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Comment by User23 | Hacker News Reader

And some Lisp implementations optimize some cons lists into vectors. The technique is called CDR encoding[1]. It's my understanding that at least the latest generation of modern CPUs actually have optimized instructions for tagged 64 bit pointers too, so this can be implemented efficiently on current hardware just as it was on Lisp machines! Of note though is that mutability complicates things, as usual.

[1] https://en.wikipedia.org/wiki/CDR_coding

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Incidentally, that link proves the point:

> In the presence of mutable objects, CDR coding becomes more complex. If a reference is updated to point to another object, but currently has an object stored in that field, the object must be relocated, along with any other pointers to it. Not only are such moves typically expensive or impossible, but over time they cause fragmentation of the store. This problem is typically avoided by using CDR coding only on immutable data structures.

This is exactly what I've been saying. Thank you for providing a formal reference to the idea.

(I'm a bit confused how we wound up talking past each other, since my original proposal was identical to CDR coding on immutable cons cells.)

It's just that since decades Lisp implementations don't do this, since most implementors don't see it providing much advantage (in speed or size) and it complicates the implementation.

Thanks for sharing this link, very useful reading

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