(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 perlsub,
perltoot, and AutoLoader for details about
the AUTOLOAD mechanism.
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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.
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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;
}
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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;
}
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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 perlbot for more details).
Use autosplit so little used or newly added functions won't be a burden to programs
that don't use them. Add test functions to the module after __END__ either using AutoSplit
or by saying:
eval join('',<main::DATA>) || die $@ unless caller();
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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.
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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
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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.
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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.
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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.
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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!
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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.
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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.
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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.
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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 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.
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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.cpan.org/modules/00modlist.long.html
http://www.cpan.org/modules/04pause.html
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or upload to one of these sites:
https://pause.kbx.de/pause/
http://pause.perl.org/pause/
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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!
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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.
