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swift-format Configuration

swift-format allows users to configure a subset of its behavior, both when used as a command line tool or as an API.

Command Line Configuration

A swift-format configuration file is a JSON file with the following top-level keys and values:

  • version (number): The version of the configuration file. For now, this should always be 1.

  • lineLength (number): The maximum allowed length of a line, in characters.

  • indentation (object): The kind and amount of whitespace that should be added when indenting one level. The object value of this property should have exactly one of the following properties:

    • spaces (number): One level of indentation is the given number of spaces.
    • tabs (number): One level of indentation is the given number of tabs.
  • tabWidth (number): The number of spaces that should be considered equivalent to one tab character. This is used during line length calculations when tabs are used for indentation.

  • maximumBlankLines (number): The maximum number of consecutive blank lines that are allowed to be present in a source file. Any number larger than this will be collapsed down to the maximum.

  • respectsExistingLineBreaks (boolean): Indicates whether or not existing line breaks in the source code should be honored (if they are valid according to the style guidelines being enforced). If this settings is false, then the formatter will be more "opinionated" by only inserting line breaks where absolutely necessary and removing any others, effectively canonicalizing the output.

  • blankLineBetweenMembers (object): Controls logic specific to the enforcement of blank lines between type members. The object value of this property has the following properties:

    • ignoreSingleLineProperties (boolean): If true, then single-line property declarations are allowed to appear consecutively without a blank line separating them.
  • lineBreakBeforeControlFlowKeywords (boolean): Determines the line-breaking behavior for control flow keywords that follow a closing brace, like else and catch. If true, a line break will be added before the keyword, forcing it onto its own line. If false (the default), the keyword will be placed after the closing brace (separated by a space).

  • lineBreakBeforeEachArgument (boolean): Determines the line-breaking behavior for generic arguments and function arguments when a declaration is wrapped onto multiple lines. If true, a line break will be added before each argument, forcing the entire argument list to be laid out vertically. If false (the default), arguments will be laid out horizontally first, with line breaks only being fired when the line length would be exceeded.

  • indentConditionalCompilationBlocks (boolean): Determines if conditional compilation blocks are indented. If this setting is false the body of #if, #elseif, and #else is not indented. Defaults to true.

TODO: Add support for enabling/disabling specific syntax transformations in the pipeline.


An example .swift-format configuration file is shown below.

    "version": 1,
    "lineLength": 100,
    "indentation": {
        "spaces": 2
    "maximumBlankLines": 1,
    "respectsExistingLineBreaks": true,
    "blankLineBetweenMembers": {
        "ignoreSingleLineProperties": true
    "lineBreakBeforeControlFlowKeywords": true,
    "lineBreakBeforeEachArgument": true

API Configuration

The SwiftConfiguration module contains a Configuration type that is equivalent to the JSON structure described above. (In fact, Configuration conforms to Codable and is how the JSON form is read from and written to disk.)

The SwiftFormatter and SwiftLinter APIs in the SwiftFormat module take a mandatory Configuration argument that specifies how the formatter should behave when acting upon source code or syntax trees.

The default initializer for Configuration creates a value equivalent to the default configuration that would be printed by invoking swift-format --mode dump-configuration. API users can also provide their own configuration by modifying this value or loading it from another source using Swift's Codable APIs.

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