6.5 Modules and Libraries of Subroutines

As you start to build a collection of subroutines, you'll find that you're copying them a lot from existing programs and pasting them into new programs. The subroutines then appear in multiple program. This makes the listings of your program code a bit verbose and repetitive. It also makes modifying a subroutine more complicated because you have to modify all the copies.

In short, subroutines are great, but if you have to keep copying them into each new program you write, it gets tiresome. So it's time to start collecting subroutines into the handy files called modules or libraries.

Here's how it works. You put all your reusable subroutines into a separate file. (Or, as you keep writing more and more code, and things get complicated, you may want to organize them into several files.) Then you just name the file in your program and presto: the subroutine's definitions all get read in, just as if they were in your program. To do this, you use the Perl built-in function use, which reads in the subroutine library file.

Let's call this module BeginPerlBioinfo.pm. You can put all your subroutine definitions into it, just as they appear in the program code. Then you can create the module by typing in the subroutine definitions as you read the book; or, more easily, it can be downloaded from the book's web site. But there is one thing to remember when creating or adding to a module: the last line in a module must be 1; or it won't work. This 1; should be the last line of the .pm file, not part of the last subroutine. If you forget this, you'll get an error message something like:

BeginPerlBioinfo.pm did not return a true value at jkl line 14.
BEGIN failed--compilation aborted at jkl line 14.

Now, to use any of the subroutines in BeginPerlBioinfo.pm, you just have to put the following statement in your code, near the top (near the use strict statement):

use BeginPerlBioinfo;

Note that .pm is left off the name on purpose: that's how Perl handles the names of modules.

There's one last thing to know about using modules to load in subroutines: the Perl program needs to know where to find the module. If you're doing all your work in one folder, everything should work okay. If Perl complains about not being able to find BeginPerlBioinfo.pm, give full pathname information to the module. If the full pathname is /home/tisdall/book/BeginPerlBioinfo.pm, then use this in your program:

use lib '/home/tisdall/book';
use BeginPerlBioinfo;

There are other ways to tell Perl where to look for modules; consult the Perl documentation for use.

Beginning in Chapter 8, I'll define subroutines and show the code, but you'll be putting them into your module and typing:

use BeginPerlBioinfo;

This module is also available for download at this book's web site.