better diagnostics on read operations from write-only

[p5sagit/p5-mst-13.2.git] / pod / perlmodlib.pod

=head1 NAME

perlmodlib - constructing new Perl modules and finding existing ones

=head1 DESCRIPTION

=head1 THE PERL MODULE LIBRARY

Many modules are included the Perl distribution.  These are described
below, and all end in F<.pm>.  You may discover compiled library
file (usually ending in F<.so>) or small pieces of modules to be
autoloaded (ending in F<.al>); these were automatically generated
by the installation process.  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, Errno, or Fcntl modules.)
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 compiler directives (pragmata) 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 lexically scoped, so an inner BLOCK may countermand them
by saying:

    no integer;
    no strict 'refs';

which lasts until the end of that BLOCK.

Some pragmas are lexically scoped--typically those that affect the
C<$^H> hints variable.  Others affect the current package instead,
like C<use vars> and C<use subs>, whic 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 attrs

set/get attributes of a subroutine

=item autouse

postpone load of modules until a function is used

=item base

Establish IS-A relationship with base class at compile time

=item blib

Use MakeMaker's uninstalled version of a package

=item constant

declare constants

=item diagnostics

Perl compiler pragma to force verbose warning diagnostics

=item fields

compile-time class fields

=item filetest

control the filetest permission operators

=item integer

compute arithmetic in integer instead of double

=item less

perl pragma to request less of something from the compiler

=item lib

manipulate @INC at compile time

=item locale

use and avoid POSIX locales for built-in operations

=item ops

restrict unsafe operations when compiling

=item overload

Package for overloading perl operations

=item re

alter regular expression behavior

=item sigtrap

enable simple signal handling

=item strict

restrict unsafe constructs

=item subs

predeclare sub names

=item utf8

turn on UTF-8 and Unicode support

=item vars

predeclare global variable names

=item vmsish

control VMS-specific language features

=item warning

control optional warnings

=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 subroutines only on demand

=item AutoSplit

split a package for autoloading

=item B

The Perl Compiler; See also L<perlcc>.

=item B::Asmdata

Autogenerated data about Perl ops, used to generate bytecode

=item B::Assembler

Assemble Perl bytecode

=item B::Bblock

Walk basic blocks

=item B::Bytecode

Perl compiler's bytecode backend

=item B::C

Perl compiler's C backend

=item B::CC

Perl compiler's optimized C translation backend

=item B::Debug

Walk Perl syntax tree, printing debug info about ops

=item B::Deparse

Perl compiler backend to produce perl code

=item B::Disassembler

Disassemble Perl bytecode

=item B::Lint

Perl lint

=item B::Showlex

Show lexical variables used in functions or files

=item B::Stackobj

Helper module for CC backend

=item B::Terse

Walk Perl syntax tree, printing terse info about ops

=item B::Xref

Generates cross reference reports for Perl programs

=item Benchmark

benchmark running times of code

=item CGI

Simple Common Gateway Interface Class

=item CGI::Apache

Make things work with CGI.pm against Perl-Apache API

=item CGI::Carp

CGI routines for writing to the HTTPD (or other) error log

=item CGI::Cookie

Interface to Netscape Cookies

=item CGI::Fast

CGI Interface for Fast CGI

=item CGI::Push

Simple Interface to Server Push

=item CGI::Switch

Try more than one constructors and return the first object available

=item CPAN

query, download and build perl modules from CPAN sites

=item CPAN::FirstTime

Utility for CPAN::Config file Initialization

=item CPAN::Nox

Wrapper around CPAN.pm without using any XS module

=item Carp

warn of errors (from perspective of caller)

=item Class::Struct

declare struct-like datatypes as Perl classes

=item Config

access Perl configuration information

=item Cwd

get pathname of current working directory

=item DB

programmatic interface to the Perl debugging API 

=item DB_File

Perl5 access to Berkeley DB version 1.x

=item Data::Dumper

stringified perl data structures, suitable for both printing and C<eval>

=item Devel::Peek

A data debugging tool for the XS programmer

=item Devel::SelfStubber

generate stubs for a SelfLoading module

=item DirHandle

supply object methods for directory handles

=item Dumpvalue

provides screen dump of Perl data.

=item DynaLoader

Dynamically load C libraries into Perl code

=item English

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

=item Env

perl module that imports environment variables

=item Errno

System errno constants

=item Exporter

Implements default import method for modules

=item ExtUtils::Command

utilities to replace common UNIX commands in Makefiles etc.

