I experimented a bit with weak hashtables and SLIME in in both Allegro and CMUCL. Here are a few things I found useful. They appear to work with both Allegro and CMUCL.

(in-package :swank-backend)

#+allegro (progn

;;; Crummy Allegro provides no way to ask if an object is a weak-vector.

(defstruct (weak-pointer (:constructor %make-weak-pointer)) pointer)

(defun make-weak-pointer (object) "Creates a new weak pointer which points to OBJECT. For portability reasons, OBJECT must not be NIL." (assert (not (null object))) (let ((wv (excl:weak-vector 1))) (setf (svref wv 0) object) (%make-weak-pointer :pointer wv)))

(defun weak-pointer-value (weak-pointer) "If WEAK-POINTER is valid, returns its value. Otherwise, returns NIL." (svref (weak-pointer-pointer weak-pointer) 0))

) ; end #+allegro progn

(definterface weak-value-gethash (ht key &optional default) "Access hash-table with weak value." (multiple-value-bind (val foundp) (gethash ht key) (if foundp (values (if (weak-pointer-p val) (weak-pointer-value val) val) t) default)))

(definterface (setf weak-value-gethash) (object ht key) "Access hash-table with weak value." (when object (setf (gethash ht key) (make-weak-pointer object))) object)

(in-package :swank)

(defun save-presented-object (object) "Save OBJECT and return the assigned id. If OBJECT was saved previously return the old id." (or (gethash object *object-to-presentation-id*) (let ((id (incf *presentation-counter*))) (setf (swank-backend::weak-value-gethash id *presentation-id-to-object*) object) (setf (gethash object *object-to-presentation-id*) id) id)))

(defun lookup-presented-object (id) "Retrieve the object corresponding to ID. The secondary value indicates the absence of an entry." (swank-backend::weak-value-gethash id *presentation-id-to-object*))

Helmut Eller replied:

The reason is probably that nobody bothered to write the code. I added the necessary bits now.

Helmut.