5 - Forth Interest Group

Simple Ob 'ect-
Oriented 6orth
C. A. Maynard
Wilson, West Australia
F
orth is a minimalist lan- lation and results in im- tional programming necesguage,
by which we proved data security, as only sary to handle a new variamean
that the core of those operations which have tion on an object, If the
the language provides facili- been designed to work with methods defined for the anties
from which the user/ the object's data structure cestor object are valid, the
programmer can build his will be executed, and there is objectwill simply obtain them
own working environment, no direct access to the data via the inheritance chain. If
It has also been described as itself. The user of an object there is the need for a different
a syntaxdirected language need have no knowledge of definition of a method for a
because, if you can define a the details of data storage or new class, that may be insyntax
which will best ex- even details of the appli- cluded in the new class and
press your needs, Forth will cable methods. He/she just will only work for the new
allow you to create a func- needs to know the valid op class of object and its
tional equivalent suitable for erations and any parameter- decendents. This ability is
programmer use.
passing requirements which called polynwqhisrn.
The latest thing in pro- may be necessary to operate Ouerlaading is the ability
gramming tools is the use of upon the object.
to have more than one defithe
objj-oriented approach, The user also needs to nition of the same method,
where data and operations know the terminology which and ensuring that the correct
upon that data are all part of fits the use of these tech- one is applied to a particular
the same 'object." This is niques. There are several object.
often refered to as encapsu- variants on the object-ori- Forth has the facilities to
create a very simple but effective
object-oriented pro-
To hide complex methods, we gramming environment. The
can set up a new vocabulary. following discussion and
development will not produce
the fastest objea-oriented
theme, but the follow- ented implementation in
ing is adequate for our needs. Forth, but will introduce the
I A c& definition s~eci- direct application of defin-
Clive Maynard is a senior lecturer in
fieS the data struciures ing and compiling words to
the Department of Computer Engineeded,
any initialization, establish an appropriate syntax.
The first requirement,
then, is to propose an appropriate
syntax to represent the
object class. (See Figure One.)
By analogy with cooking,
one should consider that
this syntax provides the
recipe to create new objeas
but that one must use the
upon an object.
recipe to make an object
Inhetftance allows the (i.e., create an instance) beuser
to minimize the addi- fore one can use it,
neering at Curtin University of ~ ~ ~ h -
nology, Western Australia, where this and the operations which
article's contents are used as part of may be performed upon the
students' Forth instruction. He also -, ~
runs Wawnic AsscciatBS. asvstems- I
1 design consultancy. He teaches real- I An instance is a ~articutime
systems using Forth on PCs and lar object of a spec& class,
on the Motorola 68HCl I, and is a co
author of The Art of Lisp Progamming and has built the data struc-
(Springer-Verlag. 1990). Clive has tures defined in the class
' developed a number of embedded &fition.
systems for industrial application.
. 1 Current interests include the develop A nretbOdis an
ment of practical, analytical tods fbr I which can be performed
predicting real-time scheduler performance
in e m ~ systems; ~ d as well
as real-time systems and Al.
Forth Dimensions 33
This syntax must provide
the compiler with all the information
needed to construct
the object. The method
names must be available for
use by any class definition,
and so will have to be defined
before use.
It will also be necessary
to provide syntadic delimiters
between the method
name and the method, and
also to separate this from the
following method name.
These methods may result
from short in-line definitions
or may need to access predefined
and hidden method
definitions. Appropriate delimiter
pairs which have been
selected are: : : and ; ; for
in-line definitions, M: andM;
for predefined methods.
Note for undentanding
thefillowing code
1. The operations are made
independent of 16-bit or
32-bit Forth implementations
by using the constant
WSIZE which returns the
number of bytes assigned
to storage of an integer
variable. This technique
reduces the efficiency of
definitions but ensures
portability, which is a
reasonable compromise.
2. Hidden methods must be
passed the address of the
beginning of data within
the object itself, and this
is shown in the stack
comments as "addr" on
top of the stack.
3. The stack comments for
the methods indicate the
parameters needed when
applying the method andl
or the results produced
by the method.
4. The class must inherit from
another class or NULL
Putting this together for
an example class which
consists of a point defined by
its x,y coordinates and a va-
riety of useful (and not-so-
useful) methods, [refer to
Figure Two].
To create an instance of
this class called PT we sim-
ply execute:
10 15 POINT PT
January 1992 February