Nice.

Just a stake, I think
((lambda (x) (x x)) (lambda (x) (x x)))
could be done with backquotes in Common Lisp or maybe Elisp.
Or something like that.
Don't trust me.

@screwlisp

@vnikolov I'm sure someone explained it to me before, but while it's trivial to write a macro that creates a quine:

CL-USER> (macrolet ((double (&body body) `'(,@body ,@body))) (double double))
(DOUBLE DOUBLE)

The problem is, I can't get that macrolet inside of the macrolet output without going via a string or something because of how commas (unquotes) work.

By definition, a quine has no normal form [late correction].
For a well-known example, a _single_ β-reduction of the (reducible) term
(λx.xx)(λx.xx)
yields itself.

Thus giving
((lambda (x) (x x)) (lambda (x) (x x)))
to Scheme doesn't terminate.
(Needs an `eval-1', maybe??)

Then
((lambda (x) (list x x)) (lambda (x) (list x x)))
works in Elisp (but obviously not Common Lisp) with lexical binding off.

Off the top of my head, I don't know how to do it with `macrolet'.

#Lisp
#Quines

@screwlisp

P.P.S.

Is this The Heart of Homoiconicity™?

A rare warranted use of `eval' (Elisp):

(let ((q '((lambda (x) (list x x)) (lambda (x) (list x x)))))
(equal q (eval q)))
=> t

after
(setq lexical-binding nil)
as needed.

#Elisp
#EmacsLisp
#Homoiconicity
#Lisp
#Quines

@screwlisp

@vnikolov @screwlisp

"Legitimate" if you think the Y combinator is a "legitimate" programming tool. :-)

I've only ever used it as a goof, with the side effect that the little joke makes people figure out that tail recursion and looping are really the same.

https://en.wikipedia.org/wiki/Fixed-point_combinator

Hmmm...

What does "legitimate" refer to here?

As regards the Y combinator, I don't mind talking about it, or what it means for a programming tool to be legitimate, but at a low rate of posting replies, unfortunately.
It is a sizable topic in any case.

@weekend_editor @screwlisp

@vnikolov @screwlisp

Sorry, my mistake. You used the word 'warranted' and I mentally massaged that into 'legitimate'. My faux pas.

What I meant by warranted/legitimate was something like a tool that a reasonably advanced programmer would find useful, efficient, and clear. Some thing that would make it into instruction books, and into widely used libraries.

I can report from personal experience that Y combinators are none of those things. :-)

(Yeah, all that just so I could make a Y combinator joke. Is this the lispy equivalent of 'dad jokes'?)