3 perlmod - Perl modules (packages)
9 Perl provides a mechanism for alternative namespaces to protect packages
10 from stomping on each other's variables. In fact, apart from certain
11 magical variables, there's really no such thing as a global variable in
12 Perl. The package statement declares the compilation unit as being in the
13 given namespace. The scope of the package declaration is from the
14 declaration itself through the end of the enclosing block (the same scope
15 as the local() operator). All further unqualified dynamic identifiers
16 will be in this namespace. A package statement affects only dynamic
17 variables--including those you've used local() on--but I<not> lexical
18 variables created with my(). Typically it would be the first declaration
19 in a file to be included by the C<require> or C<use> operator. You can
20 switch into a package in more than one place; it influences merely which
21 symbol table is used by the compiler for the rest of that block. You can
22 refer to variables and filehandles in other packages by prefixing the
23 identifier with the package name and a double colon:
24 C<$Package::Variable>. If the package name is null, the C<main> package
25 is assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
27 (The old package delimiter was a single quote, but double colon
28 is now the preferred delimiter, in part because it's more readable
29 to humans, and in part because it's more readable to B<emacs> macros.
30 It also makes C++ programmers feel like they know what's going on.)
32 Packages may be nested inside other packages: C<$OUTER::INNER::var>. This
33 implies nothing about the order of name lookups, however. All symbols
34 are either local to the current package, or must be fully qualified
35 from the outer package name down. For instance, there is nowhere
36 within package C<OUTER> that C<$INNER::var> refers to C<$OUTER::INNER::var>.
37 It would treat package C<INNER> as a totally separate global package.
39 Only identifiers starting with letters (or underscore) are stored in a
40 package's symbol table. All other symbols are kept in package C<main>,
41 including all of the punctuation variables like $_. In addition, the
42 identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC, and SIG are
43 forced to be in package C<main>, even when used for other purposes than
44 their built-in one. Note also that, if you have a package called C<m>,
45 C<s>, or C<y>, then you can't use the qualified form of an identifier
46 because it will be interpreted instead as a pattern match, a substitution,
49 (Variables beginning with underscore used to be forced into package
50 main, but we decided it was more useful for package writers to be able
51 to use leading underscore to indicate private variables and method names.
52 $_ is still global though.)
54 Eval()ed strings are compiled in the package in which the eval() was
55 compiled. (Assignments to C<$SIG{}>, however, assume the signal
56 handler specified is in the C<main> package. Qualify the signal handler
57 name if you wish to have a signal handler in a package.) For an
58 example, examine F<perldb.pl> in the Perl library. It initially switches
59 to the C<DB> package so that the debugger doesn't interfere with variables
60 in the script you are trying to debug. At various points, however, it
61 temporarily switches back to the C<main> package to evaluate various
62 expressions in the context of the C<main> package (or wherever you came
63 from). See L<perldebug>.
65 See L<perlsub> for other scoping issues related to my() and local(),
66 or L<perlref> regarding closures.
70 The symbol table for a package happens to be stored in the associative
71 array of that name appended with two colons. The main symbol table's
72 name is thus C<%main::>, or C<%::> for short. Likewise symbol table for
73 the nested package mentioned earlier is named C<%OUTER::INNER::>.
75 The value in each entry of the associative array is what you are referring
76 to when you use the C<*name> typeglob notation. In fact, the following
77 have the same effect, though the first is more efficient because it does
78 the symbol table lookups at compile time:
80 local(*main::foo) = *main::bar; local($main::{'foo'}) =
83 You can use this to print out all the variables in a package, for
84 instance. Here is F<dumpvar.pl> from the Perl library:
89 local(*stab) = eval("*${package}::");
90 while (($key,$val) = each(%stab)) {
93 print "\$$key = '$entry'\n";
98 foreach $num ($[ .. $#entry) {
99 print " $num\t'",$entry[$num],"'\n";
104 if ($key ne "${package}::" && defined %entry) {
105 print "\%$key = (\n";
106 foreach $key (sort keys(%entry)) {
107 print " $key\t'",$entry{$key},"'\n";
114 Note that even though the subroutine is compiled in package C<dumpvar>,
115 the name of the subroutine is qualified so that its name is inserted
116 into package C<main>.
