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Why LLMs Can't Write Q/Kdb+: Writing Code Right-to-Left

195 points| gabiteodoru | 7 months ago |medium.com

145 comments

order

clord|7 months ago

There is something deep in this observation. When I reflect on how I write code, sometimes it’s backwards. Sometimes I start with the data and work back through to the outer functions, unnesting as I go. Sometimes I start with the final return and work back to the inputs. I notice sometimes LLMs should work this way, but can’t. So they end up rewriting from the start.

Makes me wonder if future llms will be composing nonlinear things and be able to work in non-token-order spaces temporarily, or will have a way to map their output back to linear token order. I know nonlinear thinking is common while writing code though. current llms might be hiding a deficit by having a large and perfect context window.

hnuser123456|7 months ago

Yes, there are already diffusion language models, which start with paragraphs of gibberish and evolve them into a refined response as a whole unit.

saurik|7 months ago

The process of developing software involves this kind of non-linear code editing. When you learn to do something (and the same should go for code, even if sometimes people don't get this critical level of instruction), you don't just look at the final result: you watch people construct the result. The process of constructing code involves a temporarily linear sequence of operations on a text file, but your cursor is bouncing around as you put in commands that move your cursor through the file. We don't have the same kind of copious training data for it, but thereby what we really need to do is to train models not on code, but on all of the input that goes into a text editor. (If we concentrate on software developers that are used to do doing work entirely in a terminal this can be a bit easier, as we can then just essentially train the model on all of the keystrokes they press.)

UltraSane|7 months ago

I think long term LLMs should directly generate Abstract Syntax Trees. But this is hard now because all the training data is text code.

lelanthran|7 months ago

> Sometimes I start with the final return and work back to the inputs.

Shouldn't be hard to train a coding LLM to do this too by doubling the training time: train the LLM both forwards and backwards across the training data.

FeepingCreature|7 months ago

Another example of this is Claude placing unnecessary imports when writing Python, because it's hedge-importing modules that it suspects it might need later.

cenamus|7 months ago

Is it hedging or did the training data just have lots of unecessary imports?

trjordan|7 months ago

Seems like it could easily be training data set size as well.

I'd love to see some quantification of errors in q/kdb+ (or hebrew) vs. languages of similar size that are left-to-right.

fer|7 months ago

>Seems like it could easily be training data set size as well.

I'm convinced that's the case. On any major LLM I can carpet bomb Java/Python boilerplate without issue. For Rust, at least last time I checked, it comes up with non-existing traits, more frequent hallucinations and general struggle to use the context effectively. In agent mode it turns into a first fight with the compiler, often ending in credit destroying loops.

And don't get me started when using it for Nix...

So not surprised about something with orders of magnitude smaller public corpus.

gizmo686|7 months ago

Hebrew is still written sequentially in Unicode. The right-to-left aspect there is simply about how the characters get displayed. On mixed documents, there is U+200E and U+200F to change the text direction mid stream.

From the perspective of a LLM learning from Unicode, this would appear as a delimeter that needs to be inserted on language direction boundaries; but everything else should work the same.

MonkeyClub|7 months ago

> (or hebrew)

W.r.t. natural languages, TFA clarifies it a bit:

> And it’s not the same as translation to Arabic or Hebrew; direction here refers to the temporal order in which the tokens are produced; even for right-to-left languages, the order in which the tokens get produced remains unchanged; rather, a thin display layer handles the visual presentation.

44za12|7 months ago

That’s what I thought. Lack of training data might be a reason.

cess11|7 months ago

"Claude is aware of that, but it struggled to write correct code based on those rules"

It's actually not, and unless they in some way run a rule engine on top of their LLM SaaS stuff it seems far fetched to believe it adheres to rule sets in any way.

Local models confuse Python, Elixir, PHP and Bash when I've tried to use them for coding. They seem more stable for JS, but sometimes they slip out of that too.

Seems pretty contrived and desperate to invent transpilers from quasi-Python to other languages to try and find a software development use for LLM SaaS. Warnings about Lisp macros and other code rewrite tools ought to apply here as well. Plus, of course, the loss of 'notation as a tool of thought'.

strangescript|7 months ago

If your model is getting confused by python, its a bad model. Python is routinely the best language for all major models.

briandw|7 months ago

This is something that diffusion based models would capable of. For example diffusion-coder https://arxiv.org/abs/2506.20639 Could be trained on right to left, but it doesn't seem like they did.

catfacts|7 months ago

Cognitive load in LLMs: When LLMs are faced with syntactic complexity (Lisp/J parentheses/RL-NOP), distractors (cat facts), or unfamiliar paradigms (right-to-left evaluation), the model’s performance degrades because its "attention bandwidth" is split or overwhelmed. This mirrors human cognitive overload.

