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 builtin 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 hash of that
71 name with two colons appended. The main symbol table's name is thus
72 C<%main::>, or C<%::> for short. Likewise symbol table for the nested
73 package mentioned earlier is named C<%OUTER::INNER::>.
75 The value in each entry of the hash is what you are referring to when you
76 use the C<*name> typeglob notation. In fact, the following have the same
77 effect, though the first is more efficient because it does the symbol
78 table lookups at compile time:
80 local(*main::foo) = *main::bar;
81 local($main::{'foo'}) = $main::{'bar'};
83 You can use this to print out all the variables in a package, for
84 instance. Here is F<dumpvar.pl> from the Perl library:
89 local(*stab) = eval("*${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 Some::Module, create
226 a file called Some/Module.pm and start with this template:
228 package Some::Module; # assumes Some/Module.pm
234 use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS);
236 # set the version for version checking
238 # if using RCS/CVS, this may be preferred
239 $VERSION = do { my @r = (q$Revision: 2.21 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r }; # must be all one line, for MakeMaker
242 @EXPORT = qw(&func1 &func2 &func4);
243 %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
245 # your exported package globals go here,
246 # as well as any optionally exported functions
247 @EXPORT_OK = qw($Var1 %Hashit &func3);
251 # non-exported package globals go here
252 use vars qw(@more $stuff);
254 # initalize package globals, first exported ones
258 # then the others (which are still accessible as $Some::Module::stuff)
262 # all file-scoped lexicals must be created before
263 # the functions below that use them.
265 # file-private lexicals go here
267 my %secret_hash = ();
269 # here's a file-private function as a closure,
270 # callable as &$priv_func; it cannot be prototyped.
271 my $priv_func = sub {
275 # make all your functions, whether exported or not;
276 # remember to put something interesting in the {} stubs
277 sub func1 {} # no prototype
278 sub func2() {} # proto'd void
279 sub func3($$) {} # proto'd to 2 scalars
281 # this one isn't exported, but could be called!
282 sub func4(\%) {} # proto'd to 1 hash ref
284 END { } # module clean-up code here (global destructor)
286 Then go on to declare and use your variables in functions
287 without any qualifications.
288 See L<Exporter> and the I<Perl Modules File> for details on
289 mechanics and style issues in module creation.
291 Perl modules are included into your program by saying
299 This is exactly equivalent to
301 BEGIN { require "Module.pm"; import Module; }
305 BEGIN { require "Module.pm"; import Module LIST; }
311 is exactly equivalent to
313 BEGIN { require "Module.pm"; }
315 All Perl module files have the extension F<.pm>. C<use> assumes this so
316 that you don't have to spell out "F<Module.pm>" in quotes. This also
317 helps to differentiate new modules from old F<.pl> and F<.ph> files.
318 Module names are also capitalized unless they're functioning as pragmas,
319 "Pragmas" are in effect compiler directives, and are sometimes called
320 "pragmatic modules" (or even "pragmata" if you're a classicist).
322 Because the C<use> statement implies a C<BEGIN> block, the importation
323 of semantics happens at the moment the C<use> statement is compiled,
324 before the rest of the file is compiled. This is how it is able
325 to function as a pragma mechanism, and also how modules are able to
326 declare subroutines that are then visible as list operators for
327 the rest of the current file. This will not work if you use C<require>
328 instead of C<use>. With require you can get into this problem:
330 require Cwd; # make Cwd:: accessible
331 $here = Cwd::getcwd();
333 use Cwd; # import names from Cwd::
336 require Cwd; # make Cwd:: accessible
337 $here = getcwd(); # oops! no main::getcwd()
339 In general C<use Module ();> is recommended over C<require Module;>.
341 Perl packages may be nested inside other package names, so we can have
342 package names containing C<::>. But if we used that package name
343 directly as a filename it would makes for unwieldy or impossible
344 filenames on some systems. Therefore, if a module's name is, say,
345 C<Text::Soundex>, then its definition is actually found in the library
346 file F<Text/Soundex.pm>.
348 Perl modules always have a F<.pm> file, but there may also be dynamically
349 linked executables or autoloaded subroutine definitions associated with
350 the module. If so, these will be entirely transparent to the user of
351 the module. It is the responsibility of the F<.pm> file to load (or
352 arrange to autoload) any additional functionality. The POSIX module
353 happens to do both dynamic loading and autoloading, but the user can
354 say just C<use POSIX> to get it all.
