Smalltalk and Object Orientation: an Introduction - Free

other systems. As the browsers, inspectors and debuggers are all derived from the same source,
there is also consistency between (some of) the various implementations. In addition, the
source code for these tools is also available with the ADE.
Other languages now have similar environments (most notably Visual C++) h owever,
most of them are modeled on those found in Smalltalk implementations and few of them
illustrate any pretence of consistency between vendors.
Thus it is quite possible to say that Smalltalk is a programming language, a set of extensible classes, a
development environment or even a user interface development tool. It is, in fact, all of these.
4.3 Objects in Smalltalk
In Smalltalk everything is an object (or should be treated as an object). For example, the following
expression:
2 + 3.
should be read as the object 2 is sent the message +, with the argument the object 3. In fact in this case
for efficiency sake the + message is hard coded into the virtual machine (you will learn about this later).
However, if you search for the method + it is there, it is just that its implementation is hidden from you.
This pure object view has some interesting side effects on the language constructs such as
conditional statements. They are really messages to the objects true or false rather than being part of the
language syntax. Another example are the iterative control statements (the Smalltalk equivalent of for or
while loops in procedural languages) which are messages to numeric intervals or blocks. You will learn
more about these from Chapter 6 onwards.
4.4 History
The original goals for Smalltalk were described by Alan Kay in the early 1970s. The initial sketches
that formed the basis for Smalltalk were heavily influenced by the idea of classes as an organizing
principle (taken from Simula-67), of turtle graphics (taken from the LOGO project at MIT) and of what
is now called “direct manipulation” interfaces (inspired by the sketchpad drawing system, developed by
Ivan Sutherland at MIT Lincoln Laboratories in the early 1960s, and by Kay’s Ph.D. thesis on the
FLEX machine). 2
Between 1971 and 1975, Kay’s group at Xerox PARC designed and implemented the first real
Smalltalk language, environment and applications. This system included a number of technical
innovations:
• The language was based entirely on the Simula concepts of class and message.
• The language had no fixed syntax . Each class was responsible not only for its own behavior and
state definition, but even for parsing the token stream that followed a mention of an instance.
The innovations in the development environment were equally radical. At that time bit mapped
displays were considered expensive and arcane. Nevertheless, Kay persuaded PARC to allow him to use
bit mapped displays, which enabled Kay and his team to implement bit -mapped text in multiple sizes
and styles, using multi-windowing environments, plus high level support for bit operations and graphics
operations.
The Smalltalk -72 system further took the view that there was no reason for a separate operating
system, since the object paradigm could manage all hardware resources at least as easily as any other
approach.
By 1975-76, it had become clear that the lack of attention to issues of performance and scale were
hampering further invest igations. Kay’s group proceeded with a major redesign of all aspects of the
Smalltalk system. In the language area:
2
Note this section is based on information described in a paper by L. Peter Deutsch, called
Smalltalk, which was presented at ECOOP’89 [Deutsch 1989].
The Past, Present and Future of
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