Hashtable Vs. A-List in Scheme, Which to Choose?

As far as I can see, there is no mention of how many elements are inserted in each run? The x/horizontal-axis is not number of items, as one would expect, but is "run #" which I interpret to be "number of the run"/"run number", which says nothing about the number of elements added to the data structures. So from what is visible in the writing of post, one couldn't actually conclude the conclusion. I guess the idea is, that you must go and look at the code itself. This post could do a better job at explaining what is measured, especially since it is about a benchmark.

But aside from that: I think the answer here is not merely from a performance side of things. If there is a chance, that your program might grow to need to put more stuff in that a-list, I think you can just go for a hashtable right away and save yourself the effort to later change the code. Unless you are hyper-optimizing stuff, which most of the time we are not. I also want to note, that there is SRFI-69 [1], which has `a-list->hash-table`, in case you want to opt for a-list because of how easy they are to write in the code.

Choosing an a-list in a scenario where it might still be faster is not only a trade-off in performance, but also in maintainability. Are you ready to build an abstraction layer around a-list, so that you can later switch it out and put a hash-table in easily? If not, then the little performance cost for using a hash-table too early might be the better trade-off, compared to having to replace usages of a-list in the code later.

I think a-list is fine for things where you:

    (1) convert to hash-table anyway, with `a-list->hash-table`
    (2) construct a struct/record from it, because those usually don't have that many fields
    (3) never add to the a-list, and only have it contain stuff you manually type in the code, because those usually aren't that many either
    (4) know that the a-list will not grow much or is even static in size, and that size is small. One such example are often function (keyword) arguments.

[1]: https://srfi.schemers.org/srfi-69/srfi-69.html#lst

That's not to say performance doesn't matter anymore or that blog posts on niche topics don't matter anymore.

It's more that there are 30 opponents on all sides fighting to minimize the impact of this kind of post. CPUs are still getting faster even now despite Moore's law being dead. The business or career impact of choosing between an associative list vs hashmap in a garbage-collected language like Guile Scheme is so minimal that it's hard to quantify.

If it's in a hot enough path that it matters, it's likely that there are at least 3 things you can do within 20 minutes of work (or 5 minutes of GPU time) that will solve the problem as effectively or better.

I remember the very specific period of time when blog posts talking about functional programming for React developers were en vogue. You can speed up you Scheme app by 15%, or you can build and deploy a completely new service from scratch in Node.JS in the same amount of time.

It used to feel like code had some kind of meaning or value. Now, it's just an artifact produced as a side effect of work. But that's been a trend for a decade or so now, AI is just the latest (and most significant) iteration of it.

[0] https://www.gnu.org/software/guile/manual/html_node/VHashes....