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7.1.13 Macroless Object System

(require 'object) This is the Macroless Object System written by Wade Humeniuk (whumeniu@datap.ca). Conceptual Tributes: Yasos, MacScheme's %object, CLOS, Lack of R4RS macros.

7.1.14 Concepts

OBJECT
An object is an ordered association-list (by eq?) of methods (procedures). Methods can be added (make-method!), deleted (unmake-method!) and retrieved (get-method). Objects may inherit methods from other objects. The object binds to the environment it was created in, allowing closures to be used to hide private procedures and data.
GENERIC-METHOD
A generic-method associates (in terms of eq?) object's method. This allows scheme function style to be used for objects. The calling scheme for using a generic method is (generic-method object param1 param2 ...).
METHOD
A method is a procedure that exists in the object. To use a method get-method must be called to look-up the method. Generic methods implement the get-method functionality. Methods may be added to an object associated with any scheme obj in terms of eq?
GENERIC-PREDICATE
A generic method that returns a boolean value for any scheme obj.
PREDICATE
A object's method asscociated with a generic-predicate. Returns #t.

7.1.15 Procedures

— Function: make-object ancestor ...

Returns an object. Current object implementation is a tagged vector. ancestors are optional and must be objects in terms of object?. ancestors methods are included in the object. Multiple ancestors might associate the same generic-method with a method. In this case the method of the ancestor first appearing in the list is the one returned by get-method.

— Function: object? obj

Returns boolean value whether obj was created by make-object.

— Function: make-generic-method exception-procedure

Returns a procedure which be associated with an object's methods. If exception-procedure is specified then it is used to process non-objects.

— Function: make-generic-predicate

Returns a boolean procedure for any scheme object.

— Function: make-method! object generic-method method

Associates method to the generic-method in the object. The method overrides any previous association with the generic-method within the object. Using unmake-method! will restore the object's previous association with the generic-method. method must be a procedure.

— Function: make-predicate! object generic-preciate

Makes a predicate method associated with the generic-predicate.

— Function: unmake-method! object generic-method

Removes an object's association with a generic-method .

— Function: get-method object generic-method

Returns the object's method associated (if any) with the generic-method. If no associated method exists an error is flagged.

7.1.16 Examples

     (require 'object)
     
     (define instantiate (make-generic-method))
     
     (define (make-instance-object . ancestors)
       (define self (apply make-object
                           (map (lambda (obj) (instantiate obj)) ancestors)))
       (make-method! self instantiate (lambda (self) self))
       self)
     
     (define who (make-generic-method))
     (define imigrate! (make-generic-method))
     (define emigrate! (make-generic-method))
     (define describe (make-generic-method))
     (define name (make-generic-method))
     (define address (make-generic-method))
     (define members (make-generic-method))
     
     (define society
       (let ()
         (define self (make-instance-object))
         (define population '())
         (make-method! self imigrate!
                       (lambda (new-person)
                         (if (not (eq? new-person self))
                             (set! population (cons new-person population)))))
         (make-method! self emigrate!
                       (lambda (person)
                         (if (not (eq? person self))
                             (set! population
                                   (comlist:remove-if (lambda (member)
                                                        (eq? member person))
                                                      population)))))
         (make-method! self describe
                       (lambda (self)
                         (map (lambda (person) (describe person)) population)))
         (make-method! self who
                       (lambda (self) (map (lambda (person) (name person))
                                           population)))
         (make-method! self members (lambda (self) population))
         self))
     
     (define (make-person %name %address)
       (define self (make-instance-object society))
       (make-method! self name (lambda (self) %name))
       (make-method! self address (lambda (self) %address))
       (make-method! self who (lambda (self) (name self)))
       (make-method! self instantiate
                     (lambda (self)
                       (make-person (string-append (name self) "-son-of")
                                    %address)))
       (make-method! self describe
                     (lambda (self) (list (name self) (address self))))
       (imigrate! self)
       self)
7.1.16.1 Inverter Documentation

Inheritance:

             <inverter>::(<number> <description>)

Generic-methods

             <inverter>::value      ⇒ <number>::value
             <inverter>::set-value! ⇒ <number>::set-value!
             <inverter>::describe   ⇒ <description>::describe
             <inverter>::help
             <inverter>::invert
             <inverter>::inverter?
7.1.16.2 Number Documention

Inheritance

             <number>::()

Slots

             <number>::<x>

Generic Methods

             <number>::value
             <number>::set-value!
7.1.16.3 Inverter code
     (require 'object)
     
     (define value (make-generic-method (lambda (val) val)))
     (define set-value! (make-generic-method))
     (define invert (make-generic-method
                     (lambda (val)
                       (if (number? val)
                           (/ 1 val)
                           (error "Method not supported:" val)))))
     (define noop (make-generic-method))
     (define inverter? (make-generic-predicate))
     (define describe (make-generic-method))
     (define help (make-generic-method))
     
     (define (make-number x)
       (define self (make-object))
       (make-method! self value (lambda (this) x))
       (make-method! self set-value!
                     (lambda (this new-value) (set! x new-value)))
       self)
     
     (define (make-description str)
       (define self (make-object))
       (make-method! self describe (lambda (this) str))
       (make-method! self help (lambda (this) "Help not available"))
       self)
     
     (define (make-inverter)
       (let* ((self (make-object
                     (make-number 1)
                     (make-description "A number which can be inverted")))
              (<value> (get-method self value)))
         (make-method! self invert (lambda (self) (/ 1 (<value> self))))
         (make-predicate! self inverter?)
         (unmake-method! self help)
         (make-method! self help
                       (lambda (self)
                         (display "Inverter Methods:") (newline)
                         (display "  (value inverter) ==> n") (newline)))
         self))
     
     ;;;; Try it out
     
     (define invert! (make-generic-method))
     
     (define x (make-inverter))
     
     (make-method! x invert! (lambda (x) (set-value! x (/ 1 (value x)))))
     
     (value x)                       ⇒ 1
     (set-value! x 33)               ⇒ undefined
     (invert! x)                     ⇒ undefined
     (value x)                       ⇒ 1/33
     
     (unmake-method! x invert!)      ⇒ undefined
     
     (invert! x)                     error-->  ERROR: Method not supported: x