But now that I look closer at the ac.scm history (now ac.rkt in Anarki), I realize I was mistaken to believe Arc treated #f as a different value than nil. Turns out Arc has always equated #f and 'nil with `is`, counted them both as falsy, etc. So this library was already returning nil, from Arc's perspective.
There are some things that slip through the cracks. It looks like (type #f) has always given an "unknown type" error, as opposed to returning 'sym as it does for 'nil and '().
So with that in mind, I think it's a bug if an Arc JSON library returns 'nil or #f for JSON false, unless it returns something other than '() for JSON . To avoid collision, we could represent JSON arrays using `annotate` values rather than plain Arc lists, but I think representing JSON false and null using Arc symbols like 'false and 'null is easier.
That documentation may be wrong. On the other hand, it may be correct in the context of someone who is only using Arc, not Racket.
There are a lot of ways to conceive of what Arc "is" outside of the Racket implementations, but I think Arc implementations like Rainbow, Jarc, Arcueid, and so on tend to be inspired first by the rather small set of operations showcased in the tutorial and in arc.arc. (As the tutorial says, "The definitions in arc.arc are also an experiment in another way. They are the language spec.") Since #f isn't part of those, it's not something that an Arc implementation would necessarily focus on supporting, so there's a practical sense in which it's not a part of Arc our Arc code can rely on.
(Not that any other part of Arc is stable either.)
> Really, #t and #f are not proper Arc booleans, so it makes sense that Arc can't tell what type they are.
Really, #t and #f are not proper Arc anything, but the language apparently handles them so IMHO Arc should also be able to know what type they are. Otherwise, I fear, this will become a Hodge Podge language that will lose appeal.
Personally I don't care if Arc supports booleans. I only care that it can translate booleans (when need be) to a meaningful Arc semantic. That said, if we're going to support booleans then let's not create partial support.