356 For more information on writing extension modules, see L<perlxs>
361 Perl does not enforce private and public parts of its modules as you may
362 have been used to in other languages like C++, Ada, or Modula-17. Perl
363 doesn't have an infatuation with enforced privacy. It would prefer
364 that you stayed out of its living room because you weren't invited, not
365 because it has a shotgun.
367 The module and its user have a contract, part of which is common law,
368 and part of which is "written". Part of the common law contract is
369 that a module doesn't pollute any namespace it wasn't asked to. The
370 written contract for the module (A.K.A. documentation) may make other
371 provisions. But then you know when you C<use RedefineTheWorld> that
372 you're redefining the world and willing to take the consequences.
374 =head1 THE PERL MODULE LIBRARY
376 A number of modules are included the Perl distribution. These are
377 described below, and all end in F<.pm>. You may also discover files in
378 the library directory that end in either F<.pl> or F<.ph>. These are old
379 libraries supplied so that old programs that use them still run. The
380 F<.pl> files will all eventually be converted into standard modules, and
381 the F<.ph> files made by B<h2ph> will probably end up as extension modules
382 made by B<h2xs>. (Some F<.ph> values may already be available through the
383 POSIX module.) The B<pl2pm> file in the distribution may help in your
384 conversion, but it's just a mechanical process and therefore far from
387 =head2 Pragmatic Modules
389 They work somewhat like pragmas in that they tend to affect the compilation of
390 your program, and thus will usually work well only when used within a
391 C<use>, or C<no>. Most of these are locally scoped, so an inner BLOCK
392 may countermand any of these by saying:
397 which lasts until the end of that BLOCK.
399 Unlike the pragmas that effect the C<$^H> hints variable, the C<use
400 vars> and C<use subs> declarations are not BLOCK-scoped. They allow
401 you to predeclare a variables or subroutines within a particular
402 I<file> rather than just a block. Such declarations are effective
403 for the entire file for which they were declared. You cannot rescind
404 them with C<no vars> or C<no subs>.
406 The following pragmas are defined (and have their own documentation).
412 manipulate @INC at compile time to use MakeMaker's uninstalled version
417 force verbose warning diagnostics
421 compute arithmetic in integer instead of double
425 request less of something from the compiler
429 manipulate @INC at compile time
433 use or ignore current locale for builtin operations (see L<perllocale>)
437 restrict named opcodes when compiling or running Perl code
441 overload basic Perl operations
445 enable simple signal handling
449 restrict unsafe constructs
457 adopt certain VMS-specific behaviors
461 predeclare global variable names
465 =head2 Standard Modules
467 Standard, bundled modules are all expected to behave in a well-defined
468 manner with respect to namespace pollution because they use the
469 Exporter module. See their own documentation for details.
475 provide framework for multiple DBMs
479 load functions only on demand
483 split a package for autoloading
487 benchmark running times of code
491 interface to Comprehensive Perl Archive Network
493 =item CPAN::FirstTime
495 create a CPAN configuration file
499 run CPAN while avoiding compiled extensions
503 warn of errors (from perspective of caller)
507 declare struct-like datatypes
511 access Perl configuration information
515 get pathname of current working directory
519 access to Berkeley DB
521 =item Devel::SelfStubber
523 generate stubs for a SelfLoading module
527 supply object methods for directory handles
531 dynamically load C libraries into Perl code
535 use nice English (or awk) names for ugly punctuation variables
539 import environment variables
543 implements default import method for modules
545 =item ExtUtils::Embed
547 utilities for embedding Perl in C/C++ applications
549 =item ExtUtils::Install
551 install files from here to there
553 =item ExtUtils::Liblist
555 determine libraries to use and how to use them
557 =item ExtUtils::MM_OS2
559 methods to override Unix behaviour in ExtUtils::MakeMaker
561 =item ExtUtils::MM_Unix
563 methods used by ExtUtils::MakeMaker
565 =item ExtUtils::MM_VMS
567 methods to override Unix behaviour in ExtUtils::MakeMaker
569 =item ExtUtils::MakeMaker
571 create an extension Makefile
573 =item ExtUtils::Manifest
575 utilities to write and check a MANIFEST file
