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Introduction to Perl modules Simon Whitaker (simon@netcetera.org) May 2003 Evolving towards modularity In its simplest form, programming in a language like Perl involves writing lines of code that execute sequentially: $name = "Alice"; print "Hello $name!\n"; # prints Hello Alice! Introduction to Perl modules (SW) 2 Evolving towards modularity Writing line-by-line code works for simple tasks, but it can get unnecessarily repetitive: $name = "Alice"; print "Hello $name!\n"; $name2 = "Bob"; print "Hello $name2!\n"; $name3 = "Charlie"; print "Hello $name3!\n"; Introduction to Perl modules (SW) 3 Evolving towards modularity Problems arise when I need to revise my code. For example, imagine that I need to translate my program into French. I have to alter 3 lines of code: $name = "Alice"; print "Bonjour $name!\n"; $name2 = "Bob"; print "Bonjour $name2!\n"; $name3 = "Charlie"; print "Bonjour $name3!\n"; Introduction to Perl modules (SW) 4 Evolving towards modularity Better to encapsulate the repetitive stuff in a subroutine, which I only have to amend once: sub greet { print "Hello $_[0]\n"; } $name = "Alice"; greet($name); $name2 = "Bob"; greet($name2); Introduction to Perl modules (SW) 5 Evolving towards modularity And in French... sub greet { print "Bonjour $_[0]\n"; } $name = "Alice"; greet($name); $name2 = "Bob"; greet($name2); Introduction to Perl modules (SW) 6 Evolving towards modularity I can now use greet() anywhere in my program, making my code easier to maintain. However, what if I want to use greet() in other programs? I can copy it into each program, but then I have the same problem of having to maintain multiple instances of the same code. This is where modules come in. Introduction to Perl modules (SW) 7 What is a module? A module is a library of Perl code that can be included in your Perl program. When you include a Perl module in a program, the functionality of that module is available for you to use inside your own program. Introduction to Perl modules (SW) 8 Where do modules come from? Perl modules come from a variety of sources: • Standard modules: modules that are installed when you install Perl • CPAN: the Comprehensive Perl Archive Network – a global network of Perl module repositories • Third parties: e.g. in-house code libraries, Bioperl, etc Introduction to Perl modules (SW) 9 How to use a module To include a module in your program, use Perl's use keyword. The general form is: use Module; Typically, the module will export its most popular subroutines and variables into your program. You can then use these subroutines and variables just as if they were declared in your program. Introduction to Perl modules (SW) 10 How to use a module For example: use Hello; # use the Hello module greet("Alice"); # greet() is exported # by the Hello module Introduction to Perl modules (SW) 11 How to use a module To use subroutines or variables that aren't exported by default, precede their name with the module name followed by a double colon: use Hello; Hello::greet("Alice"); # calls greet() from the Hello module Introduction to Perl modules (SW) 12 How to use a module Alternatively, you can state explicitly which variables and subroutines you want to export when you load the module, using the general form: use Module qw( $variable subroutine ); where $variable and subroutine are the things you want to export. This form overrides the default form – only the variables and subroutines you specify are imported. Introduction to Perl modules (SW) 13 Example 1: Text::Wrap Text::Wrap is a simple but very useful module. It's part of the standard module library, which means that it's installed when you install Perl. Text::Wrap contains a subroutine called wrap() (exported by default), which wraps text to form neat paragraphs. wrap() takes three arguments: the indent string for the first line of a paragraph, the indent string for subsequent lines, and the text to be wrapped. Introduction to Perl modules (SW) 14 Example 1: Text::Wrap use Text::Wrap; $indent_first = " $indent_subsq = ""; "; $text = "For the life of me I could never understand why Mr Perkins didn't enjoy his job. He had everything a man could want: a place on the board, an absent boss and a cat named Henry."; print wrap($indent_first, $indent_subsq, $text); Introduction to Perl modules (SW) 15 Example 1: Text::Wrap Output: For the life of me I