func fabsf(_ x: Float) -> Float func fabs(_ x: Double) -> Double func fabsl(_ x: Double) -> Double func cbrtf(_ x: Float) -> Float func cbrt(_ x: Double) -> Double func cbrtl(_ x: Double) -> Double func hypotf(_ x: Float, _ y: Float) -> Float func hypot(_ x: Double, _ y: Double) -> Double func hypotl(_ x: Double, _ y: Double) -> Double func powf(_ x: Float, _ y: Float) -> Float func pow(_ x: Double, _ y: Double) -> Double func powl(_ x: Double, _ y: Double) -> Double func sqrtf(_ x: Float) -> Float func sqrt(_ x: Double) -> Double func sqrtl(_ x: Double) -> Double func ceilf(_ x: Float) -> Float func ceil(_ x: Double) -> Double func ceill(_ x: Double) -> Double func floorf(_ x: Float) -> Float func floor(_ x: Double) -> Double func floorl(_ x: Double) -> Double func roundf(_ x: Float) -> Float func round(_ x: Double) -> Double func roundl(_ x: Double) -> Double func fmodf(_ x: Float, _ y: Float) -> Float func fmod(_ x: Double, _ y: Double) -> Double func fmodl(_ x: Double, _ y: Double) -> Double func remainderf(_ x: Float, _ y: Float) -> Float func remainder(_ x: Double, _ y: Double) -> Double func remainderl(_ x: Double, _ y: Double) -> Double func fmaxf(_ x: Float, _ y: Float) -> Float func fmax(_ x: Double, _ y: Double) -> Double func fmaxl(_ x: Double, _ y: Double) -> Double func fminf(_ x: Float, _ y: Float) -> Float func fmin(_ x: Double, _ y: Double) -> Double func fminl(_ x: Double, _ y: Double) -> Double
✅	max(_:_:)
✅	min(_:_:)
✅	ObjectIdentifier
✅	OptionSet
✅	precondition
✅	preconditionFailure
✅	RandomNumberGenerator
🟠	Range Only Range<Int> is generally supported var lowerBound: Bound var upperBound: Bound func contains(_ element: Bound) -> Bool var isEmpty: Bool func map<RE>(_ transform: (Bound) throws -> RE) rethrows -> [RE]
✅	RawRepresentable
🟠	Regex init(_ string: String) func matches(_ string: String) -> [Match] func replace(_ string: String, with replacement: String) -> String
🟠	Regex.Match var count: Int subscript(index: Int) -> MatchGroup
🟠	Regex.MatchGroup var substring: Substring?
✅	Result
🟡	swap(_:_:) Does not support swapping values in arrays and other data structures
🟢	Set init() init(_ sequence: any Sequence<Element>) See Collection See SetAlgebra
🟢	SetAlgebra func contains(_ element: Element) -> Bool func union(_ other: Self) -> Self func intersection(_ other: Self) -> Self func symmetricDifference(_ other: Self) -> Self mutating func insert(_ newMember: Element) -> (inserted: Bool, memberAfterInsert: Element) mutating func remove(_ member: Element) -> Element? mutating func update(with newMember: Element) -> Element? mutating func formUnion(_ other: Self) mutating func formIntersection(_ other: Self) mutating func formSymmetricDifference(_ other: Self) func subtracting(_ other: Self) -> Self func isSubset(of other: Self) -> Bool func isDisjoint(with other: Self) -> Bool func isSuperset(of other: Self) -> Bool var isEmpty: Bool mutating func subtract(_ other: Self) func isStrictSubset(of other: Self) -> Bool func isStrictSuperset(of other: Self) -> Bool
🟢	String Kotlin strings are **not** mutable init(data: Data, encoding: StringEncoding) init(bytes: [UInt8], encoding: StringEncoding) init(contentsOf: URL) var capitalized: String var deletingLastPathComponent: String func replacingOccurrences(of search: String, with replacement: String) -> String func components(separatedBy separator: String) -> [String] func trimmingCharacters(in set: CharacterSet) -> String var utf8Data: Data func data(using: StringEncoding, allowLossyConversion: Bool = true) -> Data? var utf8: [UInt8] var utf16: [UInt8] var unicodeScalars: [UInt8] See Collection See SkipFoundation for additional string API from Foundation
✅	strlen
✅	strncmp
🟢	Substring See String
✅	SystemRandomNumberGenerator
🟡	Task init(priority: TaskPriority? = nil, operation: @escaping () async throws -> Success) static func detached(priority: TaskPriority? = nil, operation: @escaping () async -> Success) -> Task<Success, Failure> var value: Success func cancel() static func yield() async var isCancelled: Bool static var isCancelled: Bool static func checkCancellation() throws static func sleep(nanoseconds duration: UInt64) async throws static var min: UInt8
✅	TaskGroup
✅	ThrowingDiscardingTaskGroup
✅	ThrowingTaskGroup
✅	type(of:)
🟢	UInt8 static var min: UInt8 static var max: UInt8 static func random(in range: Range<UInt8>) -> UInt8 static func random(in range: Range<UInt8>, using gen: inout RandomNumberGenerator) -> UInt8
🟢	UInt16 static var min: UInt16 static var max: UInt16 static func random(in range: Range<UInt16>) -> UInt16 static func random(in range: Range<UInt16>, using gen: inout RandomNumberGenerator) -> UInt16
🟢	UInt32 static var min: UInt32 static var max: UInt32 static func random(in range: Range<UInt32>) -> UInt32 static func random(in range: Range<UInt32>, using gen: inout RandomNumberGenerator) -> UInt32
🟢	UInt Kotlin UInts are 32 bit static var min: UInt static var max: UInt static func random(in range: Range<UInt>) -> UInt static func random(in range: Range<UInt>, using gen: inout RandomNumberGenerator) -> UInt
🟢	UInt64 static var min: UInt64 static var max: UInt64 static func random(in range: Range<UInt64>) -> UInt64 static func random(in range: Range<UInt64>, using gen: inout RandomNumberGenerator) -> UInt64
✅	withDiscardingTaskGroup
✅	withTaskCancellationHandler
✅	withThrowingTaskGroup
✅	withThrowingDiscardingTaskGroup
✅	withThrowingTaskGroup