My question: is there a way to reduce cognitive load in LLMs?, one solution seems to be process the input and output format so that the LLM can use a more common format. I don't know if there is a more general solution.

Edit: Cat attack https://the-decoder.com/cat-attack-on-reasoning-model-shows-...

umanwizard|7 months ago

Isn't the whole idea of Lisp that there is _no_ syntactic complexity? Lisp programs are roughly a serialized AST.

gabiteodoru|7 months ago

I think a translation layer to a lower-density language might be a good solution; e.g. Iverson's divisible-by-11 check, 0=11|-/d, can be verbosely done in Python with

import numpy as np

def flippedSubtract(a, b): return b - a

flipSubUfunc = np.frompyfunc(flippedSubtract, 2, 1)

def isDivBy11(number): digits = list(map(int, str(number))) discriminant = flipSubUfunc.reduce(digits) return (discriminant % 11) == 0

Though Claude already understands (has already seen?) 0=11|-/d so it's hard to tell for this example

As for the cat attack, my gut feeling is that it has to do with the LLM having been trained/instructed to be kind

electroly|7 months ago

I always thought APL was written in the wrong direction. It writes like a concatenative language that's backwards--you tack things onto the front. NumPy fixes it by making the verbs all dotted function calls, effectively mirroring the order. e.g. in APL you write "10 10 ⍴ ⍳100" but in NumPy you write "np.arange(1, 101).reshape(10, 10)". Even if you don't know either language, you can tell that the APL version is the reverse of the Python version.

My hot take is that Iverson was simply wrong about this. He couldn't be expected to predict code completion and then LLMs both wanting later tokens to depend on earlier tokens. SQL messed it up, too, with "from" not coming first. If APL were developed today, I think left-to-right evaluation would have been preferred. The popularity of dotted function calls in various languages makes it reasonably clear that people like tacking things onto the end and seeing a "pipeline" form from left to right.

beagle3|7 months ago

APL was designed as a notation for math; if you pronounce it properly, it makes more sense than numpy:

The 10 by 10 reshaping of counting to 100

fwip|7 months ago

With complicated formulas, it often makes more sense and can give more guidance by first talking about the last operations to be applied. This seems to match the LLM structure, by starting by describing what we want, and then filling in the more specialized holes as we get to them. "Top-down" design vs "bottom-up".

Your insight about APL being reverse-concatenative is very cool.

yujzgzc|7 months ago

Humans can't either? I think if this convention had been more usable form of programming, we'd know by now

anonzzzies|7 months ago

Once you get used to it, traditional ways look tedious and annoying to me. I think the power is in 'once you get used to it'. That will keep out most people. See python llm implementations vs k ones as a novice and you will see verbose unreadable stuff vs line noise. When you learn the math you see verbose code where the verbose code adds nothing at all vs exactly what you would write if you could.

kragen|7 months ago

It might be a question of familiarity rather than objective usability. I'm writing this comment in Latin letters rather than Cyrillic or Hebrew because I find Latin letters much more usable than Cyrillic or Hebrew. But that's because I've been surrounded by Latin letters since I was born, and have only occasionally encountered Cyrillic or Hebrew.

I think it's obvious that Cyrillic isn't any less usable than the Latin alphabet in any objective sense. In fact, I'm using English orthography, which has all kinds of unnecessary usability problems which aren't present in any Cyrillic orthography that I know of. But familiarity is a much stronger factor; even today I can barely sound out words in Russian or Ukrainian, while English text printed in Latin letters is clearer to me than speech.

On theoretical grounds, I suspect that the APL syntax Gabi is calling RL-NOP is less usable for left-to-right readers than at least LR-NOP and maybe even conventional Please Brutally Execute My Dear Aunt Sally operator precedence. But familiarity is such a strong force that this hypothesis is very difficult to test.

The theoretical grounds are that, when reading left to right, a reader must maintain a stack of pending operators and values in their mind, unless they are saved by parentheses. (The Iverson quote disagrees with this, but I think Iverson was wrong.) Maintaining mental stacks is difficult and error-prone; this is the reason for the Tim Peters proverb, "Flat is better than nested."

