Smalltalk and Object Orientation: an Introduction - Free

9. Control and Iteration
9.1 Introduction
This chapter introduces how control and iteration are achieved in Smalltalk. To do this a number of
concepts which are u nique to Smalltalk are also discussed (such as the block object). To simplify this
process for the reader, equivalent C constructs are illustrated beside Smalltalk constructs (where they
exist).
The remainder of this chapter is structured in the following manner. Section two introduces the
concept of cascading, Section three discusses blocks and Section four describes the control structures
available in Smalltalk.
9.2 Cascading
Cascading is a syntactic form that allows multiple messages to be sent to the same object. The aim of
this is to reduce the need for tediously repeating the same object over and over again, because a whole
set of messages must be sent to that one object. The intention is to produce more concise code which is
easier to read, and to avoid the need for additional temporary variables.
Cascading is achieved by using the semicolon delimiter. This is slightly confusing for those coming
from a C or Ada background as the semicolon is used there as a expression terminato r. Instead, in
Smalltalk, it should be read as send the next message to the same object as the last message. For
example, the expressions:
can be written in a shorter form as:
Transcript cr.
Transcript tab.
Transcript show: 'Hello World'.
Transcript cr.
Transcript cr;
tab;
show: 'Hello World';
cr.
The programmer should be wary of using the cascade mechanism. Although it was intended to
simplify code, it can often have the opposite effect. For example, in a very long cascaded expression,
which appears all on the same line, it can become difficult to determine exactly what is happening. It is
therefore often the case that a set of message expressions are easier to understand when they are not
written using cascading.
9.3 Blocks
9.3.1 Block definition and evaluation
Earlier in this book you were told to treat Blocks as the same as Begin/End constructs in Pascal and
ADA or {}’s in C. This was, however, not exactly the truth. They are in fact objects in their own right
(and are instances of BlockClosure) which represent code that can be executed (or evaluated) now or at
a later date. Block expressions therefore describe deferred activities.
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