alt.hn

5/29/2026 at 12:14:29 PM

Don't Build Your Own Lisp

 https://gist.github.com/no-defun-allowed/7e3e238c959e27d4919bb4272487d7ad

by tmtvl

5/30/2026 at 1:53:34 AM

I wrote a Lisp in C and it worked out fine. It was pretty slow but had precise GC and was easy to extend. It was embedded in a commercial product that did ok.

I don't remember whether that was before or after I started absent-mindedly writing yet a different Lisp in C while hanging around RMS. He looked over my shoulder and asked what I was doing. I said writing a Lisp. He asked why I was doing that and I said everybody ought to write a Lisp. He thought about that for a few seconds, said "you should write it in assembler", and walked away.

The "Don't build your own Lisp" article seems reasonable to me and the book it negatively reviews ("Build your own Lisp", I guess) does sound bad.

by throwaway81523

5/30/2026 at 7:03:45 AM

> The author states, on how to engineer software:

    It requires faith and intuition. Faith is required to believe that if the stars align, and every incantation is correctly performed for this magical machine, the right thing will really happen. And intuition is required to work out what has gone wrong, and how to fix things when they don’t go as planned.

    Unfortunately these can’t be taught directly…
> Testing is generally preferable to faith, and testing needn’t be difficult with computers.

Sounds like the sqlite development method. Faith and tests. No architectural planning, and lots of desastrous FTS attempts.

by rurban

5/29/2026 at 12:21:26 PM

Is it too much to ask that when giving a book review, a link to the actual book title, and possibly a listing on a website, should be prominently given?

by ColinWright

5/29/2026 at 2:26:47 PM

The idea was probably that anyone reading the review would already be familiar with Build Your Own Lisp (<https://www.buildyourownlisp.com/>).

by tmtvl

5/29/2026 at 6:15:42 PM

Fair enough for the original author, but if I'd been submitting this to HN, I would probably have included this information in the text field or something.

by ameliaquining

5/29/2026 at 5:01:59 PM

I feel like third or so of this critique could be subtitled "have I mentioned that I don't like C?".

by kjs3

5/29/2026 at 7:07:34 PM

there is that. but the more detailed points are pretty spot on. the overall picture I'm left with is that if you follow these instructions you're left with a much larger, more fragile, and semantically ambiguous lisp that if you had striven for parsimony.

fwiw I think C isn't a bad source language to make a toy tiny lisp. not because any of the things the author says aren't true, but part of the lesson (or joy) here is building semantic towers. there is a kind of mystery around HLLs, where things like memory management and function application are really magical. writing a lisp is Haskell isn't pointless, but at the same time you're kind of just decorating the cake instead of baking it. doing this in C lets you know that you can build heavyweight abstractions from effectively nothing.

but the right approach is probably to abuse C to make it take the shape of a lisp rather than build out a whole C-like intermediate to bridge the gap. and it should be less than 1k lines unless you're actually trying to build something useful.

by convolvatron

5/29/2026 at 8:11:38 PM

Sure, I agree with you (and, for 2/3 of it, I don't much disagree with the OG). I just don't think articles like this hits as hard as it could when a significant portion of it is "now lemme talk about this pet peeve of mine, which I'm very committed to, even though it is at best tangential to purported topic I'm writing about". C may (or may not) be the worst of all possible languages to write a Lisp in (not touching that one :-)), but if you're gonna lecture me about writing a HLL, talk about the language, not dump a bunch of venom into an implementation detail. Or at least be up front and title it "Don't build your own Lisp in a language I hate" or something.

by kjs3

5/30/2026 at 2:55:11 AM

The dig at Kernel is kind of depressing. It's probably an evolutionary dead end, but god damn it, the world is better when people try things. Dismissing them doesn't make you cool.

by reitzensteinm

5/29/2026 at 4:59:08 PM

What if its really fun though

by mghackerlady

5/29/2026 at 9:31:59 PM

I think I’ll make a Lisp that fits in just a single sector of a floppy disk. Oh wait …

by turbostar