[xtrapolate]

> "He needs to be running a client-side service, not running whatever someone enters on his machine"

I would say that for the most part, websites such as this don't actually need a real, full-blown %s-lang compiler/VM that actually executes real code on a backend server. It would be enough to tokenize and parse things on the client's side and validate ABNF via JS. This would reduce the costs involved with running such a website, and the attack surface. If you want to get fancy, you could host an in-browser Python VM - but that's an overkill for a website such as this. Also, they're trying to support a fair bit of languages here, not all of which have browser-targeted tooling that could compile and run the code.

[wild_preference]

Then you end up with a system where someone can arrive at the right answer via AST that you didn't expect, which was a frustration when I helped students with a service like (IIRC) CodeCademy.

[cellularmitosis]

Parsing the AST is something I would love to apply to a site like 4clojure.com, to get a histogram of the "shape" of all of the submitted solutions.