Split perlmod

[perl5.git] / pod / perlmodlib.pod

=head1 NAME

perlmodlib - constructing new Perl modules and finding existing ones

=head1 DESCRIPTION

=head1 THE PERL MODULE LIBRARY

A number of modules are included the Perl distribution.  These are
described below, and all end in F<.pm>.  You may also discover files in
the library directory that end in either F<.pl> or F<.ph>.  These are old
libraries supplied so that old programs that use them still run.  The
F<.pl> files will all eventually be converted into standard modules, and
the F<.ph> files made by B<h2ph> will probably end up as extension modules
made by B<h2xs>.  (Some F<.ph> values may already be available through the
POSIX module.)  The B<pl2pm> file in the distribution may help in your
conversion, but it's just a mechanical process and therefore far from
bulletproof.

=head2 Pragmatic Modules

They work somewhat like pragmas in that they tend to affect the compilation of
your program, and thus will usually work well only when used within a
C<use>, or C<no>.  Most of these are locally scoped, so an inner BLOCK
may countermand any of these by saying:

    no integer;
    no strict 'refs';

which lasts until the end of that BLOCK.

Unlike the pragmas that effect the C<$^H> hints variable, the C<use
vars> and C<use subs> declarations are not BLOCK-scoped.  They allow
you to predeclare a variables or subroutines within a particular
I<file> rather than just a block.  Such declarations are effective
for the entire file for which they were declared.  You cannot rescind
them with C<no vars> or C<no subs>.

The following pragmas are defined (and have their own documentation).

=over 12

=item use autouse MODULE => qw(sub1 sub2 sub3)

Defers C<require MODULE> until someone calls one of the specified
subroutines (which must be exported by MODULE).  This pragma should be
used with caution, and only when necessary.

=item blib

manipulate @INC at compile time to use MakeMaker's uninstalled version
of a package

=item diagnostics

force verbose warning diagnostics

=item integer

compute arithmetic in integer instead of double

=item less

request less of something from the compiler

=item lib

manipulate @INC at compile time

=item locale

use or ignore current locale for builtin operations (see L<perllocale>)

=item ops

restrict named opcodes when compiling or running Perl code

=item overload

overload basic Perl operations

=item sigtrap

enable simple signal handling

=item strict

restrict unsafe constructs

=item subs

predeclare sub names

=item vmsish

adopt certain VMS-specific behaviors

=item vars

predeclare global variable names

=back

=head2 Standard Modules

Standard, bundled modules are all expected to behave in a well-defined
manner with respect to namespace pollution because they use the
Exporter module.  See their own documentation for details.

=over 12

=item AnyDBM_File

provide framework for multiple DBMs

=item AutoLoader

load functions only on demand

=item AutoSplit

split a package for autoloading

=item Benchmark

benchmark running times of code

=item CPAN

interface to Comprehensive Perl Archive Network

=item CPAN::FirstTime

create a CPAN configuration file

=item CPAN::Nox

run CPAN while avoiding compiled extensions

=item Carp

warn of errors (from perspective of caller)

