You make some really good points, but I have to disagree. Because of Unicode, characters aren't numbers. They have different semantic properties. However, I think arc.arc has a good idea sitting on line 2521 (of the Anarki version): "; could a string be (#\a #\b . "") ?" That is a novel idea, and probably (if more things work with improper lists) a good one. The downside would be that (isa "abc" 'string) wouldn't be true, unless we make strings (annotate "abc" 'string), but then we lose the ability to treat them as lists. Maybe we should have a retype operator, so that (retype "abc" 'string) would return an object str for which (isa str 'string) would be true, but for which all list operations (car, cdr, map, etc.) would still work (so it would probably have to return true for (isa str 'cons), too).
Well, Arc lists are (e e . nil). Why do we have a different terminator for strings?
This is where things get hairy. In Scheme lists are (e e . ()), so I would say that having strings be (#\a #\b . "") would make sense there, but Arc specifically tries to pretend that () is nil. Why should "" be different from nil, when () is nil?
As for [isa _ 'string], I propose instead the following function:
(def astring (e)
((afn (e e2)
(if
(no e)
t
; check for circularity
(is e e2)
nil
(and (acons e) (isa (car e) 'character))
(self (cdr e) (cddr e2))
; else
nil))
e (cdr e)))
Edit:
Hmm. I suppose someone will complain that you might want to differentiate between the empty string and nil. To which I respond: How is nil different from the empty list? Arc attempts to unify them; why do we want to not unify the empty string with nil? If someone says http://example.com/foobar?x= , how different is that from http://example.com/foobar ? x is still empty/nil in both cases.
As another example: table values are deleted by simply assigning nil to the value field. And yet in practice it hasn't hurt any of my use of tables; has anyone here found a real use where having a present-but-value-is-nil key in a table?
