Re: Are `eqv?' and `eq?' the same?

[Pascal J. Bourguignon]

Alexandru Cojocaru <address@hidden> writes:

> Hi,
>
> from the GUILE manual [0]:
>
>     `eq?' tests just for the same object (essentially a pointer
> comparison)
>     `eqv?' extends `eq?' to look at the value of numbers and
> characters.
>
> this is what I get:
>
>     scheme@(guile-user)> (eq? 3 (+ 1 2))
>     $1 = #t
>
> is this behavior intentional or some type of bug?

Yes.


A some implementations internalize small integers (and characters).

A lot of implementations even don't allocate small integers (fixnums) as
objects, but encode them instead as immediate values.


So:

         (let ((x 3)
               (y (+ 1 2)))
            (eq? x y))

could be represented in memory in different ways:

   +-------+
x: |  f:3  |
   +-------+
y: |  f:3  |
   +-------+



   +-------+
x: |  p0---|----------+
   +-------+          |
y: |  p0---|--------+ |
   +-------+        | |
                    | |
                    v v
               +----------+
               |    f:3   |
               +----------+

   +-------+
x: |  p1---|-----------------------------+
   +-------+                             |
y: |  p2---|--------+                    |
   +-------+        |                    |
                    |                    |
                    v                    v
               +----------+        +----------+ 
               |    f:3   |        |    f:3   |
               +----------+        +----------+

This later case would occur if the number was a bignum:
b:391278903129038129038129038901238901 for example.
(f: denotes the type tag for fixnums, b: the type tag for bignums).


Now, even if we're in the second situation, for example, we could get
it also with:  (let ((x 3) (y x)) (eq? x y)),  it is still possible that
in the process of calling a function such as eq? the argument be copied,
so that in 

   (define (eq? a b) …)

we could get a situation like:

   +-------+
a: |  p1---|-----------------------------+
   +-------+                             |
b: |  p2---|--------+                    |
   +-------+        |                    |
                    |                    |
                    v                    v
               +----------+        +----------+ 
               |    f:3   |        |    f:3   |
               +----------+        +----------+

even starting from the second or (less probable) first situation!


Now, if inside eq? we have the first or second situation, then the
result will be #t, and in the third situation, the result will be #f.


That's all you learn from eq?, which, for characters and numbers, is not
much interesting: it only tells you something about how numbers are
implemented and processed by the implementation.



-- 
__Pascal Bourguignon__
http://www.informatimago.com/