1.2 Expressions, Statements, and Side-Effects
Before we begin introduce more Perl code examples, we want to explain
the ideas of an expression and a statement, and how each
looks in Perl.
Any valid "chunk" of Perl code can be considered an expression.
That expression always evaluates to some value. Sometimes, the value to
which expression evaluates is of interest to us, and sometimes it is
not. However, we always must be aware that each expression has some
"value" that is the evaluation of that expression.
Zero or more expressions to make a statement in Perl. Statements
in Perl end with a semi-colon. For example, in the Perl code we saw
before, we turned the expression, chomp($userName), into a
statement, chomp($userName); by adding a ; to the end. If
it helps, you can think about the ;s as separating sets of
expressions that you want Perl to evaluate and execute in order.
Given that every expression, even when combined into statements,
evaluate to some value, you might be tempted to ask: What does the
expression chomp($userName) evaluate to? It turns out that
expression evaluates to the total number of characters removed from the
end of the variable $userName. This is actually one of those
cases where we are not particularly interested in the evaluation result
of the code. In this case, we were instead interested in what is called
the side-effect of the expression.
The side-effect of an expression is some change that occurs as a
result of that expression's evaluation. Often, a side-effect causes
some change in the state of the running program, such as changing the
value of a variable. In the expression chomp($userName), the
side-effect is that any newline characters are removed from the end of
the variable, @scalar{$username}.
Let's now consider a slightly more complex statement, and look for the
the expressions and side-effect. Consider the statement,
$username = <STDIN>; from our first program. In this case, we
used the expression, <STDIN> as part of a larger expression,
namely $username = <STDIN>. The expression, <STDIN>
evaluated to a scalar value, namely a string that represented a line
from the standard input. It was of particular interest to us the value
to which <STDIN> evaluated, because we wanted to save that value
in the variable, @scalar{$username}.
To cause that assignment to take place, we used the larger expression,
$username = <STDIN>. The side-effect of that larger expression
is that @scalar{$username} contains the value that <STDIN>
evaluated to. That side-effect is what we wanted in this case, and we
ignore the value to which $username = <STDIN> evaluates. (It
turns out that it evaluates to the value contained in $username
after the assignment took place.)
The concepts of statements, expressions and side-effects will become
more clear as we continue. When appropriate, we'll point out various
expression and discuss what they evaluate to, and indicate what
side-effects are of interest to us.
