Apple is rewriting Foundation in Swift

Foundation provides a base layer of functionality useful in many applications, including fundamental types for numbers, data, collections, and dates, as well as functions for task management, file system access, and more.

It is designed with these goals in mind:

  • Provide a small set of basic utility types
  • Enable a level of platform independence, to enhance portability
  • Demonstrate useful conventions that can be widely adopted by the Swift ecosystem
  • Support internationalization and localization to make software accessible around the world

Current State

This package is a work in progress that aims to build a new and unified Swift implementation of Foundation for all platforms.

It is in its early stages with many features still to be implemented.

The following types are available, with more to come later:

  • FoundationEssentials
    • AttributedString
    • Data
    • Date
    • DateInterval
    • JSONEncoder
    • JSONDecoder
    • Predicate
    • String extensions
    • UUID
  • Internationalization
    • Calendar
    • TimeZone
    • Locale
    • DateComponents
    • FormatStyle
    • ParseStrategy

Many types, including JSONEncoder, Calendar, TimeZone, and Locale are all-new Swift implementations. FormatStyle and ParseStrategy available as open source for the first time.

For internationalization support on non-Darwin platforms, we created a separate package named FoundationICU. This repository contains the necessary ICU implementations and data from the upstream Apple OSS Distribution ICU, wrapped in Swift so FoundationInternationalization can easily depend on it.

Using a common version of ICU will result in more reliable and consistent results when formatting dates, times, and numbers.

Development Focus for 2023

Quality and performance are our two most important goals for the project. Therefore, the plans for the first half of 2023 are continuing refinement of the core API, adding to our suites of unit and performance tests, and expanding to other platforms where possible, using the most relevant code from swift-corelibs-foundation.

Later this year, the porting effort will continue. It will bring high quality Swift implementations of additional important Foundation API such as URL, Bundle, FileManager, FileHandle, Process, SortDescriptor, SortComparator and more.

Building and Testing

Building the Foundation package requires the under-development Swift 5.9 toolchain on or later than May 3rd 2023 (607f4eb), on macOS and Linux.


macOS Ventura 13.3.1 is the minimum supported version.

  • Download the latest Xcode from the App Store
  • Download and install the Swift 5.9 toolchain for Xcode from
  • Launch Xcode and select the downloaded 5.9 toolchains via Xcode > Toolchains
  • Open Package.swift and select Debug > Test



Being written in Swift, this new implementation provides some major benefits over the previous C and Objective-C versions.

Locale, TimeZone and Calendar no longer require bridging from Objective-C. Common tasks like getting a fixed Locale are an order of magnitude faster for Swift clients. Calendar‘s ability to calculate important dates can take better advantage of Swift’s value semantics to avoid intermediate allocations, resulting in over a 20% improvement in some benchmarks. Date formatting using FormatStyle also has some major performance upgrades, showing a massive 150% improvement in a benchmark of formatting with a standard date and time template.

Even more exciting are the improvements to JSON decoding in the new package. Foundation has a brand-new Swift implementation for JSONDecoder and JSONEncoder, eliminating costly roundtrips to and from the Objective-C collection types. The tight integration of parsing JSON in Swift for initializing Codable types improves performance, too. In benchmarks parsing test data, there are improvements in decode time from 200% to almost 500%.


The success of the Swift language is an example of what’s possible when a community comes together with a shared interest.

For Foundation, our goal is to create the best fundamental data types and internationalization features, and make them available to Swift developers everywhere. It will take advantage of emerging features in the language as they are added, and enable library and app authors to build higher level API with confidence.

Moving Foundation into this future requires not only an improved implementation, but also an improved process for using it outside of Apple’s platforms. Therefore, Foundation now has a path for the community to add new API for the benefit of Swift developers on every platform.

The Foundation package is an independent project in its early incubation stages. Inspired by the workgroups in the Swift project, it has a workgroup to (a) oversee community API proposals and (b) to closely coordinate with developments in the Swift project and Apple platforms. In the future, we will explore how to sunset the existing swift-corelibs-foundation and migrate to using the new version of Foundation created by this project.

The workgroup meets regularly to review proposals, look at emerging trends in the Swift ecosystem, and discuss how the library can evolve to best meet our common goals.

Foundation Framework and Foundation Package

The Swift code in the package is the core of the Foundation framework that ships on macOS, iOS, and other Apple platforms. As new Swift implementations of Foundation API are implemented in the package, Apple will use those implementations in the framework as well.

The Foundation framework may have the occasional need to add Darwin-specific API, but our goal is to share as much code and API between all platforms as possible. In cases where platform-specific code is needed within a single source file, a compiler directive is used to include or exclude it.


Foundation welcomes contributions from the community, including bug fixes, tests, documentation, and ports to new platforms.

The project uses the Swift forums for discussion and GitHub Issues for tracking bugs, feature requests, and other work.

Please see the CONTRIBUTING document for more information, including the process for accepting community contributions for new API in Foundation.

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