alt.hn

6/21/2026 at 3:36:33 PM

(How to Write a (Lisp) Interpreter (In Python)) (2010)

 https://norvig.com/lispy.html

by tosh

6/21/2026 at 7:18:12 PM

Is it classics day or something? (Fine with us!)

Related:

(How to Write a (Lisp) Interpreter (In Python)) (2010) - https://news.ycombinator.com/item?id=39665939 - March 2024 (91 comments)

(How to Write a (Lisp) Interpreter (In Python)) (2010) - https://news.ycombinator.com/item?id=30443949 - Feb 2022 (9 comments)

(How to Write a (Lisp) Interpreter (In Python)) - https://news.ycombinator.com/item?id=30327437 - Feb 2022 (3 comments)

(How to Write a (Lisp) Interpreter (In Python)) - https://news.ycombinator.com/item?id=26036431 - Feb 2021 (1 comment)

How to Write a Lisp Interpreter In Python (2010) - https://news.ycombinator.com/item?id=20590439 - Aug 2019 (29 comments)

How to Write a Lisp Interpreter in Python (2010) - https://news.ycombinator.com/item?id=12777852 - Oct 2016 (28 comments)

How to Write a Lisp Interpreter in Python (2010) - https://news.ycombinator.com/item?id=7825054 - May 2014 (41 comments)

(How to Write a ((Better) Lisp) Interpreter (in Python)) - https://news.ycombinator.com/item?id=1746916 - Oct 2010 (10 comments)

(How to Write a (Lisp) Interpreter (in Python)) - https://news.ycombinator.com/item?id=1745322 - Sept 2010 (39 comments)

by dang

6/21/2026 at 9:37:14 PM

I want to see Dang’s list of fav classics :)

by azhenley

6/21/2026 at 9:45:13 PM

Sadly, https://news.ycombinator.com/favorites?id=dang is underutilized.

by fragmede

6/22/2026 at 12:45:09 AM

Yikes, I just learned from looking there that misclick favorites are a thing. Hastily un-favorited some nasty stuff.

It would be fun to build up a list of real favorites - just the sort of thing there never seems to be time for...

by dang

6/23/2026 at 10:48:12 AM

If favoriting didn't take an extra click, it would be easier to build up that list HINT HINT

by fragmede

6/21/2026 at 7:48:29 PM

I feel like we need a vibe coded guide in this modern day and age.

by Izmaki

6/21/2026 at 4:25:46 PM

If you ever wondered how to write a programming language, this is probably the best resource to get started (and then of course Crafting Interpreters).

See also part 2 https://norvig.com/lispy2.html

by chombier

6/21/2026 at 7:10:27 PM

I've always found MAL ("Make-A-Lisp" https://github.com/kanaka/mal) a bit more approachable, probably because I was out after creating my own programming language before I've written much Python. It's language agnostic, and really easy to follow along with most programming languages out there, explaining everything as you progress.

That it's language agnostic and somehow matters feels weird now a lot of time (and experience I suppose) afterwards, but back when I only knew 1-2 languages by heart, also having to face understanding Python at the same time from Norvigs guide/reference made it slightly more complicated for me.

I use this as a litmus test now when coming across new languages (implementing MAL in the new language), as it's such an easy approach to practically test large parts of the new language, and there is always host-language-specific tricks you can learn along the way.

by embedding-shape

6/21/2026 at 3:57:11 PM

(how-to in-python (write (interpreter lisp)))

by zahlman

6/21/2026 at 4:44:36 PM

Yes, but to be fair, you only have a couple minutes to fight the HN title regex.

by consumer451

6/21/2026 at 8:57:55 PM

I've been working on a similar (ish) project for a while: Ribbit (https://github.com/udem-dlteam/ribbit). Supports a full R4RS REPL (with tail-calls) in the same sizes as Lispy (8Kb for JavaScript and 6.5Kb for x86)!

by leonardool

6/21/2026 at 3:48:44 PM

I can't recommend highly enough to implement a simple lisp (or a forth).

Illuminating experience and it will also help you see (among many other things) the parentheses in a different light.

by tosh

6/21/2026 at 4:08:12 PM

First day of paradigms course in the 2000s and prof says "if your opinion of Scheme is too many parentheses, then you're an idiot."

Needless to say that was my opinion and every day I think, more and more, how right he was.

(later I did make some gui apps that included scripting and chose s-expr syntax because of how simple it is to implement it)

by stdatomic

6/21/2026 at 4:23:27 PM

There are two problems with Lisp parentheses in my opinion:

1) Humans are not that equipped to handle that level of nesting without some other aid, this is why Lisp code is usually indented.