=item ExtUtils::Embed

Utilities for embedding Perl in C/C++ applications

=item ExtUtils::Install

install files from here to there

=item ExtUtils::Installed

Inventory management of installed modules

=item ExtUtils::Liblist

determine libraries to use and how to use them

=item ExtUtils::MM_OS2

methods to override UN*X behavior in ExtUtils::MakeMaker

=item ExtUtils::MM_Unix

methods used by ExtUtils::MakeMaker

=item ExtUtils::MM_VMS

methods to override UN*X behavior in ExtUtils::MakeMaker

=item ExtUtils::MM_Win32

methods to override UN*X behavior in ExtUtils::MakeMaker

=item ExtUtils::MakeMaker

create an extension Makefile

=item ExtUtils::Manifest

utilities to write and check a MANIFEST file

=item ExtUtils::Miniperl

write the C code for perlmain.c

=item ExtUtils::Mkbootstrap

make a bootstrap file for use by DynaLoader

=item ExtUtils::Mksymlists

write linker options files for dynamic extension

=item ExtUtils::Packlist

manage .packlist files

=item ExtUtils::testlib

add blib/* directories to @INC

=item Fatal

replace functions with equivalents which succeed or die

=item Fcntl

load the C Fcntl.h defines

=item File::Basename

split a pathname into pieces

=item File::Compare

Compare files or filehandles

=item File::Copy

Copy files or filehandles

=item File::DosGlob

DOS like globbing and then some

=item File::Find

traverse a file tree

=item File::Path

create or remove a series of directories

=item File::Spec

portably perform operations on file names

=item File::Spec::Functions

portably perform operations on file names

=item File::Spec::Mac

File::Spec for MacOS

=item File::Spec::OS2

methods for OS/2 file specs

=item File::Spec::Unix

methods used by File::Spec

=item File::Spec::VMS

methods for VMS file specs

=item File::Spec::Win32

methods for Win32 file specs

=item File::stat

by-name interface to Perl's built-in 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

Perl5 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::Dir

supply object methods for directory handles

=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::Poll

Object interface to system poll call

=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 IO::Socket::INET

Object interface for AF_INET domain sockets

=item IO::Socket::UNIX

Object interface for AF_UNIX domain sockets

=item IPC::Msg

SysV Msg IPC object class

=item IPC::Open2

open a process for both reading and writing

=item IPC::Open3

open a process for reading, writing, and error handling

=item IPC::Semaphore

SysV Semaphore IPC object class

=item IPC::SysV

SysV IPC constants

=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 Math::Trig

trigonometric functions

=item NDBM_File

Tied access to ndbm files

=item Net::Ping

check a remote host for reachability

=item Net::hostent

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

=item Net::netent

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

=item Net::protoent

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

=item Net::servent

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

=item O

Generic interface to Perl Compiler backends

=item Opcode

Disable named opcodes when compiling perl code

=item POSIX

Perl interface to IEEE Std 1003.1

=item Pod::Html

module to convert pod files to HTML

=item Pod::Text

convert POD data to formatted ASCII text

=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

Perl interface to the UNIX syslog(3) calls

=item Term::Cap

Perl termcap interface

=item Term::Complete

Perl word completion module

=item Term::ReadLine

Perl interface to various C<readline> packages. 

=item Test

provides a simple framework for writing test scripts

=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 or array of arrays

=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 Thread 

multithreading

=item Thread::Queue 

thread-safe queues

=item Thread::Semaphore 

thread-safe semaphores

=item Thread::Signal 

Start a thread which runs signal handlers reliably

=item Thread::Specific 

thread-specific keys

=item Tie::Array

base class for tied arrays

=item Tie::Handle

base class definitions for tied handles

=item Tie::Hash, Tie::StdHash

base class definitions for tied hashes

=item Tie::RefHash

use references as hash keys

=item Tie::Scalar, Tie::StdScalar

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 built-in gmtime() function

=item Time::localtime

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

=item User::pwent

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

=back

To find out I<all> modules installed on your system, including
those without documentation or outside the standard release, 
jus tdo 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 you do not have a B<find>
program, you can use the Perl B<find2perl> program instead, which
generates Perl code as output you can run through perl.  If you
have a B<man> program but it doesn't find your modules, you'll have
to fix your manpath.  See L<perl> for details.  If you have no
system B<man> command, you might try the B<perldoc> program.

=head2 Extension Modules

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

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 on CPAN (described below), or using web search engines
like Alta Vista or Deja News.