118 Assignment to a typeglob performs an aliasing operation, i.e.,
122 causes variables, subroutines, and file handles accessible via the
123 identifier C<richard> to also be accessible via the identifier C<dick>. If
124 you want to alias only a particular variable or subroutine, you can
125 assign a reference instead:
129 makes $richard and $dick the same variable, but leaves
130 @richard and @dick as separate arrays. Tricky, eh?
132 This mechanism may be used to pass and return cheap references
133 into or from subroutines if you won't want to copy the whole
137 *some_hash = fn( \%another_hash );
139 local *hashsym = shift;
140 # now use %hashsym normally, and you
141 # will affect the caller's %another_hash
142 my %nhash = (); # do what you want
146 On return, the reference will overwrite the hash slot in the
147 symbol table specified by the *some_hash typeglob. This
148 is a somewhat tricky way of passing around references cheaply
149 when you won't want to have to remember to dereference variables
152 Another use of symbol tables is for making "constant" scalars.
154 *PI = \3.14159265358979;
156 Now you cannot alter $PI, which is probably a good thing all in all.
158 You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and
159 package the *foo symbol table entry comes from. This may be useful
160 in a subroutine which is passed typeglobs as arguments
162 sub identify_typeglob {
164 print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
166 identify_typeglob *foo;
167 identify_typeglob *bar::baz;
171 You gave me main::foo
174 The *foo{THING} notation can also be used to obtain references to the
175 individual elements of *foo, see L<perlref>.
177 =head2 Package Constructors and Destructors
179 There are two special subroutine definitions that function as package
180 constructors and destructors. These are the C<BEGIN> and C<END>
181 routines. The C<sub> is optional for these routines.
183 A C<BEGIN> subroutine is executed as soon as possible, that is, the
184 moment it is completely defined, even before the rest of the containing
185 file is parsed. You may have multiple C<BEGIN> blocks within a
186 file--they will execute in order of definition. Because a C<BEGIN>
187 block executes immediately, it can pull in definitions of subroutines
188 and such from other files in time to be visible to the rest of the
191 An C<END> subroutine is executed as late as possible, that is, when the
192 interpreter is being exited, even if it is exiting as a result of a
193 die() function. (But not if it's is being blown out of the water by a
194 signal--you have to trap that yourself (if you can).) You may have
195 multiple C<END> blocks within a file--they will execute in reverse
196 order of definition; that is: last in, first out (LIFO).
198 Inside an C<END> subroutine C<$?> contains the value that the script is
199 going to pass to C<exit()>. You can modify C<$?> to change the exit
200 value of the script. Beware of changing C<$?> by accident (e.g.,, by
201 running something via C<system>).
203 Note that when you use the B<-n> and B<-p> switches to Perl, C<BEGIN>
204 and C<END> work just as they do in B<awk>, as a degenerate case.
208 There is no special class syntax in Perl, but a package may function
209 as a class if it provides subroutines that function as methods. Such a
210 package may also derive some of its methods from another class package
211 by listing the other package name in its @ISA array.
213 For more on this, see L<perlobj>.
217 A module is just a package that is defined in a library file of
218 the same name, and is designed to be reusable. It may do this by
219 providing a mechanism for exporting some of its symbols into the symbol
220 table of any package using it. Or it may function as a class
221 definition and make its semantics available implicitly through method
222 calls on the class and its objects, without explicit exportation of any
223 symbols. Or it can do a little of both.
225 For example, to start a normal module called Fred, create
226 a file called Fred.pm and put this at the start of it:
231 use vars qw(@ISA @EXPORT @EXPORT_OK);
233 @EXPORT = qw(&func1 &func2);
234 @EXPORT_OK = qw($sally @listabob %harry &func3);
235 use vars qw($sally @listabob %harry);
237 Then go on to declare and use your variables in functions
238 without any qualifications.
239 See L<Exporter> and the I<Perl Modules File> for details on
240 mechanics and style issues in module creation.
242 Perl modules are included into your program by saying
250 This is exactly equivalent to
252 BEGIN { require "Module.pm"; import Module; }
256 BEGIN { require "Module.pm"; import Module LIST; }
262 is exactly equivalent to
264 BEGIN { require "Module.pm"; }
266 All Perl module files have the extension F<.pm>. C<use> assumes this so
267 that you don't have to spell out "F<Module.pm>" in quotes. This also
268 helps to differentiate new modules from old F<.pl> and F<.ph> files.