=item Class::Struct

declare struct-like datatypes

=item Config

access Perl configuration information

=item Cwd

get pathname of current working directory

=item DB_File

access to Berkeley DB

=item Devel::SelfStubber

generate stubs for a SelfLoading module

=item DirHandle

supply object methods for directory handles

=item DynaLoader

dynamically load C libraries into Perl code

=item English

use nice English (or awk) names for ugly punctuation variables

=item Env

import environment variables

=item Exporter

implements default import method for modules

=item ExtUtils::Embed

utilities for embedding Perl in C/C++ applications

=item ExtUtils::Install

install files from here to there

=item ExtUtils::Liblist

determine libraries to use and how to use them

=item ExtUtils::MM_OS2

methods to override Unix behaviour in ExtUtils::MakeMaker

=item ExtUtils::MM_Unix

methods used by ExtUtils::MakeMaker

=item ExtUtils::MM_VMS

methods to override Unix behaviour in ExtUtils::MakeMaker

=item ExtUtils::MakeMaker

create an extension Makefile

=item ExtUtils::Manifest

utilities to write and check a MANIFEST file

=item ExtUtils::Mkbootstrap

make a bootstrap file for use by DynaLoader

=item ExtUtils::Mksymlists

write linker options files for dynamic extension

=item ExtUtils::testlib

add blib/* directories to @INC

=item Fcntl

load the C Fcntl.h defines

=item File::Basename

split a pathname into pieces

=item File::CheckTree

run many filetest checks on a tree

=item File::Compare

compare files or filehandles

=item File::Copy

copy files or filehandles

=item File::Find

traverse a file tree

=item File::Path

create or remove a series of directories

=item File::stat

by-name interface to Perl's builtin stat() functions

=item FileCache

keep more files open than the system permits

=item FileHandle

supply object methods for filehandles

=item FindBin

locate directory of original perl script

=item GDBM_File

access to the gdbm library

=item Getopt::Long

extended processing of command line options

=item Getopt::Std

process single-character switches with switch clustering

=item I18N::Collate

compare 8-bit scalar data according to the current locale

=item IO

load various IO modules

=item IO::File

supply object methods for filehandles

=item IO::Handle

supply object methods for I/O handles

=item IO::Pipe

supply object methods for pipes

=item IO::Seekable

supply seek based methods for I/O objects

=item IO::Select

OO interface to the select system call

=item IO::Socket

object interface to socket communications

=item IPC::Open2

open a process for both reading and writing

=item IPC::Open3

open a process for reading, writing, and error handling

=item Math::BigFloat

arbitrary length float math package

=item Math::BigInt

arbitrary size integer math package

=item Math::Complex

complex numbers and associated mathematical functions

=item NDBM_File

tied access to ndbm files

=item Net::Ping

Hello, anybody home?

=item Net::hostent

by-name interface to Perl's builtin gethost*() functions

=item Net::netent

by-name interface to Perl's builtin getnet*() functions

=item Net::protoent

by-name interface to Perl's builtin getproto*() functions

=item Net::servent

by-name interface to Perl's builtin getserv*() functions

=item Opcode

disable named opcodes when compiling or running perl code

=item Pod::Text

convert POD data to formatted ASCII text

=item POSIX

interface to IEEE Standard 1003.1

=item SDBM_File

tied access to sdbm files

=item Safe

compile and execute code in restricted compartments

=item Search::Dict

search for key in dictionary file

=item SelectSaver

save and restore selected file handle

=item SelfLoader

load functions only on demand

=item Shell

run shell commands transparently within perl

=item Socket

load the C socket.h defines and structure manipulators

=item Symbol

manipulate Perl symbols and their names

=item Sys::Hostname

try every conceivable way to get hostname

=item Sys::Syslog

interface to the Unix syslog(3) calls

=item Term::Cap

termcap interface

=item Term::Complete

word completion module

=item Term::ReadLine

interface to various C<readline> packages

=item Test::Harness

run perl standard test scripts with statistics

=item Text::Abbrev

create an abbreviation table from a list

=item Text::ParseWords

parse text into an array of tokens

=item Text::Soundex

implementation of the Soundex Algorithm as described by Knuth

=item Text::Tabs

expand and unexpand tabs per the Unix expand(1) and unexpand(1)

=item Text::Wrap

line wrapping to form simple paragraphs

=item Tie::Hash

base class definitions for tied hashes

=item Tie::RefHash

base class definitions for tied hashes with references as keys

=item Tie::Scalar

base class definitions for tied scalars

=item Tie::SubstrHash

fixed-table-size, fixed-key-length hashing

=item Time::Local

efficiently compute time from local and GMT time

=item Time::gmtime

by-name interface to Perl's builtin gmtime() function

=item Time::localtime

by-name interface to Perl's builtin localtime() function

=item Time::tm

internal object used by Time::gmtime and Time::localtime

=item UNIVERSAL

base class for ALL classes (blessed references)

=item User::grent

by-name interface to Perl's builtin getgr*() functions

=item User::pwent

by-name interface to Perl's builtin getpw*() functions

=back

To find out I<all> the modules installed on your system, including
those without documentation or outside the standard release, do this:

    find `perl -e 'print "@INC"'` -name '*.pm' -print

They should all have their own documentation installed and accessible via
your system man(1) command.  If that fails, try the I<perldoc> program.

