@node perlmod, perlform, perlsub, Top
@unnumbered Perl modules (packages)


@unnumberedsec NAME

perlmod - Perl modules (packages)

@unnumberedsec DESCRIPTION

@unnumberedsubsec Packages

Perl provides a mechanism for alternative namespaces to protect packages
from stomping on each other@'s variables.  In fact, apart from certain
magical variables, there@'s really no such thing as a global variable in
Perl.  The package statement declares the compilation unit as being in the
given namespace.  The scope of the package declaration is from the
declaration itself through the end of the enclosing block (the same scope
as the local() operator).  All further unqualified dynamic identifiers
will be in this namespace.  A package statement only affects dynamic
variables--including those you@'ve used local() on--but @emph{not} lexical
variables created with my().  Typically it would be the first declaration
in a file to be included by the require or use operator.  You can
switch into a package in more than one place; it merely influences which
symbol table is used by the compiler for the rest of that block.  You can
refer to variables and filehandles in other packages by prefixing the
identifier with the package name and a double colon:
@code{$Package::Variable}.  If the package name is null, the @code{main} package
is assumed.  That is, @code{$::sail} is equivalent to @code{$main::sail}.

(The old package delimiter was a single quote, but double colon
is now the preferred delimiter, in part because it@'s more readable
to humans, and in part because it@'s more readable to @strong{emacs} macros.
It also makes C++ programmers feel like they know what@'s going on.)

Packages may be nested inside other packages: @code{$OUTER::INNER::var}.  This
implies nothing about the order of name lookups, however.  All symbols
are either local to the current package, or must be fully qualified
from the outer package name down.  For instance, there is nowhere
within package @code{OUTER} that @code{$INNER::var} refers to @code{$OUTER::INNER::var}.
It would treat package @code{INNER} as a totally separate global package.

Only identifiers starting with letters (or underscore) are stored in a
package@'s symbol table.  All other symbols are kept in package @code{main},
including all of the punctuation variables like $_.  In addition, the
identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC and SIG are
forced to be in package @code{main}, even when used for other purposes than
their built-in one.  Note also that, if you have a package called @code{m},
s or @code{y}, then you can@'t use the qualified form of an identifier
because it will be interpreted instead as a pattern match, a substitution,
or a translation.

(Variables beginning with underscore used to be forced into package
main, but we decided it was more useful for package writers to be able
to use leading underscore to indicate private variables and method names.
$_ is still global though.)

Eval()ed strings are compiled in the package in which the eval() was
compiled.  (Assignments to @code{$SIG@{@}}, however, assume the signal
handler specified is in the @code{main} package.  Qualify the signal handler
name if you wish to have a signal handler in a package.)  For an
example, examine @file{perldb.pl} in the Perl library.  It initially switches
to the @code{DB} package so that the debugger doesn@'t interfere with variables
in the script you are trying to debug.  At various points, however, it
temporarily switches back to the @code{main} package to evaluate various
expressions in the context of the @code{main} package (or wherever you came
from).  See @xref{perldebug,Perldebug},.

See @xref{perlsub,Perlsub}, for other scoping issues related to my() and local(), 
or @xref{perlref,Perlref}, regarding closures.

@unnumberedsubsec Symbol Tables

The symbol table for a package happens to be stored in the associative
array of that name appended with two colons.  The main symbol table@'s
name is thus @code{%main::}, or @code{%::} for short.  Likewise symbol table for
the nested package mentioned earlier is named @code{%OUTER::INNER::}.

The value in each entry of the associative array is what you are referring
to when you use the @code{*name} typeglob notation.  In fact, the following
have the same effect, though the first is more efficient because it does
the symbol table lookups at compile time:

@example
local(*main::foo) = *main::bar; local($main::@{@'foo@'@}) =
$main::@{@'bar@'@};
@end example

You can use this to print out all the variables in a package, for
instance.  Here is @file{dumpvar.pl} from the Perl library:

@example
package dumpvar;
sub main::dumpvar @{
    ($package) = @@_;
    local(*stab) = eval("*$@{package@}::");
    while (($key,$val) = each(%stab)) @{
	   local(*entry) = $val;
	   if (defined $entry) @{
	       print "\$$key = @'$entry@'\n";
	   @}
@end example

@example
if (defined @@entry) @{
    print "\@@$key = (\n";
    foreach $num ($[ .. $#entry) @{
		   print "  $num\t@'",$entry[$num],"@'\n";
    @}
    print ")\n";
@}
@end example

@example
if ($key ne "$@{package@}::" && defined %entry) @{
    print "\%$key = (\n";
    foreach $key (sort keys(%entry)) @{
		   print "  $key\t@'",$entry@{$key@},"@'\n";
    @}
    print ")\n";
@}
       @}
   @}
@end example

Note that even though the subroutine is compiled in package @code{dumpvar},
the name of the subroutine is qualified so that its name is inserted
into package @code{main}.

