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| ANSI Common Lisp 5 Data and Control Flow 5.3 Dictionary of Data and Control Flow
5.3.63 get-setf-expansion | Function
  Syntax: get-setf-expansion place `&optional` environment vars, vals, store-vars, writer-form, reader-form Arguments and Values: place - a place. environment - an environment object. vars, vals, store-vars, writer-form, reader-form - a setf expansion. Description: Determines five values constituting the setf expansion for place in environment; see Section 5.1.1.2 Setf Expansions. If environment is not supplied or nil, the environment is the null lexical environment. Examples:
       (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)))
See Also: defsetf, define-setf-expander, setf Notes: 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)`. Allegro CL Implementation Details: None.

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