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get-setf-expansion place &optional environment
⇒ vars, vals, store-vars, writer-form, reader-form
place—a place.
environment—an environment object.
vars, vals, store-vars, writer-form, reader-form—a setf expansion.
Determines five values constituting the setf expansion for place in environment; see Setf Expansions.
If environment is not supplied or nil, the environment is the null lexical environment.
(get-setf-expansion 'x) ⇒ NIL, NIL, (#:G0001), (SETQ X #:G0001), X
;;; This macro is like POP
(defmacro xpop (place &environment env)
(multiple-value-bind (dummies vals new setter getter)
(get-setf-expansion place env)
`(let* (,@(mapcar #'list dummies vals) (,(car new) ,getter))
(if (cdr new) (error "Can't expand this."))
(prog1 (car ,(car new))
(setq ,(car new) (cdr ,(car new)))
,setter))))
(defsetf frob (x) (value)
`(setf (car ,x) ,value)) ⇒ FROB
;;; The following is an error; an error might be signaled at macro expansion time
(flet ((frob (x) (cdr x))) ;Invalid
(xpop (frob z)))
defsetf , define-setf-expander , setf
Any compound form is a valid place, since any compound form whose operator f has no setf expander are expanded into a call to (setf f).