Protobj was a Scheme exercise, and is packaged for those curious, not intended for practical use. Protobj was written primarily as a syntax-rules learning exercise, and secondarily because people asked about prototype object models for Scheme from time to time.

Protobj implements a simple prototype-delegation object model, somewhat similar to that of Self, and also related to those of SLIB object and OScheme. 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.

Protobj was originally writtin in R5RS, SRFI-9, SRFI-23, and SRFI-39.

2Tour🔗 i

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):
> (definea(%))
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:
> (!ax1)
Get a’s slot x’s value:
> (?ax)
1
Bind b to a clone of a:
> (defineb(%a))
Get b’s slot x’s value, which is inherited from a:
> (?bx)
1
Set a’s slot x’s value to 42, and observe that b inherits the new value:
> (!ax42)
> (?ax)
42
> (?bx)
42
Set b’s slot x’s value to 69, and observe that a retains its own x value although ’s x value has been changed:
> (!bx69)
> (?ax)
42
> (?bx)
69
Add to a an xplus slot containing a closure that implements a method of the object:
> (!axplus(lambda($n)(+(?$x)n)))
Apply the method to the a and b objects (b inherits any new slots added to a):
> (@axplus7)
49
> (@bxplus7)
76
Observe the shorthand syntax for applying methods to an object multiple times, with the syntax having the value of the lastmost application:
> (@a(xplus1000)(xplus7))
49
Bind to c an object that clones a and adds slot y with value 101:
> (definec(%a(y101)))
Get the values of both the x and y slots of c:
> (?cxy)
42101
Finally, bind d to a clone of a that overrides a’s x slot:
> (defined(%a(x1)(y2)(z3)))
> (?dxyz)
123

3Basic Interface🔗 i

The basic interface of Protobj is a set of procedures.

procedure
( object?x)→boolean?
x:any/c

Predicate for whether or not x is a Protobj object.

procedure
( object-parentobj)→(or/c#fobject?)
obj:object?

Yields the parent object of object obj.

procedure
( object-set!objslot-symbolval)→void?
obj:object?
slot-symbol:symbol?
val:any/c

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

procedure
( object-getobjslot-symbol)→object?
obj:object?
slot-symbol:symbol?

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.

procedure
( object-getobjslot-symbolnoslot-thunk)→any/c
obj:object?
slot-symbol:symbol?
noslot-thunk:(->any)

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.

procedure
( object-applyobjslot-symbolarg...)→any
obj:object?
slot-symbol:symbol?
arg:any/c

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.

procedure
( object-apply/noslot-thunk obj
noslot-thunk
slot-symbol
arg...) → any
obj:object?
noslot-thunk:(->any)
slot-symbol:symbol?
arg:any/c

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

procedure
( object-raw-clone/no-slots-copyobj)→void?
obj:object?
( object-raw-clone/copy-immed-slotsobj)→void?
obj:object?
( object-raw-clone/copy-all-slotsobj)→void?
obj:object?

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

parameter
( current-root-object)→object?
(current-root-objectobj)→void?
obj:object?

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.

4Terse Syntax🔗 i

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

syntax
( ^obj)

obj : object?

Parent of obj.

syntax
( !objslotval)
(!obj(slotval)...)

obj : object?
slot : identifier?
val : any/c

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.

syntax
( ?objslot...)

obj : object
slot : identifier?

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

syntax
( @objslotarg...)
(@obj(slotarg...)...)

obj : object?
slot : identifier?
arg : any/c

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.

syntax
( %obj(slotval)...)
(%)

obj : object?
slot : identifier?
val : any/c

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

5Known Issues🔗 i

Needs more units tests.

6History🔗 i

Version 2:0 — 2016年02月29日
- Moving from PLaneT to new package system.
Version 1:2
2011年11月08日
- Fixed object? not being exported. (Thanks to Shviller for reporting.)
Version 1:1 — 2009年03月03日
- License is now LGPL 3.
- Converted to authors new Scheme administration system.
- Changed slot lists and slot pairs to be explicitly mutable, for PLT 4.x.
Version 1:0 — 2005年06月19日
- Fixed bug in %protobj:apply* (thanks to Benedikt Rosenau for reporting).
- Changed $ syntax to ?, so that $ could be used for “self” in methods.
- Documentation changes.
Version 0.1 — 2005年01月05日
- Initial release.

7Legal🔗 i

Copyright 2005, 2009, 2011, 2016 Neil 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 3 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/licenses/ for details. For other licenses and consulting, please contact the author.

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