Generalized Variables

Generally, any values or attributes are represented in slots of objects (or in stack frames). To retrieve and alter the value of a slot, two primitive operations, access and update, must be provided. Instead of defining two distinct primitives for every slot of objects, EusLisp, like Common Lisp, provides uniform update operations based on the generalized variable concept. In this concept, a common form is recognized either as a value access form or as a slot location specifier. Thus, you only need to remember accessing form for each slot and update is achieved by setf macro used in conjunction with the access form. For example, (car x) can be used to replace the value in the car slot of x when used with setf as in (setf (car '(a b) 'c), as well as to take the car value out of the list.

This method is also applicable to all the user defined objects. When a class or a structure is defined, the access and update forms for each slot are automatically defined. Each of those forms is defined as a macro whose name is the concatenation of the class name and slot name. For example, car of a cons can be addressed by (cons-car '(a b c)).

(defclass person :super object :slots (name age))
(defclass programmer :super person :slots (language machine))
(setq x (instantiate programmer))
(setf (programmer-name x) "MATSUI"
      (person-age x) 30)
(incf (programmer-age x))
(programmer-age x)   --> 31
(setf (programmer-language x) 'EUSLISP
      (programmer-machine x) 'SUN4)

Array elements can be accessed in the same manner.

(setq a (make-array '(3 3) :element-type :float))
(setf (aref a 0 0) 1.0 (aref a 1 1) 1.0 (aref a 2 2) 1.0)
a --> #2f((1.0 0.0 0.0) (0.0 1.0 0.0) (0.0 0.0 1.0))

(setq b (instantiate bit-vector 10))  --> #*0000000000
(setf (bit b 5) 1)
b --> #*0000010000

In order to define special setf methods for particular objects, defsetf macro is provided.

(defsetf symbol-value set)
(defsetf get (sym prop) (val) `(putprop ,sym ,val ,prop))