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Why Clojure?

This is going to be the first post in a series about Clojure and how to get started with it. In this post, I’ll talk about what Clojure is, why it’s interesting to learn, and how to get a Clojure environment set up on your system. In subsequent posts, I’ll dive into the basics of Clojure and give you some pointers for how to get started with it.

What is Clojure?

Clojure is a functional programming language suitable for general purpose programming tasks. It’s a dialect of Lisp which runs on top of the Java virtual machine. Clojure was developed by Rich Hickey in 2007, and is actively maintained by a community overseen by Rich.

How and why did you learn about Clojure?

I admit I’m a bit late to the Clojure party, relatively speaking. I first heard of Clojure while reading The Unicorn Project, a novel of DevOps and software development by Gene Kim. Maxine, the protagonist of the novel, is a fan of Clojure, and reading the novel convinced me to give it a look.

Some of the things about Clojure that I found interesting included:

  1. Clojure is a language in the Lisp family. Although I’m not religious about my development tools, as I’ve mentioned before, I am a fan of Emacs, which I’ve used on and off since college. I think you can’t really use Emacs for very long without learning at least a bit of Emacs Lisp, so I was already familiar with Lisp languages. This does shorten the learning curve a bit.

  2. Clojure provides a REPL, and a lot of development happens there. I’ve long been a fan of languages with good REPLs, and the way they enable a sort of interactive, exploratory style of code development. And the REPL is pretty central to both the “how” and “why” of Clojure development.

  3. Clojure is cross-platform, and sits on top of the JVM. This gives it a lot of power and flexibility. There’s also a large and active community and ecosystem of Clojure add-ons and stuff. And the cross-platform, JVM-based environemnt makes getting an environment set up easy.

  4. Clojure is a functional programming language. There’s a lot of reasons why functional programming is a desirable thing, but as Maxine notes in The Unicorn Project, one of the most important is that data in Clojure is immutable. This prevents a lot of the bugs that come up in non-functional languages. My general agnosticism about tools means I don’t necessarily think Clojure is the tool for every problem, but I do think it’s a viable solution for a lot of stuff and therefore worth having in my toolbelt.

  5. Clojure has a browser-based scripting version. It’s called ClojureScript, and ClojureScript can be compiled to standard Javascript. This gives you a unified and coherent platform for client, server, and desktop development.

  6. Clojure is a new, fun thing that I didn’t already know. Hey, I’m a nerd and I have ADHD, so I’m susceptible to the “ooh, shiny” syndrome. What can I say?

Getting Clojure set up on your system

Luckily, getting a Clojure environment set up on your system is relatively easy, thanks to development environments like Leiningen. Here’s what you need to do to get Clojure running on your system:

  1. Install prerequisites. You’ll need to make sure you have a Java Virtual Machine on your system. JDK 8 or JDK 11 are the supported versions, and changes in later versions will produce random weird errors. I found that I needed to use one of the official Oracle builds because the OpenJDK version is missing some SSL certificates, and that breaks the downloading of Clojure extensions and libraries. Installing git and a good terminal program is also a good idea. (I’m using tilix on Linux. I suggest Windows Terminal on Windows, and iTerm2 on OS X.)

  2. Download and install Leiningen. This is super easy and mostly automated. The leiningen website has instructions. You’ll need to make sure the JDK tools are installed and in your path for this to work.

  3. Check that your setup is working. Run the command lein version. You should see a message like this:

         Leiningen 2.9.3 on Java 1.8.0_251 Java HotSpot(TM) 64-Bit Server VM
  4. Set up your editor/IDE. I’ll talk about this in the next section, but using an editor with good Clojure integration is super important for a good development experience.

Clojure editor/IDE integration

As I mentioned above, one of the features of Clojure that makes developing with it so fun – and powerful – is that a lot of development is done in an exploratory fashion with the REPL. We’ll see this in subsequent installments of my tutorial. But what this means is that you want to configure your editor with good Clojure integration, including an integrated REPL. Here are some choices you can use:

  • Emacs users should install CIDER. There’s some customization you might want to do, but the CIDER installation guide will get you going.
  • Visual Studio Code users can pick from a few extensions, but the most well–integrated and easy-to-install choice I’ve found is Calva.
  • vi/vim users might want to start here. You might also find tmux and tmuxinator to be helpful tools.
  • Sublime Text users would do well to take a look at Greg Williams’s guide.
  • Atom users might benefit from Jacek Schae’s guide.
  • If you want a purpose–built IDE for Clojure development, take a look at JetBrains Cursive and Nightcode. I haven’t used either of these, but they seem to be highly regarded.
  • If you use another editor or development tool, you’re on your own, I’m afraid. But you should look for something that lets you run Leiningen commands, has a built–in terminal or shell, and that has integration with the Clojure REPL.

Some add-ons for Leiningen

There are a couple of add-ons for Leiningen that I’ve found are super helpful from a development perspective:

  • lein-cprint colorizes the output in the Leiningen REPL and makes it easier to read.

  • debux gives you some easy trace-based debugging helpers.

  • lein-autoreload automatically reloads the REPL when you make changes to your source files.

  • rebel-readline adds command history and editing to the Clojure REPL.

You can install these by creating a new file in $HOME/.lein/profiles.clj and adding the following to it:

    :plugins [
      [cider/cider-nrepl "0.24.0"]
      [lein-cprint "1.3.3"]
      [philoskim/debux "0.6.5"]
      [lein-autoreload "0.1.1"]
      [com.jakemccrary/lein-test-refresh "0.24.1"]

    :dependencies [
      [com.bhauman/rebel-readline "0.1.4"]

    :aliases {
      "rebl" ["trampoline" "run" "-m" "rebel-readline.main"]

Once you’ve created this file, run the command lein deps to install all the bits and pieces.

That’s it! You’re up and running. If you want to play a bit before my next tutorial, you can spin up a REPL with lein repl and try some of the examples from the Programming at the REPL guide.

This post is licensed under CC BY 4.0 by the author.

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