Perl Module Basics

Packages may not seem immediately useful, but they form the basis for 

modular programming in Perl by providing encapsulation. Since each package 

forms a separate namespace for variables and subroutines, a package is free to 

implement its functionality without concern for the rest of the program. For 

example, let's say I'd like to override Perl's logarithmic function, log(),

3

 inside 

my package Acme::PotatoPeeler:

package Acme::PotatoPeeler;

sub log {

   print STDERR "PotatoPeeler Says: $_[0]\n";

}

If packages didn't provide encapsulation, I would have just overridden the loga 

rithm function for the entire program, and the stock simulation algorithms in 

Acme::StockPicker wouldn't work so well! Of course, if that's what you really want, 

you can do that too. I'll explain how to use packages to  redefine the world  later.

Symbol Tables

Packages work through the magic of symbol tables. Each package has a hash asso 

ciated with it called a symbol table. For every symbol in the package, there is a key 

in the hash. The value stored in the hash is a typeglob

4

 containing the value for 

the symbol.

Why doesn't the hash directly store the value of the variable? Perl supports, for 

better or worse, variables of different types with the same name you can have a 

scalar named $foo, an array named @foo, and a hash named %foo in the same package. 

The typeglob provides the level of indirection necessary to make this work.

Most compilers for other languages use symbol tables to keep track of variables 

declared in a program. Perl is unique in that it exposes its symbol tables to the pro 

grammer for examination and even manipulation at runtime. You can refer to a 

symbol table hash by using the package name plus a trailing package specifier, 

:: (double colon). Here's an example that prints out a sorted list of symbols for the 

File::Find package:

use File::Find;

print "$_\n" for sort keys %File::Find::;

3. Didn't know you could do that? I'll explain in more depth later in the  Exporting  section.

4. There isn't room here to dip into the arcane world of typeglobs. Suffice it to say that, outside 

of some useful idioms that I'll cover later, you can use them to do some really odd things that 

are probably best left undone.

25

25