CLHS: Function GET-SETF-EXPANSION

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Function GET-SETF-EXPANSION

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)))

Affected By: None.

Exceptional Situations: None.

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`)`.

The following X3J13 cleanup issues, not part of the specification, apply to this section:

- GET-SETF-METHOD-ENVIRONMENT:ADD-ARG
- SETF-METHOD-VS-SETF-METHOD:RENAME-OLD-TERMS

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