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  * May 26, 2025
  * blog

Hacker News now runs on top of Common Lisp

Hacker News was written in the Arc lisp dialect, a dialect created by
Paul Graham. Arc was implemented on top of Racket, but that has now
changed. HN runs on top of SBCL since a few months.

But why? For performance reasons.

    I recently noticed that Hacker News no longer uses paging for
    long threads. In the past, when a discussion grew large, we had
    to click "More" to load the next page of comments, and dang would
    occasionally post helpful tips to remind us about this feature.
    Was there an announcement regarding this change? Has anyone else
    still seen paging recently? I'd love to know more
    details--especially the technical aspects or considerations that
    went into the decision.

Answer:

    It's because Clarc is finally out.

dang, Sept. 2024

    [Clarc] is much faster and also will easily let HN run on
    multiple cores. It's been in the works for years, mainly because
    I rarely find time to work on it, but it's all pretty close to
    done.

dang, 2022

How it's done:

    there's now an Arc-to-JS called Lilt, and an Arc-to-Common Lisp
    called Clarc. In order to make those easier to develop, we
    reworked the lower depths of the existing Arc implementation to
    build Arc up in stages. The bottom one is called arc0, then arc1
    is written in arc0, and arc2 in arc1. The one at the top (arc2, I
    think) is full Arc. This isn't novel, but it makes
    reimplementation easier since you pack as much as possible in the
    later stages, and only arc0 needs to be written in the underlying
    system (Racket, JS, or CL).

dang, 2019

But Clarc's code isn't released, although it could be done:

    open-sourcing the Arc implementation (i.e. Clarc) would be much
    easier [than the HN site]. The way to do it would be to port the
    original Arc release (http://arclanguage.org/) to Clarc. It
    includes a sample application which is an early version of HN,
    scrubbed of anything HN- or YC-specific.

dang, day of this post

Releasing the new HN code base however wouldn't work:

    Much of the HN codebase consists of anti-abuse measures that
    would stop working if people knew about them. Unfortunately.
    separating out the secret parts would by now be a lot of work.
    The time to do it will be if and when we eventually release the
    alternative Arc implementations we've been working on.

https://news.ycombinator.com/item?id=21546438

Congrats for the successful "splash-free" transition though.

---------------------------------------------------------------------

  * edit 10PM UTC+2: added a quote to clarify about open-sourcing the
    HN code ("wouldn't work") VS Clarc itself ("much easier").

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