Assignment to a typeglob performs an aliasing operation, i.e.,

@example
*dick = *richard;
@end example

causes variables, subroutines and file handles accessible via the
identifier @code{richard} to also be accessible via the identifier @code{dick}.  If
you only want to alias a particular variable or subroutine, you can
assign a reference instead:

@example
*dick = \$richard;
@end example

makes $richard and $dick the same variable, but leaves
 @@richard and @@dick as separate arrays.  Tricky, eh?

This mechanism may be used to pass and return cheap references
into or from subroutines if you won@'t want to copy the whole
thing.

@example
%some_hash = ();
*some_hash = fn( \%another_hash );
sub fn @{
	local *hashsym = shift;
	# now use %hashsym normally, and you
	# will affect the caller@'s %another_hash
	my %nhash = (); # do what you want
	return \%nhash; 
@}
@end example

On return, the reference wil overwrite the hash slot in the
symbol table specified by the *some_hash typeglob.  This
is a somewhat tricky way of passing around references cheaply
when you won@'t want to have to remember to dereference variables
explicitly.

Another use of symbol tables is for making "constant"  scalars.

@example
*PI = \3.14159265358979;
@end example

Now you cannot alter $PI, which is probably a good thing all in all.

@unnumberedsubsec Package Constructors and Destructors

There are two special subroutine definitions that function as package
constructors and destructors.  These are the BEGIN and END
routines.  The sub is optional for these routines.

A BEGIN subroutine is executed as soon as possible, that is, the
moment it is completely defined, even before the rest of the containing
file is parsed.  You may have multiple BEGIN blocks within a
file--they will execute in order of definition.  Because a BEGIN
block executes immediately, it can pull in definitions of subroutines
and such from other files in time to be visible to the rest of the
file.

An END subroutine is executed as late as possible, that is, when the
interpreter is being exited, even if it is exiting as a result of a
die() function.  (But not if it@'s is being blown out of the water by a
signal--you have to trap that yourself (if you can).)  You may have
multiple END blocks within a file--they will execute in reverse
order of definition; that is: last in, first out (LIFO).

Inside an END subroutine $? contains the value that the script is
going to pass to @code{exit()}.  You can modify $? to change the exit
value of the script.  Beware of changing $? by accident (eg, by
running something via system).

Note that when you use the -n and -p switches to Perl, BEGIN
and END work just as they do in @strong{awk}, as a degenerate case.

@unnumberedsubsec Perl Classes

There is no special class syntax in Perl, but a package may function
as a class if it provides subroutines that function as methods.  Such a
package may also derive some of its methods from another class package
by listing the other package name in its @@ISA array.  

For more on this, see @xref{perlobj,Perlobj},.

@unnumberedsubsec Perl Modules

A module is just a package that is defined in a library file of
the same name, and is designed to be reusable.  It may do this by
providing a mechanism for exporting some of its symbols into the symbol
table of any package using it.  Or it may function as a class
definition and make its semantics available implicitly through method
calls on the class and its objects, without explicit exportation of any
symbols.  Or it can do a little of both.

For example, to start a normal module called Fred, create
a file called Fred.pm and put this at the start of it:

@example
package Fred;
use Exporter ();
@@ISA       = qw(Exporter);
@@EXPORT    = qw(func1 func2);
@@EXPORT_OK = qw($sally @@listabob %harry func3);
@end example

Then go on to declare and use your variables in functions
without any qualifications.
See @xref{(pm.info)Exporter,Exporter}, and the @emph{Perl Modules File} for details on 
mechanics and style issues in module creation.

Perl modules are included into your program by saying

@example
use Module;
@end example

or

@example
use Module LIST;
@end example

This is exactly equivalent to

@example
BEGIN @{ require "Module.pm"; import Module; @}
@end example

or

@example
BEGIN @{ require "Module.pm"; import Module LIST; @}
@end example

As a special case

@example
use Module ();
@end example

is exactly equivalent to

@example
BEGIN @{ require "Module.pm"; @}
@end example

All Perl module files have the extension @file{.pm}.  use assumes this so
that you don@'t have to spell out "@file{Module.pm}" in quotes.  This also
helps to differentiate new modules from old @file{.pl} and @file{.ph} files.
Module names are also capitalized unless they@'re functioning as pragmas,
"Pragmas" are in effect compiler directives, and are sometimes called
"pragmatic modules" (or even "pragmata" if you@'re a classicist).

