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).
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.
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)!
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.
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)
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.
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.
@bananaflag, the x versus (x) distinction is also what makes this evaluator so small: the AST uses atom versus list as the dispatch boundary, so grouping and application deliberately share syntax. An infix parser moves that complexity into precedence and associativity rules; it does not eliminate it. Indentation is still essential, but that is a tooling and display issue rather than a grammar issue.
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.
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
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. ;-)
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.
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.
> 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.
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.
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.
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.
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)
I feel like we need a vibe coded guide in this modern day and age.
I want to see Dang’s list of fav classics :)
Sadly, https://news.ycombinator.com/favorites?id=dang is underutilized.
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...
It’s not like they pay you to browse Hacker News all day!
https://news.ycombinator.com/item?id=8123327
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
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.
Similarly - 80 line LISP interpreter with integers, lexical scope, first class functions
https://leontrolski.github.io/interpreter.html
(how-to in-python (write (interpreter lisp)))
Yes, but to be fair, you only have a couple minutes to fight the HN title regex.
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)!
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
And in the other direction, here's a Python interpreter written in Lisp: https://github.com/metawilm/cl-python
Probably way faster as well.
also in the interop topic:
Common Lisp for Python: https://github.com/marcoheisig/cl4py
call Python from CL: https://github.com/digikar99/py4cl2-cffi
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.
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)
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
so, instead of
you'd prefer
is that right?
Emacs vs vim, go!
vs
(aar (bar1 1 2 3) (bar2 1 2 3) (bar3 (car1 2 3)(car2)(car3)))
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.
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
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.
@bananaflag, the x versus (x) distinction is also what makes this evaluator so small: the AST uses atom versus list as the dispatch boundary, so grouping and application deliberately share syntax. An infix parser moves that complexity into precedence and associativity rules; it does not eliminate it. Indentation is still essential, but that is a tooling and display issue rather than a grammar issue.
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()
Writing a Lisp is one of my favorite projects. I try to do it every year or two, taking a different approach each time.
The one where you replaced parentheses with the crying laughing emojis was definitely the worst.
This sounds amazing.
I enjoyed thinking about it. Maybe replace if...else with great...sad etc
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.
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
I made a rusty version of this. No linked lists, but that's not important in the grand Scheme of things.
https://wmedrano.dev/post/2025/11/15/lisp-1
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
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. ;-)
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.
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
or you could just use Raku and its “surprisingly good lisp impression”:
https://www.codesections.com/blog/raku-lisp-impression/
article to follow between all the ai noises these days
(2010)?
Appreciated the title xD
;;;; Not sorry
)
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.
?
Why did you replace your very similar comment with "--" just to post essentially the same thing again?
Man these kind of resources have aged really bad in the age of AI.
How so?
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.
> 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.
How do you mean “these kind”?
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.
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.
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.