Sunday, January 8, 2012

Clojure Extension for Chrome

Do you like Clojure? Do you often read blogs and articles about it? I know I do. And every time I see an interesting Clojure snippet I rush to open my REPL to try it out. To do that I have to execute following steps:
1. Press Logo key to open my console.
2. cd to any project
3. lein repl
4. Copy the code snippet from the webpage and paste it in the REPL.
5. Wow, cool!
Your own steps to run the code may be different and probably more efficient. However, in any case, they probably involve opening a REPL and copy pasting code from the page into it.

What if you do this way too often to repeat all the steps? What if you are just plain lazy? All of us Clojurians know that lazy is good, therefore I decided to jump on the problem and try to make it easier. What if, I asked myself, I could just select a piece of Clojure code right in my browser, right click it and choose something like Eval to get my result back. For that functionality I would need to write a browser plugin, specifically a Chrome extension.

 As it turned out, writing Chrome extensions is a pretty easy task. It is not that much different than writing regular web application - write some HTML files, put some JavaScript in it to do the work and an Ajax call to communicate with the backend service. The only significant difference is in various security rules which the extension has to satisfy. Anyway, the official documentation is pretty good and the examples are even better.
As for the backend service, I used the source of the popular online REPL tryclojure written by Anthony Grimes and modified it slightly to replace Noir framework with an old version of Compojure which I already used in some of my older apps. Then I unmodified it because Noir is way better.

Anyway, after a few afternoons of hacking, results of which can be seen on goranjovic/chromeclojure on GitHub if anyone cares, I published the app. Clojure backend is hosted on Heroku on a free plan and the Chrome extension itself is where all Chrome extensions live - on Google's Chrome Web Store . Click on the link, install it and try it out on this very blog post. I included some Clojure snippets below just for that purpose.

A simple snippet that calculates the meaning of life:


Go on, select the snippet, right click it and choose Eval as Clojure. If everything is installed and working properly you should see a Chrome notification containing result 42. If you, for whichever reason, don't really like notifications and find them annoying you can change the extension to use plain old alerts to show the result. To do this:
1. Open Wrench Menu > Tools > Extensions
2. Find Clojure Extension
3. Click the Options link
4. Choose between offered response methods and click Save. Currently it is just notification and alert, but more options may come in future releases.

A somewhat less simple snippet which converts number 12345 into a sequence of its digits in base 12:



Again, use the extension to eval the code. It should return (7 1 8 9).

It is also possible to evaluate several forms sequentially, which is useful if you have one or several def-ed functions and one call which evaluates what is needed. This seems useful for the previous example. Select both Clojure forms and evaluate them as before.



Of course, you don't have to evaluate them both at once. You could have evaluated the function definition first and then the function call in a different request. Each user has a session and can use it for defs within the time limit.

Naturally, not everything can be evaluated this way. For example if you try to evaluate the following snippet you will get an error, since require isn't allowed for security reasons and the jar is probably unavailable to the backend service.



So, unfortunately it is not possible to use this feature to evaluate snippets like the ones on Noir website. But, then again sometimes it's better not to be lazy and fire up leiningen.

I hope you like my extension and find it useful in your browsing. If you find any bugs feel free to report them on the project's Issues page. If you have any idea on how to extend the plugin functionality feel free to comment or fork my repo and add them yourself.

Happy hacking!

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