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# perlmodlib

- NAME
- DESCRIPTION
- THE PERL MODULE LIBRARY
  - Pragmatic Modules
  - Standard Modules
  - Extension Modules
- CPAN
- Modules: Creation, Use, and Abuse
  - Guidelines for Module Creation
  - Guidelines for Converting Perl 4 Library Scripts into Modules
  - Guidelines for Reusing Application Code
- NOTE

## NAME

perlmodlib - constructing new Perl modules and finding existing ones

## DESCRIPTION

## THE PERL MODULE LIBRARY

Many modules are included the Perl distribution. These are described below, and all end in `.pm`. You may discover compiled library file (usually ending in `.so`) or small pieces of modules to be autoloaded (ending in `.al`); these were automatically generated by the installation process. You may also discover files in the library directory that end in either `.pl` or `.ph`. These are old libraries supplied so that old programs that use them still run. The `.pl` files will all eventually be converted into standard modules, and the `.ph` files made by h2ph will probably end up as extension modules made by h2xs. (Some `.ph` values may already be available through the POSIX, Errno, or Fcntl modules.) The pl2pm file in the distribution may help in your conversion, but it's just a mechanical process and therefore far from bulletproof.

### 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 use, or no.  Most of these
    are lexically scoped, so an inner BLOCK may countermand them
    by saying:

        no integer;
        no strict 'refs';
        no warnings;

which lasts until the end of that BLOCK.

Some pragmas are lexically scoped--typically those that affect the `$^H` hints variable. Others affect the current package instead, like `use vars` and `use subs`, which allow you to predeclare a variables or subroutines within a particular file rather than just a block. Such declarations are effective for the entire file for which they were declared. You cannot rescind them with `no vars` or `no subs`.

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

attributes
  Get/set subroutine or variable attributes
attrs
  Set/get attributes of a subroutine (deprecated)
autouse
  Postpone load of modules until a function is used
base
  Establish IS-A relationship with base class at compile time
blib
  Use MakeMaker's uninstalled version of a package
bytes
  Force byte semantics rather than character semantics
charnames
  Define character names for `\N{named}` string literal escape.
constant
  Declare constants
diagnostics
  Perl compiler pragma to force verbose warning diagnostics
fields
  Compile-time class fields
filetest
  Control the filetest permission operators
integer
  Compute arithmetic in integer instead of double
less
  Request less of something from the compiler
lib
  Manipulate @INC at compile time
locale
  Use and avoid POSIX locales for built-in operations
open
  Set default disciplines for input and output
ops
  Restrict unsafe operations when compiling
overload
  Package for overloading perl operations
re
  Alter regular expression behaviour
sigtrap
  Enable simple signal handling
strict
  Restrict unsafe constructs
subs
  Predeclare sub names
utf8
  Enable/disable UTF-8 in source code
vars
  Predeclare global variable names (obsolete)
warnings
  Control optional warnings
warnings::register
  Warnings import function

