A test runner for compiler-like Swift projects

What's New


  • Update SwiftPM dependency.


Lite is a test runner for compiler-like Swift programs. It is structured similarly to LLVM's lit except with way fewer configuration options. Its advantage is being easily usable from a Swift project.


To use lite as a testing tool, you'll need to add Lite as a dependency in your Package.swift file.

.package(url: "", from: "0.0.1")

Then, you'll need to add a target called lite to your Package.swift that depends on the Lite support library, LiteSupport.

.target(name: "lite", dependencies: ["LiteSupport"])

Making a lite Target

From that target's main.swift, make a call to runLite(substitutions:pathExtensions:testDirPath:testLinePrefix:parallelismLevel:). This call is the main entry point to lite's test running.

It takes 5 arguments:

Argument Description
substitutions The mapping of substitutions to make inside each run line. A substitution looks for a string beginning with '%' and replaces that whole string with the substituted value.
pathExtensions The set of path extensions that Lite should search for when discovering tests.
testDirPath The directory in which Lite should look for tests. Lite will perform a deep search through this directory for all files whose extension exists in pathExtensions and which have valid RUN lines.
testLinePrefix The prefix before RUN: in a file. This is almost always your specific langauge's line comment syntax.
parallelismLevel Specifies the amount of parallelism to apply to the test running process. Default value is .none, but you can provide .automatic to use the available machine cores, or .explicit(n) to specify an explicit number of parallel tests

Note: An example consumer of Lite exists in this repository as lite-test.

Once you've defined that, you're ready to start running your tester!

You can run it standalone or via CI using:

swift run lite

Defining Tests

Lite tests are expressed as bash shell commands written inside comments in your source file (usually at the top).

These commands can be very simple:

// RUN: %my-prog %s

Or very complex:

// RUN: %my-prog %s --arg1 foo --directory %T --output-file %t && diff %T/foo.out %t

You can also have multiple RUN lines in one file, which will all use the same set of substitutions.

Lite comes with 4 standard substitutions:

Substitution Value
%s The current source file path, quoted.
%S The directory in which the current source file resides, quoted.
%T A temporary directory which will be created when substituted.
%t A temporary file (multiple %ts will reference the same file), in the directory specified with %T, quoted.


Harlan Haskins (@harlanhaskins)

Robert Widmann (@codafi)


This project is released under the MIT license, a copy of which is avaialable in this repository.


  • Swift Tools 5.0.0
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Last updated: Wed Jan 17 2024 02:07:44 GMT-1000 (Hawaii-Aleutian Standard Time)