=head1 CPAN

CPAN stands for Comprehensive Perl Archive Network; it's a globally
replicated trove of Perl materials, including documentation, style
guides, tricks and trap, alternate ports to non-Unix systems and
occasional binary distributions for these.   Search engines for
CPAN can be found at http://cpan.perl.com/ and at
http://theory.uwinnipeg.ca/mod_perl/cpan-search.pl .

Most importantly, CPAN includes around a thousand unbundled modules,
some of which require a C compiler to build.  Major categories of
modules are:

=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

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/
                   ftp://ftpza.co.za/pub/mirrors/cpan/

=item Asia

   China           ftp://freesoft.cei.gov.cn/pub/languages/perl/CPAN/
   Hong Kong       ftp://ftp.hkstar.com/pub/CPAN/
   Israel          ftp://bioinfo.weizmann.ac.il/pub/software/perl/CPAN/
   Japan           ftp://ftp.dti.ad.jp/pub/lang/CPAN/
                   ftp://ftp.jaist.ac.jp/pub/lang/perl/CPAN/
                   ftp://ftp.lab.kdd.co.jp/lang/perl/CPAN/
                   ftp://ftp.meisei-u.ac.jp/pub/CPAN/
                   ftp://ftp.ring.gr.jp/pub/lang/perl/CPAN/
                   ftp://mirror.nucba.ac.jp/mirror/Perl/
   Singapore       ftp://ftp.nus.edu.sg/pub/unix/perl/CPAN/
   South Korea     ftp://ftp.bora.net/pub/CPAN/
                   ftp://ftp.nuri.net/pub/CPAN/
   Taiwan          ftp://ftp.wownet.net/pub2/PERL/
                   ftp://ftp1.sinica.edu.tw/pub1/perl/CPAN/
   Thailand        ftp://ftp.cs.riubon.ac.th/pub/mirrors/CPAN/
                   ftp://ftp.nectec.or.th/pub/mirrors/CPAN/

=item Australasia

   Australia       ftp://cpan.topend.com.au/pub/CPAN/
                   ftp://ftp.labyrinth.net.au/pub/perl/CPAN/
                   ftp://ftp.sage-au.org.au/pub/compilers/perl/CPAN/
                   ftp://mirror.aarnet.edu.au/pub/perl/CPAN/
   New Zealand     ftp://ftp.auckland.ac.nz/pub/perl/CPAN/
                   ftp://sunsite.net.nz/pub/languages/perl/CPAN/

Central America

   Costa Rica      ftp://ftp.ucr.ac.cr/pub/Unix/CPAN/

=item Europe

   Austria         ftp://ftp.tuwien.ac.at/pub/languages/perl/CPAN/
   Belgium         ftp://ftp.kulnet.kuleuven.ac.be/pub/mirror/CPAN/
   Bulgaria        ftp://ftp.ntrl.net/pub/mirrors/CPAN/
   Croatia         ftp://ftp.linux.hr/pub/CPAN/
   Czech Republic  ftp://ftp.fi.muni.cz/pub/perl/
                   ftp://sunsite.mff.cuni.cz/Languages/Perl/CPAN/
   Denmark         ftp://sunsite.auc.dk/pub/languages/perl/CPAN/
   Estonia         ftp://ftp.ut.ee/pub/languages/perl/CPAN/
   Finland         ftp://ftp.funet.fi/pub/languages/perl/CPAN/
   France          ftp://ftp.lip6.fr/pub/perl/CPAN/
                   ftp://ftp.oleane.net/pub/mirrors/CPAN/
                   ftp://ftp.pasteur.fr/pub/computing/CPAN/
   Germany         ftp://ftp.archive.de.uu.net/pub/CPAN/
                   ftp://ftp.gmd.de/packages/CPAN/
                   ftp://ftp.gwdg.de/pub/languages/perl/CPAN/
                   ftp://ftp.leo.org/pub/comp/programming/languages/script/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/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/
   Ireland         ftp://sunsite.compapp.dcu.ie/pub/perl/
   Italy           ftp://cis.uniRoma2.it/CPAN/
                   ftp://ftp.flashnet.it/pub/CPAN/
                   ftp://ftp.unina.it/pub/Other/CPAN/
                   ftp://ftp.unipi.it/pub/mirror/perl/CPAN/
   Netherlands     ftp://ftp.cs.uu.nl/mirror/CPAN/
                   ftp://ftp.EU.net/packages/cpan/
                   ftp://ftp.nluug.nl/pub/languages/perl/CPAN/
   Norway          ftp://ftp.uit.no/pub/languages/perl/cpan/
                   ftp://sunsite.uio.no/pub/languages/perl/CPAN/
   Poland          ftp://ftp.man.szczecin.pl/pub/perl/CPAN/
                   ftp://ftp.man.torun.pl/pub/doc/CPAN/
                   ftp://ftp.pk.edu.pl/pub/lang/perl/CPAN/
                   ftp://sunsite.icm.edu.pl/pub/CPAN/
   Portugal        ftp://ftp.ci.uminho.pt/pub/mirrors/cpan/
                   ftp://ftp.ist.utl.pt/pub/CPAN/
                   ftp://ftp.ua.pt/pub/CPAN/
   Romania         ftp://ftp.dntis.ro/pub/mirrors/perl-cpan/
                   ftp://ftp.dnttm.ro/pub/CPAN/
    Russia         ftp://ftp.chg.ru/pub/lang/perl/CPAN/
                   ftp://ftp.sai.msu.su/pub/lang/perl/CPAN/
   Slovakia        ftp://ftp.entry.sk/pub/languages/perl/CPAN/
   Slovenia        ftp://ftp.arnes.si/software/perl/CPAN/
   Spain           ftp://ftp.etse.urv.es/pub/perl/
                   ftp://ftp.rediris.es/mirror/CPAN/
   Sweden          ftp://ftp.sunet.se/pub/lang/perl/CPAN/
   Switzerland     ftp://sunsite.cnlab-switch.ch/mirror/CPAN/
   Turkey          ftp://sunsite.bilkent.edu.tr/pub/languages/CPAN/
   United Kingdom  ftp://ftp.demon.co.uk/pub/mirrors/perl/CPAN/
                   ftp://ftp.flirble.org/pub/languages/perl/CPAN/
                   ftp://ftp.plig.org/pub/CPAN/
                   ftp://sunsite.doc.ic.ac.uk/packages/CPAN/
                   ftp://unix.hensa.ac.uk/mirrors/perl-CPAN/

