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swift-format

swift-format provides the formatting technology for SourceKit-LSP and the building blocks for doing code formatting transformations.

This package can be used as a command line tool or linked into other applications as a Swift Package Manager dependency and invoked via an API.

NOTE: No default Swift code style guidelines have yet been proposed. The style that is currently applied by swift-format is just one possibility, and the code is provided so that it can be tested on real-world code and experiments can be made by modifying it.

Matching swift-format to Your Swift Version

swift-format depends on SwiftSyntax and the standalone parsing library that is distributed as part of the Swift toolchain, so you should check out and build swift-format from the release tag or branch that is compatible with the version of Swift you are using.

The major and minor version components of swift-format and SwiftSyntax must be the same—this is expressed in the SwiftSyntax dependency in Package.swift—and those version components must match the Swift toolchain that is installed and used to build and run the formatter:

Xcode Release Swift Version swift-format Branch / Tags
Swift at main main
Xcode 13.3 Swift 5.6 release/5.6 / 0.50600.x
Xcode 13.0–13.2 Swift 5.5 swift-5.5-branch / 0.50500.x
Xcode 12.5 Swift 5.4 swift-5.4-branch / 0.50400.x
Xcode 12.0–12.4 Swift 5.3 swift-5.3-branch / 0.50300.x
Xcode 11.4–11.7 Swift 5.2 swift-5.2-branch / 0.50200.x
Xcode 11.0–11.3 Swift 5.1 swift-5.1-branch

For example, if you are using Xcode 13.3 (Swift 5.6), you will need swift-format 0.50600.0.

Getting swift-format

If you are mainly interested in using swift-format (rather than developing it), then once you have identified the version you need, you can check out the source and build it using the following commands:

VERSION=0.50600.0  # replace this with the version you need
git clone https://github.com/apple/swift-format.git
cd swift-format
git checkout "tags/$VERSION"
swift build -c release

Note that the git checkout command above will leave the repository in a "detached HEAD" state. This is fine if building and running the tool is all you want to do.

Once the build has finished, the swift-format executable will be located at .build/release/swift-format.

To test that the formatter was built succesfully and is compatible with your Swift toolchain, you can also run the following command:

swift test --parallel

We recommend using the --parallel flag to speed up the test run since there are a large number of tests.

Command Line Usage

The general invocation syntax for swift-format is as follows:

swift-format [SUBCOMMAND] [OPTIONS...] [FILES...]

The tool supports a number of subcommands, each of which has its own options and are described below. Descriptions of the subcommands that are available can also be obtained by running swift-format --help, and the description of a specific subcommand can be obtained by using the --help flag after the subcommand name; for example, swift-format lint --help.

Formatting

swift-format [format] [OPTIONS...] [FILES...]

The format subcommand formats one or more Swift source files (or source code from standard input if no file paths are given on the command line). Writing out the format subcommand is optional; it is the default behavior if no other subcommand is given.

This subcommand supports all of the common lint and format options, as well as the formatting-only options below:

  • -i/--in-place: Overwrites the input files when formatting instead of printing the results to standard output. No backup of the original file is made before it is overwritten.

Linting

swift-format lint [OPTIONS...] [FILES...]

The lint subcommand checks one or more Swift source files (or source code from standard input if no file paths are given on the command line) for style violations and prints diagnostics to standard error for any violations that are detected.

This subcommand supports all of the common lint and format options, as well as the linting-only options below:

  • -s/--strict: If this option is specified, lint warnings will cause the tool to exit with a non-zero exit code (failure). By default, lint warnings do not prevent a successful exit; only fatal errors (for example, trying to lint a file that does not exist) cause the tool to exit unsuccessfully.

Options Supported by Formatting and Linting

The following options are supported by both the format and lint subcommands:

  • --assume-filename <path>: The file path that should be used in diagnostics when linting or formatting from standard input. If this option is not provided, then <stdin> will be used as the filename printed in diagnostics.

  • --color-diagnostics/--no-color-diagnostics: By default, swift-format will print diagnostics in color if standard error is connected to a terminal and without color otherwise (for example, if standard error is being redirected to a file). These flags can be used to force colors on or off respectively, regardless of whether the output is going to a terminal.

  • --configuration <file>: The path to a JSON file that contains configurable settings for swift-format. If omitted, a default configuration is use (which can be seen by running swift-format dump-configuration).

  • --ignore-unparsable-files: If this option is specified and a source file contains syntax errors or can otherwise not be parsed successfully by the Swift syntax parser, it will be ignored (no diagnostics will be emitted and it will not be formatted). Without this option, an error will be emitted for any unparsable files.

  • -p/--parallel: Process files in parallel, simultaneously across multiple cores.

  • -r/--recursive: If specified, then the tool will process .swift source files in any directories listed on the command line and their descendants. Without this flag, it is an error to list a directory on the command line.

Viewing the Default Configuration

swift-format dump-configuration

The dump-configuration subcommand dumps the default configuration in JSON format to standard output. This can be used to simplify generating a custom configuration, by redirecting it to a file and editing it.

Configuring the Command Line Tool

For any source file being checked or formatted, swift-format looks for a JSON-formatted file named .swift-format in the same directory. If one is found, then that file is loaded to determine the tool's configuration. If the file is not found, then it looks in the parent directory, and so on.

If no configuration file is found, a default configuration is used. The settings in the default configuration can be viewed by running swift-format --mode dump-configuration, which will dump it to standard output.

If the --configuration <file> option is passed to swift-format, then that configuration will be used unconditionally and the file system will not be searched.

See Documentation/Configuration.md for a description of the configuration file format and the settings that are available.

Miscellaneous

Running swift-format -v or swift-format --version will print version information about swift-format version and then exit.

API Usage

swift-format can be easily integrated into other tools written in Swift. Instead of invoking the formatter by spawning a subprocess, users can depend on swift-format as a Swift Package Manager dependency and import the SwiftFormat module, which contains the entry points into the formatter's diagnostic and correction behavior.

Formatting behavior is provided by the SwiftFormatter class and linting behavior is provided by the SwiftLinter class. These APIs can be passed either a Swift source file URL or a Syntax node representing a SwiftSyntax syntax tree. The latter capability is particularly useful for writing code generators, since it significantly reduces the amount of trivia that the generator needs to be concerned about adding to the syntax nodes it creates. Instead, it can pass the in-memory syntax tree to the SwiftFormat API and receive perfectly formatted code as output.

Please see the documentation in the SwiftFormatter and SwiftLinter classes for more information about their usage.

Checking Out the Source Code for Development

The main branch is used for development. Pull requests should be created to merge into the main branch; changes that are low-risk and compatible with the latest release branch may be cherry-picked into that branch after they have been merged into main.

If you are interested in developing swift-format, there is additional documentation about that here.