=head2 Extension Modules

Extension modules are written in C (or a mix of Perl and C) and may be
statically linked or in general are
dynamically loaded into Perl if and when you need them.  Supported
extension modules include the Socket, Fcntl, and POSIX modules.

Many popular C extension modules do not come bundled (at least, not
completely) due to their sizes, volatility, or simply lack of time for
adequate testing and configuration across the multitude of platforms on
which Perl was beta-tested.  You are encouraged to look for them in
archie(1L), the Perl FAQ or Meta-FAQ, the WWW page, and even with their
authors before randomly posting asking for their present condition and
disposition.

=head1 CPAN

CPAN stands for the Comprehensive Perl Archive Network.  This is a globally
replicated collection of all known Perl materials, including hundreds
of unbundled modules.  Here are the major categories of modules:

=over

=item *
Language Extensions and Documentation Tools

=item *
Development Support

=item *
Operating System Interfaces

=item *
Networking, Device Control (modems) and InterProcess Communication

=item *
Data Types and Data Type Utilities

=item *
Database Interfaces

=item *
User Interfaces

=item *
Interfaces to / Emulations of Other Programming Languages

=item *
File Names, File Systems and File Locking (see also File Handles)

=item *
String Processing, Language Text Processing, Parsing, and Searching

=item *
Option, Argument, Parameter, and Configuration File Processing

=item *
Internationalization and Locale

=item *
Authentication, Security, and Encryption

=item *
World Wide Web, HTML, HTTP, CGI, MIME

=item *
Server and Daemon Utilities

=item *
Archiving and Compression

=item *
Images, Pixmap and Bitmap Manipulation, Drawing, and Graphing

=item *
Mail and Usenet News

=item *
Control Flow Utilities (callbacks and exceptions etc)

=item *
File Handle and Input/Output Stream Utilities

=item *
Miscellaneous Modules

=back

The registered CPAN sites as of this writing include the following.
You should try to choose one close to you:

=over

=item *
Africa

    South Africa    ftp://ftp.is.co.za/programming/perl/CPAN/

=item *
Asia

    Hong Kong       ftp://ftp.hkstar.com/pub/CPAN/
    Japan           ftp://ftp.jaist.ac.jp/pub/lang/perl/CPAN/
                    ftp://ftp.lab.kdd.co.jp/lang/perl/CPAN/
    South Korea     ftp://ftp.nuri.net/pub/CPAN/
    Taiwan          ftp://dongpo.math.ncu.edu.tw/perl/CPAN/
                    ftp://ftp.wownet.net/pub2/PERL/

=item *
Australasia

    Australia       ftp://ftp.netinfo.com.au/pub/perl/CPAN/
    New Zealand     ftp://ftp.tekotago.ac.nz/pub/perl/CPAN/

