The purpose of
the Smalltalk project is to provide computer support for the creative spirit in
everyone. Our work flows from a vision that includes a creative individual and
the best computing hardware available. We have chosen to concentrate on two
principle areas of research: a language of description (programming language)
that serves as an interface between the models in the human mind and those in
computing hardware, and a language of interaction (user interface) that matches
the human communication system to that of the computer. Our work has followed a
two- to four-year cycle that can be seen to parallel the scientific method:
Modularity: No component in a complex system should depend on the
internal details of any other component.
 |
| Figure 2: System complexity. As the number of components in
a system increases, the chances for unwanted interaction increase rapidly.
Because of this, a computer language should be designed to minimize the
possibilities of such interdependence. |
This principle is depicted in figure
2. If there are N components in a system, then there are roughly
N-squared potential dependencies between them. If computer systems are
ever to be of assistance in complex human tasks, they must be designed to
minimize such interdependence. The message-sending metaphor provides modularity
by decoupling the intent of a message (embodied in its name) from the
method used by the recipient to carry out the intent. Structural
information is similarly protected because all access to the internal state of
an object is through this same message interface.
The
complexity of a system can often be reduced by grouping similar components. Such
grouping is achieved through data typing in conventional programming languages,
and through classes in Smalltalk. A class describes other objects --
their internal state, the message protocol they recognize, and the internal
methods for responding to those messages. The objects so described are called
instances of that class. Even classes themselves fit into this framework;
they are just instances of class Class, which describes the appropriate
protocol and implementation for object description.
- Classification: A language must provide a means for classifying
similar objects, and for adding new classes of objects on equal footing with
the kernel classes of the system.
Classification is the
objectification of nessness. In other words, when a human sees a chair,
the experience is taken both literally an "that very thing" and abstractly as
"that chair-like thing". Such abstraction results from the marvelous ability of
the mind to merge "similar" experience, and this abstraction manifests itself as
another object in the mind, the Platonic chair or chairness.
Classes are the chief mechanism for extension in
Smalltalk. For instance, a music system would be created by adding new classes
that describe the representation and interaction protocol of Note,
Melody, Score, Timbre, Player, and so on.
The "equal footing" clause of the above principle is important because it
insures that the system will be used as it was designed. In other words, a
melody could be represented as an ad hoc collection of Integers
representing pitch, duration, and other parameters, but if the language can
handle Notes as easily as Integers, then the user will
naturally describe a melody as a collection of Notes. At each stage of
design, a human will naturally choose the most effective representation if the
system provides for it. The principle of modularity has an interesting
implication for the procedural components in a system:
- Polymorphism: A program should specify only the behavior of
objects, not their representation.
A
conventional statement of this principle is that a program should never declare
that a given object is a SmallInteger or a LargeInteger, but
only that it responds to integer protocol. Such generic description is crucial
to models of the real world.
Consider an automobile
traffic simulation. Many procedures in such a system will refer to the various
vehicles involved. Suppose one wished to add, say, a street sweeper. Substantial
amounts of computation (in the form of recompiling) and possible errors would be
involved in making this simple extension if the code depended on the objects it
manipulates. The message interface establishes an ideal framework for such an
extension. Provided that street sweepers support the same protocol as all other
vehicles, no changes are needed to include them in the simulation:
- Factoring: Each independent component in a system would appear
in only one place.
There are many reasons for this principle.
First of all, it saves time, effort, and space if additions to the system need
only be made in one place. Second, users can more easily locate a component that
satisfies a given need. Third, in the absence of proper factoring, problems
arise in synchronizing changes and ensuring that all interdependent components
are consistent. You can see that a failure in factoring amounts to a violation
of modularity.
Smalltalk encourages well-factored designs
through inheritance. Every class inherits behavior from its superclass.
This inheritance extends through increasingly general classes, ultimately ending
with class Object which describes the default behavior of all objects
in the system. In our traffic simulation above, StreetSweeper (and all
other vehicle classes) would be described as a subclass of a general
Vehicle class, thus inheriting appropriate default behavior and
avoiding repetition of the same concepts in many different places. Inheritance
illustrates a further pragmatic benefit of factoring:
- Leverage: When a system is well factored, great leverage is
available to users and implementers alike.
Take
the case of sorting an ordered collection of objects. In Smalltalk, the user
would define a message called sort in the class
OrderedCollection. When this has been done, all forms of ordered
collections in the system will instantly acquire this new capability through
inheritance. As an aside, it is worth noting that the same method can
alphabetize text as well as sort numbers, since comparison protocol is
recognized by the classes which support both text and numbers.
