Hacker News Reader

Comment by Nesco | Hacker News Reader

LLMs are autoregressive models. However, the notion of order in ASTs might be nonexistent, especially for parallel branches of computation/control flow. You could attempt to untangle each branch into N sequences, but this would erase control-flow information.

Even when there is an objective ordering of the children of every node, you still have four traversal options: {preorder, postorder} × {BF, DF}.

Note: For children lacking an objective ordering, you might apply generic rules to define a traversal order, but you’d end up with as many depth-first traversals as there are possible orders—essentially a crude heuristic. If you want the evaluation order to be dynamic at each step (e.g., using RL), the complexity grows geometrically worse. That’s been my experience tinkering with a custom AST DSL for ARC-AGI.

View on HN

Cool to hear you've worked on ARC-AGI — I poked with it too. You’re totally right about the messy traversal space, especially with parallel branches. What feels ambiguous at the token level becomes structured ambiguity in the AST — and that’s progress.

My hunch is that LLMs don’t need to solve the whole traversal space — they just need a clean, abstract interface. Even parallel branches can be normalized into a schema that the model can reason over consistently. And in practice, you rarely need full recursion or a complete tree walk to understand a node — but having that option unlocks deeper comprehension when it counts.

This kind of structural understanding would also massively improve Copilot-style tools, especially for less popular libraries where token-level familiarity breaks down. If models could reason over types and structure instead of guessing based on frequency, completions would be a lot more reliable outside the top 1% of APIs.

> LLMs are autoregressive models.

Most LLMs are autoregressive models, but exceptions exist, e.g., Mercury [0] is a diffusion LLM.

[0] https://www.inceptionlabs.ai/news

Well, from my very limited comprehension of diffusion models, they apply to fixed length structure, mostly from a continuous space. Maybe a way to make them work with tree structures could be found - that's no trivial task

Autoregressive LLMs don't usually work on tree structures, they work on capped-length linear token sequences, which are isomorphic to fixed-length sequences.

I'm not sure why you think working on tree structures rather than fixed length sequences would be necessary for diffusion language models—which, again, actually exist; aside from Mercury which is proprietary, there is also LLaDA: https://ml-gsai.github.io/LLaDA-demo/

Reply on news.ycombinator.com