Version 0.2, 2005-06-19, http://www.neilvandyke.org/protobj/
by
Neil W. Van Dyke
<neil@neilvandyke.org>
Copyright © 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
<http://www.gnu.org/copyleft/lesser.html> for details. For
other license options and consulting, contact the author.
Protobj is a Scheme library that implements a simple prototype-delegation
object model, somewhat similar to that of
Self, and also
related to those of
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, Protobj 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 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.
Protobj requires R5RS, SRFI-9, SRFI-23, and SRFI-39.
The following is a quick tour of Protobj using the terse special syntax.
a to the new object that is created by cloning the default root
object (% is special syntax for invoking the clone method):
(define a (%))
a is an object and that a's parent is the default
root object:
(object? a) => #t
(eq? (^ a) (current-root-object)) => #t
a a slot named x with value 1:
(! a x 1)
a's slot x's value:
(? a x) => 1
b to a clone of a:
(define b (% a))
b's slot x's value, which is inherited from a:
(? b x) => 1
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
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
a an xplus slot containing a closure that implements a
method of the object:
(! a xplus (lambda ($ n) (+ (? $ x) n)))
a and b objects (b inherits
any new slots added to a):
(@ a xplus 7) => 49
(@ b xplus 7) => 76
(@ a (xplus 1000) (xplus 7)) => 49
101:
(define c (% a (y 101)))
x and y slots of c:
(? c x y) => 42 101
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.
Sets the slot identified by symbol slot-symbol in object obj to value
val.
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.
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.
Applies the method (closure) in the slot named byslot-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.
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.
These procedures implement different ways of cloning an object, and are generally bound as
clonemethods in root objects./no-slots-copydoes not copy any slots,/copy-immed-slotscopes immediate slots, and/copy-all-slotscopies 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-copyin itscloneslot.
Since Protobj's raison d'etre was to play with syntax, here it is. Note that slot names are never quoted.
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.
Yields the values of the given slots of obj. If more than one slot is given, a multiple-value return is used.
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.
Clones object obj, binding any given slots to respective given vals.
The Protobj test suite can be enabled by editing the source code file and loading Testeez.
%protobj:apply* (thanks to Benedikt Rosenau for
reporting). Changed $ syntax to ?, so that $ could be
used for “self” in methods. Documentation changes.