### 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.

AnyDBM_File
  Provide framework for multiple DBMs
AutoLoader
  Load subroutines only on demand
AutoSplit
  Split a package for autoloading
B
  The Perl Compiler
B::Asmdata
  Autogenerated data about Perl ops, used to generate bytecode
B::Assembler
  Assemble Perl bytecode
B::Bblock
  Walk basic blocks
B::Bytecode
  Perl compiler's bytecode backend
B::C
  Perl compiler's C backend
B::CC
  Perl compiler's optimized C translation backend
B::Concise
  Walk Perl syntax tree, printing concise info about ops
B::Debug
  Walk Perl syntax tree, printing debug info about ops
B::Deparse
  Perl compiler backend to produce perl code
B::Disassembler
  Disassemble Perl bytecode
B::Lint
  Perl lint
B::Showlex
  Show lexical variables used in functions or files
B::Stackobj
  Helper module for CC backend
B::Stash
  Show what stashes are loaded
B::Terse
  Walk Perl syntax tree, printing terse info about ops
B::Xref
  Generates cross reference reports for Perl programs
Benchmark
  Benchmark running times of Perl code
ByteLoader
  Load byte compiled perl code
CGI
  Simple Common Gateway Interface Class
CGI::Apache
  Backward compatibility module for CGI.pm
CGI::Carp
  CGI routines for writing to the HTTPD (or other) error log
CGI::Cookie
  Interface to Netscape Cookies
CGI::Fast
  CGI Interface for Fast CGI
CGI::Pretty
  Module to produce nicely formatted HTML code
CGI::Push
  Simple Interface to Server Push
CGI::Switch
  Backward compatibility module for defunct CGI::Switch
CPAN
  Query, download and build perl modules from CPAN sites
CPAN::FirstTime
  Utility for CPAN::Config file Initialization
CPAN::Nox
  Wrapper around CPAN.pm without using any XS module
Carp
  Warn of errors (from perspective of caller)
Carp::Heavy
  Carp guts
Class::Struct
  Declare struct-like datatypes as Perl classes
Cwd
  Get pathname of current working directory
DB
  Programmatic interface to the Perl debugging API (draft, subject to
DB_File
  Perl5 access to Berkeley DB version 1.x
Devel::SelfStubber
  Generate stubs for a SelfLoading module
DirHandle
  Supply object methods for directory handles
Dumpvalue
  Provides screen dump of Perl data.
English
  Use nice English (or awk) names for ugly punctuation variables
Env
  Perl module that imports environment variables as scalars or arrays
Exporter
  Implements default import method for modules
Exporter::Heavy
  Exporter guts
ExtUtils::Command
  Utilities to replace common UNIX commands in Makefiles etc.
ExtUtils::Embed
  Utilities for embedding Perl in C/C++ applications
ExtUtils::Install
  Install files from here to there
ExtUtils::Installed
  Inventory management of installed modules
ExtUtils::Liblist
  Determine libraries to use and how to use them
ExtUtils::MM_Cygwin
  Methods to override UN*X behaviour in ExtUtils::MakeMaker
ExtUtils::MM_OS2
  Methods to override UN*X behaviour in ExtUtils::MakeMaker
ExtUtils::MM_Unix
  Methods used by ExtUtils::MakeMaker
ExtUtils::MM_VMS
  Methods to override UN*X behaviour in ExtUtils::MakeMaker
ExtUtils::MM_Win32
  Methods to override UN*X behaviour in ExtUtils::MakeMaker
ExtUtils::MakeMaker
  Create an extension Makefile
ExtUtils::Manifest
  Utilities to write and check a MANIFEST file
ExtUtils::Mkbootstrap
  Make a bootstrap file for use by DynaLoader
ExtUtils::Mksymlists
  Write linker options files for dynamic extension
ExtUtils::Packlist
  Manage .packlist files
ExtUtils::testlib
  Add blib/* directories to @INC
Fatal
  Replace functions with equivalents which succeed or die
Fcntl
  Load the C Fcntl.h defines
File::Basename
  Split a pathname into pieces
File::CheckTree
  Run many filetest checks on a tree
File::Compare
  Compare files or filehandles
File::Copy
  Copy files or filehandles
File::DosGlob
  DOS like globbing and then some
File::Find
  Traverse a file tree
File::Path
  Create or remove directory trees
File::Spec
  Portably perform operations on file names
File::Spec::Epoc
  Methods for Epoc file specs
File::Spec::Functions
  Portably perform operations on file names
File::Spec::Mac
  File::Spec for MacOS
File::Spec::OS2
  Methods for OS/2 file specs
File::Spec::Unix
  Methods used by File::Spec
File::Spec::VMS
  Methods for VMS file specs
File::Spec::Win32
  Methods for Win32 file specs
