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?
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.
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.