[labria]

  (define eval-expr
    (lambda (expr env)
      (pmatch expr
        [`,x (guard (symbol? x))
          (env x)]
        [`(lambda (,x) ,body)
          (lambda (arg)
            (eval-expr body (lambda (y)
                              (if (eq? x y)
                                  arg
                                  (env y)))))]
        [`(,rator ,rand)
         ((eval-expr rator env)
          (eval-expr rand env))])))

There's an amazing talk about it: https://www.youtube.com/watch?v=OyfBQmvr2Hc

[estomagordo]

Great. In 90 minutes I may be able to know what you're talking about.

[estomagordo]

Is it a complete lisp interpreter, written in lisp?

[Globz]

Yes, it uses pmatch behind the scene to match your expr against the evaluator.

https://github.com/webyrd/quines/blob/master/pmatch.scm

I have to agree, this like rank #1 in my book!

[lioeters]

This is exactly what came to mind when seeing the question.

I was just recently (re-)reading an article that goes in depth:

Lisp as the Maxwell’s equations of software

http://www.michaelnielsen.org/ddi/lisp-as-the-maxwells-equat...

[chombier]

Once you've seen this one, it's difficult not to rank it first :)

[quickthrower2]

Y Combinator?