LispKit Object


Library (lispkit object) implements a simple, delegation-based object system for LispKit. It provides procedural and declarative interfaces for objects and classes. The class system is optional. It mostly provides means to define and manage new object types and construct objects using object constructors.


Introduction


Similar to other Scheme and Lisp-based object systems, methods of objects are defined in terms of object/class-specific specializations of generic procedures. A generic procedure consists of methods for the various objects/classes it supports. A generic procedure performs a dynamic dispatch on the first parameter (the self parameter) to determine the applicable method.

Generic procedures


Generic procedures can be defined using the define-generic form. Here is an example which defines three generic methods, one with only a self parameter, and two with three parameters self, x and y. The last generic procedure definition includes a default method which is applicable to all objects for which there is no specific method. When a generic procedure without default is applied to an object that does not define its own method implementation, an error gets signaled.

(define-generic (point-coordinates self))
(define-generic (set-point-coordinates! self x y))
(define-generic (point-move! self x y)
  (let ((coord (point-coordinate self)))
    (set-point-coordinate! self (+ (car coord) x) (+ (cdr coord) y))))


Objects


An object encapsulates a list of methods each implementing a generic procedure. These methods are regular closures which can share mutable state. Objects do not have an explicit notion of a field or slot as in other Scheme or Lisp-based object systems. Fields/slots need to be implemented via generic procedures and method implementations sharing state. Here is an example explaining this approach:

(define (make-point x y)
  (object ()
    ((point-coordinates self) (cons x y))
    ((set-point-coordinates! self nx ny) (set! x nx) (set! y ny))
    ((object->string self) (string-append (object->string x) "/" (object->string y)))))


This is a function creating new point objects. The x and y parameters of the constructor function are used for representing the state of the point object. The created point objects implement three generic procedures: point-coordinates, set-point-coordinates, and object->string. The latter procedure is defined directly by the library and, in general, used for creating a string representation of any object. By implementing the object->string method, the behavior gets customized for the object.

The following lines of code illustrate how point objects can be used:

(define pt (make-point 25 37))
pt                                              => #object:#<box (...)>
(object->string pt)                             => "25/37"
(point-coordinates pt)                          => (25 . 37)
(set-point-coordinates! pt 5 6)
(object->string pt)                             => "5/6"
(point-coordinates pt)                          => (5 . 6)


Inheritance


The LispKit object system supports inheritance via delegation. The following code shows how colored points can be implemented by delegating all point functionality to the previous implementation and by simply adding only color-related logic.

(define-generic (point-color self) #f)
(define (make-colored-point x y color)
  (object ((super (make-point x y)))
    ((point-color self) color)
    ((object->string self)
       (string-append (object->string color) ":" (invoke (super object->string) self)))))


The object created in function make-colored-point inherits all methods from object super which gets set to a new point object. It adds a new method to generic procedure point-color and redefines the object->string method. The redefinition is implemented in terms of the inherited object->string method for points. The form invoke can be used to refer to overridden methods in delegatee objects. Thus, (invoke (super object->string) self) calls the object->string method of the super object but with the identity (self) of the colored point.

The following interaction illustrates the behavior:

(define cpt (make-colored-point 100 50 'red))
(point-color cpt)                               => red
(point-coordinates cpt)                         => (100 . 50)
(set-point-coordinates! cpt 101 51)
(object->string cpt)                            => "red:101/51"


Objects can delegate functionality to multiple delegatees. The order in which they are listed determines the methods which are being inherited in case there are conflicts, i.e. multiple delegatees implement a method for the same generic procedure.

Classes


Classes add syntactic sugar, simplying the creation and management of objects. They play the following role in the object-system of LispKit:

  1. A class defines a constructor for objects represented by this class.
  2. Each class defines an object type, which can be used to distinguish objects created by the same constructor and supporting the same methods.
  3. A class can inherit functionality from several other classes, making it easy to reuse functionality.
  4. Classes are first-class objects supporting a number of class-related procedures.

The following code defines a point class with similar functionality as above:

(define-class (point x y) ()
  (object ()
    ((point-coordinates self) (cons x y))
    ((set-point-coordinates! self nx ny) (set! x nx) (set! y ny))
    ((object->string self) (string-append (object->string x) "/" (object->string y)))))


Instances of this class are created by using the generic procedure make-instance which is implemented by all class objects:

(define pt2 (make-instance point 82 10))
pt2                                             => #point:#<box (...)>
(object->string pt2)                            => "82/10"


Each object created by a class implements a generic procedure object-class referring to the class of the object. Since classes are objects themselves we can obtain their name with generic procedure class-name:

(object-class pt2)                              => #class:#<box (...)>
(class-name (object-class pt2))                 => point
(instance-of? point pt2)                        => #t
(instance-of? point pt)                         => #f


Generic procedure instance-of? can be used to determine whether an object is a direct or indirect instance of a given class. The last two lines above show that pt2 is an instance of point, but pt is not, even though it is functionally equivalent.

The following definition re-implements the colored point example from above using a class:

(define-class (colored-point x y color) (point)
  (if (or (< x 0) (< y 0))
      (error "coordinates are negative: ($0; $1)" x y))
  (object ((super (make-instance point x y)))
    ((point-color self) color)
    ((object->string self)
       (string-append (object->string color) ":" (invoke (super object->string) self)))))


The following lines illustrate the behavior of colored-point objects vs point objects:

(point-color cpt2)                              => blue
(point-coordinates cpt2)                        => (128 . 256)
(set-point-coordinates! cpt2 64 32)
(object->string cpt2)                           => "blue:64/32"
(instance-of? point cpt2)                       => #t
(instance-of? colored-point cpt2)               => #t
(instance-of? colored-point cpt)                => #f
(class-name (object-class cpt2))                => colored-point

Procedural object interface


(object? expr)     [procedure]

(make-object)     [procedure]
(make-object delegate ...)

(method obj generic)     [procedure]

(object-methods obj)     [procedure]

(add-method! obj generic method)     [procedure]

(delete-method! obj generic)     [procedure]

(make-generic-procedure ...)     [procedure]


Declarative object interface


(object ...)     [syntax]

(define-generic ...)     [syntax]

(invoke ...)     [syntax]


Procedural class interface


(class? expr)     [procedure]

root     [object]

(make-class name superclasses constructor)     [procedure]

Instance methods


(object-class self)     [generic procedure]

(object-equal? self obj)     [generic procedure]

(object->string self)     [generic procedure]

Class methods


(class-name self)     [generic procedure]

(class-direct-superclasses self)     [generic procedure]

(subclass? self other)     [generic procedure]

(make-instance self . args)     [generic procedure]

(instance-of? self obj)     [generic procedure]


Declarative class interface


(define-class ...)     [syntax]