File::Temp
  Return name and handle of a temporary file safely
File::stat
  By-name interface to Perl's built-in stat() functions
FileCache
  Keep more files open than the system permits
FileHandle
  Supply object methods for filehandles
FindBin
  Locate directory of original perl script
GDBM_File
  Perl5 access to the gdbm library.
Getopt::Long
  Extended processing of command line options
Getopt::Std
  Process single-character switches with switch clustering
I18N::Collate
  Compare 8-bit scalar data according to the current locale
IO
  Load various IO modules
IPC::Open2
  Open a process for both reading and writing
IPC::Open3
  Open a process for reading, writing, and error handling
Math::BigFloat
  Arbitrary length float math package
Math::BigInt
  Arbitrary size integer math package
Math::Complex
  Complex numbers and associated mathematical functions
Math::Trig
  Trigonometric functions
Net::Ping
  Check a remote host for reachability
Net::hostent
  By-name interface to Perl's built-in gethost*() functions
Net::netent
  By-name interface to Perl's built-in getnet*() functions
Net::protoent
  By-name interface to Perl's built-in getproto*() functions
Net::servent
  By-name interface to Perl's built-in getserv*() functions
O
  Generic interface to Perl Compiler backends
Opcode
  Disable named opcodes when compiling perl code
Pod::Checker
  Check pod documents for syntax errors
Pod::Find
  Find POD documents in directory trees
Pod::Html
  Module to convert pod files to HTML
Pod::InputObjects
  Objects representing POD input paragraphs, commands, etc.
Pod::LaTeX
  Convert Pod data to formatted Latex
Pod::Man
  Convert POD data to formatted *roff input
Pod::ParseUtils
  Helpers for POD parsing and conversion
Pod::Parser
  Base class for creating POD filters and translators
Pod::Plainer
  Perl extension for converting Pod to old style Pod.
Pod::Select
  Extract selected sections of POD from input
Pod::Text
  Convert POD data to formatted ASCII text
Pod::Text::Color
  Convert POD data to formatted color ASCII text
Pod::Text::Overstrike
  Convert POD data to formatted overstrike text
Pod::Text::Termcap
  Convert POD data to ASCII text with format escapes
Pod::Usage
  Print a usage message from embedded pod documentation
SDBM_File
  Tied access to sdbm files
Safe
  Compile and execute code in restricted compartments
Search::Dict
  Search for key in dictionary file
SelectSaver
  Save and restore selected file handle
SelfLoader
  Load functions only on demand
Shell
  Run shell commands transparently within perl
Socket
  Load the C socket.h defines and structure manipulators
Symbol
  Manipulate Perl symbols and their names
Term::ANSIColor
  Color screen output using ANSI escape sequences
Term::Cap
  Perl termcap interface
Term::Complete
  Perl word completion module
Term::ReadLine
  Perl interface to various `readline` packages. If
Test
  Provides a simple framework for writing test scripts
Test::Harness
  Run perl standard test scripts with statistics
Text::Abbrev
  Create an abbreviation table from a list
Text::ParseWords
  Parse text into an array of tokens or array of arrays
Text::Soundex
  Implementation of the Soundex Algorithm as Described by Knuth
Text::Wrap
  Line wrapping to form simple paragraphs
Thread
  Manipulate threads in Perl (EXPERIMENTAL, subject to change)
Thread::Queue
  Thread-safe queues
Thread::Semaphore
  Thread-safe semaphores
Thread::Signal
  Start a thread which runs signal handlers reliably
Thread::Specific
  Thread-specific keys
Tie::Array
  Base class for tied arrays
Tie::Handle
  Base class definitions for tied handles
Tie::Hash
  Base class definitions for tied hashes
Tie::RefHash
  Use references as hash keys
Tie::Scalar
  Base class definitions for tied scalars
Tie::SubstrHash
  Fixed-table-size, fixed-key-length hashing
Time::Local
  Efficiently compute time from local and GMT time
Time::gmtime
  By-name interface to Perl's built-in gmtime() function
Time::localtime
  By-name interface to Perl's built-in localtime() function
Time::tm
  Internal object used by Time::gmtime and Time::localtime
UNIVERSAL
  Base class for ALL classes (blessed references)
User::grent
  By-name interface to Perl's built-in getgr*() functions
User::pwent
  By-name interface to Perl's built-in getpw*() functions