269 Module names are also capitalized unless they're functioning as pragmas,
270 "Pragmas" are in effect compiler directives, and are sometimes called
271 "pragmatic modules" (or even "pragmata" if you're a classicist).
273 Because the C<use> statement implies a C<BEGIN> block, the importation
274 of semantics happens at the moment the C<use> statement is compiled,
275 before the rest of the file is compiled. This is how it is able
276 to function as a pragma mechanism, and also how modules are able to
277 declare subroutines that are then visible as list operators for
278 the rest of the current file. This will not work if you use C<require>
279 instead of C<use>. With require you can get into this problem:
281 require Cwd; # make Cwd:: accessible
282 $here = Cwd::getcwd();
284 use Cwd; # import names from Cwd::
287 require Cwd; # make Cwd:: accessible
288 $here = getcwd(); # oops! no main::getcwd()
290 In general C<use Module ();> is recommended over C<require Module;>.
292 Perl packages may be nested inside other package names, so we can have
293 package names containing C<::>. But if we used that package name
294 directly as a filename it would makes for unwieldy or impossible
295 filenames on some systems. Therefore, if a module's name is, say,
296 C<Text::Soundex>, then its definition is actually found in the library
297 file F<Text/Soundex.pm>.
299 Perl modules always have a F<.pm> file, but there may also be dynamically
300 linked executables or autoloaded subroutine definitions associated with
301 the module. If so, these will be entirely transparent to the user of
302 the module. It is the responsibility of the F<.pm> file to load (or
303 arrange to autoload) any additional functionality. The POSIX module
304 happens to do both dynamic loading and autoloading, but the user can
305 say just C<use POSIX> to get it all.
307 For more information on writing extension modules, see L<perlxs>
312 Perl does not enforce private and public parts of its modules as you may
313 have been used to in other languages like C++, Ada, or Modula-17. Perl
314 doesn't have an infatuation with enforced privacy. It would prefer
315 that you stayed out of its living room because you weren't invited, not
316 because it has a shotgun.
318 The module and its user have a contract, part of which is common law,
319 and part of which is "written". Part of the common law contract is
320 that a module doesn't pollute any namespace it wasn't asked to. The
321 written contract for the module (A.K.A. documentation) may make other
322 provisions. But then you know when you C<use RedefineTheWorld> that
323 you're redefining the world and willing to take the consequences.
325 =head1 THE PERL MODULE LIBRARY
327 A number of modules are included the Perl distribution. These are
328 described below, and all end in F<.pm>. You may also discover files in
329 the library directory that end in either F<.pl> or F<.ph>. These are old
330 libraries supplied so that old programs that use them still run. The
331 F<.pl> files will all eventually be converted into standard modules, and
332 the F<.ph> files made by B<h2ph> will probably end up as extension modules
333 made by B<h2xs>. (Some F<.ph> values may already be available through the
334 POSIX module.) The B<pl2pm> file in the distribution may help in your
335 conversion, but it's just a mechanical process and therefore far from
338 =head2 Pragmatic Modules
340 They work somewhat like pragmas in that they tend to affect the compilation of
341 your program, and thus will usually work well only when used within a
342 C<use>, or C<no>. Most of these are locally scoped, so an inner BLOCK
343 may countermand any of these by saying:
348 which lasts until the end of that BLOCK.
350 Unlike the pragmas that effect the C<$^H> hints variable, the C<use
351 vars> and C<use subs> declarations are not BLOCK-scoped. They allow
352 you to pre-declare a variables or subroutines within a particular
353 I<file> rather than just a block. Such declarations are effective
354 for the entire file for which they were declared. You cannot rescind
355 them with C<no vars> or C<no subs>.
357 The following pragmas are defined (and have their own documentation).
363 manipulate @INC at compile time to use MakeMaker's uninstalled version
368 force verbose warning diagnostics
372 compute arithmetic in integer instead of double
376 request less of something from the compiler
380 manipulate @INC at compile time
384 use or ignore current locale for built-in operations (see L<perllocale>)
