The pain

Look at a typical blog post in clojure[script]. The post usually presents a couple of code snippets. As I see it, there are two pains with code snippets:

  1. they contain the input and the output but not the actual evaluation of the input
  2. it’s impossible for the reader to modify the output

The forgotten dream

A long time ago, all the developers had a common dream. The dream was about interactivity, liveness, evaluation…

But we put this dream aside - because the browser understands only javascript.

And after a while, we even forgot that we ever had this dream.

Still, there are some people that didn’t forget this dream, like Alan Kay:

Question: Well, look at Wikipedia — it’s a tremendous collaboration.

Alan Kay: It is, but go to the article on Logo, can you write and execute Logo programs? Are there examples? No. The Wikipedia people didn’t even imagine that, in spite of the fact that they’re on a computer.

Here is the full interview of Alan Kay. (Thanks @fasihsignal for bringing this quote to our awareness.)


The klipse plugin

The klipse plugin is a small step toward this dream: it is a javascript tag that transforms static clojure code snippets of an html page to live and interactive snippets.

Klipse is written in clojurescript, it uses replumb for code evaluation (self-hosted cljs) and CodeMirror for text editing.

For instance, let’s have on this page a static code snippet with (map inc [1 2 3]):

(map inc [1 2 3])
;(2 3 4)

(This blog is written with jekyll: the kramdown plugin helps a lot in beautifying the code snippets.)

And now, we are going to klipsify this code snippet:

(map inc [1 2 3])

Feel free to edit the code above: it’s interactive => it evaluates again as you type.

All I had to do in order to klipsify my code snippet, was to set the language-klipse class (configurable) to the appropriate html element.

See it by yourself: here is the source of this page:

<p>And now, we are going to <strong>klipsify</strong> this code snippet:</p>

<pre><code class="language-klipse">(map inc [1 2 3])

Ah! I forgot to mention that I had to remove ;(2 3 4) from the code snippet.

Before dealing about integration of the klipse plugin on a web page, let’s enjoy another klipse snippet for generating a lazy Fibonacci sequence:

(def fib-seq-seq
  ((fn fib [a b] 
       (lazy-seq (cons a (fib b (+ a b)))))
     0 1))
(take 13 fib-seq-seq)


All you need to do in order to integrate the klipse plugin to your blog (or any other web page), is to add this javascript tag to your web page:

<link rel="stylesheet" type="text/css" href="">

    window.klipse_settings = {
        selector: '.language-klipse'// css selector for the html elements you want to klipsify
<script src=""></script>

By the way, this is exactly what we did on the page that you are currently reading.

Feel free to contact us for further explanations:

Other languages

The klipse plugin is designed as a platform that could support any language that has a client-side evaluator, by writing modules to the klipse plugin. Currently, there are modules available for the following languages:

Limitations and issues

There are a couple of limitations and issues with the klipse plugin:

  • The javascript is quite big (around 1.2MB zipped, 9.4MB unzipped). Currently self-host clojurescript doesn’t support advanced compilation. We did our best not to impact the page loading time.

  • You can require clojure libraries in your code snippet but the code might take a while to execute, as the code of the library is JIT loaded from github. Here is an example with clojure.set:

(ns my.set
  (:require [clojure.set :refer [map-invert]]))

(map-invert {:klipse "snippet" :dreaming "interactivity"})
  • When you require a clojure library, klipse will try to load the library from several repos. As a consequence, if you open the browser dev tools, you’ll see a lot of 404s.

  • Most of the clojure code is valid clojurescript code and will be evaluated properly by self-host clojurescript. But there are a lot of things that are not (yet) implemented in clojurescript: refs, agents, Ratio, BigDecimal, BigInteger, Character literals, runtime enforcement of arity when calling a fn… (see the full list here: Differences between Clojure and Clojurescript).

  • Requiring custom clojure[script] libraries in a klipse snippet is not yet available.

Share your thoughts (and your dreams) in the comments below.

Clojurescript rocks!