Using Common Lisp CLOS objects as keys in a hashtable?

Question

I'd like to use Common Lisp CLOS objects as keys in a hashtable. I thought it would be as simple as this:

(defclass my-class () ((a :accessor a :initarg a)))

(defun my-class= (my-instance-1 my-instance-2)
(equal (a my-instance-1) (a my-instance-2)))

(defparameter my-hash-table (make-hash-table :test #'my-class=))

Checking out the Common Lisp Hyperspec, it seems I can only use eq, eql, equal, or equalp to test equality.

Is there any way I can do this? Or is this just a really stoopid thing to do, and that's why the standard doesn't allow it?

Answer 1

Common Lisp standard does not provide any mechanism to provide additional test functions for hash-tables (beyond standard ones). You have 2 options:

1. Use genhash genhash which is portable hash-table implementation (not compatible with built-in ones)
2. Use non-standard extensions:
3. SBCL has sb-ext:define-hash-table-test function (documentation)
4. Clisp has a similar function ext:define-hash-table-test (documentation)
5. Allegro and Lispworks accept non-standard values for :test argument and has :hash-function argument (Allegro, Lispworks).