577 =item ExtUtils::Mkbootstrap
579 make a bootstrap file for use by DynaLoader
581 =item ExtUtils::Mksymlists
583 write linker options files for dynamic extension
585 =item ExtUtils::testlib
587 add blib/* directories to @INC
591 load the C Fcntl.h defines
595 split a pathname into pieces
597 =item File::CheckTree
599 run many filetest checks on a tree
603 compare files or filehandles
607 copy files or filehandles
615 create or remove a series of directories
619 by-name interface to Perl's builtin stat() functions
623 keep more files open than the system permits
627 supply object methods for filehandles
631 locate directory of original perl script
635 access to the gdbm library
639 extended processing of command line options
643 process single-character switches with switch clustering
647 compare 8-bit scalar data according to the current locale
651 load various IO modules
655 supply object methods for filehandles
659 supply object methods for I/O handles
663 supply object methods for pipes
667 supply seek based methods for I/O objects
671 OO interface to the select system call
675 object interface to socket communications
679 open a process for both reading and writing
683 open a process for reading, writing, and error handling
687 arbitrary length float math package
691 arbitrary size integer math package
695 complex numbers and associated mathematical functions
699 tied access to ndbm files
707 by-name interface to Perl's builtin gethost*() functions
711 by-name interface to Perl's builtin getnet*() functions
715 by-name interface to Perl's builtin getproto*() functions
719 by-name interface to Perl's builtin getserv*() functions
723 disable named opcodes when compiling or running perl code
727 convert POD data to formatted ASCII text
731 interface to IEEE Standard 1003.1
735 tied access to sdbm files
739 compile and execute code in restricted compartments
743 search for key in dictionary file
747 save and restore selected file handle
751 load functions only on demand
755 run shell commands transparently within perl
759 load the C socket.h defines and structure manipulators
763 manipulate Perl symbols and their names
767 try every conceivable way to get hostname
771 interface to the Unix syslog(3) calls
779 word completion module
783 interface to various C<readline> packages
787 run perl standard test scripts with statistics
791 create an abbreviation table from a list
793 =item Text::ParseWords
795 parse text into an array of tokens
799 implementation of the Soundex Algorithm as described by Knuth
803 expand and unexpand tabs per the Unix expand(1) and unexpand(1)
807 line wrapping to form simple paragraphs
811 base class definitions for tied hashes
815 base class definitions for tied hashes with references as keys
819 base class definitions for tied scalars
821 =item Tie::SubstrHash
823 fixed-table-size, fixed-key-length hashing
827 efficiently compute time from local and GMT time
831 by-name interface to Perl's builtin gmtime() function
833 =item Time::localtime
835 by-name interface to Perl's builtin localtime() function
839 internal object used by Time::gmtime and Time::localtime
843 base class for ALL classes (blessed references)
847 by-name interface to Perl's builtin getgr*() functions
851 by-name interface to Perl's builtin getpw*() functions
855 To find out I<all> the modules installed on your system, including
856 those without documentation or outside the standard release, do this:
858 find `perl -e 'print "@INC"'` -name '*.pm' -print
860 They should all have their own documentation installed and accessible via
861 your system man(1) command. If that fails, try the I<perldoc> program.
863 =head2 Extension Modules
865 Extension modules are written in C (or a mix of Perl and C) and may be
866 statically linked or in general are
867 dynamically loaded into Perl if and when you need them. Supported
868 extension modules include the Socket, Fcntl, and POSIX modules.
870 Many popular C extension modules do not come bundled (at least, not
871 completely) due to their sizes, volatility, or simply lack of time for
872 adequate testing and configuration across the multitude of platforms on
873 which Perl was beta-tested. You are encouraged to look for them in
874 archie(1L), the Perl FAQ or Meta-FAQ, the WWW page, and even with their
875 authors before randomly posting asking for their present condition and
880 CPAN stands for the Comprehensive Perl Archive Network. This is a globally
881 replicated collection of all known Perl materials, including hundreds
882 of unbundled modules. Here are the major categories of modules:
