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defstruct 154
The body forms can be written as if the variables in storing-function-or-args
were bound to subforms of the call on access-fn and the store-variables were
bound to the second subform of setf. However, this is not actually the
case. During the evaluation of the body forms, these variables are bound
to names of temporary variables, generated as if by gensym or gentemp,
that will be bound by the expansion of setf to the values of those subforms.
This binding permits the body forms to be writeen without regard for order
of evaluation. defsetf arranges for the temporary variables to be optimized
out of the final results in cases where that is possible. In other words, an
attempt is made by defsetf to generate the best code possible.
Note that the code generated by the body forms must include provision for
returning the correct value (the value of store-variables). This is handled
by the body forms rather than by defsetf because in many cases this value
can be returned at no extra cost, by calling a function that simultaneously
stores into the generalized variable and returns the correct value.
Here is an example of the complex form of defsetf.
,
I -
(defsetf subseq (sequence start &optional end) (new-sequence)
'(progn (replace ,sequence ,new-sequence
:start1 ,start :end1 ,end)
,new-sequence»
For even more complex operations on setf: See the macro
define-setf-method, page 141.
defstruct options &body items Macro
defstruct defines a record-structure data type. A call to defstruct looks
like:
(defstruct (name option-l option-2 ... )
slot-description-l
slot-description-2
... )
name must be a symbol; it is the name of the structure. It is given a
si:defstruct-description property that describes the attributes and elements
of the structure; this is intended to be used by programs that examine
other Lisp programs and that want to display the contents of structures in
a helpful way. name is used for other things; for more information: See
the section "Named Structures" in Symbolics Common Lisp: Language Concepts.
Because evaluation of a defstruct form causes many functions and macros
to be defined, you must take care not to define the same name with two