388 restrict named opcodes when compiling or running Perl code
392 overload basic Perl operations
396 enable simple signal handling
400 restrict unsafe constructs
404 pre-declare sub names
408 adopt certain VMS-specific behaviors
412 pre-declare global variable names
416 =head2 Standard Modules
418 Standard, bundled modules are all expected to behave in a well-defined
419 manner with respect to namespace pollution because they use the
420 Exporter module. See their own documentation for details.
426 provide framework for multiple DBMs
430 load functions only on demand
434 split a package for autoloading
438 benchmark running times of code
442 interface to Comprehensive Perl Archive Network
444 =item CPAN::FirstTime
446 create a CPAN configuration file
450 run CPAN while avoiding compiled extensions
454 warn of errors (from perspective of caller)
456 =item Class::Template
458 struct/member template builder
462 access Perl configuration information
466 get pathname of current working directory
470 access to Berkeley DB
472 =item Devel::SelfStubber
474 generate stubs for a SelfLoading module
478 supply object methods for directory handles
482 dynamically load C libraries into Perl code
486 use nice English (or awk) names for ugly punctuation variables
490 import environment variables
494 implements default import method for modules
496 =item ExtUtils::Embed
498 utilities for embedding Perl in C/C++ applications
500 =item ExtUtils::Install
502 install files from here to there
504 =item ExtUtils::Liblist
506 determine libraries to use and how to use them
508 =item ExtUtils::MM_OS2
510 methods to override UN*X behaviour in ExtUtils::MakeMaker
512 =item ExtUtils::MM_Unix
514 methods used by ExtUtils::MakeMaker
516 =item ExtUtils::MM_VMS
518 methods to override UN*X behaviour in ExtUtils::MakeMaker
520 =item ExtUtils::MakeMaker
522 create an extension Makefile
524 =item ExtUtils::Manifest
526 utilities to write and check a MANIFEST file
528 =item ExtUtils::Mkbootstrap
530 make a bootstrap file for use by DynaLoader
532 =item ExtUtils::Mksymlists
534 write linker options files for dynamic extension
536 =item ExtUtils::testlib
538 add blib/* directories to @INC
542 load the C Fcntl.h defines
546 split a pathname into pieces
548 =item File::CheckTree
550 run many filetest checks on a tree
554 compare files or filehandles
558 copy files or filehandles
566 create or remove a series of directories
570 by-name interface to Perl's built-in stat() functions
574 keep more files open than the system permits
578 supply object methods for filehandles
582 locate directory of original perl script
586 access to the gdbm library
590 extended processing of command line options
594 process single-character switches with switch clustering
598 compare 8-bit scalar data according to the current locale
602 load various IO modules
606 supply object methods for filehandles
610 supply object methods for I/O handles
614 supply object methods for pipes
618 supply seek based methods for I/O objects
622 OO interface to the select system call
626 object interface to socket communications
630 open a process for both reading and writing
634 open a process for reading, writing, and error handling
638 arbitrary length float math package
642 arbitrary size integer math package
646 complex numbers and associated mathematical functions
650 tied access to ndbm files
658 by-name interface to Perl's built-in gethost*() functions
662 by-name interface to Perl's built-in getnet*() functions
666 by-name interface to Perl's built-in getproto*() functions
670 by-name interface to Perl's built-in getserv*() functions
674 disable named opcodes when compiling or running perl code
678 convert POD data to formatted ASCII text
682 interface to IEEE Standard 1003.1
686 tied access to sdbm files
690 compile and execute code in restricted compartments
694 search for key in dictionary file
698 save and restore selected file handle
702 load functions only on demand
706 run shell commands transparently within perl
710 load the C socket.h defines and structure manipulators
714 manipulate Perl symbols and their names
718 try every conceivable way to get hostname
722 interface to the UNIX syslog(3) calls
730 word completion module
734 interface to various C<readline> packages
738 run perl standard test scripts with statistics
742 create an abbreviation table from a list
744 =item Text::ParseWords
746 parse text into an array of tokens
750 implementation of the Soundex Algorithm as described by Knuth
754 expand and unexpand tabs per the unix expand(1) and unexpand(1)
758 line wrapping to form simple paragraphs
762 base class definitions for tied hashes
766 base class definitions for tied hashes with references as keys
770 base class definitions for tied scalars
772 =item Tie::SubstrHash
774 fixed-table-size, fixed-key-length hashing
778 efficiently compute time from local and GMT time
782 by-name interface to Perl's built-in gmtime() function
784 =item Time::localtime
786 by-name interface to Perl's built-in localtime() function
790 internal object used by Time::gmtime and Time::localtime
794 base class for ALL classes (blessed references)
798 by-name interface to Perl's built-in getgr*() functions
802 by-name interface to Perl's built-in getpw*() functions
806 To find out I<all> the modules installed on your system, including
807 those without documentation or outside the standard release, do this:
809 find `perl -e 'print "@INC"'` -name '*.pm' -print
811 They should all have their own documentation installed and accessible via
812 your system man(1) command. If that fails, try the I<perldoc> program.
