[racket] clarification for beginners please

From: Danny Yoo (dyoo at hashcollision.org)
Date: Thu Apr 25 14:59:04 EDT 2013

    Probably a crab would be filled with a sense of personal outrage
    if it could hear us class it without ado or apology as a crustacean,
    and thus dispose of it.  "I am no such thing, it would say: I am
    MYSELF, MYSELF alone."

                  - William James, The Varieties of Religious Experience

eq? is a function for letting us distinguish different crabs that would
otherwise look the same to us.

Let's say that we have two crabs c1 and c2.

;; In BSL:
(define-struct crab (name))

(define c1 (make-crab "joe"))
(define c2 (make-crab "joe"))

As it stands, we may want to say they are the same because they "look" the
same --- they have the same structural shape.  equal? tells us that they
look the same.

> (equal? c1 c2)

But as William James notes, these crabs might be outraged at our lack of
crustacean discrimination.  They may argue: we may look alike, but we are
different crabs.  How shall we judge?

> (eq? c1 c2)

A crab is itself, itself alone:

> (eq? c1 c1)
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.racket-lang.org/users/archive/attachments/20130425/0f123c0d/attachment-0001.html>

Posted on the users mailing list.