Chapter 10. Subroutines - Perl Cd Bookshelf [Electronic resources] نسخه متنی

Chapter 10. Subroutines

Contents:

Introduction

Accessing Subroutine Arguments

Making Variables Private to a Function

Creating Persistent Private Variables

Determining Current Function Name

Passing Arrays and Hashes by Reference

Detecting Return Context

Passing by Named Parameter

Skipping Selected Return Values

Returning More Than One Array or Hash

Returning Failure

Prototyping Functions

Handling Exceptions

Saving Global Values

Redefining a Function

Trapping Undefined Function Calls with AUTOLOAD

Nesting Subroutines

Writing a Switch Statement

Program: Sorting Your Mail

W. H. Auden, "Three Songs for St Cecilia's Day"

Composing mortals with immortal fire.

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.