get-setf-expansion

ToC

Allegro CL

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.