=item *
Europe

    Austria         ftp://ftp.tuwien.ac.at/pub/languages/perl/CPAN/
    Belgium         ftp://ftp.kulnet.kuleuven.ac.be/pub/mirror/CPAN/
    Czech Republic  ftp://sunsite.mff.cuni.cz/Languages/Perl/CPAN/
    Denmark         ftp://sunsite.auc.dk/pub/languages/perl/CPAN/
    Finland         ftp://ftp.funet.fi/pub/languages/perl/CPAN/
    France          ftp://ftp.ibp.fr/pub/perl/CPAN/
                    ftp://ftp.pasteur.fr/pub/computing/unix/perl/CPAN/
    Germany         ftp://ftp.gmd.de/packages/CPAN/
                    ftp://ftp.leo.org/pub/comp/programming/languages/perl/CPAN/
                    ftp://ftp.mpi-sb.mpg.de/pub/perl/CPAN/
                    ftp://ftp.rz.ruhr-uni-bochum.de/pub/CPAN/
                    ftp://ftp.uni-erlangen.de/pub/source/Perl/CPAN/
                    ftp://ftp.uni-hamburg.de/pub/soft/lang/perl/CPAN/
    Greece          ftp://ftp.ntua.gr/pub/lang/perl/
    Hungary         ftp://ftp.kfki.hu/pub/packages/perl/CPAN/
    Italy           ftp://cis.utovrm.it/CPAN/
    the Netherlands ftp://ftp.cs.ruu.nl/pub/PERL/CPAN/
                    ftp://ftp.EU.net/packages/cpan/
    Norway          ftp://ftp.uit.no/pub/languages/perl/cpan/
    Poland          ftp://ftp.pk.edu.pl/pub/lang/perl/CPAN/
                    ftp://sunsite.icm.edu.pl/pub/CPAN/
    Portugal        ftp://ftp.ci.uminho.pt/pub/lang/perl/
                    ftp://ftp.telepac.pt/pub/CPAN/
    Russia          ftp://ftp.sai.msu.su/pub/lang/perl/CPAN/
    Slovenia        ftp://ftp.arnes.si/software/perl/CPAN/
    Spain           ftp://ftp.etse.urv.es/pub/mirror/perl/
                    ftp://ftp.rediris.es/mirror/CPAN/
    Sweden          ftp://ftp.sunet.se/pub/lang/perl/CPAN/
    UK              ftp://ftp.demon.co.uk/pub/mirrors/perl/CPAN/
                    ftp://sunsite.doc.ic.ac.uk/packages/CPAN/
                    ftp://unix.hensa.ac.uk/mirrors/perl-CPAN/

=item *
North America

    Ontario         ftp://ftp.utilis.com/public/CPAN/
                    ftp://enterprise.ic.gc.ca/pub/perl/CPAN/
    Manitoba        ftp://theory.uwinnipeg.ca/pub/CPAN/
    California      ftp://ftp.digital.com/pub/plan/perl/CPAN/
                    ftp://ftp.cdrom.com/pub/perl/CPAN/
    Colorado        ftp://ftp.cs.colorado.edu/pub/perl/CPAN/
    Florida         ftp://ftp.cis.ufl.edu/pub/perl/CPAN/
    Illinois        ftp://uiarchive.uiuc.edu/pub/lang/perl/CPAN/
    Massachusetts   ftp://ftp.iguide.com/pub/mirrors/packages/perl/CPAN/
    New York        ftp://ftp.rge.com/pub/languages/perl/
    North Carolina  ftp://ftp.duke.edu/pub/perl/
    Oklahoma        ftp://ftp.ou.edu/mirrors/CPAN/
    Oregon          http://www.perl.org/CPAN/
                    ftp://ftp.orst.edu/pub/packages/CPAN/
    Pennsylvania    ftp://ftp.epix.net/pub/languages/perl/
    Texas           ftp://ftp.sedl.org/pub/mirrors/CPAN/
                    ftp://ftp.metronet.com/pub/perl/

=item *
South America

    Chile           ftp://sunsite.dcc.uchile.cl/pub/Lang/perl/CPAN/

=back

For an up-to-date listing of CPAN sites,
see F<http://www.perl.com/perl/CPAN> or F<ftp://ftp.perl.com/perl/>.

=head1 Modules: Creation, Use, and Abuse

(The following section is borrowed directly from Tim Bunce's modules
file, available at your nearest CPAN site.)

Perl implements a class using a package, but the presence of a
package doesn't imply the presence of a class.  A package is just a
namespace.  A class is a package that provides subroutines that can be
used as methods.  A method is just a subroutine that expects, as its
first argument, either the name of a package (for "static" methods),
or a reference to something (for "virtual" methods).

A module is a file that (by convention) provides a class of the same
name (sans the .pm), plus an import method in that class that can be
called to fetch exported symbols.  This module may implement some of
its methods by loading dynamic C or C++ objects, but that should be
totally transparent to the user of the module.  Likewise, the module
might set up an AUTOLOAD function to slurp in subroutine definitions on
demand, but this is also transparent.  Only the F<.pm> file is required to
exist.  See L<perlsub>, L<perltoot>, and L<AutoLoader> for details about 
the AUTOLOAD mechanism.

