How to start writing Lein Plugins?

Leiningen is powerful and build tool for Clojure. I have been using lein mainly for my clojure apps, and it had all been only for lein test, lein run, lein clean, lein uberjar.

Recently, I was trying to CICD my clojure application for Lambda function and I was unable to upload uberjar with my lein deploy to the repository. So, I started looking into code to get it deploy with uberjar. But I ended up writing my own custom plugin to create uberjar and deploy uberjar to my repository. So, here I am sharing how easy it is to start writing lein plugins.

Before starting to write one, let’s see how lein invokes custom plugins. When you write lein plugin, lein will basically call your plugin method. So let’s say we write lein say-phrase, lein will call say-phrase function from say-phrase.clj and passes project.clj converted to {} and optional args that we might provide. Ok, let’s get our hands dirty, we will create a plugin, that will take a optional argument.

$ lein say-phrase
Hello World!
$ lein say-phrase morning
Good Morning!

So, this is how structure of our project directory should look. I am naming the project directory as “say-phrase-plugin”.

say-phrase-plugin
|__project.clj
|__src
  |__leiningen
     |__say_phrase.clj

Yup, that easy-peazy. You need to put your plugin name file in src/leiningen folder. That’s how leiningen will locate your plugin file. You might also want to look at how are built-in leiningen plugins are located.

Also, there is another way to create. Yes, you guessed it right, just use

lein new plugin say-phrase --to-dir say-phrase-template

Ok, let’s take a look at project.clj. You will need :eval-in-leiningen true to be set in your project.clj.

Now, let’s take a look at src/leiningen/say_phrase.clj

(ns leiningen.say-phrase)
(defn say-phrase
  "I don't do a lot."
  [project & args]
  (println "Hi!"))

We will be writing our code in say-phrase function. We use condp to check phrase and use leiningen.core.main/info to print out to console. Avoid using println , use leiningen.core.main/warn, leiningen.core.main/info, and leiningen.core.main/debug.

And, that’s it. To test it locally, you can change the version to 0.1.0 instead of 0.1.0-SNAPSHOT, and run below command.

$ lein install
 Created /<path-to-dir>/say-phrase-template/target/say-phrase-0.1.0.jar
 Wrote /<path-to-dir>/say-phrase-template/pom.xml
Installed jar and pom into local repo.

To use say-phrase plugin in your existing app, add [say-phrase "0.1.0"] to :plugins section. And you are all set.

$ cd <to-existing-app>
$ lein say-phrase
 Hello World!
$ lein say-phrase morning
 Good Morning!

Now, if you are all happy with your plugin and you want to push it to Clojar, use lein deploy clojar, it will prompt you for your username and password.

$ cd <to-say-phrase-template>
$ lein deploy clojars

And your plugin is deployed to Clojar.

Hope this helps and you get start with writing many more plugins and publish!