could never understand why Mr Perkins didn't enjoy his job. He had everything a man could want: a place on the board, an absent boss and a cat named Henry. Introduction to Perl modules (SW) 16 Example 1: Text::Wrap Text::Wrap also has a variable called $columns, which determines how many columns my text wraps to. Unlike wrap(), $columns isn't exported from Text::Wrap by default, so I refer to it here using its fully qualified name: # start of program as before $Text::Wrap::columns = 20; print wrap($indent_first, $indent_subsq, $text); Introduction to Perl modules (SW) 17 Example 1: Text::Wrap Output: For the life of me I could never understand why Mr Perkins didn't enjoy his job. He had everything a man could want: a place on the board, an absent boss and a cat named Henry. Introduction to Perl modules (SW) 18 3 benefits of using modules 1. Saves time and effort 2. Greater portability 3. Modularity Introduction to Perl modules (SW) 19 Benefit 1: Saves time and effort There are thousands of Perl modules out there, covering a huge range of common (and not so common) tasks. Don't re-invent the wheel! If you need to grab data from a web page, create PDF files on the fly or connect to a database from your Perl program, I have great news for you – someone else has already done the hard work. Better still, they'll share it with you! Introduction to Perl modules (SW) 20 Benefit 2: Greater portability Perl modules are generally written with portability in mind. Again – let someone else do the hard work! For example: imagine that you want to move a file from within your Perl program. Perl doesn't have a built-in move function, so you might be tempted to write: `mv oldfile newfile`; Introduction to Perl modules (SW) 21 Benefit 2: Greater portability This works fine – on UNIX. But if you try to run the program on Windows, or another operating system that doesn't have a mv command, it fails. This, however, is portable across OSs: use File::Copy; move("oldfile", "newfile"); Introduction to Perl modules (SW) 22 Benefit 3: Modularity As the name suggests, modules are modular! With modules you can parcel bits of code that do specific tasks (e.g. wrapping text) into discrete bundles, which can be re-used elsewhere. If the code needs to be updated you only need to change the module, not dozens of Perl programs. Introduction to Perl modules (SW) 23 A brief word about objects From time to time you'll come across objectoriented (OO) Perl modules. It's not important at this stage to understand much about OO programming or how it works, but it helps to be familiar with the syntax it uses so that you feel comfortable using OO Perl modules. Introduction to Perl modules (SW) 24 A brief word about objects Objects represent complex entities, such as people, books or DNA sequences. Generally, you create objects using new: $alice = new Student(); and interact with them using methods, which work just like subroutines: print $alice->gradeAverage(); Introduction to Perl modules (SW) 25 Example 2: Net::FTP Net::FTP is an object-oriented Perl module that allows you to access FTP servers. As usual, you start by importing the module: use Net::FTP; Then you create a new Net::FTP object: $ftp = new Net::FTP("ftp.mysite.com"); Introduction to Perl modules (SW) 26 Example 2: Net::FTP Now you can interact with your new object: $ftp->login("username", "password"); $ftp->cwd("/pub"); $ftp->get("mrperkins.doc"); $ftp->quit(); Introduction to Perl modules (SW) 27 Module documentation When you install Perl modules, you get access to documentation for that module. To read the documentation for a module, use the perldoc command: $ perldoc File::Copy Alternatively, on Unix you can use the man command. On Windows you will have HTML documentation available – there's a link in the Start menu. Introduction to Perl modules (SW) 28 Further reading For the official low-down on modules: $ perldoc perlmod For a list of standard modules: $ perldoc perlmodlib To read in the bath: Learning Perl Objects, References & Modules by Randal L. Schwartz with Tom Phoenix. June 2003 (est.) Introduction to Perl modules (SW) 29 Summary • Modules are libraries of code that you can insert into your Perl programs • Modules save you time and effort, by providing easy access to common tasks • Use the perldoc command to get documentation on a Perl module • Search CPAN for general-purpose modules Introduction to Perl modules (SW) 30