i think the most interesting design detail of lua tables is just glossed over as "nil-holes" in this article. namely, that nil values do not exist. there is no table.delete(key) method, you just zero out the value and the key stops existing. the same thing is true for any variable, if you set it to nil it ceases to be. i find that implementation fascinating.
this post was submitted on 12 Feb 2026
53 points (100.0% liked)
Programming
26088 readers
160 users here now
Welcome to the main community in programming.dev! Feel free to post anything relating to programming here!
Cross posting is strongly encouraged in the instance. If you feel your post or another person's post makes sense in another community cross post into it.
Hope you enjoy the instance!
Rules
Rules
- Follow the programming.dev instance rules
- Keep content related to programming in some way
- If you're posting long videos try to add in some form of tldr for those who don't want to watch videos
Wormhole
Follow the wormhole through a path of communities !webdev@programming.dev
founded 2 years ago
MODERATORS
Except it's ass if you want to do non-destructive data processing of arbitrary structures and your input and output might have null as a value. You can't just know about fields a, b, and c of the table and leave everything else as it is, you need to know the whole structure and make sure you write null in the output for fields that have nil in them.
Or, more realistically, use libraries that implement null as custom user data.
Iirc Roberto Ierusalimschy even considered introducing a null value in one of the recent versions, of course confusingly named ‘undefined’ — but changed his mind. Perhaps it's for the better than to have such a backwards name for it.
yeah that's probably when you should drop down to C.
Eh, dkjson implements null as an object with a metatable function that encodes it back as "null". Hopefully it's considered equal to itself in comparisons.
Dkjson is fast enough for most scripting purposes. OTOH cjson's userdata null is supported by some other libraries that deal with data structures.
Of course, there's a problem then that various libs may have their own nulls, not equal to each other. There's even a lib that tries to marry some of them.
interesting! it should be equal since it's always just a pointer to that same table.
I just never tried, so amn't entirely sure.
Iirc I needed a json lib on Windows, or was just fed up with compiling things for some reason, and used dkjson instead of cjson. It turned out to be more than adequate, as is pretty typical for Lua-only code. Although it can use the LPeg lib to speed up parsing.
yeah there's a lot of regex in there so i definitely get wanting some compiled parts.
I assume they did that for performance reasons, because removing entries is slow probably? From user perspective, it would have made it more sense to remove the key instead defining it as nil and then expecting the user to handle the nil. The key does not stop existing, right? I don't program in Lua.
Nil doesn't exist as a value in Lua, because a variable or a field that don't exist are indistinguishable from a nil value, since they all return nil when evaluating. In particular, this leads to the situation that you can't have a table where some of the fields are assigned nil as the value, because those fields effectively don't exist.
Ah I see, that makes it clear. I guess it is a "good enough" solution where it doesn't matter in real world.
I'm guessing it's more of a stylistic choice. Lisps typically work the same way, except they can retrieve the full map structure even if some fields contain nil as the value.
Unfortunately, Lua's approach hinders exchanging structures with null values with other environments: see my comment here.
the key does stop existing, insofar as referring to any name that is not defined in lua results in nil. which includes variables, since they are defined in a global table. i don't actually know what happens internally.
Such a strange website. Pretty decent content, but just lying around as HTML/js/css files without a coherent layout to tie them all together? Reminds me of good ol' internet.
Yes, I like it. It makes only one (big) mistake: a horizontal table-of-contents. Nobody does that. You can put it on the left, or above the text, but... not like that.
wdym coherent layout? this thing looks more coherent than any reactwebbloatapp i have seen
it is also very mobile friendly, no lag while scrolling because react is being the bloated garbage it is
I mean overall website, not the page itself. The page is super, but if you fiddle with the url a bit, the site has more nice content. But each page is different and seemingly don’t connect from other pages.
I'm guessing the author didn't want to mess with a framework every time they post, with different page layouts they might've evolved over the years. Though the absence of site-wide navigation is still baffling.
Experience has shown that having a map as your only data structure is definitely a mistake. It's much better to support real arrays too. I doubt it would have made the implementation significantly more complex either (maybe even simpler for luajit).
Idk, PHP chugs on splendidly with arrays that combine both arrays and maps. I regret to say that PHP is faster than some better languages like Python and Ruby, and arrays are the workhorse structure there.
any benchmarks to support this claim?
Feel free to look them up, I'm not preventing you from doing that.
I've had my own field experience with evaluating PHP vs Python, and know who came out on top.
Right but it's fast(ish) in spite of that. It would still be better with separate types.
Weird choice of lifestyle to complain about the two fastest scripting languages being ‘fast-ish’. While you complain, the world is using Lua for embedded scripts and game programming, and PHP for the web.
People straight up write libraries in Lua instead of C, because the performance difference is unnoticeable. But apparently it's only ‘fastish’.