Common Lisp the Language, 2nd Edition

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19.7.3. Other Aspects of Explicitly Specified Structures

The :initial-offset option allows one to specify that slots be allocated beginning at a representational element other than the first. For example, the form

(defstruct (binop (:type list) (:initial-offset 2)) 
  (operator '? :type symbol) 
  operand-1 
  operand-2)

would result in the following behavior for make-binop:

(make-binop :operator '+ :operand-1 'x :operand-2 5) 
   => (nil nil + x 5) 

(make-binop :operand-2 4 :operator '*) 
   => (nil nil * nil 4)

The selectors binop-operator, binop-operand-1, and binop-operand-2 would be essentially equivalent to caddr, cadddr, and car of cddddr, respectively. Similarly, the form

(defstruct (binop (:type list) :named (:initial-offset 2)) 
  (operator '? :type symbol) 
  operand-1 
  operand-2)

would result in the following behavior for make-binop:

(make-binop :operator '+ :operand-1 'x :operand-2 5) 
   => (nil nil binop + x 5) 

(make-binop :operand-2 4 :operator '*) 
   => (nil nil binop * nil 4)

If the :include is used with the :type option, then the effect is first to skip over as many representation elements as needed to represent the included structure, then to skip over any additional elements specified by the :initial-offset option, and then to begin allocation of elements from that point. For example:

(defstruct (binop (:type list) :named (:initial-offset 2)) 
  (operator '? :type symbol) 
  operand-1 
  operand-2) 

(defstruct (annotated-binop (:type list) 
                            (:initial-offset 3) 
                            (:include binop)) 
  commutative associative identity) 

(make-annotated-binop :operator '* 
                      :operand-1 'x 
                      :operand-2 5 
                      :commutative t 
                      :associative t 
                      :identity 1) 
   => (nil nil binop * x 5 nil nil nil t t 1)

The first two nil elements stem from the :initial-offset of 2 in the definition of binop. The next four elements contain the structure name and three slots for binop. The next three nil elements stem from the :initial-offset of 3 in the definition of annotated-binop. The last three list elements contain the additional slots for an annotated-binop.

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