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

### 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.

## CPAN

CPAN stands for Comprehensive Perl Archive Network; it's a globally replicated trove of Perl materials, including documentation, style guides, tricks and traps, 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:

- Language Extensions and Documentation Tools
- Development Support
- Operating System Interfaces
- Networking, Device Control (modems) and InterProcess Communication
- Data Types and Data Type Utilities
- Database Interfaces
- User Interfaces
- Interfaces to / Emulations of Other Programming Languages
- File Names, File Systems and File Locking (see also File Handles)
- String Processing, Language Text Processing, Parsing, and Searching
- Option, Argument, Parameter, and Configuration File Processing
- Internationalization and Locale
- Authentication, Security, and Encryption
- World Wide Web, HTML, HTTP, CGI, MIME
- Server and Daemon Utilities
- Archiving and Compression
- Images, Pixmap and Bitmap Manipulation, Drawing, and Graphing
- Mail and Usenet News
- Control Flow Utilities (callbacks and exceptions etc)
- File Handle and Input/Output Stream Utilities
- Miscellaneous Modules

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

Africa

          South Africa   ftp://ftp.is.co.za/programming/perl/CPAN/
                         ftp://ftp.saix.net/pub/CPAN/
                         ftp://ftp.sun.ac.za/CPAN/
                         ftp://ftpza.co.za/pub/mirrors/cpan/

Asia

          China          ftp://freesoft.cei.gov.cn/pub/languages/perl/CPAN/
          Hong Kong      ftp://ftp.pacific.net.hk/pub/mirror/CPAN/
          Indonesia      ftp://malone.piksi.itb.ac.id/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/
          Saudi-Arabia   ftp://ftp.isu.net.sa/pub/CPAN/
          Singapore      ftp://ftp.nus.edu.sg/pub/unix/perl/CPAN/
          South Korea    ftp://ftp.bora.net/pub/CPAN/
                         ftp://ftp.kornet.net/pub/CPAN/
                         ftp://ftp.nuri.net/pub/CPAN/
          Taiwan         ftp://coda.nctu.edu.tw/computer-languages/perl/CPAN/
                         ftp://ftp.ee.ncku.edu.tw/pub3/perl/CPAN/
                         ftp://ftp1.sinica.edu.tw/pub1/perl/CPAN/
          Thailand       ftp://ftp.nectec.or.th/pub/mirrors/CPAN/

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/

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.grolier.fr/pub/perl/CPAN/
                         ftp://ftp.lip6.fr/pub/perl/CPAN/
                         ftp://ftp.oleane.net/pub/mirrors/CPAN/
                         ftp://ftp.pasteur.fr/pub/computing/CPAN/
                         ftp://ftp.uvsq.fr/pub/perl/CPAN/
          German         ftp://ftp.gigabell.net/pub/CPAN/
          Germany        ftp://ftp.archive.de.uu.net/pub/CPAN/
                         ftp://ftp.freenet.de/pub/ftp.cpan.org/pub/
                         ftp://ftp.gmd.de/packages/CPAN/
                         ftp://ftp.gwdg.de/pub/languages/perl/CPAN/

  ftp://ftp.leo.org/pub/comp/general/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/
  Germany ftp://ftp.archive.de.uu.net/pub/CPAN/
  ftp://ftp.freenet.de/pub/ftp.cpan.org/pub/
  ftp://ftp.gmd.de/packages/CPAN/
  ftp://ftp.gwdg.de/pub/languages/perl/CPAN/

  ftp://ftp.leo.org/pub/comp/general/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/
  Iceland ftp://ftp.gm.is/pub/CPAN/
  Ireland ftp://cpan.indigo.ie/pub/CPAN/
  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.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.torun.pl/pub/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.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.mirror.ac.uk/sites/ftp.funet.fi/pub/languages/perl/CPAN/
  ftp://ftp.plig.org/pub/CPAN/
  ftp://sunsite.doc.ic.ac.uk/packages/CPAN/