814 =head2 Extension Modules
816 Extension modules are written in C (or a mix of Perl and C) and get
817 dynamically loaded into Perl if and when you need them. Supported
818 extension modules include the Socket, Fcntl, and POSIX modules.
820 Many popular C extension modules do not come bundled (at least, not
821 completely) due to their sizes, volatility, or simply lack of time for
822 adequate testing and configuration across the multitude of platforms on
823 which Perl was beta-tested. You are encouraged to look for them in
824 archie(1L), the Perl FAQ or Meta-FAQ, the WWW page, and even with their
825 authors before randomly posting asking for their present condition and
830 CPAN stands for the Comprehensive Perl Archive Network. This is a globally
831 replicated collection of all known Perl materials, including hundreds
832 of unbundled modules. Here are the major categories of modules:
837 Language Extensions and Documentation Tools
843 Operating System Interfaces
846 Networking, Device Control (modems) and InterProcess Communication
849 Data Types and Data Type Utilities
858 Interfaces to / Emulations of Other Programming Languages
861 File Names, File Systems and File Locking (see also File Handles)
864 String Processing, Language Text Processing, Parsing, and Searching
867 Option, Argument, Parameter, and Configuration File Processing
870 Internationalization and Locale
873 Authentication, Security, and Encryption
876 World Wide Web, HTML, HTTP, CGI, MIME
879 Server and Daemon Utilities
882 Archiving and Compression
885 Images, Pixmap and Bitmap Manipulation, Drawing, and Graphing
891 Control Flow Utilities (callbacks and exceptions etc)
894 File Handle and Input/Output Stream Utilities
897 Miscellaneous Modules
901 The registered CPAN sites as of this writing include the following.
902 You should try to choose one close to you:
907 ftp://ftp.sterling.com/programming/languages/perl/
910 ftp://ftp.sedl.org/pub/mirrors/CPAN/
913 ftp://ftp.uoknor.edu/mirrors/CPAN/
916 ftp://ftp.delphi.com/pub/mirrors/packages/perl/CPAN/
919 ftp://uiarchive.cso.uiuc.edu/pub/lang/perl/CPAN/
922 ftp://ftp.cis.ufl.edu/pub/perl/CPAN/
925 ftp://ftp.switch.ch/mirror/CPAN/
928 ftp://ftp.sunet.se/pub/lang/perl/CPAN/
931 ftp://ftp.ci.uminho.pt/pub/lang/perl/
934 ftp://ftp.cs.ruu.nl/pub/PERL/CPAN/
937 ftp://ftp.demon.co.uk/pub/mirrors/perl/CPAN/
940 ftp://ftp.rz.ruhr-uni-bochum.de/pub/programming/languages/perl/CPAN/
943 ftp://ftp.leo.org/pub/comp/programming/languages/perl/CPAN/
946 ftp://ftp.pasteur.fr/pub/computing/unix/perl/CPAN/
949 ftp://ftp.ibp.fr/pub/perl/CPAN/
952 ftp://ftp.funet.fi/pub/languages/perl/CPAN/
955 ftp://ftp.tekotago.ac.nz/pub/perl/CPAN/
958 ftp://ftp.mame.mu.oz.au/pub/perl/CPAN/
961 ftp://coombs.anu.edu.au/pub/perl/
964 ftp://dongpo.math.ncu.edu.tw/perl/CPAN/
967 ftp://ftp.lab.kdd.co.jp/lang/perl/CPAN/
970 ftp://ftp.is.co.za/programming/perl/CPAN/
974 For an up-to-date listing of CPAN sites,
975 see F<http://www.perl.com/perl/CPAN> or F<ftp://ftp.perl.com/perl/>.
977 =head1 Modules: Creation, Use, and Abuse
979 (The following section is borrowed directly from Tim Bunce's modules
980 file, available at your nearest CPAN site.)