Because the use statement implies a BEGIN block, the importation
of semantics happens at the moment the use statement is compiled,
before the rest of the file is compiled.  This is how it is able
to function as a pragma mechanism, and also how modules are able to
declare subroutines that are then visible as list operators for
the rest of the current file.  This will not work if you use require
instead of use.  With require you can get into this problem:

@example
require Cwd;		# make Cwd:: accessible
$here = Cwd::getcwd();	
@end example

@example
use Cwd;			# import names from Cwd:: 
$here = getcwd();
@end example

@example
require Cwd;	    	# make Cwd:: accessible
$here = getcwd(); 		# oops! no main::getcwd()
@end example

In general @code{use Module ();} is recommended over @code{require Module;}.

Perl packages may be nested inside other package names, so we can have
package names containing @code{::}.  But if we used that package name
directly as a filename it would makes for unwieldy or impossible
filenames on some systems.  Therefore, if a module@'s name is, say,
@code{Text::Soundex}, then its definition is actually found in the library
file @file{Text/Soundex.pm}.

Perl modules always have a @file{.pm} file, but there may also be dynamically
linked executables or autoloaded subroutine definitions associated with
the module.  If so, these will be entirely transparent to the user of
the module.  It is the responsibility of the @file{.pm} file to load (or
arrange to autoload) any additional functionality.  The POSIX module
happens to do both dynamic loading and autoloading, but the user can
just say @code{use POSIX} to get it all.

For more information on writing extension modules, see @xref{perlxs,Perlxs},
and @xref{perlguts,Perlguts},.

@unnumberedsec 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 (AKA documentation) may make other
provisions.  But then you know when you @code{use RedefineTheWorld} that
you@'re redefining the world and willing to take the consequences.

@unnumberedsec THE PERL MODULE LIBRARY

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

@unnumberedsubsec Pragmatic Modules

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

@example
no integer;
no strict @'refs@';
@end example

which lasts until the end of that BLOCK.

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

@table @asis
@item diagnostics
Pragma to produce enhanced diagnostics

@item integer
Pragma to compute arithmetic in integer instead of double

@item less
Pragma to request less of something from the compiler

@item ops
Pragma to restrict use of unsafe opcodes

@item overload
Pragma for overloading operators 

@item sigtrap
Pragma to enable stack backtrace on unexpected signals

@item strict
Pragma to restrict unsafe constructs

@item subs
Pragma to predeclare sub names

@item vars
Pragma to predeclare global symbols

@end table
@unnumberedsubsec 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.

@table @asis
@item AnyDBM_File
provide framework for multiple DBMs

@item AutoLoader
load functions only on demand

@item AutoSplit
split a package for autoloading

@item Benchmark
benchmark running times of code

@item Carp
warn of errors (from perspective of caller)

@item Config
access Perl configuration option

@item Cwd
get pathname of current working directory

@item DB_File
Perl access to Berkeley DB

@item Devel::SelfStubber
generate stubs for a SelfLoading module

@item DynaLoader
Dynamically load C libraries into Perl code

@item English
use nice English (or @strong{awk}) names for ugly punctuation variables

@item Env
perl module that imports environment variables

@item Exporter
provide import/export controls for Perl modules

@item ExtUtils::Liblist
determine libraries to use and how to use them

@item ExtUtils::MakeMaker
create an extension Makefile

@item ExtUtils::Manifest
utilities to write and check a MANIFEST file

@item ExtUtils::Mkbootstrap
make a bootstrap file for use by DynaLoader

@item ExtUtils::Miniperl
!!!GOOD QUESTION!!!

@item Fcntl
load the C Fcntl.h defines

@item File::Basename
parse file specifications

@item File::CheckTree
run many filetest checks on a tree

@item File::Find
traverse a file tree

@item FileHandle
supply object methods for filehandles

@item File::Path
create or remove a series of directories

@item Getopt::Long
extended getopt processing

@item Getopt::Std
Process single-character switches with switch clustering

@item I18N::Collate
compare 8-bit scalar data according to the current locale

@item IPC::Open2
a process for both reading and writing

@item IPC::Open3
open a process for reading, writing, and error handling

@item Net::Ping
check a host for upness

@item POSIX
Perl interface to IEEE Std 1003.1

@item SelfLoader
load functions only on demand

@item Safe
Creation controlled compartments in which perl code can be evaluated.

@item Socket
load the C socket.h defines and structure manipulators

@item Test::Harness
run perl standard test scripts with statistics

@item Text::Abbrev
create an abbreviation table from a list

@end table
To find out all the modules installed on your system, including
those without documentation or outside the standard release, do this:

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

They should all have their own documentation installed and accessible via
your system man(1) command.  If that fails, try the @emph{perldoc} program.