=head2 Guidelines for Module Creation

=over 4

=item Do similar modules already exist in some form?

If so, please try to reuse the existing modules either in whole or
by inheriting useful features into a new class.  If this is not
practical try to get together with the module authors to work on
extending or enhancing the functionality of the existing modules.
A perfect example is the plethora of packages in perl4 for dealing
with command line options.

If you are writing a module to expand an already existing set of
modules, please coordinate with the author of the package.  It
helps if you follow the same naming scheme and module interaction
scheme as the original author.

=item Try to design the new module to be easy to extend and reuse.

Use blessed references.  Use the two argument form of bless to bless
into the class name given as the first parameter of the constructor,
e.g.,:

 sub new {
	my $class = shift;
	return bless {}, $class;
 }

or even this if you'd like it to be used as either a static
or a virtual method.

 sub new {
	my $self  = shift;
	my $class = ref($self) || $self;
	return bless {}, $class;
 }

Pass arrays as references so more parameters can be added later
(it's also faster).  Convert functions into methods where
appropriate.  Split large methods into smaller more flexible ones.
Inherit methods from other modules if appropriate.

Avoid class name tests like: C<die "Invalid" unless ref $ref eq 'FOO'>.
Generally you can delete the "C<eq 'FOO'>" part with no harm at all.
Let the objects look after themselves! Generally, avoid hard-wired
class names as far as possible.

Avoid C<$r-E<gt>Class::func()> where using C<@ISA=qw(... Class ...)> and
C<$r-E<gt>func()> would work (see L<perlbot> for more details).

Use autosplit so little used or newly added functions won't be a
burden to programs which don't use them. Add test functions to
the module after __END__ either using AutoSplit or by saying:

 eval join('',<main::DATA>) || die $@ unless caller();

Does your module pass the 'empty subclass' test? If you say
"C<@SUBCLASS::ISA = qw(YOURCLASS);>" your applications should be able
to use SUBCLASS in exactly the same way as YOURCLASS.  For example,
does your application still work if you change:  C<$obj = new YOURCLASS;>
into: C<$obj = new SUBCLASS;> ?

Avoid keeping any state information in your packages. It makes it
difficult for multiple other packages to use yours. Keep state
information in objects.

Always use B<-w>. Try to C<use strict;> (or C<use strict qw(...);>).
Remember that you can add C<no strict qw(...);> to individual blocks
of code which need less strictness. Always use B<-w>. Always use B<-w>!
Follow the guidelines in the perlstyle(1) manual.

=item Some simple style guidelines

The perlstyle manual supplied with perl has many helpful points.

Coding style is a matter of personal taste. Many people evolve their
style over several years as they learn what helps them write and
maintain good code.  Here's one set of assorted suggestions that
seem to be widely used by experienced developers:

Use underscores to separate words.  It is generally easier to read
$var_names_like_this than $VarNamesLikeThis, especially for
non-native speakers of English. It's also a simple rule that works
consistently with VAR_NAMES_LIKE_THIS.

Package/Module names are an exception to this rule. Perl informally
reserves lowercase module names for 'pragma' modules like integer
and strict. Other modules normally begin with a capital letter and
use mixed case with no underscores (need to be short and portable).

You may find it helpful to use letter case to indicate the scope
or nature of a variable. For example:

 $ALL_CAPS_HERE   constants only (beware clashes with perl vars)
 $Some_Caps_Here  package-wide global/static
 $no_caps_here    function scope my() or local() variables

Function and method names seem to work best as all lowercase.
e.g., C<$obj-E<gt>as_string()>.

You can use a leading underscore to indicate that a variable or
function should not be used outside the package that defined it.

=item Select what to export.

Do NOT export method names!

Do NOT export anything else by default without a good reason!

Exports pollute the namespace of the module user.  If you must
export try to use @EXPORT_OK in preference to @EXPORT and avoid
short or common names to reduce the risk of name clashes.

