[cffi-devel] setf-expander of FOREIGN-SLOT-VALUE

[Marco Gidde]

The setf-expansion for nested FOREIGN-SLOT-VALUE calls ist not quite
correct:

CFFI> (macroexpand '(setf (foreign-slot-value
			   (foreign-slot-value ptr `struct1 'slot1)
			   `struct2 'slot2) val))
(LET* ()
  (MULTIPLE-VALUE-BIND
      (#:G1997)
      VAL
    (PROGN
     (FOREIGN-SLOT-SET #:G1997 (FOREIGN-SLOT-VALUE PTR) 'STRUCT2 'SLOT2)
     #:G1997)))
T
                               ^^^^^^^^^^^^^^^^^^^^^^^^


Defining the expander like

(define-setf-expander foreign-slot-value (ptr type slot-name &environment env)
  (multiple-value-bind (temps vals stores store-form access-form)
      (get-setf-expansion ptr env)
    (declare (ignore store-form stores))
    (let ((type-tmp (gensym "TYPE-"))
          (slot-name-tmp (gensym "SLOT-NAME-"))
          (store (gensym "STORE-")))
      (values
       `(,type-tmp ,slot-name-tmp , at temps)
       `(,type ,slot-name , at vals)
       `(,store)
       `(progn
         (foreign-slot-set ,store ,access-form ,type-tmp ,slot-name-tmp)
         ,store)
       `(foreign-slot-value ,access-form ,type-tmp ,slot-name-tmp)))))

should do the job a bit better, though it might be optimized in some
way (this is my first try to define a setf expander).

Btw. the manual contains an example use of FOREIGN-SLOT-VALUE with the
simplified syntax

(foreign-slot-value ptr 'struct 'slot-1 'slot-2)

I think CFFI has all the informations at hand to handle this, so will
it be implemented one day?


Regards,

Marco Gidde