This Skip framework is available at, which can be checked out and tested with skip test once Skip is installed.


Swift standard library for Skip apps.


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.


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.


  • 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 Tests for the current test run status.


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.



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.


The following table shows SkipLib’s current test status. Test contributions are welcome.

Test Case Swift Kotlin
ArrayTests testAdd PASS PASS
ArrayTests testAddDidSet PASS PASS
ArrayTests testAppend PASS PASS
ArrayTests testAppendDidSet PASS PASS
ArrayTests testArrayIndex PASS PASS
ArrayTests testArrayLiteralInit PASS PASS
ArrayTests testArrayReferences PASS PASS
ArrayTests testDeepNestedArrays PASS PASS
ArrayTests testInsert PASS PASS
ArrayTests testNestedSubscriptDidSet PASS PASS
ArrayTests testOptionalElements PASS PASS
ArrayTests testSubscriptDidSet PASS PASS
CollectionsTests testCompactMap PASS PASS
CollectionsTests testDictionaryForEach PASS PASS
CollectionsTests testDoubleStride PASS PASS
CollectionsTests testEmptyStride PASS PASS
CollectionsTests testEnumerated PASS PASS
CollectionsTests testFilter PASS PASS
CollectionsTests testFilterMapReduce PASS PASS
CollectionsTests testFirstLast PASS PASS
CollectionsTests testFlatMap PASS PASS
CollectionsTests testIndices PASS PASS
CollectionsTests testJoin PASS PASS
CollectionsTests testLazyFilterMap PASS SKIP
CollectionsTests testMap PASS PASS
CollectionsTests testReadSlice PASS PASS
CollectionsTests testReduce PASS PASS
CollectionsTests testRemoveFirst PASS PASS
CollectionsTests testSort PASS PASS
CollectionsTests testStride PASS PASS
CollectionsTests testStrideThrough PASS PASS
CollectionsTests testWriteSlice PASS PASS
CollectionsTests testZipCompactMap PASS SKIP
ConcurrencyTests testAsyncLet PASS PASS
ConcurrencyTests testAsyncSequence PASS PASS
ConcurrencyTests testMainActor PASS PASS
ConcurrencyTests testSimpleValue PASS PASS
ConcurrencyTests testTaskCancelWithExcepti PASS PASS
ConcurrencyTests testTaskCancelWithoutExce PASS PASS
ConcurrencyTests testTaskIsCancelled PASS PASS
ConcurrencyTests testThrowsException PASS PASS
DictionaryTests testDictionaryLiteralInit PASS PASS
DictionaryTests testDictionaryReferences PASS PASS
DictionaryTests testIterate PASS PASS
DictionaryTests testKeysValues PASS PASS
DictionaryTests testNestedSubscriptDidSet PASS PASS
DictionaryTests testPopFirst PASS PASS
DictionaryTests testSubscriptDefaultValue PASS PASS
DictionaryTests testSubscriptDidSet PASS PASS
GlobalsTests testFatalError PASS PASS
GlobalsTests testSwap PASS PASS
KotlinInteropTests testCustomKotlinConversio PASS PASS
MathTests testCeil PASS PASS
MathTests testEulersNumbers PASS PASS
MathTests testFloor PASS PASS
MathTests testHypot PASS PASS
MathTests testInfinity PASS PASS
MathTests testMath PASS PASS
MathTests testNaN PASS PASS
MathTests testPow PASS PASS
MathTests testRound PASS PASS
MathTests testSpecialNumbers PASS PASS
MathTests testSqrt PASS PASS
NumberTests testEquatable PASS PASS
NumberTests testFixedWidthIntegers PASS PASS
NumberTests testFloatingPoint PASS PASS
NumberTests testHashable PASS PASS
NumberTests testIntegers PASS PASS
NumberTests testMinMax PASS PASS
NumberTests testNumberConversions PASS PASS
NumberTests testNumberInitializers PASS PASS
NumberTests testNumberMinMax PASS PASS
NumberTests testUnsignedIntegers PASS PASS
OptionSetTests testContains PASS PASS
OptionSetTests testInsert PASS PASS
OptionalTests testOptionalMap PASS PASS
ResultTests testFailure PASS PASS
ResultTests testSuccess PASS PASS
SetTests testCustomHashable PASS PASS
SetTests testInit PASS PASS
SetTests testInsert PASS PASS
SetTests testIntersection PASS PASS
SetTests testIsDisjoint PASS PASS
SetTests testPopFirst PASS PASS
SetTests testRemove PASS PASS
SetTests testSubsetSuperset PASS PASS
SetTests testSymmetricDifference PASS PASS
SetTests testUnion PASS PASS
SkipLibTests testFatalError PASS PASS
SkipLibTests testSkipLib PASS PASS
SkipLibTests testUnitTests PASS PASS
StringTests testCharacterFunctions PASS PASS
StringTests testCreation PASS PASS
StringTests testFirstDropFirst PASS PASS
StringTests testManipulation PASS PASS
StringTests testMultibyteCharacterFun PASS PASS
StringTests testMultlineStrings PASS PASS
StringTests testSlice PASS PASS
StringTests testSplitJoin PASS PASS
StringTests testStringSearching PASS PASS
StringTests testUnicodeStrings PASS SKIP
StructTests testNestedStructDidSet PASS PASS
StructTests testStructMutate PASS PASS
StructTests testStructMutateDidSet PASS PASS
StructTests testStructReferences PASS PASS
    100% 97%