2) Parentheses aren't just about grouping, and this is unintuitive. For example, x is not the same as (x). This is a bit like in set theory where x is not the same as {x}, but parentheses do not look like the kind of sign that would work like that.

by bananaflag

6/22/2026 at 12:37:38 AM

I thought parentheses were fairly intuitive. They are not for grouping, more like representing an AST.

For other languages, similarly, x is not the same as x()

by wmedrano

6/21/2026 at 5:58:57 PM

[flagged]

by jimmypk

6/21/2026 at 4:15:06 PM

main problem isn't brackets themselves - it's that they're too on the right

had brackets been displayed as curly braces in C - everything would look much more manageable

by NooneAtAll3

6/21/2026 at 4:43:05 PM

I changed my opinion about parens when I stopped formatting like C, and used indent rather than parens to denote blocks. That is, a large amount of them at the end is totally fine.

by eska

6/21/2026 at 4:37:21 PM

so, instead of

    (foo (bar (1 2 3))
you'd prefer

    {
      foo {
        bar {
          1
          2
          3
        }
      }
    }
is that right?

by phpnode

6/21/2026 at 6:39:50 PM 

    ( aar
      (bar1 1 2 3)
      (bar2 1 2 3)
      (bar3 
         (car1 2 3)
         (car2)
         (car3)
      )
    )
vs

(aar (bar1 1 2 3) (bar2 1 2 3) (bar3 (car1 2 3)(car2)(car3)))

by NooneAtAll3

6/21/2026 at 4:57:45 PM

Emacs vs vim, go!

by irishcoffee

6/22/2026 at 12:06:09 AM

I actually find wisp [0] harder to read/write than just plain Scheme.

But there's quite a few little reader projects that swap out the first layer of syntax for another. Lisp isn't entirely tied to one representation.

[0] https://www.draketo.de/software/wisp

by shakna

6/21/2026 at 4:09:15 PM

Writing a Lisp is one of my favorite projects. I try to do it every year or two, taking a different approach each time.

by azhenley

6/21/2026 at 4:49:01 PM

The one where you replaced parentheses with the crying laughing emojis was definitely the worst.

by onraglanroad

6/21/2026 at 11:04:42 PM

This sounds amazing.

by all2

6/22/2026 at 5:55:06 PM

I enjoyed thinking about it. Maybe replace if...else with great...sad etc

by onraglanroad

6/22/2026 at 6:39:42 AM

Classics!

There is a different take on Lisp in Python - fakelisp. It's literally Lisp in Python, not an interpreter, but a syntax sugar library that allows embedding Lisp snippets into valid Python.

https://codeberg.org/okaleniuk/fakelisp

    from fakelisp import *

    # And now you can start mixing Python and LISP
    X = (BEGIN
        (SET (F) (LAMBDA (X)
            (IF (EQ (X) (1))
                (1)
                (MUL (X) (F (SUB (X) (1)))))))

        (LIST (F (4)) (42)))

    # Back to Python any time
    print "x: ", str(X)

by okaleniuk

6/22/2026 at 12:15:37 AM

Strangely, Peter's 1987 Ph.D. thesis cites itself (reference 90), but with the year being off by one (1986).

Writing a LISP in Python is only for educational use, or to have a boostrap LISP that you can write a better (faster) LISP in.

by jll29

6/21/2026 at 7:36:29 PM

My Lisp from 1975 was actually used in real world and highly lucrative. Gemini could read the source code, but it told that my code was piece of shit and cannot be implemented in 64-bit world without drastic changes, so it made an example. But that version was just too advanced and too complex as a study subject. There are already enuff good Lisps in the world, methinks.

https://github.com/timonoko/nokolisp

by timonoko

6/21/2026 at 8:44:39 PM

Interestingly enough, linguists also use Lisp-like parentheses or brackets to annotate sentence structures. Trees and brackets are isomorphic, as both phrase structure grammarians and the original SICP lectures pointed out.

The brackets in the title sentence would look a lot different though. ;-)

by adamddev1

6/22/2026 at 6:29:52 AM

I'll put in a plug for David Beazly's SICP course. While we didn't build a full Lisp interpreter, we built something similar over a week-long hands-on course. I believe his github for the course is private and only available to students. https://www.dabeaz.com/sicp.html

by avilay

6/21/2026 at 8:32:45 PM

Depressing to think that AI will be doing most of this in the future. Sharing it freely in the internet, basically ensures AI can copy it well.

by sashank_1509

6/21/2026 at 6:22:55 PM

I actually perfected the Norvig Lisp at one time. It has compiler to python and just everything. Those very few here that can actually read code, understand why this project soon exploded into biggest piece of odorous excrement.

https://github.com/timonoko/nokolis.py

by timonoko

6/21/2026 at 4:46:47 PM

There are edge cases where this fails, but `def parse(s): return json.loads('['+re.sub('([")])\s*(["(])','\g<1>,\g<2>',re.sub('[^()\s]+','"\g<0>"',s)).replace('(','[').replace(')',']')+']')` is a surprisingly robust lisp parser.

