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perlmodlib - constructing new Perl modules and finding existing ones

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perlmodlib - constructing new Perl modules and finding existing ones


perlmodlib - constructing new Perl modules and finding existing ones



A number of modules are included the Perl distribution. These are described below, and all end in .pm. 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 module.) 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 pragmas 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 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 $^H hints variable, the use vars and use subs declarations are not BLOCK-scoped. They 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).

use autouse MODULE => qw(sub1 sub2 sub3)
Defers 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.

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

force verbose warning diagnostics

compute arithmetic in integer instead of double

request less of something from the compiler

manipulate @INC at compile time

use or ignore current locale for builtin operations (see the perllocale manpage)

restrict named opcodes when compiling or running Perl code

overload basic Perl operations

enable simple signal handling

restrict unsafe constructs

predeclare sub names

adopt certain VMS-specific behaviors

predeclare global variable names

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.

provide framework for multiple DBMs

load functions only on demand

split a package for autoloading

benchmark running times of code

interface to Comprehensive Perl Archive Network

create a CPAN configuration file

run CPAN while avoiding compiled extensions

warn of errors (from perspective of caller)

declare struct-like datatypes

access Perl configuration information

get pathname of current working directory

access to Berkeley DB

generate stubs for a SelfLoading module

supply object methods for directory handles

dynamically load C libraries into Perl code

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

import environment variables

implements default import method for modules

utilities for embedding Perl in C/C++ applications

install files from here to there

determine libraries to use and how to use them

methods to override Unix behaviour in ExtUtils::MakeMaker

methods used by ExtUtils::MakeMaker

methods to override Unix behaviour in ExtUtils::MakeMaker

create an extension Makefile

utilities to write and check a MANIFEST file

make a bootstrap file for use by DynaLoader

write linker options files for dynamic extension

add blib/* directories to @INC

load the C Fcntl.h defines

split a pathname into pieces

run many filetest checks on a tree

compare files or filehandles

copy files or filehandles

traverse a file tree

create or remove a series of directories

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

keep more files open than the system permits

supply object methods for filehandles

locate directory of original perl script

access to the gdbm library

extended processing of command line options

process single-character switches with switch clustering

compare 8-bit scalar data according to the current locale

load various IO modules

supply object methods for filehandles

supply object methods for I/O handles

supply object methods for pipes

supply seek based methods for I/O objects

OO interface to the select system call

object interface to socket communications

open a process for both reading and writing

open a process for reading, writing, and error handling

arbitrary length float math package

arbitrary size integer math package

complex numbers and associated mathematical functions

simple interface to parts of Math::Complex for those who need trigonometric functions only for real numbers

tied access to ndbm files

Hello, anybody home?

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

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

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

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

disable named opcodes when compiling or running perl code

convert POD data to formatted ASCII text

interface to IEEE Standard 1003.1

tied access to sdbm files

compile and execute code in restricted compartments

search for key in dictionary file

save and restore selected file handle

load functions only on demand

run shell commands transparently within perl

load the C socket.h defines and structure manipulators

manipulate Perl symbols and their names

try every conceivable way to get hostname

interface to the Unix syslog(3) calls

termcap interface

word completion module

interface to various readline packages

run perl standard test scripts with statistics

create an abbreviation table from a list

parse text into an array of tokens

implementation of the Soundex Algorithm as described by Knuth

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

line wrapping to form simple paragraphs

base class definitions for tied hashes

base class definitions for tied hashes with references as keys

base class definitions for tied scalars

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

efficiently compute time from local and GMT time

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

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

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

base class for ALL classes (blessed references)

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

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

To find out 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 perldoc program.

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.


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:

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

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

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.
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 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 ``@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 which need less strictness. Always use -w. Always use -w! Follow the guidelines in the perlstyle(1) manual.

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.

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:

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 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.

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


or upload to one of these sites:


and notify <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!

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.

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 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?

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:

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 which 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)' ...
     perl -mModule::Name ...    (in perl5.002 or higher)


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.

Source: Perl manual pages Copyright: Larry Wall, et al.

(Corrections, notes, and links courtesy of RocketAware.com)

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