What follows are examples of "
Hello, World" implemented in different styles of modules. It must be understood that a module is not necessary in Perl; functions and code can be defined and used anywhere. This is just for example purposes. Contrast with
Java where a class is always necessary. A real "Hello, World" function would be written like so: sub hello { "Hello, world!\n" } print hello(); or simply printed in one line: print "Hello, world!\n";
Procedural example Here is "Hello, World" implemented as a procedural module with a customizable target for the greeting, just to make things interesting. Also included is a short script to illustrate the module's use.
hello_world.pl • !/usr/bin/env perl • Loads the module and imports any functions into our namespace • (defaults to "main") exported by the module. Hello::World exports • hello() by default. Exports can usually be controlled by the caller. use Hello::World; print hello(); # prints "Hello, world!\n" print hello("Milky Way"); # prints "Hello, Milky Way!\n"
Hello/World.pm • "package" is the namespace where the module's functionality/data resides. • It dictates the name of the file if you want it to be "use"d. • If more than one word, it constrains the location of the module. package Hello::World; • By default Perl allows you to use variables without declaring • them. This may be convenient for short scripts and one-liners. • But in a longer unit of code such as a module it is wise to declare • your variables both to catch typos and to constrain their • accessibility appropriately from outside the module. The strict pragma • forces you to declare your variables. use strict; • Similarly, Perl does not issue most compiler or run-time warnings by default. • More complicated scripts, such as most modules, will usually find them very • helpful for debugging. The warnings pragma turns on optional warnings. use warnings; • A module's version number is stored in $ModuleName::VERSION; certain • forms of the "use" built-in depend on this variable being defined. our $VERSION = '1.00'; • Inherit from the "Exporter" module which handles exporting functions. • Most procedural modules make use of this. use base 'Exporter'; • When the module is invoked, export, by default, the function "hello" into • the namespace of the using code. our @EXPORT = qw(hello); • Lines starting with an equal sign indicate embedded POD • documentation. POD sections end with an =cut directive, and can • be intermixed almost freely with normal code. =head1 NAME Hello::World - An encapsulation of a common output message =head1 SYNOPSIS use Hello::World; print hello(); print hello("Milky Way"); =head1 DESCRIPTION This is a procedural module which gives you the famous "Hello, world!" message, and it’s even customizable! =head2 Functions The following functions are exported by default =head3 hello print hello(); print hello($target); Returns the famous greeting. If a C is given it will be used, otherwise "world" is the target of your greeting. =cut • define the function hello(). sub hello { my $target = shift; $target = 'world' unless defined $target; return "Hello, $target!\n"; } =head1 AUTHOR Joe Hacker =cut • A Perl module must end with a true value or else it is considered not to • have loaded. By convention this value is usually 1 though it can be • any true value. A module can end with false to indicate failure but • this is rarely used and it would instead die() (exit with an error). 1; ---- Since Hello/World.pm is not in your @INC path, you must specify . on the command line to run the above example: perl -I. hello_world.pl
Object-oriented example Here's an example of the same thing done in an object-oriented style. The advantage of an OO module is that each object can be configured independently from other objects.
hello_world.pl • !/usr/bin/env perl use Hello::World; my $hello = Hello::World->new; $hello->print; # prints "Hello, world!\n" $hello->target("Milky Way"); $hello->print; # prints "Hello, Milky Way!\n" my $greeting = Hello::World->new(target => "Pittsburgh"); $greeting->print; # prints "Hello, Pittsburgh!\n" $hello->print; # still prints "Hello, Milky Way!\n"
Hello/World.pm • In Perl there is no special 'class' definition. A namespace is a class. package Hello::World; use strict; use warnings; our $VERSION = "1.00"; =head1 NAME Hello::World - An encapsulation of a common output message =head1 SYNOPSIS use Hello::World; my $hello = Hello::World->new(); $hello->print; =head1 DESCRIPTION This is an object-oriented library which can print the famous "H.W." message. =head2 Methods =head3 new my $hello = Hello::World->new(); my $hello = Hello::World->new( target => $target ); Instantiates an object which holds a greeting message. If a C is given it is passed to Ctarget >>. =cut • The constructor of an object is called new() by convention. Any • method may construct an object and you can have as many as you like. sub new { my($class, %args) = @_; my $self = bless({}, $class); my $target = exists $args{target} ? $args{target} : "world"; $self->{target} = $target; return $self; } =head3 target my $target = $hello->target; $hello->target($target); Gets and sets the current target of our message. =cut sub target { my $self = shift; if ( @_ ) { my $target = shift; $self->{target} = $target; } return $self->{target}; } =head3 to_string my $greeting = $hello->to_string; Returns the $greeting as a string =cut sub to_string { my $self = shift; return "Hello, $self->{target}!"; } =head3 print $hello->print; Outputs the greeting to STDOUT =cut sub print { my $self = shift; print $self->to_string(), "\n"; } =head1 AUTHOR Joe Hacker =cut 1; ---- ==Perl packages and namespaces==