Topics

Collections

Collections are perhaps the most complex part of the Swift standard library, and of SkipLib. Swift's comprehensive collection protocols allow Array, Set, Dictionary, String, and other types to all share a common set of API, including iteration, map, reduce, and much more.

Corresponding Kotlin types - List, Set, Map, String, etc - do not share a similarly rich API set. As a result, SkipLib must duplicate collection protocol implementations in both Collections.kt and String.kt, and must duplicate SetAlgebra implementations in both Set.kt and OptionSet.kt.

See the explanatory comments in Collections.kt for more information on the design of SkipLib's internal collections support.

Codable

Skip is able to synthesize default Codable conformance for the Android versions of your Swift types. The Android versions will encode and decode exactly like their Swift source types. Skip also supports your custom CodingKeys as well as your custom encode(to:) and init(from:) functions for encoding and decoding.

There are, however, a few restrictions:

• Skip cannot synthesize Codable conformance for enums that are not RawRepresentable. You must implement the required protocol functions yourself.

• If you implement your own encode function or init(from:) decoding constructor and you use CodingKeys, you must declare your own CodingKeys enum. You cannot rely on the synthesized enum.

• Array, Set, and Dictionary are fully supported, but nesting of these types is limited. So for example Skip can encode and decode Array<MyCodableType> and Dictionary<String, MyCodableType>, but not Array<Dictionary<String, MyCodableType>>. Two forms of container nesting are currently supported: arrays-of-arrays - e.g. Array<Array<MyCodableType>> - and dictionaries-of-array-values - e.g. Dictionary<String, Array<MyCodableType>>. In practice, other nesting patters are rare.

• When calling decode, you must supply a concrete type literal to decode. This applies to both top-level Decoders like JSONDecoder as well as containers like KeyedDecodingContainer. The following will work:

  let object = try decoder.decode(MyType.self, from: jsonData) 

  But these examples will not work:

  let type = MyType.self
  let object = try decoder.decode(type, from: jsonData)
  
  // T is a generic type
  let object = try decoder.decode(T.self, from: jsonData)

It is common for developers to take advantage of Decodable-typed generic functions to be able to decode arbitrary types, so this last limitation is the most onerous. You must consider it when writing your decoding code, and it often requires refactoring existing decoding code being ported to Skip.

One mechanism to ease this restriction and allow you to decode unknown generic types is to write inline decoding functions that take advantage of Kotlin's reified types. Inline functions, however, come with their own limitations and tradeoffs. You can read more about this topic in the Kotlin language documentation. Skip automatically converts any Swift function with the @inline(__always) attribute into a Kotlin inline function with reified generics.

For example, a function like the following will work with Skip, so long as you call it with a concrete Response type or with a generic Response type from another inline function:

@inline(__always) public func send<R: Request, Response: Decodable>(request: R) async throws -> Response {
    let data = try await download(request: request)
    return try jsonDecoder.decode(Response.self, from: data)
}

It transpiles to code like:

inline suspend fun <reified R, reified Response> send(request: R): Response where R: Request, Response: Decodable {
    val data = download(request = request)
    return jsonDecoder.decode(Response::class, from = data) 
}