How to use the command 'lein' (with examples)

Leiningen, commonly referred to as lein, is a powerful command-line tool used for managing Clojure projects. It simplifies the tasks of project creation, dependency management, running code, and packaging projects for deployment. With declarative configuration, lein makes it easy to maintain standardized build environments for Clojure applications, facilitating smooth development and deployment processes. This guide will walk you through various use cases of the lein command with detailed explanations and examples.

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

Code:

lein new template_name project_name

Motivation for using the example:
Generating project scaffolding is often the first step in starting a new software project. By using a predefined template, developers can ensure consistency across multiple projects, allowing for a standardized structure that adheres to best practices. This command simplifies the setup process, enabling developers to focus more on coding rather than configuration.

Explanation of arguments:

• new: This argument tells lein to create a new project.
• template_name: The name of a predefined template that provides the basic file structure and configurations. This could be app for a standard application, lib for a library, etc.
• project_name: The name of the new project you want to create, which serves as both the directory name and the project identifier.

Example output:
After executing the command, the terminal might display something like:

Generating a project called project_name based on the template template_name.
Created a new project in directory ./project_name

This indicates that a new directory named project_name has been created, containing all the requisite files and folders as defined by the selected template.

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

Code:

lein repl

Motivation for using the example:
The REPL (Read-Eval-Print Loop) is an interactive programming environment that allows developers to test code snippets, experiment with libraries, and rapidly prototype features. By starting a REPL session, you can immediately evaluate your Clojure expressions and inspect their results, facilitating development that is more exploratory and iterative in nature.

Explanation of arguments:

• repl: This argument starts an interactive session where Clojure code can be entered and evaluated.

Example output:
On running the command, the terminal should enter a REPL session displaying:

nREPL server started on port 7888 on host 127.0.0.1 - nrepl://127.0.0.1:7888
REPL-y 0.4.3, nREPL 0.6.0
Clojure 1.10.1
user=> 

This output confirms that lein has initiated a REPL session, ready for your code inputs.

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

Code:

lein run args

Motivation for using the example:
Executing the main function is crucial for running a Clojure application. This command allows developers to quickly test their application end-to-end with specified arguments without setting up additional build scripts. It simplifies the workflow in development, especially when automating tasks or running demos.

Explanation of arguments:

• run: This tells lein to execute the main function of the project.
• args: These optional arguments are passed directly to the main function, allowing for parameterization and dynamic control over application behavior.

Example output:
When this command is executed, the following might appear depending on how the main function is defined:

Running the main function with arguments: args
Application started with argument: args

This output signals that the main function has been executed correctly, with the arguments influencing the behavior as expected.

Use case 4: Run the project’s tests

Code:

lein test

Motivation for using the example:
Automated testing is a cornerstone of modern software development practices. Running tests regularly ensures that code changes do not break existing functionality, thereby maintaining code quality. With lein test, developers can easily execute all tests in the project, streamline the continuous integration process, and obtain immediate feedback on code correctness.

Explanation of arguments:

• test: This command instructs lein to discover and execute all test functions defined in the project.

Example output:
After running the command, a potential output might look like:

Running namespace:project-name.core-test
Ran 3 tests containing 15 assertions.
0 failures, 0 errors.

This output provides a summary of test execution, including the number of tests run, assertions made, and the absence of any failures or errors, indicating test success.

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

Code:

lein uberjar

Motivation for using the example:
Creating an executable jar file, often referred to as an uberjar, bundles all project files, resources, and dependencies into a single file. This packaging simplifies deployment, as the application and its dependencies can be transferred and executed on different environments without setup conflicts, ensuring consistency across various stages of deployment.

Explanation of arguments:

• uberjar: This command tells lein to compile the project and package it, along with all its dependencies, into a standalone jar file.

Example output:
Upon execution, the output might look like:

Compiling project-name.core
Created /path/to/project/target/uberjar/project-name-0.1.0-SNAPSHOT-standalone.jar

This confirms that the project has been successfully compiled into an executable jar file, ready for deployment or distribution.

Conclusion:

Through these use cases, we’ve explored various functionalities offered by the lein command, showcasing its power in managing Clojure projects effectively. Whether you’re initializing a new project, experimenting with code in a REPL, running applications, testing code, or packaging software for deployment, lein provides a streamlined approach, making it an invaluable tool for Clojure developers.