TOPLAP (http://toplap.org/) is a decent example of this but they are focused on audio/visual entertainment only.

Basically I'm looking for Twitch for programmers.

https://github.com/robpike/ivy

    interactive := false
    if name == "-" {
        interactive = true
        fd = os.Stdin
    } else {
        interactive = false
        fd, err = os.Open(name)
    }

Now he is scratching his itch and making nice slides to boot.

However, it would have been simpler for him to have typed "python" at the commandline... Python ints are arb precision :)

He isn't implementing a bignum package, this is all about the APL interpreter.

Big ratios:

    [2]> (/ 5723475972349572938475982734985723984759823749587 7239457928374598723984759237459723497598234)
    5723475972349572938475982734985723984759823749587/7239457928374598723984759237459723497598234

    [4]> (* 572934759237495723482374957239475982374957239847523485 (sqrt -1))
    #C(0 572934759237495723482374957239475982374957239847523485)

    [8]> (SETF (EXT:LONG-FLOAT-DIGITS) 1000)
    1000
    [9]> (sqrt 2.0l0)
    1.4142135623730950488016887242096980785696718753769480731766797379907324784621070388503875343276415727350138462309122970249248360558507372126441214970999358314132226659275055927557999505011527820605714701095599716059702745345968620147285174186408891986095523292304843087143214508397626036279952514079896872534L0

Graham later made Arc, the language in which HN is written. To the best of my knowledge Arc's only major project is in fact Hacker News.

So, essentially, yes, you're correct.

largely written in a few million lines of Lisp...

Implementing a BigNum calculator using APL concepts is one way to play. I do this all the time. I'll implement something that will never be in production somewhere or useful to anybody purely for the learning experience and challenge of it.

[EDIT: spelling]

Python 2 longs or Python 3 ints ;)