The problem here is that the example of empty-vector equivalence isn't clear about why the equivalence is unspecified. Is it because they are literals? Or because they're empty? I'm pretty sure the intention is the former, but there's still an ambivalence.
Everything else is consistent in saying that empty vectors are not eqv? unless they happen to have the same memory address. Which is not the case here: we're deliberately equating empty vectors because they are functionally equivalent, and apparently this is not permitted by R7RS (modulo the ambivalence). I think that the interpretation of empty vectors as eqv? is a valid point of view, despite the standards. That said, our implementation should adhere to the standards. On Sun, Nov 6, 2016 at 5:11 AM, Alex Shinn <alexsh...@gmail.com> wrote: > On Sat, Nov 5, 2016 at 6:29 PM, Chris Hanson <c...@chris-hanson.org> wrote: > >> I can understand why it acts that way, since two empty vectors are >> equivalent for all intents and purposes. >> >> Either way, eqv? and eqv-hash must agree, so one of them has to be >> changed. >> >> The eqv? of empty vectors seems to be false according to R7RS. Except >> that the section on eqv? is a little ambiguous around exactly this point, >> so maybe not. >> > > The spec of eqv? is identical to the R5RS, except for the > equivalence of numbers which is based on the R6RS > (but fixing the broken wording of R6RS), to be a sort of > "operational equivalence" which distinguishes, e.g. +0.0 > and -0.0. > > For empty vectors, all three specs give the example: > > (eqv? '#() '#()) => <unspecified> > > The specs all give special license for constants to make > it clear this is unspecified. The definition of `vector' and > `make-vector' otherwise say they return "newly allocated" > vectors, suggesting false would be required, but the > discussion of storage model in section 3.4 of the R7RS > goes further to allow optimizing this case: > > [...] It is also understood that empty strings, empty vectors, > and empty bytevectors, which contain no locations, > may or may not be newly allocated. > > -- > Alex > > On Fri, Nov 4, 2016 at 5:35 PM, Taylor R Campbell <campb...@mumble.net> >> wrote: >> >>> (let ((u (vector)) >>> (v (vector))) >>> (list (list 'eq (eq? u v) (eq-hash u) (eq-hash v) >>> (= (eq-hash u) (eq-hash v))) >>> (list 'eqv (eqv? u v) (eqv-hash u) (eqv-hash v) >>> (= (eqv-hash u) (eqv-hash v))))) >>> ;Value 15: ((eq #f 107689176 107689168 #f) (eqv #t 107689176 107689168 >>> #f)) >>> >>> Oops. >>> >>> Why do we treat empty vectors as eqv? If we do, eqv-hash needs to be >>> made to agree. >>> >>> _______________________________________________ >>> MIT-Scheme-devel mailing list >>> MIT-Scheme-devel@gnu.org >>> https://lists.gnu.org/mailman/listinfo/mit-scheme-devel >>> >> >> >> _______________________________________________ >> MIT-Scheme-devel mailing list >> MIT-Scheme-devel@gnu.org >> https://lists.gnu.org/mailman/listinfo/mit-scheme-devel >> >> >
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