펄 모듈

**펄 모듈**(Perl module)은 펄 언어의 소프트웨어 구성 요소이다.

모듈은 마치 자바 패키지처럼 소스 코드가 *CGI*, *Net:FTP*, *XML::Parser*와 같은 이름공간을 정의하기 위한 펄 매커니즘인 패키지 안에 있을 수 있도록 정의한다. 즉, 파일 구조는 이름공간 구조를 미러링한다. (예: Net::FTP를 위한 소스 코드는 Net/FTP.pm에 속한다)

## 예제
"Hello, World!"의 예는 여러 스타일로 구현할 수 있다. 펄에서는 모듈이 반드시 필요한 것이 아니다. 함수와 코드는 어디에서든 정의하여 사용할 수 있다. 아래는 단지 예를 들기 위한 부분이다. 클래스가 꼭 필요한 자바와 다르다. "Hello, World"의 함수는 다음과 같이 작성할 수 있다:

```perl
sub hello { "Hello, world!\n" }
print hello();
```


아니면 단순히 한 줄로도 가능하다:

```perl
print "Hello, world!\n";
```


### 절차적 예제
절차적 모듈로 구현한 "Hello, World"의 예는 다음과 같다. 모듈 이용을 기술하는 짧은 스크립트도 포함되어 있다.

*hello_world.pl*
----

```perl
#!/usr/bin/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*
----

```perl
# "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<$target> 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 <joe@joehacker.org>

  =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;
```

----

### 객체 지향 예제
동일하지만 객제 지향 스타일로 된 예는 다음과 같다. 객체 지향 모듈의 이점은 각 객체가 다른 객체와 독립적으로 구성될 수 있다는 점이다.

*hello_world.pl*
----

```perl
#!/usr/bin/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*
----

```perl
# 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<$target> is
  given it is passed to C<< $hello->target >>.

  =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 <joe@joehacker.org>

  =cut

  1;
```

----

## 같이 보기
* CPAN

## 외부 링크
*  Perl modules (packages and symbol tables)

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