I suspect that operator precedence might be superior for two reasons:

1. It more often avoids parentheses, which are extra symbols to recognize and correctly pair up in your mind.

2. The meaning of high-precedence subexpressions like `x×b` are almost context-independent—although an exponentiation operator or something like a C struct field selector could still follow `b` and change its meaning, following multiplications, divisions, additions, subtractions, or comparisons will not, and preceding additions, subtractions, or comparisons also will not. I conjecture that this facilitates subconscious pattern recognition.

But the familiarity factor enormously outweighs these theoretical considerations for me.

adrian_b|7 months ago

The popularity of a convention has no relationship with its usability.

Everybody learns in school the traditional convention for writing mathematical expressions.

It appears that for most people it is difficult or impossible to unlearn later such a convention, even if they encounter a superior convention.

On the other hand, I am among those fewer for which this is not true, so when I have first read the book "A Programming Language" of K. Iverson, on which the later APL language and its successors have been based, I have immediately recognized that the Iverson convention is much better than the school convention, and I have no trouble in using it.

When reading a program written with the Iverson convention, you still read from left to right, but you typically do not read until the end of the line, but only as much of the left part as necessary to understand the purpose of the line. (Because the right operand of any operator is everything that follows it until the end of the line, and the details of that computation may be irrelevant. With school notation, when searching where a variable has been modified and how, you must jump between the beginning of the line and the end of the line, to find the last operations that have generated the stored value, when reading and understanding the complete expression would be a waste of time.)

The original motivation of the Iverson convention, which remains very important, was to give a useful meaning for a sequence of identical non-commutative operators, e.g. subtraction and division. This is particularly desirable when the operators are used in vector reductions.

(With school notation, a0 - a1 - a2 - ... - an is seldom a useful expression, but with the Iverson convention it becomes alternate sum, which is needed very frequently. Similarly for division.)

maest|7 months ago

I think there is a reason for this, but maybe not a good one.

1. Function application should be left to right, e.g. `sqrt 4`

2. Precedence order should be very simple. In k, everything has the same precedence order (with the exceptions of brackets)

1 + 2 forces you to have this right to left convention, annoyingly.

Fwiw, I think 2 is great and I would rather give up 1 than 2. However, writing function application as `my_fun arg` is a very strong convention.

layer8|7 months ago

Even if you aren’t used to it, you’d be able to reason yourself through it, knowing how the language works, and would be aware that you need to reason through it. And it isn’t that LLMs don’t know that the language works that way, if you ask them about it. It also isn’t that they aren’t able to reason through it, if you ask them to do so. It’s that they lack awareness when to switch modes, lack the ability to have their knowledge interrupt their “intuitive” output and instead start reasoning about how to proceed.

vessenes|7 months ago

Interesting. Upshot - right to left eval means you generally must start at the end, or at least hold an expression in working memory - LLMs - not so good at this.

I wonder if diffusion models would be better at this; most start out as sequential token generators and then get finetuned.

roschdal|7 months ago

I can write code right-to-left, I simply choose to not do it.

kelas|7 months ago

no, it wasn't your choice how you were taught to read and write something like this:

1|2*3>>4+5

in C and k, this expression should hopefully evaluate to 1, but this is just a lucky coincidence: reading and writing these two expressions are wildly different in complexity in those two languages. if you're not sure what i mean, ask your local LLM to explain why that is, but make sure you're sitting down. what you'll discover is that what you think you "simply chose to do" is not what you're actually doing.

while it is true that you can write code anyway you deem fit, i'm afraid you're a bit confused about the actual direction you're forced to think you chose to write it.

but once you're there, it suddenly gets a lot less complicated, and - miraculosly - doesn't cancel out or mess up your previous beliefs and habits.

k/q, of apl heritage, are beautiful - first and foremost because they're simple to write and simple to read.

awsanswers|7 months ago

I fully discount the right to left thing.

There is not enough q/kdb full source code "out there" that would have made it into the LLM training data. It tends to be used in secretive environments and can be very site specific in convention. I bet a purpose built small fine tune with real + synthetic data would be enough to get something generating better Q code.

impossiblefork|7 months ago

I think in the long run the sensible way to deal with this kind of monitoring is either shared-IP web endpoints for European ISPs, or per-connection random IPv6 addresses, reallocated continuously.