Generally anything not exported is still accessible from outside the
module using the ModuleName::item_name (or C<$blessed_ref-E<gt>method>)
syntax.  By convention you can use a leading underscore on names to
indicate informally that they are 'internal' and not for public use.

(It is actually possible to get private functions by saying:
C<my $subref = sub { ... };  &$subref;>.  But there's no way to call that
directly as a method, because a method must have a name in the symbol
table.)

As a general rule, if the module is trying to be object oriented
then export nothing. If it's just a collection of functions then
@EXPORT_OK anything but use @EXPORT with caution.

=item Select a name for the module.

This name should be as descriptive, accurate, and complete as
possible.  Avoid any risk of ambiguity. Always try to use two or
more whole words.  Generally the name should reflect what is special
about what the module does rather than how it does it.  Please use
nested module names to group informally or categorize a module.
There should be a very good reason for a module not to have a nested name.
Module names should begin with a capital letter.

Having 57 modules all called Sort will not make life easy for anyone
(though having 23 called Sort::Quick is only marginally better :-).
Imagine someone trying to install your module alongside many others.
If in any doubt ask for suggestions in comp.lang.perl.misc.

If you are developing a suite of related modules/classes it's good
practice to use nested classes with a common prefix as this will
avoid namespace clashes. For example: Xyz::Control, Xyz::View,
Xyz::Model etc. Use the modules in this list as a naming guide.

If adding a new module to a set, follow the original author's
standards for naming modules and the interface to methods in
those modules.

To be portable each component of a module name should be limited to
11 characters. If it might be used on MS-DOS then try to ensure each is
unique in the first 8 characters. Nested modules make this easier.

=item Have you got it right?

How do you know that you've made the right decisions? Have you
picked an interface design that will cause problems later? Have
you picked the most appropriate name? Do you have any questions?

The best way to know for sure, and pick up many helpful suggestions,
is to ask someone who knows. Comp.lang.perl.misc is read by just about
all the people who develop modules and it's the best place to ask.

All you need to do is post a short summary of the module, its
purpose and interfaces. A few lines on each of the main methods is
probably enough. (If you post the whole module it might be ignored
by busy people - generally the very people you want to read it!)

Don't worry about posting if you can't say when the module will be
ready - just say so in the message. It might be worth inviting
others to help you, they may be able to complete it for you!

=item README and other Additional Files.

It's well known that software developers usually fully document the
software they write. If, however, the world is in urgent need of
your software and there is not enough time to write the full
documentation please at least provide a README file containing:

=over 10

=item *
A description of the module/package/extension etc.

=item *
A copyright notice - see below.

=item *
Prerequisites - what else you may need to have.

=item *
How to build it - possible changes to Makefile.PL etc.

=item *
How to install it.

=item *
Recent changes in this release, especially incompatibilities

=item *
Changes / enhancements you plan to make in the future.

=back

If the README file seems to be getting too large you may wish to
split out some of the sections into separate files: INSTALL,
Copying, ToDo etc.

=over 4

=item Adding a Copyright Notice.

How you choose to license your work is a personal decision.
The general mechanism is to assert your Copyright and then make
a declaration of how others may copy/use/modify your work.

Perl, for example, is supplied with two types of licence: The GNU
GPL and The Artistic Licence (see the files README, Copying, and
Artistic).  Larry has good reasons for NOT just using the GNU GPL.

My personal recommendation, out of respect for Larry, Perl, and the
perl community at large is to state something simply like:

 Copyright (c) 1995 Your Name. All rights reserved.
 This program is free software; you can redistribute it and/or
 modify it under the same terms as Perl itself.

This statement should at least appear in the README file. You may
also wish to include it in a Copying file and your source files.
Remember to include the other words in addition to the Copyright.

=item Give the module a version/issue/release number.

To be fully compatible with the Exporter and MakeMaker modules you
should store your module's version number in a non-my package
variable called $VERSION.  This should be a floating point
number with at least two digits after the decimal (i.e., hundredths,
e.g, C<$VERSION = "0.01">).  Don't use a "1.3.2" style version.
See Exporter.pm in Perl5.001m or later for details.