=item North America

   Alberta         ftp://sunsite.ualberta.ca/pub/Mirror/CPAN/
    California     ftp://cpan.nas.nasa.gov/pub/perl/CPAN/
                   ftp://ftp.cdrom.com/pub/perl/CPAN/
                   ftp://ftp.digital.com/pub/plan/perl/CPAN/
   Colorado        ftp://ftp.cs.colorado.edu/pub/perl/CPAN/
   Florida         ftp://ftp.cise.ufl.edu/pub/perl/CPAN/
   Illinois        ftp://uiarchive.uiuc.edu/pub/lang/perl/CPAN/
   Indiana         ftp://csociety-ftp.ecn.purdue.edu/pub/CPAN/
                   ftp://ftp.uwsg.indiana.edu/pub/perl/CPAN/
   Manitoba        ftp://theory.uwinnipeg.ca/pub/CPAN/
   Massachusetts   ftp://ftp.ccs.neu.edu/net/mirrors/ftp.funet.fi/pub/languages/perl/CPAN/
                   ftp://ftp.iguide.com/pub/mirrors/packages/perl/CPAN/
    Mexico         ftp://ftp.msg.com.mx/pub/CPAN/
    Minnesota      ftp://ftp.midearthbbs.com/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/
    Ontario        ftp://ftp.crc.ca/pub/packages/lang/perl/CPAN/
   Oregon          ftp://ftp.orst.edu/pub/packages/CPAN/
   Pennsylvania    ftp://ftp.epix.net/pub/languages/perl/
   Texas           ftp://ftp.sedl.org/pub/mirrors/CPAN/
   Utah            ftp://mirror.xmission.com/CPAN/
   Virginia        ftp://ftp.perl.org/pub/perl/CPAN/
                   ftp://ruff.cs.jmu.edu/pub/CPAN/
    Washington     ftp://ftp-mirror.internap.com/pub/CPAN/
                   ftp://ftp.spu.edu/pub/CPAN/

=item South America

   Brazil          ftp://cpan.if.usp.br/pub/mirror/CPAN/
    Chile          ftp://sunsite.dcc.uchile.cl/pub/Lang/perl/CPAN/

=back

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

Always use B<-w>.  

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 that 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 that need less strictness.  

Always use B<-w>.  

Follow the guidelines in the perlstyle(1) manual.

Always use B<-w>.

=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 L<Exporter> 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, register the module with CPAN.  You should 
include details of its location in your announcement.

Some notes about ftp archives: Please use a long descriptive file
name that 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.
Otherwise try to add a mechanism to revert to the
old behavior 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 Perl applications that 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 that 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.