Basically, to make the IP no longer be PII.

gowld|7 months ago

LLMs are already solving this problem using the "thinking" phase. They don't just one-shot an attempt at the output. The left-to-right narrative thinking process edits multiple drafts of the code they eventually output.

aghilmort|7 months ago

most mainstream models are decoders vs. encoders-decoders, diffusers, etc. and lack reversible causal reasoning, which of course can be counter-intuitive since it doesn’t feel that way when models can regenerate prior content

some hacks for time / position/ space flipping the models:

- test spate of diffusion models emerging. pro is faster, con is smaller context, ymmv is if trained on that language &/or context large enough to ICL lang booster info

- exploit known LTL tricks that may work there’s bunch of these

- e.g., tell model to gen drafts in some sort RPN variant of lang, if tests tell it to simulate creating such a fork of this and then gen clean standard form at end

- have it be explicit about leapfrogging recall and reasoning, eg be excessively verbose with comments can regex strip later

- have it build a stack / combo of the RPN & COT & bootstrapping its own ICL

- exploit causal markers - think tags that can splinter time - this can really boost any of the above methods - eg give each instance of things disjoint time tags, A1 vs K37 for numbered instances of things that share a given space - like a time GUID

- use orthogonal groups of such tags to splinter time and space recall and reasoning in model, to include seemingly naive things like pass 1 etc

- our recent arXiv paper on HDRAM / hypertokens pushes causal markers to classic-quantum holographic extreme and was built for this, next version will be more accessible

- the motivators are simple - models fork on prefix-free modulo embedding noise, so the more you make prefix-free, the better the performance, there’s some massive caveats on how to do this perfectly which is exactly our precise work - think 2x to 10x gain on model and similar on reasoning, again ymmv as we update preprint, post second paper that makes baseline better, prep git release etc to make it tons easier to get better recall and exploit same to get better reasoning by making it possible for any model to do the equivalent of arbitrary RPN

- our future state is exactly this a prompt compiler for exactly this use case - explainable time-independent computation in any model

helsinki|7 months ago

My curmudgeonly genius Q/Kdb+ programmer of a co-worker, whom claims to be immune to the impact of LLMs, is going to be fucking pissed when he hears about Qython.

gabiteodoru|7 months ago

:D Well I'm still building Qython, but if your colleague has some example code snippets they think particularly difficult to translate, I'd love to take on the challenge!

nxobject|7 months ago

Incidentally, I've had the same thing too with Lisps on both o-series and smaller Claude models - always a mismatched paren or two.

knome|7 months ago

don't plan on it staying that way. I used to toss wads of my own forth-like language into LLMs to see what kinds of horrible failure modes the latest model would have in parsing and generating such code.

at first they were hilariously bad, then just bad, then kind of okay, and now anthropic's claude4opus reads and writes it just fine.

sitkack|7 months ago

How much incontext documentation for your language are you giving it, or does it just figure it out?

am17an|7 months ago

Another quirk inserting random whitespace when generating code. Seem to be tokens for different lengths of whitespace

grej|7 months ago

This is, in part, one of the reasons why I am interested in the emerging diffusion based text generation models.

i000|7 months ago

R has right assigment `1 -> x` LLMs seem to enjoy it a bit too much.

leprechaun1066|7 months ago

It's not because of the left of right evaluation. If the difference was that simple, most humans, let alone LLMs, wouldn't struggle with picking up q when they come from the common languages.

Usually when someone solves problems with q, they don't use the way one would for Python/Java/C/C++/C#/etc.

This is probably a poor example, if I asked someone to write a function to create an nxn identity matrix for a given number the non-q solution would probably involve some kind of nested loop that checks if i==j and assigns 1, otherwise assigns 0.

In q you'd still check equivalence, but instead of looping, you generate a list of numbers as long as the given dimension and then compare each item of the list to itself:

  {x=/:x:til x}3
An LLM that's been so heavily trained on an imperative style will likely struggle to solve similar (and often more complex) problems in a standard q manner.

wat10000|7 months ago

A human can deal with right-to-left evaluation by moving the cursor around to write in that direction. An LLM can’t do that on its own. A human given an editor that can only append would struggle too.

gabiteodoru|7 months ago

Or, even better, also from the cookbook: {(2#x)#1,x#0} But this really borders on obfuscation :P

rob_c|7 months ago

Same reason the same models don't fundamentally understand all languages. They're not trained to. Frankly the design changes to get this to work in training is minimal but this isn't the way English works so expect most of the corporate LLM to struggle because that's where the interest and money is.

