How to use the command lein (with examples)

Leiningen is a build automation and dependency management tool for Clojure projects. It helps manage Clojure projects with a declarative configuration. This article provides examples of different use cases for the lein command.

Use case 1: Generate scaffolding for a new project based on a template

Code:

lein new template_name project_name

Motivation: When starting a new Clojure project, it can be time-consuming to set up the initial project structure and dependencies. Leiningen simplifies this process by providing a template-based project generation command.

Explanation:

• template_name: The name of the template to be used for creating the project. Leiningen comes with built-in templates such as app, lib, helloworld, etc.
• project_name: The name of the project to be generated. This will determine the project directory structure and the project’s namespace.

Example output:

Generated project 'myproject' based on the 'app' template.

Use case 2: Start a REPL session either with the project or standalone

Code:

lein repl

Motivation: A REPL (Read-Eval-Print Loop) session is an interactive environment to experiment with Clojure code. It allows for quick code evaluation and testing. Leiningen provides a convenient command to start a REPL session.

Explanation: No additional arguments are required for this command. It starts a REPL session either with the project’s dependencies or in a standalone manner.

Example output:

nREPL server started on port 54321 on host 127.0.0.1 - nrepl://127.0.0.1:54321

Use case 3: Run the project’s -main function with optional args

Code:

lein run args

Motivation: For projects with a defined -main function, this command allows you to run the project as an executable. It is useful during development or when the project needs to be run on demand.

Explanation:

• args: Optional arguments to be passed to the -main function. These arguments can be accessed within the project’s code.

Example output:

Hello, World!

Use case 4: Run the project’s tests

Code:

lein test

Motivation: Testing is an important aspect of software development. Leiningen provides a command to easily run the tests defined in a Clojure project.

Explanation: No additional arguments are required for this command. It automatically discovers and runs all the test namespaces in the project.

Example output:

Ran 5 tests containing 10 assertions.
0 failures, 0 errors.

Use case 5: Package up the project files and all its dependencies into a jar file

Code:

lein uberjar

Motivation: When the project is ready for deployment, it needs to be packaged as a standalone executable jar file. Leiningen simplifies this process by providing an uberjar command.

Explanation: No additional arguments are required for this command. It compiles the project’s source files, gathers all the dependencies, and packages everything into a single jar file.

Example output:

Created uberjar target/myproject-0.1.0-SNAPSHOT-standalone.jar

Conclusion:

Leiningen is a powerful tool for managing Clojure projects. It provides a set of intuitive commands to automate common project-related tasks like project generation, REPL sessions, running the project, testing, and packaging. With the examples provided in this article, you’ll be able to leverage Leiningen effectively for your Clojure projects.