University of Virginia, Department of Computer Science
CS655: Programming Languages
Design Principles Behind Smalltalk
Daniel H. H. Ingalls
Learning Research Group
Xerox Palo Alto Research Center
BYTE Magazine, August 1981. (c) by The McGraw-Hill Companies, Inc., NY.
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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:
The Smalltalk-80 system marks our fifth time through this
cycle. In this article, I present some of the general principles we have
observed in the course of our work. While the presentation frequently touches on
Smalltalk "motherhood", the principles themselves are more general and should
prove useful in evaluating other systems and in guiding future work.
- Build an application program within the current system (make an
- Based on that experience, redesign the language (formulate a theory)
- Build a new system based on the new design (make a prediction that can be
Just to get warmed up, I'll start with a principle that
is more social than technical and that is largely responsible for the particular
bias of the Smalltalk project:
The point here is
that the human potential manifests itself in individuals. To realize this
potential, we must provide a medium that can be mastered by a single individual.
Any barrier that exists between the user and some part of the system will
eventually be a barrier to creative expression. Any part of the system that
cannot be changed or that is not sufficiently general is a likely source of
impediment. If one part of the system works differently from all the rest, that
part will require additional effort to control. Such an added burden may detract
from the final result and will inhibit future endeavors in that area. We can
thus infer a general principle of design:
- Personal Mastery:
If a system is to serve the creative spirit, it must be
entirely comprehensible to a single individual.
- Good Design:
A system should be built with a minimum set of unchangeable
parts; those parts should be as general as possible; and all parts of the
system should be held in a uniform framework.
In designing a language for
use with computers, we do not have to look far to find helpful hints. Everything
we know about how people think and communicate is applicable. The mechanisms of
human thought and communication have been engineered for millions of years, and
we should respect them as being of sound design. Moreover, since we must work
with this design for the next million years, it will save time if we make our
computer models compatible with the mind, rather that the other way around.
1 illustrates the principle components in our discussion. A person is
presented as having a body and a mind. The body is the site of primary
experience, and, in the context of this discussion, it is the physical channel
through which the universe is perceived and through which intentions are carried
out. Experience is recorded and processed in the mind. Creative thought (without
going into its mechanism) can be viewed as the spontaneous appearance of
information in the mind. Language is the key to that information:
The interaction between two individuals is
represented in figure
1 as two arcs. The solid arc represents explicit communication: the actual
words and movements uttered and perceived. The dashed arc represents implicit
communication: the shared culture and experience that form the context of the
explicit communication. In human interaction, much of the actual communication
is achieved through reference to shared context, and human language is built
around such allusion. This is the case with computers as well.
- Purpose of Language: To provide a framework
It is no coincidence that a computer can be viewed as one
of the participants in figure
1. In this case, the "body" provides for visual display of information and
for sensing input from a human user. The "mind" of a computer includes the
internal memory and processing elements and their contents. Figure
1 shows that several different issues are involved in the design of a
This fact is responsible for the difficulty of
explaining Smalltalk to people who view computer languages in a more restricted
sense. Smalltalk is not simply a better way of organizing procedures or a
different technique for storage management. It is not just an extensible
hierarchy of data types, or a graphical user interface. It is all of these
things and anything else that is needed to support the interactions shown in figure
- Scope: The
design of a language for using computers must deal with internal models,
external media, and the interaction between these in both the human and the
The scope of language design. Communication between two
people (or between one person and a computer) includes communication on
two levels. Explicit communication includes the information that is
transmitted in a given message. Implicit communication includes the
relevant assumptions common to the two
The mind observes a vast universe of experience, both immediate and recorded.
One can derive a sense of oneness with the universe simply by letting this
experience be, just as it is. However, if one wishes to participate, literally
to take a part, in the universe, one must draw distinctions. In so
doing one identifies an object in the universe, and simultaneously all the
rest becomes not-that-object. Distinction by itself is a start, but the
process of distinguishing does not get any easier. Every time you want to talk
about "that chair over there", you must repeat the entire processes of
distinguishing that chair. This is where the act of reference comes in: we can
associate a unique identifier with an object, and, from that time on, only the
mention of that identifier is necessary to refer to the original object.
