Writing Custom Plugins for Leiningen: Enhance Your Clojure Workflow

10.3.2 Writing Custom Plugins

Leiningen is a powerful build automation tool for Clojure, widely used for managing dependencies, running tests, packaging applications, and much more. However, there are times when the built-in functionality of Leiningen might not meet the specific needs of your project or organization. In such cases, writing custom plugins can be an effective way to extend Leiningen’s capabilities. This section will guide you through the process of creating, defining tasks, and deploying custom Leiningen plugins.

Plugin Structure

Creating a custom Leiningen plugin involves setting up a new Clojure project with a specific structure that Leiningen recognizes as a plugin. This structure allows you to define new tasks and extend the functionality of Leiningen in a modular way.

Setting Up a New Plugin Project

To start, you’ll need to create a new Clojure project that will serve as your plugin. This can be done using Leiningen itself:

lein new lein-plugin my-awesome-plugin

This command creates a new project with the necessary directory structure and files. Here’s what the basic structure looks like:

my-awesome-plugin/
├── project.clj
├── src/
│   └── my_awesome_plugin/
│       └── core.clj
└── README.md

• project.clj: This file contains the project metadata and dependencies. For a plugin, it should also specify the :eval-in-leiningen key.
• src/my_awesome_plugin/core.clj: This is where you’ll define the logic for your plugin.
• README.md: A place to document your plugin’s functionality and usage.

Configuring project.clj

The project.clj file for a plugin needs to specify that it should be evaluated in the context of Leiningen. Here’s an example configuration:

(defproject my-awesome-plugin "0.1.0-SNAPSHOT"
  :description "A Leiningen plugin to do awesome things"
  :url "http://example.com/my-awesome-plugin"
  :license {:name "Eclipse Public License"
            :url "http://www.eclipse.org/legal/epl-v10.html"}
  :dependencies [[org.clojure/clojure "1.10.3"]]
  :eval-in-leiningen true)

The key :eval-in-leiningen true is crucial as it tells Leiningen to evaluate the plugin in its own context.

Tasks Definition

Once your plugin project is set up, the next step is to define the tasks that your plugin will perform. Tasks in Leiningen are essentially functions that can be invoked from the command line.

Defining a New Task

Tasks are defined as functions within your plugin’s namespace. Here’s an example of a simple task definition:

(ns my-awesome-plugin.core)

(defn my-task
  "A simple task that prints a message."
  [project & args]
  (println "Hello from my awesome plugin!"))

In this example, my-task is a function that takes a project argument and any additional arguments (& args). The project argument is a map representing the current project’s configuration, which can be used to access project-specific settings.

Registering the Task

To make the task available to Leiningen, you need to register it in the project.clj file:

(defproject my-awesome-plugin "0.1.0-SNAPSHOT"
  :description "A Leiningen plugin to do awesome things"
  :url "http://example.com/my-awesome-plugin"
  :license {:name "Eclipse Public License"
            :url "http://www.eclipse.org/legal/epl-v10.html"}
  :dependencies [[org.clojure/clojure "1.10.3"]]
  :eval-in-leiningen true
  :leiningen {:hooks [my-awesome-plugin.core/my-task]})

With this setup, you can now run your task using Leiningen:

lein my-task

Advanced Task Functionality

Tasks can be as simple or complex as needed. They can interact with the file system, run shell commands, or even modify the project configuration. Here’s an example of a more complex task that interacts with the file system:

(ns my-awesome-plugin.core
  (:require [clojure.java.io :as io]))

(defn clean-temp-files
  "Cleans up temporary files in the project's target directory."
  [project & args]
  (let [target-dir (io/file (:target-path project))]
    (doseq [file (file-seq target-dir)]
      (when (.isFile file)
        (println "Deleting" (.getName file))
        (.delete file)))))

This task cleans up temporary files in the project’s target directory, demonstrating how tasks can perform file operations.

Deployment

Once your plugin is developed, you may want to share it with your team or the broader Clojure community. Deployment involves packaging your plugin and making it available for others to use.

Packaging the Plugin

Leiningen plugins are typically distributed as JAR files. You can package your plugin using the lein jar command:

lein jar

This command creates a JAR file in the target directory of your plugin project.

Distributing the Plugin

There are several ways to distribute your plugin:

1. Local Installation: You can install the plugin locally by placing the JAR file in the ~/.lein/plugins directory. This is useful for testing or internal team use.

2. Maven Repository: For broader distribution, you can publish your plugin to a Maven repository. This allows others to include your plugin as a dependency in their project.clj files. To publish, you’ll need to configure your project with the repository details and use the lein deploy command.

3. GitHub or Other VCS: You can also distribute your plugin by hosting the source code on a platform like GitHub. Users can clone the repository and build the plugin themselves.

Example: Publishing to Clojars

Clojars is a popular community repository for Clojure libraries and plugins. Here’s how you can publish your plugin to Clojars:

1. Create an Account: Sign up for an account on Clojars.

2. Configure project.clj: Add the Clojars repository to your project.clj:

   :repositories [["clojars" {:url "https://repo.clojars.org/"}]]
   

3. Deploy the Plugin: Use the lein deploy clojars command to publish your plugin:

   lein deploy clojars
   

   You’ll be prompted for your Clojars credentials during the deployment process.

Best Practices and Common Pitfalls

When developing Leiningen plugins, consider the following best practices and be aware of common pitfalls:

• Documentation: Provide clear documentation for your plugin, including usage instructions and examples. This helps users understand how to integrate and use your plugin effectively.

• Testing: Thoroughly test your plugin to ensure it works as expected across different environments and project configurations.

• Versioning: Use semantic versioning to communicate changes and compatibility. This is especially important if your plugin is widely used.

• Dependencies: Be mindful of the dependencies your plugin introduces. Ensure they do not conflict with common project dependencies.

• Performance: Optimize your plugin for performance, especially if it will be used in large projects or CI/CD pipelines.

Conclusion

Writing custom Leiningen plugins is a powerful way to tailor the Clojure build process to your specific needs. By understanding the plugin structure, defining tasks, and deploying your plugin, you can significantly enhance your development workflow. Whether you’re automating repetitive tasks, integrating with external tools, or enforcing project standards, custom plugins can provide the flexibility and functionality you need.

Quiz Time!