An interactive problem website for Clojure beginners

Security-minded folks might be interested in the "clojail" sandboxing library (https://github.com/cognitivedissonance/clojail/blob/master/s...) that this webapp uses (https://github.com/dbyrne/4clojure/blob/74f381701b62a12e2bfb...). clojail's README contains some caveats worth reading.

Why not link to the site directly? https://www.4clojure.com/

I thought the repo is more interesting for hackers. I watched it besides solving some problems.

I thought this site was very cool, but some of the problems could be explained more clearly. I was getting things without any idea why my answers were incorrect.