We have said that a computer system should provide
models that are compatible with those in the mind. Therefore:
The Smalltalk storage manager provides an
object-oriented model of memory for the entire system. Uniform reference is
achieved simply by associating a unique integer with every object in the system.
This uniformity is important because it means that variables in the system can
take on widely differing values and yet can be implemented as simple memory
cells. Objects are created when expressions are evaluated, and they can be
passed around by uniform reference, so that no provision for their storage is
necessary in the procedures that manipulate them. When all references to an
object have disappeared from the system, the object itself vanishes, and its
storage is reclaimed. Such behavior is essential to full support of the object
- Objects: A computer language should support the concept of
"object" and provide a uniform means for referring to the objects in its
A way to find out if a language is
working well is to see if programs look like they are doing what they are doing.
If they are sprinkled with statements that relate to the management of storage,
then their internal model is not well matched to that of humans. Can you imagine
having to prepare someone for each thing you tell them or having to inform them
when you are through with a given topic and that it can be forgotten?
- Storage Management:
To be truly "object-oriented", a computer system must provide
automatic storage management.
Each object in our universe has a life of its own.
Similarly, the brain provides for independent processing along with storage of
each mental object. This suggests a third principle of design:
Just as programs get messy if
object storage is dealt with explicitly, control in the system becomes
complicated if processing is performed extrinsically. Let us consider the
process of adding 5 to a number. In most computer systems, the compiler figures
out what kind of number it is and generates code to add 5 to it. This is not
good enough for an object-oriented system because the exact kind of number
cannot be determined by the compiler (more on this later). A possible solution
is to call a general addition routine that examines the type of the arguments to
determine the approximate action. This is not a good approach because it means
that this critical routine must be edited by novices who just want to
experiment with their own class of numbers. It is also a poor design because
intimate knowledge about the internals of objects is sprinkled throughout the
- Messages: Computing should be
viewed as an intrinsic capability of objects that can be uniformly invoked by
Smalltalk provides a much cleaner solution: it
sends the name of the desired operation, along with any arguments, as a
message to the number, with the understanding that the receiver knows
best how to carry out the desired operation. Instead of a bit-grinding processor
raping and plundering data structures, we have a universe of well-behaved
objects that courteously ask each other to carry out their various desires. The
transmission of messages is the only process that is carried on outside of
objects and this is as it should be, since messages travel between objects. The
principle of good design can be restated for languages:
Examples of success in this area include LISP,
which is built on the model of linked structures; APL, which is built on the
model of arrays; and Smalltalk, which is built on the model of communicating
objects. In each case, large applications are viewed in the same way as the
fundamental units from which the system is built. In Smalltalk especially, the
interaction between the most primitive objects is viewed in the same way as the
highest-level interaction between the computer and its user. Every object in
Smalltalk, even a lowly integer, has a set of messages, a protocol, that
defines the explicit communication to which that object can respond. Internally,
objects may have local storage and access to other shared information which
comprise the implicit context of all communication. For instance, the message +
5 (add five) carries an implicit assumption that the augend is the present value
of the number receiving the message.
- Uniform Metaphor: A language should be designed around a
powerful metaphor that can be uniformly applied in all areas.
Modularity: No component in a complex system should depend on the
internal details of any other component.
metaphor provides a framework in which complex systems can be built. Several
related organizational principles contribute to the successful management of
complexity. To begin with:
|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.
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 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.
- 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.
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:
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.
- Polymorphism: A program should specify only the behavior of
objects, not their representation.
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:
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
- Factoring: Each independent component in a system would appear
in only one place.
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:
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.
- Leverage: When a system is well factored, great leverage is
available to users and implementers alike.
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:
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.
- Virtual Machine: A virtual machine specification establishes a
framework for the application of technology.
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:
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
- Reactive Principle: Every component accessible to the user
should be able to present itself in a meaningful way for observation and
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:
some examples of conventional operating system components that have been
naturally incorporated into the Smalltalk language:
- Operating System: An operating system is a collection of things
that don't fit into a language. There shouldn't be one.
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
- 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
- File system -- Included in the normal framework through objects such as
Files and Directories with message protocols that support
- 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
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
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:
the clock ticks, better and better computer support for the creative spirit is
evolving. Help is on the way.
- Natural Selection: Languages and systems that are of sound
design will persist, to be supplanted only by better ones.