@unnumberedsubsec Extension Modules

Extension modules are written in C (or a mix of Perl and C) and get
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 size, 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.

@unnumberedsec CPAN

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

@itemize @bullet
@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
@end itemize

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

@itemize @bullet
@item ftp://ftp.sterling.com/programming/languages/perl/
@item ftp://ftp.sedl.org/pub/mirrors/CPAN/
@item ftp://ftp.uoknor.edu/mirrors/CPAN/
@item ftp://ftp.delphi.com/pub/mirrors/packages/perl/CPAN/
@item ftp://uiarchive.cso.uiuc.edu/pub/lang/perl/CPAN/
@item ftp://ftp.cis.ufl.edu/pub/perl/CPAN/
@item ftp://ftp.switch.ch/mirror/CPAN/
@item ftp://ftp.sunet.se/pub/lang/perl/CPAN/
@item ftp://ftp.ci.uminho.pt/pub/lang/perl/
@item ftp://ftp.cs.ruu.nl/pub/PERL/CPAN/
@item ftp://ftp.demon.co.uk/pub/mirrors/perl/CPAN/
@item ftp://ftp.rz.ruhr-uni-bochum.de/pub/programming/languages/perl/CPAN/
@item ftp://ftp.leo.org/pub/comp/programming/languages/perl/CPAN/
@item ftp://ftp.pasteur.fr/pub/computing/unix/perl/CPAN/
@item ftp://ftp.ibp.fr/pub/perl/CPAN/
@item ftp://ftp.funet.fi/pub/languages/perl/CPAN/
@item ftp://ftp.tekotago.ac.nz/pub/perl/CPAN/
@item ftp://ftp.mame.mu.oz.au/pub/perl/CPAN/
@item ftp://coombs.anu.edu.au/pub/perl/
@item ftp://dongpo.math.ncu.edu.tw/perl/CPAN/
@item ftp://ftp.lab.kdd.co.jp/lang/perl/CPAN/
@item ftp://ftp.is.co.za/programming/perl/CPAN/
@end itemize

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

@unnumberedsec Modules: Creation, Use and Abuse

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

Perl 5 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.

@unnumberedsubsec Guidelines for Module Creation

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

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

@item Try to design the new module to be easy to extend and reuse.
Use blessed references.  Use the two argument form of bless to bless
into the class name given as the first parameter of the constructor,
e.g.:

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

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

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

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: @code{die "Invalid" unless ref $ref eq @'FOO@'}.
Generally you can delete the "@code{eq @'FOO@'}" part with no harm at all.
Let the objects look after themselves! Generally, avoid hardwired
class names as far as possible.

Avoid @code{$r->Class::func()} where using @code{@@ISA=qw(... Class ...)} and
@code{$r->func()} would work (see @xref{perlbot,Perlbot}, for more details).

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

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

Does your module pass the @'empty sub-class@' test? If you say
"@code{@@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:  @code{$obj = new YOURCLASS;}
into: @code{$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 @code{use strict;} (or @code{use strict qw(...);}).
Remember that you can add @code{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.

@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:

@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
@end example

Function and method names seem to work best as all lowercase.
E.g., @code{$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.

@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 @code{$blessed_ref->method})
syntax.  By convention you can use a leading underscore on names to
informally indicate that they are @'internal@' and not for public use.

(It is actually possible to get private functions by saying:
@code{my $subref = sub @{ ... @};  &$subref;}.  But there@'s no way to call that
directly as a method, since 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 informally group or categorise a module.
A module should have a very good reason 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 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:

@itemize @bullet
@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.
@end itemize

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.

@table @asis
@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 license: The GNU
GPL and The Artistic License (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 simply state something like:

@example
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.
@end example

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 $VERSION.  This should be a valid floating point 
number with at least two digits after the decimal (ie hundredths,
e.g, @code{$VERSION = "0.01"}).  Don@'t use a "1.3.2" style version.
See Exporter.pm in Perl5.001m or later for details.

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

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

If possible you should place the module into a major ftp archive and
include details of it@'s 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

@example
http://franz.ww.tu-berlin.de/modulelist
@end example

or upload to one of these sites: 

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

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!

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

@end table
@end table
@unnumberedsubsec Guidelines for Converting Perl 4 Library Scripts into Modules

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

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

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

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

@itemize @bullet
@item Adds the standard Module prologue lines
@item Converts package specifiers from @' to ::
@item Converts die(...) to croak(...)
@item Several other minor changes
@end itemize

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!

@end table
@unnumberedsubsec Guidelines for Reusing Application Code

@table @asis
@item Complete applications rarely belong in the Perl Module Library.
@itemx Many applications contain some perl code which could be reused.
Help save the world! Share your code in a form that makes it easy
to reuse.

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

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

@end table