It may be handy to add a function or method to retrieve the number.
Use the number in announcements and archive file names when
releasing the module (ModuleName-1.02.tar.Z).
See perldoc ExtUtils::MakeMaker.pm for details.

=item How to release and distribute a module.

It's good idea to post an announcement of the availability of your
module (or the module itself if small) to the comp.lang.perl.announce
Usenet newsgroup.  This will at least ensure very wide once-off
distribution.

If possible you should place the module into a major ftp archive and
include details of its location in your announcement.

Some notes about ftp archives: Please use a long descriptive file
name which includes the version number. Most incoming directories
will not be readable/listable, i.e., you won't be able to see your
file after uploading it. Remember to send your email notification
message as soon as possible after uploading else your file may get
deleted automatically. Allow time for the file to be processed
and/or check the file has been processed before announcing its
location.

FTP Archives for Perl Modules:

Follow the instructions and links on

   http://franz.ww.tu-berlin.de/modulelist

or upload to one of these sites:

   ftp://franz.ww.tu-berlin.de/incoming
   ftp://ftp.cis.ufl.edu/incoming

and notify <F<upload@franz.ww.tu-berlin.de>>.

By using the WWW interface you can ask the Upload Server to mirror
your modules from your ftp or WWW site into your own directory on
CPAN!

Please remember to send me an updated entry for the Module list!

=item Take care when changing a released module.

Always strive to remain compatible with previous released versions
(see 2.2 above) Otherwise try to add a mechanism to revert to the
old behaviour if people rely on it. Document incompatible changes.

=back

=back

=head2 Guidelines for Converting Perl 4 Library Scripts into Modules

=over 4

=item There is no requirement to convert anything.

If it ain't broke, don't fix it! Perl 4 library scripts should
continue to work with no problems. You may need to make some minor
changes (like escaping non-array @'s in double quoted strings) but
there is no need to convert a .pl file into a Module for just that.

=item Consider the implications.

All the perl applications which make use of the script will need to
be changed (slightly) if the script is converted into a module.  Is
it worth it unless you plan to make other changes at the same time?

=item Make the most of the opportunity.

If you are going to convert the script to a module you can use the
opportunity to redesign the interface. The 'Guidelines for Module
Creation' above include many of the issues you should consider.

=item The pl2pm utility will get you started.

This utility will read *.pl files (given as parameters) and write
corresponding *.pm files. The pl2pm utilities does the following:

=over 10

=item *
Adds the standard Module prologue lines

=item *
Converts package specifiers from ' to ::

=item *
Converts die(...) to croak(...)

=item *
Several other minor changes

=back

Being a mechanical process pl2pm is not bullet proof. The converted
code will need careful checking, especially any package statements.
Don't delete the original .pl file till the new .pm one works!

=back

=head2 Guidelines for Reusing Application Code

=over 4

=item Complete applications rarely belong in the Perl Module Library.

=item Many applications contain some perl code which could be reused.

Help save the world! Share your code in a form that makes it easy
to reuse.

=item Break-out the reusable code into one or more separate module files.

=item Take the opportunity to reconsider and redesign the interfaces.

=item In some cases the 'application' can then be reduced to a small

fragment of code built on top of the reusable modules. In these cases
the application could invoked as:

     perl -e 'use Module::Name; method(@ARGV)' ...
or
     perl -mModule::Name ...    (in perl5.002 or higher)

=back

=head1 NOTE

Perl does not enforce private and public parts of its modules as you may
have been used to in other languages like C++, Ada, or Modula-17.  Perl
doesn't have an infatuation with enforced privacy.  It would prefer
that you stayed out of its living room because you weren't invited, not
because it has a shotgun.

The module and its user have a contract, part of which is common law,
and part of which is "written".  Part of the common law contract is
that a module doesn't pollute any namespace it wasn't asked to.  The
written contract for the module (A.K.A. documentation) may make other
provisions.  But then you know when you C<use RedefineTheWorld> that
you're redefining the world and willing to take the consequences.