I think it was implemented in PLT Scheme before it stopped being a Scheme implementation and became Racket.

Hacker News runs on a language that runs on Racket? Which language is that?

Arc Lisp[0]. This forum was concieved as a MVP for the language.

Anarki, a divergent open source fork, can be found here[1].

[0] http://arclanguage.org/

[1] https://github.com/arclanguage/anarki

I thought that was Arc?

It is. Arc itself is written in Racket.

Ok, so pretty good proof that racket is a working programming language programming language then :-)