Re: Surprising behavior of eq?

[John Cowan <cowan@ccil.org>]

What's happening is that you are trying to compare strings with an
inappropriate predicate.  Eqv? tests for *identity* (being the same
object), whereas equal? tests for *equality* (havng the same structure).
Therefore, (equal? "foo" "foo") will always return #t, and almost all the
time that's the predicate you should use.

The question then is: when are two strings identical and not merely equal?
In the general case, they are identical if they are the result of a single
constructor.  Thus given:

(define a (string #\a #\b #\c))
(define b a)

then (eqv? a b) => #t

But given

(define c (string #\a #\b #c))
(define d (string #\a #\b #c))

then (eqv? c  d) => #f

Finally, we have to deal with string literals.  If there is more than one
appearance of "xyz" in code, they are all equal, but are they identical?
The answer is that it is implementation-dependent.  A Scheme is free to
consolidate all equal literals into the same object.  It looks like Guile
does so when they both appear in the same top-level expression, but *not*
in different top-level expressions.  So given

(define e "xyz")
(define f "xyz")

then (eqv? e f) => #f

But (let ((g "xyz") (h "xyz"))  (eqv? g h)) => #t, because Guile
consolidates the two equal literals into a single object.  If you try this
on Chibi Scheme, you'll get #f, because Chibi does *not* consolidate string
literals.

I hope that is helpful.




On Sun, Sep 20, 2020 at 9:11 AM Zelphir Kaltstahl <
zelphirkaltstahl@posteo.de> wrote:

> And I've noticed something more about equality stuff in the context of
> tests:
>
> ~~~~
> (eqv? "a" "a")
> $3 = #t
>
> ;; but
>
> (define char->string
>   (λ (c)
>     (list->string
>      (list c))))
>
> (import
>   ;; unit tests
>   (srfi srfi-64))
>
> (test-begin "string-utils-test")
>
> (test-group
>  "char-to-string-test"
>
>  (test-eqv "char->string converts a character to a string"
>    "a"
>    (char->string #\a)))
>
> (test-end "string-utils-test")
>
> %%%% Starting test string-utils-test  (Writing full log to
> "string-utils-test.log")
> $2 = ("string-utils-test")
> :19: FAIL char->string converts a character to a string
> # of unexpected failures  1
> ~~~~
>
> So while (eqv? ...) gives the correct (?) result, the test procedure
> (test-eqv ...) which seems to indicate using (eqv? ...) via its name
> does not think of the two strings as equivalent.
>
>
> On 20.09.20 14:19, Zelphir Kaltstahl wrote:
> > Sorry, I misclicked "send" when I wanted to further edit my e-mail ...
> >
> > My Guile version is:
> >
> > ~~~~
> > (version)
> > $6 = "3.0.4"
> > ~~~~
> >
> > On 20.09.20 14:16, Zelphir Kaltstahl wrote:
> >> Hello Guile users,
> >>
> >> I just noticed something weird about eq?.
> >>
> >> My Guile version is:
> >>
> >>
> >> I get the different results, depending on whether I define some
> >> bindings in a let or using define:
> >>
> >> (In Emacs Geiser:)
> >>
> >> ~~~~
> >> (define x '(10 9))
> >> (define y '(10 9))
> >> (eq? x y)
> >> $2 = #f
> >>
> >> (let ([x '(10 9)]
> >>       [y '(10 9)])
> >>      (eq? x y))
> >> $3 = #t
> >> ~~~~
> >>
> >> Is this intentional or a bug?
> >>
> >> I first noticed something strange when writing the following code:
> >>
> >> ~~~~DEFINITION~~~~
> >> (define make-multiple-list-remover
> >>   (λ (equal-proc)
> >>     (λ (lst unwanted)
> >>       (let loop ([remaining-list lst])
> >>         (cond
> >>          [(null? remaining-list)
> >>           '()]
> >>          [(equal-proc (car remaining-list) unwanted)
> >>           (loop (cdr remaining-list))]
> >>          [else
> >>           (cons (car remaining-list)
> >>                 (loop (cdr remaining-list)))])))))
> >> ~~~~
> >>
> >> ~~~~TEST~~~~
> >> (let ([a '(9 10)]
> >>       [b '(9 10)])
> >>   (test-equal "make-multiple-list-remover-03"
> >>     `(1 2 (3) (4) ,a)
> >>     ((make-multiple-list-remover eq?)
> >>      `(a b (c) (d) ,a) b)))
> >> ~~~~
> >>
> >> I was wondering, why the test fails. I think (eq? ...) should not be
> >> able to see the equivalence of both lists a and b, just like when
> >> defined using (define ...).
> >>
> >> I can also run it in the REPL and see the difference:
> >>
> >> ~~~~
> >> (define a '(9 10))
> >> (define b '(9 10))
> >> ((make-multiple-list-remover eq?)
> >>  `(a b (c) (d) ,a) b)
> >> $4 = (a b (c) (d) (9 10))
> >>
> >> (let ([a '(9 10)]
> >>       [b '(9 10)])
> >>   ((make-multiple-list-remover eq?)
> >>    `(a b (c) (d) ,a) b))
> >> $5 = (a b (c) (d))
> >> ~~~~
> >>
> >> Somehow the bindings of let seem to be different from the bindings
> >> created using define. What about using define inside let?
> >>
> >> ~~~~
> >>
> >> ~~~~
> >> --
> >> repositories: https://notabug.org/ZelphirKaltstahl
> > Somehow the bindings of let seem to be different from the bindings
> > created using define. What about using define inside let?
> >
> > ~~~~
> > (let ([unrelated 'bla])
> >   (define a '(9 10))
> >   (define b '(9 10))
> >   ((make-multiple-list-remover eq?)
> >    `(a b (c) (d) ,a) b))
> > $7 = (a b (c) (d))
> > ~~~~
> >
> > So there the define usage also differs from when I use define on the top
> > level. Perhaps that is the difference? On which level the bindings are
> > defined?
> >
> > Regards,
> > Zelphir
> >
> --
> repositories: https://notabug.org/ZelphirKaltstahl
>
>