The benefits of structure for implementers are obvious.
To begin with, there will be fewer primitives to implement. For instance, all
graphics in Smalltalk are performed with a single primitive operation. With only
one task to do, an implementer can bestow loving attention on every instruction,
knowing that each small improvement in efficiency will be amplified throughout
the system. It is natural to ask what set of primitive operations would be
sufficient to support an entire computing system. The answer to this question is
called a virtual machine specification:
- Virtual Machine: A virtual machine specification establishes a
framework for the application of technology.
The Smalltalk virtual
machine establishes an object-oriented model for storage, a message-oriented
model for processing, and a bitmap model for visual display of information.
Through the use of microcode, and ultimately hardware, system performance can be
improved dramatically without any compromise to the other virtues of the system.
User Interface
A user
interface is simply a language in which most of the communication is visual.
Because visual presentation overlaps heavily with established human culture,
esthetics plays a very important role in this area. Since all capability of a
computer system is ultimately delivered through the user interface, flexibility
is also essential here. An enabling condition for adequate flexibility of a user
interface can be stated as an object-oriented principle:
- Reactive Principle: Every component accessible to the user
should be able to present itself in a meaningful way for observation and
manipulation.
This criterion is well supported by the model of
communicating objects. By definition, each object provides an appropriate
message protocol for interaction. This protocol is essentially a microlanguage
particular to just that kind of object. At the level of the user interface, the
appropriate language for each object on the screen is presented visually (as
text, menus, pictures) and sensed through keyboard activity and the use of a
pointing device.
It should be noted that operating
systems seem to violate this principle. Here the programmer has to depart from
an otherwise consistent framework of description, leave whatever context has
been built up, and deal with an entirely different and usually very primitive
environment. This need not be so:
- Operating System: An operating system is a collection of things
that don't fit into a language. There shouldn't be one.
Here are
some examples of conventional operating system components that have been
naturally incorporated into the Smalltalk language:
- Storage management -- Entirely automatic. Objects are created by a message
to their class and reclaimed when no further references to them exist.
Expansion of the address space through virtual memory is similarly
transparent.
- File system -- Included in the normal framework through objects such as
Files and Directories with message protocols that support
file access.
- Display handling -- The display is simply an instance of class
Form, which is continually visible, and the graphical manipulation
messages defined in that class are used to change the visible image.
- Keyboard Input -- The user input devices are similarly modeled as objects
with appropriate messages for determining their state or reading their history
as a sequence of events.
- Access to subsystems -- Subsystems are naturally incorporated as
independent objects within Smalltalk: there they can draw on the large
existing universe of description, and those that involve interaction with the
user can participate as components in the user interface.
- Debugger -- The state of the Smalltalk processor is accessible as an
instance of class Process that owns a chain of stack frames. The
debugger is just a Smalltalk subsystem that has access to manipulate the state
of a suspended process. It should be noted that nearly the only run-time error
that can occur in Smalltalk is for a message not to be recognized by its
receiver.
Smalltalk has no "operating system" as
such. The necessary primitive operations, such as reading a page from the disk,
are incorporated as primitive methods in response to otherwise normal Smalltalk
messages.
Future Work
As might be
expected, work remains to be done on Smalltalk. The easiest part to describe
is the continued application of the principles in this paper. For example, the
Smalltalk-80 system falls short in its factoring because it supports only
hierarchical inheritance. Future Smalltalk systems will generalize this model
to arbitrary (multiple) inheritance. Also, message protocols have not been
formalized. The organization provides for protocols, but it is currently only
a matter of style for protocols to be consistent from one class to another.
This can be remedied easily by providing proper protocol objects that can be
consistently shared. This will then allow formal typing of variables by
protocol without losing the advantages of polymorphism.
The other remaining work is less easy to articulate. There are clearly other
aspects to human thought that have not been addressed in this paper. These
must be identified as metaphors that can complement the existing models of the
language.
Sometimes the advance of computer systems
seems depressingly slow. We forget that steam engines were high-tech to our
grandparents. I am optimistic about the situation. Computer systems are, in
fact, getting simpler and, as a result, more usable. I would like to close
with a general principle which governs this process:
- Natural Selection: Languages and systems that are of sound
design will persist, to be supplanted only by better ones.
Even as
the clock ticks, better and better computer support for the creative spirit is
evolving. Help is on the way.
