chickadee » protobj



Prototype-Delegation Object Model in Scheme


Neil van Dyke


To use the definitions described in this document import its bindings by evaluating the (import protobj) form in the current scope.

Protobj is a Scheme library that implements a simple prototype-delegation object model, somewhat similar to that of [Self], and also related to [SLIB-Object] and [OScheme]. Protobj was written mainly as a syntax-rules learning exercise, but also because people ask about prototype object models for Scheme from time to time. Like most object systems, it should be regarded as an amusement. The Protobj library defines both a verbose set of procedures, and terse special syntax.

Protobj is based on objects with named slots that can contain arbitrary values. Object have immediate slots, and single parent objects from which additional slots are inherited. When setting in a child object a slot inherited from the parent, a new immediate slot is created in the child so that the parent is unaffected and the slot is no longer inherited.

Methods are simply closures stored in slots. When a method is applied, the first term of the closure is the receiver object. Unlike Self, getting the contents of the slot is distinguished from invoking a method contained in the slot. This distinction was made due to the way first-class closures are often used in Scheme.

An object is cloned by invoking the clone method. The default root object's clone method creates a new child object without any immediate slots, rather than copying any slots. This behavior can be overridden to always copy certain slots, to copy immediate slots, or to copy all inherited slots. An overriding clone method can be implemented to apply its parent's clone method to itself and then set certain slots in the new child appropriately.

The following is a quick tour of Protobj using the terse special syntax.

Bind a to the new object that is created by cloning the default root object (% is special syntax for invoking the clone method):

 (define a (%))

Verify that a is an object and that a's parent is the default root object:

 (object? a) => #t
 (eq? (^ a) (current-root-object)) => #t

Add to a a slot named x with value 1:

 (! a x 1)

Get a's slot x's value:

 (? a x) => 1

Bind b to a clone of a:

 (define b (% a))

Get b's slot x's value, which is inherited from a:

 (? b x) => 1

Set a's slot x's value to 42, and observe that b inherits the new value:

 (! a x 42)
 (? a x) => 42
 (? b x) => 42

Set b's slot x's value to 69, and observe that a retains its own x value although b's x value has been changed:

 (! b x 69)
 (? a x) => 42
 (? b x) => 69

Add to a an xplus slot containing a closure that implements a method of the object:

 (! a xplus (lambda (self n) (+ (? self x) n)))

Apply the method to the a and b objects (b inherits any new slots added to a):

 (@ a xplus 7) => 49
 (@ b xplus 7) => 76

Observe the shorthand syntax for applying methods to an object multiple times, with the syntax having the value of the lastmost application:

 (@ a (xplus 1000) (xplus 7)) => 49

Bind to c an object that clones a and adds slot y with value 101:

 (define c (% a (y 101)))

Get the values of both the x and y slots of c:

 (? c x y) => 42 101

Finally, bind d to a clone of a that overrides a's x slot:

 (define d (% a (x 1) (y 2) (z 3)))
 (? d x y z) => 1 2 3

The basic interface of Protobj is a set of procedures.

object? xprocedure

Predicate for whether or not x is a Protobj object.

object-parent objprocedure

Yields the parent object of object obj.

object-set! obj slot-symbol valprocedure

Sets the slot identified by symbol slot-symbol in object obj to value val.

object-get obj slot-symbolprocedure

Yields the value of slot named by symbol slot-symbol in object obj (immediate or inherited). If no slot of that name exists, an error is signaled.

object-get obj slot-symbol noslot-thunkprocedure

Yields the value of slot named by symbol slot-symbol in object obj (immediate or inherited), if any such slot exists. If no slot of that name exists, then yields the value of applying closure noslot-thunk.

(object-apply obj slot-symbol { arg }*)procedure

Applies the method (closure) in the slot named by slot-symbol of object obj. The first term of the method is obj, and one or more arg are the remaining terms. If no such slot exists, an error is signaled.

(object-apply/noslot-thunk obj noslot-thunk slot-symbol { arg }*)procedure

Like object-apply, except that, if the slot does not exist, instead of signalling an error, the value is the result of applying noslot-thunk.

object-raw-clone/no-slots-copy objprocedure
object-raw-clone/copy-immed-slots objprocedure
object-raw-clone/copy-all-slots objprocedure

These procedures implement different ways of cloning an object, and are generally bound as clone methods in root objects. /no-slots-copy does not copy any slots, /copy-immed-slots copes immediate slots, and /copy-all-slots copies all slots including inherited ones.


Parameter for the default root object. The initial value is a root object that has object-raw-clone/no-slots-copy in its clone slot.

Since Protobj's raison d'etre was to play with syntax, here it is. Note that slot names are never quoted.

(^ obj)syntax

Parent of obj.

(! obj slot val)syntax
(! obj)syntax

Sets object obj's slot slot's value to val. In the second form of this syntax, multiple slots of obj may be set at once, and are set in the order given.

(? obj { slot }+)syntax

Yields the values of the given slots of obj. If more than one slot is given, a multiple-value return is used.

(@ obj slot { arg }*)syntax
(@ obj { (slot { arg }* ) }+)syntax

Applies obj's slot method, with obj as the first term and args as the remaining terms. In the second form of this syntax, multiple methods may be applied, and the value is the value of the last method application.

(% [ obj { (slot val) }* ])syntax

Clones object obj, binding any given slots to respective given vals.

You can override the method print to customize printing of objects:

(define x (%))
(! x print
     (lambda (self #!optional (port (current-output-port)))
       (fprintf port "#<my object>")))


Free Software Foundation, "GNU Lesser General Public License," Version

2.1, February 1999, 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.

Anselm Baird-Smith, "OScheme."
David Ungar and Randall B. Smith, "Self: The Power of Simplicity," Lisp and Symbolic Computation, 4, 3, 1991.

Wade Humeniuk, "Macroless Object System," SLIB object.

Richard Kelsey, "Defining Record Types," SRFI 9, 9 September 1999.

Stephan Houben, "Error reporting mechanism," SRFI 23, 26 April 2001.

Marc Feeley, "Parameter objects," SRFI 39, 30 June 2003.

Neil W. Van Dyke, "Testeez: Simple Test Mechanism for Scheme," Version 0.1.



 Copyright (c) 2005 Neil W. Van Dyke.  This program is Free
 Software; you can redistribute it and/or modify it under the terms of the
 GNU Lesser General Public License as published by the Free Software
 Foundation; either version 2.1 of the License, or (at your option) any
 later version.  This program is distributed in the hope that it will be
 useful, but without any warranty; without even the implied warranty of
 merchantability or fitness for a particular purpose.  See the GNU Lesser
 General Public License [LGPL] for details.  For other license options and
 commercial consulting, contact the author. 

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