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2 points by evanrmurphy 3457 days ago | link | parent

> But isomorphic to a hash table?

I'm talking about the core notion of a hash table. It's composed of key-value pairs, not key-value "lists of two". :P This is a restatement of bogomipz's point made elsewhere in this thread.

> Arc doesn't have enough table support.

If alists were better supported, you could use them in place of tables in every case except where the utmost efficiency is required.

But I'm not sure it's even correct to frame this as an axioms vs. efficiency debate. Something I've learned from PicoLisp is that heterogeneous data structures slow down the general case by complicating memory allocation and garbage allocation. PicoLisp manages to be a fast interpreter (say what?), in part because it uses the cons cell for everything [1].

> I'd much prefer to write '((a 1) (b 2)) rather than '((a . 1) (b . 2)) and destructure using (let (k v) ...) rather than (let (k . v) ...).

I think this is a cosmetic issue that has to do only with our visual representation of cons pairs and Arc's incumbent ssyntax.

For example, if you changed the ssyntax so that a.b expanded to (a . b) instead of (a b), then these snippets would be more pleasant to write: '(a.1 b.2) and (let k.v ...) . I'm not actually proposing this particular solution, but it should illustrate my point that the issue is only syntactic/cosmetic.

> How are those related? [...] Did you mean to say that you suspect some change regarding dotted lists (or just the way we look at them) will help with both alists and these other cases?

Well I did say I still have some details to work out. ;)

I think your paraphrase is accurate. A "change regarding dotted lists (or just the way we look at them)" is what I was trying to express with "judicious use of conses" in the grandparent.



1 point by rocketnia 3457 days ago | link

IMO, a pair is a list of two. What would you propose using for a triple or a singleton?


1 point by evanrmurphy 3457 days ago | link

A pair is a list of two in English because English lists aren't nil-terminated. But Arc lists are, so we're talking about the difference between (key . val) and (key . (val . nil)).

I don't have a great answer to your triple/singleton question yet except to ask that you consider the following:

- The fundamental data structure of lisp is the cons pair, so perhaps pairs warrant some special treatment over singletons, triples, etc.

- The demand for associative arrays in general-purpose programming is far greater than that for any kind of triple-based data structure, which is why tables have their own type in Arc to begin with

Update: Cons pairs are so powerful that we've used them as the base for almost our entire language. And yet the associative array structure (which screams "pair"!) that we've made from them (i.e. alists) is so inadequate that we all outsource that functionality to tables instead. Around tables we've then developed the conveniences for syntax, etc.... Doesn't this seem a bit kludgy for The Hundred-year Language?


2 points by rocketnia 3456 days ago | link

The main advantage of cons pairs, in my mind, is that they're all the same size, so it's easier to reason about them and memory-manage them on a low level. They're also just as powerful as they need to be to support an expressive language. But that doesn't make them ideal abstractions for exploratory programming, especially when an equivalent abstraction in the same language takes fewer characters to type out and is even better supported thanks to 'map, 'any, etc.


1 point by evanrmurphy 3456 days ago | link

Yes, that makes sense. I may have gone somewhat overboard / overly dramatic in this subthread. :) I think I mostly just want alists to be more convenient. Need to think about this more...


1 point by rocketnia 3456 days ago | link

I've been overly dramatic here too. I mostly wanted to help you make sure you were on a path that held water while giving you some hooks to convince me by... but I brought some external pet peeves into the mix and got worked up. XP Please do continue with your train of thought. ^^ Here's hoping the train mixes underwater hooks, or something.


1 point by evanrmurphy 3456 days ago | link

Awesome, thank you. :)