Examples of Setf Expansions (ANSI and GNU Common Lisp Document)

5.1.1.4 Examples of Setf Expansions

Examples of the contents of the constituents of setf expansions follow.

For a variable x:

  ()              ;list of temporary variables  
  ()              ;list of value forms          
  (g0001)         ;list of store variables      
  (setq x g0001)  ;storing form                 
  x               ;accessing form               

  Figure 5–3: Sample Setf Expansion of a Variable

For (car exp):

  (g0002)                             ;list of temporary variables  
  (exp)                               ;list of value forms          
  (g0003)                             ;list of store variables      
  (progn (rplaca g0002 g0003) g0003)  ;storing form                 
  (car g0002)                         ;accessing form               

           Figure 5–4: Sample Setf Expansion of a CAR Form

For (subseq seq s e):

  (g0004 g0005 g0006)         ;list of temporary variables  
  (seq s e)                   ;list of value forms          
  (g0007)                     ;list of store variables      
  (progn (replace g0004 g0007 :start1 g0005 :end1 g0006) g0007) 
                              ;storing form                 
  (subseq g0004 g0005 g0006)  ; accessing form              

     Figure 5–5: Sample Setf Expansion of a SUBSEQ Form

In some cases, if a subform of a place is itself a place, it is necessary to expand the subform in order to compute some of the values in the expansion of the outer place. For (ldb bs (car exp)):

  (g0001 g0002)            ;list of temporary variables  
  (bs exp)                 ;list of value forms          
  (g0003)                  ;list of store variables      
  (progn (rplaca g0002 (dpb g0003 g0001 (car g0002))) g0003) 
                           ;storing form                 
  (ldb g0001 (car g0002))  ; accessing form              

     Figure 5–6: Sample Setf Expansion of a LDB Form