by joshuamorton

6/21/2026 at 3:55:21 PM

(2010)?

by e12e

6/21/2026 at 3:51:26 PM

Appreciated the title xD

by urcite_ty_kokos

6/21/2026 at 9:40:29 PM

article to follow between all the ai noises these days

by ljcoco

6/21/2026 at 11:40:28 PM

;;;; Not sorry

)

by mathnode

6/21/2026 at 9:35:22 PM

[flagged]

by unfirehose

6/21/2026 at 4:46:53 PM

[flagged]

by timonoko

6/21/2026 at 5:10:33 PM

There is always someone better than you at almost everything you do, this is statistical reality.

If all you care about is the artifact and not the path, there is no reason to do anything.

Use the tool to better yourself, your understanding and push the limits of what is possible. If a Lisp in assembly with GC is now hello world, change what a hard project is.

I see this attitude a lot, and I think it is rooted in a sort of self-centered elitism. Anyone can do it, so why do it? Instead you could have the AI teach you how to implement it yourself with a deep understanding that no human, even if you paid them, would put up with.

But sure, get depressed. But why tho?

by genxy

6/21/2026 at 11:53:30 PM

> I see this attitude a lot, and I think it is rooted in a sort of self-centered elitism. Anyone can do it, so why do it?

The undercurrent is that nobody will be able to make a career out of it anymore. Sure, programming is intellectually stimulating, but it's questionable whether it'll lead to highly compensated jobs now.

I'm a hobbyist who loves computers so I'm still going to do it. I have been entertaining thoughts of doing it professionally though. The market isn't encouraging at all.

by matheusmoreira

6/22/2026 at 7:59:51 AM

It changes what you can do and how you do it, but it still needs someone with skills to drive it.

And it does hit a capability plateau, you cannot just say, "make me a compiler for xyz". Your prompt has to be at least 8 sentences. A skilled programmer and a skilled user of AI can do amazing things. But you need to be both. Lay persons cranking out zombie react sites shouldn't be scaring anyone. It is good to have a half functioning prototype, it allows everyone to have something to bounce ideas off of.

by genxy

6/21/2026 at 4:59:51 PM

is learning how to accomplish or understand something boring

just because someone or something else does it better?

by tosh

6/21/2026 at 5:45:13 PM

It's funny, in the 8-bit days a lot of us learned programming for its own sake without much expectation it'd be lucrative. Took ~50 years to get back to that spirit as the default.

by abecedarius

6/21/2026 at 5:12:00 PM

It depends why you're doing it. Are you doing it for the product or the process? (Of course, they're not mutually exclusive.) I do it for the fun of building, in which case AI is irrelevant.

by Lyngbakr

6/21/2026 at 4:58:17 PM

I mean it’s still worth doing, even if AI can do it. But I definitely empathize with that bit of AI ennui.

by chamomeal

6/21/2026 at 6:03:33 PM

?

by timonoko

6/21/2026 at 6:05:45 PM

Why did you replace your very similar comment with "--" just to post essentially the same thing again?

by Jtsummers

6/21/2026 at 5:07:02 PM

Man these kind of resources have aged really bad in the age of AI.

by RedCinnabar

6/21/2026 at 5:11:12 PM

Why would AI make these age worse than, say, libraries or languages becoming obsolete?

I don't think a good learning resource gets worse just because there's a newer alternative.

by Crespyl

6/21/2026 at 5:38:40 PM

> I don’t think a good learning resource gets worse[...]

Probably not, but they become irrelevant. The other day I found an old programming book at my parents’ and while it was still a terrific resource, I couldn’t image anyone learning a language from a book nowadays.

AI is doing the same thing but 100 times effectively than anything else.

by RedCinnabar

6/21/2026 at 5:20:35 PM

How do you mean “these kind”?

by incanus77

6/21/2026 at 5:31:54 PM

Blog tutorials, guides, programming books and youtube tutorials. They are completely irrelevant in a time where you have a personal tutor willing to explain every single detail of a subject.

by RedCinnabar

6/22/2026 at 12:06:22 AM

Relevance is overrated. I've been writing for myself. Writing articles about the implementation of my programming language helps crystallize my knowledge. It's been remarkably effective at ensuring I won't simply forget the subject matter in the future. The fact other humans might enjoy reading my articles is just a nice bonus.

by matheusmoreira

6/21/2026 at 6:24:26 PM

That's like saying your grandfather is irrelevant now that he's spawned children and grandchildren. Good luck to those personal tutors without this source material.

by macintux

6/21/2026 at 5:09:21 PM

How so?

by jgalt212