Surprising behavior of eq?

[Zelphir Kaltstahl <zelphirkaltstahl@posteo.de>]

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?

~~~~

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-- 
repositories: https://notabug.org/ZelphirKaltstahl