10. Subroutines

10.0. Introduction

To avoid the dangerous practice of copying and pasting code throughout a program, your larger programs will probably reuse chunks of code with subroutines. We'll use the terms subroutine and function interchangeably, because Perl doesn't distinguish between the two any more than C does. Even object-oriented methods are just subroutines that are called using a special syntax, described in Chapter 13, Classes, Objects, and Ties.

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 your subroutines are declared. These 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, Packages, Libraries, and Modules. They can even be created on the fly using eval or the AUTOLOAD mechanism, or generated using closures, which can be used as function templates.

If you are familiar with other programming languages, several characteristics of Perl's functions may surprise you if you're not prepared. Most of the recipes in this chapter illustrate how to take advantage of  - and be aware of  - these properties.

• Perl functions have no formal, named parameters, but this is not necessarily a bad thing. See Recipes 10.1 and 10.7.

• All variables are global unless declared otherwise. See Recipes 10.2, 10.3, and 10.13 for details.

• Passing or returning more than one array or hash normally causes them to lose their separate identities. See Recipes 10.5, 10.8, 10.9, and 10.11 to avoid this.

• A function can know whether it was called in list or scalar context, how many arguments it was called with, and even the name of the function that called it. See Recipes 10.4 and 10.6 to find out how.

• Perl's undef value can be used to indicate an error condition since no valid string or number ever has that value. 10.10 covers subtle pitfalls with undef you should avoid, and 10.12 shows how to deal with other catastrophic conditions.

• Perl supports interesting operations on functions you might not see in other languages, like anonymous functions, creating functions on the fly, and calling them indirectly using function pointers. See Recipes 10.14 and 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.