982 Perl implements a class using a package, but the presence of a
983 package doesn't imply the presence of a class. A package is just a
984 namespace. A class is a package that provides subroutines that can be
985 used as methods. A method is just a subroutine that expects, as its
986 first argument, either the name of a package (for "static" methods),
987 or a reference to something (for "virtual" methods).
989 A module is a file that (by convention) provides a class of the same
990 name (sans the .pm), plus an import method in that class that can be
991 called to fetch exported symbols. This module may implement some of
992 its methods by loading dynamic C or C++ objects, but that should be
993 totally transparent to the user of the module. Likewise, the module
994 might set up an AUTOLOAD function to slurp in subroutine definitions on
995 demand, but this is also transparent. Only the .pm file is required to
998 =head2 Guidelines for Module Creation
1002 =item Do similar modules already exist in some form?
1004 If so, please try to reuse the existing modules either in whole or
1005 by inheriting useful features into a new class. If this is not
1006 practical try to get together with the module authors to work on
1007 extending or enhancing the functionality of the existing modules.
1008 A perfect example is the plethora of packages in perl4 for dealing
1009 with command line options.
1011 If you are writing a module to expand an already existing set of
1012 modules, please coordinate with the author of the package. It
1013 helps if you follow the same naming scheme and module interaction
1014 scheme as the original author.
1016 =item Try to design the new module to be easy to extend and reuse.
1018 Use blessed references. Use the two argument form of bless to bless
1019 into the class name given as the first parameter of the constructor,
1024 return bless {}, $class;
1027 or even this if you'd like it to be used as either a static
1028 or a virtual method.
1032 my $class = ref($self) || $self;
1033 return bless {}, $class;
1036 Pass arrays as references so more parameters can be added later
1037 (it's also faster). Convert functions into methods where
1038 appropriate. Split large methods into smaller more flexible ones.
1039 Inherit methods from other modules if appropriate.
1041 Avoid class name tests like: C<die "Invalid" unless ref $ref eq 'FOO'>.
1042 Generally you can delete the "C<eq 'FOO'>" part with no harm at all.
1043 Let the objects look after themselves! Generally, avoid hardwired
1044 class names as far as possible.
1046 Avoid C<$r-E<gt>Class::func()> where using C<@ISA=qw(... Class ...)> and
1047 C<$r-E<gt>func()> would work (see L<perlbot> for more details).
1049 Use autosplit so little used or newly added functions won't be a
1050 burden to programs which don't use them. Add test functions to
1051 the module after __END__ either using AutoSplit or by saying:
1053 eval join('',<main::DATA>) || die $@ unless caller();
1055 Does your module pass the 'empty sub-class' test? If you say
1056 "C<@SUBCLASS::ISA = qw(YOURCLASS);>" your applications should be able
1057 to use SUBCLASS in exactly the same way as YOURCLASS. For example,
1058 does your application still work if you change: C<$obj = new YOURCLASS;>
1059 into: C<$obj = new SUBCLASS;> ?
1061 Avoid keeping any state information in your packages. It makes it
1062 difficult for multiple other packages to use yours. Keep state
1063 information in objects.
1065 Always use B<-w>. Try to C<use strict;> (or C<use strict qw(...);>).
1066 Remember that you can add C<no strict qw(...);> to individual blocks
1067 of code which need less strictness. Always use B<-w>. Always use B<-w>!
1068 Follow the guidelines in the perlstyle(1) manual.
1070 =item Some simple style guidelines
1072 The perlstyle manual supplied with perl has many helpful points.
1074 Coding style is a matter of personal taste. Many people evolve their
1075 style over several years as they learn what helps them write and
1076 maintain good code. Here's one set of assorted suggestions that
1077 seem to be widely used by experienced developers:
1079 Use underscores to separate words. It is generally easier to read
1080 $var_names_like_this than $VarNamesLikeThis, especially for
1081 non-native speakers of English. It's also a simple rule that works
1082 consistently with VAR_NAMES_LIKE_THIS.
1084 Package/Module names are an exception to this rule. Perl informally
1085 reserves lowercase module names for 'pragma' modules like integer
1086 and strict. Other modules normally begin with a capital letter and
1087 use mixed case with no underscores (need to be short and portable).
1089 You may find it helpful to use letter case to indicate the scope
1090 or nature of a variable. For example:
1092 $ALL_CAPS_HERE constants only (beware clashes with perl vars)
1093 $Some_Caps_Here package-wide global/static
1094 $no_caps_here function scope my() or local() variables
1096 Function and method names seem to work best as all lowercase.
1097 e.g.,, C<$obj-E<gt>as_string()>.
