Function

Signature: delivery-shaker-weak-pointer pointing accessor &key setter remover dead-value pointed

Used to make a pointer from one object to another weak object during the shaking operation. The operations of delivery-shaker-weak-pointer are:

1. At the time it is called it computes the setter and remover if these are not given, and stores all its arguments for the shaker.
2. Before the shaker starts, the shaker finds the value of the pointed object (if this is not given) using the accessor , and stores weak pointers to the pointing object and the pointed object. It then uses the remover to remove the pointer from the pointing object.
3. After the main shaking operation, for each pair of pointing/pointed objects it checks if both have survived the shaking. If they did, it stores a pointer to the pointed object in pointing using the setter .

Arguments:

pointing

The pointing object. Because of the way delivery-shaker-weak-pointer is defined, you are free to use your own notion of pointing, for example, it may be the key in a hash-table .

accessor

The accessor that is called with the pointing object. It returns the pointed object. The accessor is used for two purposes:

1. getting the pointed object if it is not given.

2. computing the setter if it is not given.

If both :pointed and :setter are passed to delivery-shaker-weak-pointer , the accessor is not used. The accessor can be one of:

A symbol. This specifies a function that is called with the pointing object as its argument.

A list starting with a symbol. In this case the car of the list is called with the pointing object as its first argument, and the cdr forming the rest of the arguments, that is:

(apply (car accessor ) pointing (cdr accessor ))

For example, if the accessor is (slot-value name) , the call is (slot-value pointing name) , and

(aref 1 2) => (aref pointing 1 2).

setter

If the setter is not given, it is computed by the system using the accessor and the same expansion that setf would use. If it is given, it has the same properties as the accessor , except that in the call the pointed object is inserted before all the arguments. That is, if the setter is (set-something name ), the call is (set-something pointed pointing name ) . In addition, where the accessor accepts a symbol, the setter also accepts a function object.

remover

Default value t , which means use the setter . This is used to remove the pointer from the pointing object. It is called exactly like the setter , except that the first argument is dead-value , rather than pointed.

pointed

This gives the value of the pointed object. If it is not given, the accessor is used to get the pointed object.

dead-value

Default value nil . This the value that is stored by the remover in the pointing value before starting the shaking. Note that if the pointed object is shaken, the pointing object is left with the dead-value .

Note that between the calls to the remover and the setter (steps 2 and 3 above), the pointing object points to the wrong thing (the dead-value ). This may cause problems if the object is used by the system during the shaking (this does not happen unless you access objects which you should not access), or if you define more than one delivery-shaker-weak-pointer on the same object, and one of these uses a slot that has been defined by the other. Thus you have to make sure that you do not cause this situation.

Example 1 :

Suppose the keys of *my-hash-table* are conses of an object and a number, and it is desired to remove from *my-hash-table* those entries where the car is not pointed to from anywhere else. This can be done by something like this :

;;;;--------------------------------------------------------

;; This will eliminate all the entries where the car is nil

(defun clean-my-hash-table (table)

  (maphash (lambda (x y) 

             (declare (ignore y))

             (unless (car x) (remhash x table)))

           table))

;; this will cause the car of any entry where the car is not

;; pointed to from another object to change to nil

(defun shake-my-hash-table ()

  (maphash #'(lambda (x y) (declare (ignore y))

               (delivery-shaker-weak-pointer x 'car))

           *my-hash-table*)

;;this will cause clean-my-hash-table to be called later

;; in the shaking, provided *my-hash-table* is still alive.

(delivery-shaker-cleanup *my-hash-table* 'clean-my-hash-table))

;; call this function at delivery time

(define-action "Delivery Actions" "shake my hash table"

             'shake-my-hash-table) 

;;;;----------------------------------------------------------

If the car can be nil , the code above removes some entries it should not. In this case the appropriate lines should be changed to:

(delivery-shaker-weak-pointer x 'car :dead-value 'my-dead-value))

and

(when (eq (car x) 'my-dead-value) (remhash x table))

[ This assumes there are no entries where the car is my-dead-value .]

Note that the cleanup function is not going to be called unless the hash table actually survives the shaking operation.

Example 2:

The value of *aaa* is a list of objects of type a-struct , which has a slot called name , which points to a symbol. We want to get rid of any of these structures if the symbol is not pointed to by some other object.

Implementation A:

Make the pointers from the structures to the names be weak, and have the cleanup function throw away any structure where the name becomes nil .

(defun clean-*aaa* ()

  (loop for a on *aaa*)

(delivery-shaker-weak-pointer a 'a-struct-name))

  (delivery-shaker-cleanup 

   '*aaa* 

   #'(lambda (symbol) 

       (set symbol

             (remove-if-not 'a-struct-name 

               (symbol-value symbol ) )))))

(define-action "Delivery Actions" "Clean aaa" 'clean-*aaa*)

Implementation B:

Make a pointer from the symbol to the structure, and make *aaa* point weakly to the names, and set *aaa* to nil . The remover and accessor do nothing, and the setter is defined to restore *aaa* . This implementation does not use the cleanup function.

(defun clean-*aaa* ()

  (let ((setter #'(lambda  (name symbol)

                    (set symbol (nconc 

                                (symbol-value symbol) 

                                  (list(get name 'a-struct))) )

                                (remprop name 'a-struct)))) 

    (dolist (x *aaa* () 

      (let  ((name  (a-struct-name x)))

        (setf (get name 'a-struct)  x)

        (delivery-shaker-weak-pointer '*aaa* nil

                                      :remover nil 

                                      :pointed name

                                      :setter setter)))

    (setq *aaa* nil)))

(define-action "Delivery actions" "Clean aaa" 'clean-*aaa*)