10.0. Introduction
To
avoid the dangerous practice of copying and pasting code, larger
programs reuse chunks of code as subroutines and functions. We'll use
the terms subroutine and
function interchangeably because Perl doesn't
distinguish between the two. Even object-oriented methods are just
subroutines that are called using a special syntax, described in
Chapter 13.
A subroutine is
declared with the sub keyword. Here's a simple
subroutine definition:
sub hello {
$greeted++; # global variable
print "hi there!\n";
}
The typical way of calling that subroutine is:
hello( ); # call subroutine hello with no arguments/parameters
Because Perl compiles your program before executing it, it doesn't
matter where subroutines are declared. Definitions don't have to be
in the same file as your main program. They can be pulled in from
other files using the do,
require, or use operators, as
described in Chapter 12. They can even be created
on the fly using eval or AUTOLOAD, or generated
using closures, which can act as function
templates.
If you are familiar with other programming languages, several
characteristics of Perl's functions may surprise you if you're
unprepared for them. Most recipes in this chapter illustrate how to
be aware of—and to take advantage of—these properties.
-
Perl functions have no formal, named parameters, but this is not
necessarily a bad thing. See Recipe 10.1 and
Recipe 10.7.
-
All variables are global unless declared otherwise. See Recipe 10.2, Recipe 10.3, and Recipe 10.13 for details.
-
Passing or returning more than one array or hash normally causes them
to lose their separate identities. See Recipe 10.5, Recipe 10.8, Recipe 10.9, and Recipe 10.11 to avoid
this.
-
A function can know in which context it was called, how many
arguments it was called with, and even which other function called
it. See Recipe 10.4 and Recipe 10.6 to find out how.
-
Perl's undef value can be used to signal an error
return from the function because no valid string, number, or
reference ever has that value. Recipe 10.10
covers subtle pitfalls with undef you should
avoid, and Recipe 10.12 shows how to deal with
other catastrophic conditions.
-
Perl supports interesting operations on functions that you might not
see in other languages, such as anonymous functions, creating
functions on the fly, and calling them indirectly using function
pointers. See Recipe 10.14 and Recipe 10.16 for these esoteric topics.
-
Calling a function as $x =
&func; does not supply any arguments, but
rather provides direct access to its caller's @_
array! If you omit the ampersand and use either func(
) or func, then a new and empty
@_ is provided instead.
-
Historically, Perl hasn't provided a construct like C's
switch or the shell's case for
multiway branching. The switch function shown in
Recipe 10.17 takes care of that for you.