1099 You can use a leading underscore to indicate that a variable or
1100 function should not be used outside the package that defined it.
1102 =item Select what to export.
1104 Do NOT export method names!
1106 Do NOT export anything else by default without a good reason!
1108 Exports pollute the namespace of the module user. If you must
1109 export try to use @EXPORT_OK in preference to @EXPORT and avoid
1110 short or common names to reduce the risk of name clashes.
1112 Generally anything not exported is still accessible from outside the
1113 module using the ModuleName::item_name (or C<$blessed_ref-E<gt>method>)
1114 syntax. By convention you can use a leading underscore on names to
1115 indicate informally that they are 'internal' and not for public use.
1117 (It is actually possible to get private functions by saying:
1118 C<my $subref = sub { ... }; &$subref;>. But there's no way to call that
1119 directly as a method, because a method must have a name in the symbol
1122 As a general rule, if the module is trying to be object oriented
1123 then export nothing. If it's just a collection of functions then
1124 @EXPORT_OK anything but use @EXPORT with caution.
1126 =item Select a name for the module.
1128 This name should be as descriptive, accurate, and complete as
1129 possible. Avoid any risk of ambiguity. Always try to use two or
1130 more whole words. Generally the name should reflect what is special
1131 about what the module does rather than how it does it. Please use
1132 nested module names to group informally or categorize a module.
1133 There should be a very good reason for a module not to have a nested name.
1134 Module names should begin with a capital letter.
1136 Having 57 modules all called Sort will not make life easy for anyone
1137 (though having 23 called Sort::Quick is only marginally better :-).
1138 Imagine someone trying to install your module alongside many others.
1139 If in any doubt ask for suggestions in comp.lang.perl.misc.
1141 If you are developing a suite of related modules/classes it's good
1142 practice to use nested classes with a common prefix as this will
1143 avoid namespace clashes. For example: Xyz::Control, Xyz::View,
1144 Xyz::Model etc. Use the modules in this list as a naming guide.
1146 If adding a new module to a set, follow the original author's
1147 standards for naming modules and the interface to methods in
1150 To be portable each component of a module name should be limited to
1151 11 characters. If it might be used on DOS then try to ensure each is
1152 unique in the first 8 characters. Nested modules make this easier.
1154 =item Have you got it right?
1156 How do you know that you've made the right decisions? Have you
1157 picked an interface design that will cause problems later? Have
1158 you picked the most appropriate name? Do you have any questions?
1160 The best way to know for sure, and pick up many helpful suggestions,
1161 is to ask someone who knows. Comp.lang.perl.misc is read by just about
1162 all the people who develop modules and it's the best place to ask.
1164 All you need to do is post a short summary of the module, its
1165 purpose and interfaces. A few lines on each of the main methods is
1166 probably enough. (If you post the whole module it might be ignored
1167 by busy people - generally the very people you want to read it!)
1169 Don't worry about posting if you can't say when the module will be
1170 ready - just say so in the message. It might be worth inviting
1171 others to help you, they may be able to complete it for you!
1173 =item README and other Additional Files.
1175 It's well known that software developers usually fully document the
1176 software they write. If, however, the world is in urgent need of
1177 your software and there is not enough time to write the full
1178 documentation please at least provide a README file containing:
1183 A description of the module/package/extension etc.
1186 A copyright notice - see below.
1189 Prerequisites - what else you may need to have.
1192 How to build it - possible changes to Makefile.PL etc.
1198 Recent changes in this release, especially incompatibilities
1201 Changes / enhancements you plan to make in the future.
1205 If the README file seems to be getting too large you may wish to
1206 split out some of the sections into separate files: INSTALL,
1211 =item Adding a Copyright Notice.
1213 How you choose to license your work is a personal decision.
1214 The general mechanism is to assert your Copyright and then make
1215 a declaration of how others may copy/use/modify your work.
1217 Perl, for example, is supplied with two types of license: The GNU
1218 GPL and The Artistic License (see the files README, Copying, and
1219 Artistic). Larry has good reasons for NOT just using the GNU GPL.
1221 My personal recommendation, out of respect for Larry, Perl, and the
1222 perl community at large is to state something simply like:
1224 Copyright (c) 1995 Your Name. All rights reserved.
1225 This program is free software; you can redistribute it and/or
1226 modify it under the same terms as Perl itself.
