Re: Surprising behavior of eq?

[John Cowan]

On Sun, Sep 20, 2020 at 11:37 AM Zelphir Kaltstahl <
zelphirkaltstahl@posteo.de> wrote:

> "This is where the eqv? predicate comes into picture. The eqv? is exactly
> the same as the eq? predicate, except that it will always return #t for
> same primitive values."
>
> Of course SO is not a standard. Either it is simply wrong, or I
> misunderstood "primitive values" in that phrase. I thought: "Ah strings are
> a primitive value, so eqv? should work in all cases when comparing
> strings." However, this has been debunked.
>
Strings tend to be treated as primitive values, but technically they are
compound values, just as much as vectors are.  And like vectors, you can
change the characters of a string without changing its identity.

> The only thing I do not quite understand yet is: What is the difference
> between (eqv? ...) and (eq? ...) then?
>
Efficiency only.

> If (eqv? ...) is only #t if things get consolidated in the same store
> place, would that not be the same as pointer equivalence?
>
Eq? is allowed to return #f on identical-looking characters and numbers
because they may or may not involve allocation.  Characters and fixnums
(integers up to about 2^60) are generally not allocated and eq? will return
#t on them, but larger integers and other types almost certainly will not:

(let ((i (* 48923498234892340 78902372789023)) (j (* 48923498234892340
78902372789023))) (eq? i j)) => #f

That's because the values of both i and j have to be allocated, and
therefore aren't represented by the same pointers.  But unlike strings, you
can't mutate the digits of an integer, so providing eqv? gives us an
abstract notion of identity.  The Scheme standards simplify the situation
by saying that eq? may return #f even when eqv? returns #t in the cases of
numbers and characters.

Because of these strange effects, I recommend (and this is just me) not
using eq? unless you can prove that you need the additional effi.

Note that (eq? 5 5.0) and (eqv? 5 5.0) are both #f, so if you want to
compare for numeric equality you should use (= 5 5.0) => #t.  The reason
they are not identical is that they behave differently:  (/ 5.0 2) => 2.5,
whereas (/ 5 2) => 5/2.

Here's an example that shows us that Guile does not consolidate number
literals, although it could:

(eqv? 3860180095872584138419938783820 3860180095872584138419938783820) =>#f

(In R7RS, procedures that are eqv? aren't necessarily eq? either, and in
R6RS even procedures that seem totally identical may be neither eq? nor
eqv?.  That's another story.)