Give it time until we have true globally multi lingual models for superior context awareness.

strangescript|7 months ago

A byte tokenized model is naturally 100% multi-lingual in all languages in its data set. There just isn't a lot of reason for teams to spend the extra training time to build that sort of model.

sitkack|7 months ago

Ordering issues can be overcome by allowing the model to think in one direction and then reverse the output once it has created it.

aardvark179|7 months ago

How do they do with lisps?

kelas|7 months ago

(en passant, k is arguably more lispy than some lisps. for a lisp guy, the first cultural shock is usually the absence of 99% of superfluous parens)

as for LLM copilots and the quality of their lisp: why you'd expect them to excel in lisp better than they lisp in excel, pardon the pun?

kelas|7 months ago

curiously enough,

this thread made me revisit some past conversations with people like atw, nsl and aab with regard to possible ways to expose humans to the way rivers flow in k/q/apl land. the choices are limited, and decision takes some agony:

a) if you don't want your audience to close the tab right away, you'd say "a k expression is written, read and evaluated strictly right to left unless the precedence is explicitly overridden by parens, and this works better than you think, no worries, you'll come around. by the way, parens are evil, avoid them if you can".

b) if your intent is to retain a sharper crowd who went to yale or something, you'd say "a k expression is to be understood right of left", and throw them a freebie in form of a prompt for their local LLM in order to get lit. the magic sequence is just "f g h x leibniz".

for my own selfish reasons, i always chose the former, and it seems to perform better than the latter, proof:

https://github.com/kparc/ksimple

https://github.com/kparc/kcc

still, neither approach is anywhere near the chances of successfuly explaining which way to write python code to a 5yo kid, especially its precedence rules, which are much more intuitive (lol).

to explain the same thing to an LLM is not much different, really. all you need to do is to depress your 0yo kid with an obscene amount of _quality_ python code, of which there is no shortage. obviously, the more python code is fed to LLMs, the more humans will paste more LLM-generated python code, to be fed back to LLMs, ad lemniscate.

(and don't mind the future tense, we are already there)

============

so this is why LLMs can't write k/q/apl. first, they haven't seen enough of it. second, they are helpless to understand the meaning of a quote which was once chosen to helm a book known as SICP, not to mention countless human counterparts who came across it earlier, to the same effect:

"I think that it's extraordinarily important that we in computer science keep fun in computing. When it started out it was an awful lot of fun. Of course the paying customers got shafted every now and then and after a while we began to take their complaints seriously. We began to feel as if we really were responsible for the successful error-free perfect use of these machines. I don’t think we are. I think we're responsible for stretching them setting them off in new directions and keeping fun in the house. I hope the field of computer science never loses its sense of fun. Above all I hope we don’t become missionaries. Don't feel as if you're Bible salesmen. The world has too many of those already. What you know about computing other people will learn. Don’t feel as if the key to successful computing is only in your hands. What's in your hands I think and hope is intelligence: the ability to see the machine as more than when you were first led up to it that you can make it more."

― Alan J. Perlis

gabiteodoru|7 months ago

Those github links are so cool, thanks for sharing! :)

ape4|7 months ago

I read the other day here that the new Apple AI can write out-of-order. Maybe it can do this.

tantalor|7 months ago

Languages that are difficult for LLM to read & write are also difficult for the general public. These languages have always had poor uptake and never reach critical mass, or are eventually replaced by better languages.

Language designers would be smart to recognize this fact and favor making their languages more LLM friendly. This should also make them more human friendly.

markerz|7 months ago

I actually think Ruby on Rails is incredibly difficult for LLMs to write because of how many implicit "global state" things occur. I'm always surprised how productive people are with it, but people are productive with it for sure.

jdiff|7 months ago

I don't find this to be true. There are languages that are difficult to wrap your head around initially, but that turn out to be delightfully productive with a few adjustments to the mental model. Adjustments that LLMs don't have the training data for.

That says nothing about the language at all, actually. Just that it's small and easily confused for something more idiomatic to a newbie.

Tainnor|7 months ago

This argument in favour of mediocrity and catering to the lowest common denominator is one of the key reasons why I dislike people who want to shove LLMs into everything (including art).