1228 This statement should at least appear in the README file. You may
1229 also wish to include it in a Copying file and your source files.
1230 Remember to include the other words in addition to the Copyright.
1232 =item Give the module a version/issue/release number.
1234 To be fully compatible with the Exporter and MakeMaker modules you
1235 should store your module's version number in a non-my package
1236 variable called $VERSION. This should be a floating point
1237 number with at least two digits after the decimal (i.e., hundredths,
1238 e.g, C<$VERSION = "0.01">). Don't use a "1.3.2" style version.
1239 See Exporter.pm in Perl5.001m or later for details.
1241 It may be handy to add a function or method to retrieve the number.
1242 Use the number in announcements and archive file names when
1243 releasing the module (ModuleName-1.02.tar.Z).
1244 See perldoc ExtUtils::MakeMaker.pm for details.
1246 =item How to release and distribute a module.
1248 It's good idea to post an announcement of the availability of your
1249 module (or the module itself if small) to the comp.lang.perl.announce
1250 Usenet newsgroup. This will at least ensure very wide once-off
1253 If possible you should place the module into a major ftp archive and
1254 include details of its location in your announcement.
1256 Some notes about ftp archives: Please use a long descriptive file
1257 name which includes the version number. Most incoming directories
1258 will not be readable/listable, i.e., you won't be able to see your
1259 file after uploading it. Remember to send your email notification
1260 message as soon as possible after uploading else your file may get
1261 deleted automatically. Allow time for the file to be processed
1262 and/or check the file has been processed before announcing its
1265 FTP Archives for Perl Modules:
1267 Follow the instructions and links on
1269 http://franz.ww.tu-berlin.de/modulelist
1271 or upload to one of these sites:
1273 ftp://franz.ww.tu-berlin.de/incoming
1274 ftp://ftp.cis.ufl.edu/incoming
1276 and notify upload@franz.ww.tu-berlin.de.
1278 By using the WWW interface you can ask the Upload Server to mirror
1279 your modules from your ftp or WWW site into your own directory on
1282 Please remember to send me an updated entry for the Module list!
1284 =item Take care when changing a released module.
1286 Always strive to remain compatible with previous released versions
1287 (see 2.2 above) Otherwise try to add a mechanism to revert to the
1288 old behaviour if people rely on it. Document incompatible changes.
1294 =head2 Guidelines for Converting Perl 4 Library Scripts into Modules
1298 =item There is no requirement to convert anything.
1300 If it ain't broke, don't fix it! Perl 4 library scripts should
1301 continue to work with no problems. You may need to make some minor
1302 changes (like escaping non-array @'s in double quoted strings) but
1303 there is no need to convert a .pl file into a Module for just that.
1305 =item Consider the implications.
1307 All the perl applications which make use of the script will need to
1308 be changed (slightly) if the script is converted into a module. Is
1309 it worth it unless you plan to make other changes at the same time?
1311 =item Make the most of the opportunity.
1313 If you are going to convert the script to a module you can use the
1314 opportunity to redesign the interface. The 'Guidelines for Module
1315 Creation' above include many of the issues you should consider.
1317 =item The pl2pm utility will get you started.
1319 This utility will read *.pl files (given as parameters) and write
1320 corresponding *.pm files. The pl2pm utilities does the following:
1325 Adds the standard Module prologue lines
1328 Converts package specifiers from ' to ::
1331 Converts die(...) to croak(...)
1334 Several other minor changes
1338 Being a mechanical process pl2pm is not bullet proof. The converted
1339 code will need careful checking, especially any package statements.
1340 Don't delete the original .pl file till the new .pm one works!
1344 =head2 Guidelines for Reusing Application Code
1348 =item Complete applications rarely belong in the Perl Module Library.
1350 =item Many applications contain some perl code which could be reused.
1352 Help save the world! Share your code in a form that makes it easy
1355 =item Break-out the reusable code into one or more separate module files.
1357 =item Take the opportunity to reconsider and redesign the interfaces.
1359 =item In some cases the 'application' can then be reduced to a small
1361 fragment of code built on top of the reusable modules. In these cases
1362 the application could invoked as:
1364 perl -e 'use Module::Name; method(@ARGV)' ...
1366 perl -mModule::Name ... (in perl5.002)