North America

          Alberta        ftp://sunsite.ualberta.ca/pub/Mirror/CPAN/
          California     ftp://cpan.nas.nasa.gov/pub/perl/CPAN/
                         ftp://cpan.valueclick.com/CPAN/
                         ftp://ftp.cdrom.com/pub/perl/CPAN/
                         http://download.sourceforge.net/mirrors/CPAN/
          Colorado       ftp://ftp.cs.colorado.edu/pub/perl/CPAN/
          Florida        ftp://ftp.cise.ufl.edu/pub/perl/CPAN/
          Georgia        ftp://ftp.twoguys.org/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/
          Kentucky       ftp://ftp.uky.edu/CPAN/
          Manitoba       ftp://theoryx5.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/
          New York       ftp://ftp.deao.net/pub/CPAN/
                         ftp://ftp.rge.com/pub/languages/perl/
          North Carolina ftp://ftp.duke.edu/pub/perl/
          Nova Scotia    ftp://cpan.chebucto.ns.ca/pub/CPAN/
          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/
          Tennessee      ftp://ftp.sunsite.utk.edu/pub/CPAN/
          Texas          ftp://ftp.sedl.org/pub/mirrors/CPAN/
                         ftp://jhcloos.com/pub/mirror/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.llarian.net/pub/CPAN/
                         ftp://ftp.spu.edu/pub/CPAN/

South America

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

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

## 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 `.pm` file is required to exist. See the perlsub manpage, the perltoot manpage, and the AutoLoader manpage for details about the AUTOLOAD mechanism.

### Guidelines for Module Creation

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.

Try to design the new module to be easy to extend and reuse.
  Try to `use warnings;` (or `use warnings qw(...);`). Remember that you can add `no warnings qw(...);` to individual blocks of code that need less warnings.

      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: `die "Invalid" unless ref $ref eq 'FOO'`. Generally you can delete the `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 `$r->Class::func()` where using `@ISA=qw(... Class ...)` and `$r->func()` would work (see the perlbot manpage 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 `@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: `$obj = new YOURCLASS;` into: `$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 -w.

  Try to `use strict;` (or `use strict qw(...);`). Remember that you can add `no strict qw(...);` to individual blocks of code that need less strictness.

  Always use -w.

  Follow the guidelines in the perlstyle(1) manual.

  Always use -w.

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., `$obj->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.

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 `$blessed_ref->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: `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.

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.

  If developing modules for private internal or project specific use, that will never be released to the public, then you should ensure that their names will not clash with any future public module. You can do this either by using the reserved Local::* category or by using a category name that includes an underscore like Foo_Corp::*.

  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.

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!

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:
  - A description of the module/package/extension etc.
  - A copyright notice - see below.
  - Prerequisites - what else you may need to have.
  - How to build it - possible changes to Makefile.PL etc.
  - How to install it.
  - Recent changes in this release, especially incompatibilities
  - Changes / enhancements you plan to make in the future.
  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.
  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.

  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 $VERSION. This should be a floating point number with at least two digits after the decimal (i.e., hundredths, e.g, `$VERSION = "0.01"`). Don't use a "1.3.2" style version. See the Exporter manpage 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.

  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://www.perl.com/CPAN/modules/00modlist.long.html
           http://www.perl.com/CPAN/modules/04pause.html

        or upload to one of these sites:

           https://pause.kbx.de/pause/
           http://pause.perl.org/pause/

    and notify <modules@perl.org>.

    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!

  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.

### Guidelines for Converting Perl 4 Library Scripts into Modules

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.
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?
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.
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:
  - Adds the standard Module prologue lines
  - Converts package specifiers from ' to ::
  - Converts die(...) to croak(...)
  - Several other minor changes
  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!

### Guidelines for Reusing Application Code

- Complete applications rarely belong in the Perl Module Library.
- 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.

- Break-out the reusable code into one or more separate module files.
- Take the opportunity to reconsider and redesign the interfaces.
- 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)

## 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 `use RedefineTheWorld` that you're redefining the world and willing to take the consequences.

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