Modules and Their Functional Interfaces (Perl & LWP)

G.2. Modules and Their Functional Interfaces

Modules are the main way that Perl provides for bundling up code for later use by yourself or others. As I'm sure you can't help noticing from reading The Perl Journal, CPAN (the Comprehensive Perl Archive Network) is the repository for modules (or groups of modules) that others have written, to do anything from composing music to accessing web pages. A good deal of those modules even come with every installation of Perl.

One module that you may have used before, and which is fairly typical in its interface, is Text::Wrap. It comes with Perl, so you don't even need to install it from CPAN. You use it in a program of yours, by having your program code say early on:

use Text::Wrap;

and after that, you can access a function called wrap, which inserts line-breaks in text that you feed it, so that the text will be wrapped to 72 (or however many) columns.

The way this use Text::Wrap business works is that the module Text::Wrap exists as a file Text/Wrap.pm somewhere in one of your library directories. That file contains Perl code[6] which, among other things, defines a function called Text::Wrap::wrap, and then exports that function, which means that when you say wrap after having said use Text::Wrap, you'll be actually calling the Text::Wrap::wrap function. Some modules don't export their functions, so you have to call them by their full name, like Text::Wrap::wrap(parameters).

[6]And mixed in with the Perl code, there's documentation, which is what you read with perldoc Text::Wrap. The perldoc program simply ignores the code and formats the documentation text, whereas use Text::Wrap loads and runs the code while ignoring the documentation.

Regardless of whether the typical module exports the functions it provides, a module is basically just a container for chunks of code that do useful things. The way the module allows for you to interact with it, is its interface. And when, like with Text::Wrap, its interface consists of functions, the module is said to have a functional interface.[7]

[7]The term "function" (and therefore "functional") has various senses. I'm using the term here in its broadest sense, to refer to routines—bits of code that are called by some name and take parameters and return some value.

Using modules with functional interfaces is straightforward—instead of defining your own "wrap" function with sub wrap { ... }, you entrust use Text::Wrap to do that for you, along with whatever other functions its defines and exports, according to the module's documentation. Without too much bother, you can even write your own modules to contain your frequently used functions; I suggest having a look at the perlmod manpage for more leads on doing this.