887 Language Extensions and Documentation Tools
893 Operating System Interfaces
896 Networking, Device Control (modems) and InterProcess Communication
899 Data Types and Data Type Utilities
908 Interfaces to / Emulations of Other Programming Languages
911 File Names, File Systems and File Locking (see also File Handles)
914 String Processing, Language Text Processing, Parsing, and Searching
917 Option, Argument, Parameter, and Configuration File Processing
920 Internationalization and Locale
923 Authentication, Security, and Encryption
926 World Wide Web, HTML, HTTP, CGI, MIME
929 Server and Daemon Utilities
932 Archiving and Compression
935 Images, Pixmap and Bitmap Manipulation, Drawing, and Graphing
941 Control Flow Utilities (callbacks and exceptions etc)
944 File Handle and Input/Output Stream Utilities
947 Miscellaneous Modules
951 The registered CPAN sites as of this writing include the following.
952 You should try to choose one close to you:
959 South Africa ftp://ftp.is.co.za/programming/perl/CPAN/
964 Hong Kong ftp://ftp.hkstar.com/pub/CPAN/
965 Japan ftp://ftp.jaist.ac.jp/pub/lang/perl/CPAN/
966 ftp://ftp.lab.kdd.co.jp/lang/perl/CPAN/
967 South Korea ftp://ftp.nuri.net/pub/CPAN/
968 Taiwan ftp://dongpo.math.ncu.edu.tw/perl/CPAN/
969 ftp://ftp.wownet.net/pub2/PERL/
974 Australia ftp://ftp.netinfo.com.au/pub/perl/CPAN/
975 New Zealand ftp://ftp.tekotago.ac.nz/pub/perl/CPAN/
980 Austria ftp://ftp.tuwien.ac.at/pub/languages/perl/CPAN/
981 Belgium ftp://ftp.kulnet.kuleuven.ac.be/pub/mirror/CPAN/
982 Czech Republic ftp://sunsite.mff.cuni.cz/Languages/Perl/CPAN/
983 Denmark ftp://sunsite.auc.dk/pub/languages/perl/CPAN/
984 Finland ftp://ftp.funet.fi/pub/languages/perl/CPAN/
985 France ftp://ftp.ibp.fr/pub/perl/CPAN/
986 ftp://ftp.pasteur.fr/pub/computing/unix/perl/CPAN/
987 Germany ftp://ftp.gmd.de/packages/CPAN/
988 ftp://ftp.leo.org/pub/comp/programming/languages/perl/CPAN/
989 ftp://ftp.mpi-sb.mpg.de/pub/perl/CPAN/
990 ftp://ftp.rz.ruhr-uni-bochum.de/pub/CPAN/
991 ftp://ftp.uni-erlangen.de/pub/source/Perl/CPAN/
992 ftp://ftp.uni-hamburg.de/pub/soft/lang/perl/CPAN/
993 Greece ftp://ftp.ntua.gr/pub/lang/perl/
994 Hungary ftp://ftp.kfki.hu/pub/packages/perl/CPAN/
995 Italy ftp://cis.utovrm.it/CPAN/
996 the Netherlands ftp://ftp.cs.ruu.nl/pub/PERL/CPAN/
997 ftp://ftp.EU.net/packages/cpan/
998 Norway ftp://ftp.uit.no/pub/languages/perl/cpan/
999 Poland ftp://ftp.pk.edu.pl/pub/lang/perl/CPAN/
1000 ftp://sunsite.icm.edu.pl/pub/CPAN/
1001 Portugal ftp://ftp.ci.uminho.pt/pub/lang/perl/
1002 ftp://ftp.telepac.pt/pub/CPAN/
1003 Russia ftp://ftp.sai.msu.su/pub/lang/perl/CPAN/
1004 Slovenia ftp://ftp.arnes.si/software/perl/CPAN/
1005 Spain ftp://ftp.etse.urv.es/pub/mirror/perl/
1006 ftp://ftp.rediris.es/mirror/CPAN/
1007 Sweden ftp://ftp.sunet.se/pub/lang/perl/CPAN/
1008 UK ftp://ftp.demon.co.uk/pub/mirrors/perl/CPAN/
1009 ftp://sunsite.doc.ic.ac.uk/packages/CPAN/
1010 ftp://unix.hensa.ac.uk/mirrors/perl-CPAN/
1015 Ontario ftp://ftp.utilis.com/public/CPAN/
1016 ftp://enterprise.ic.gc.ca/pub/perl/CPAN/
1017 Manitoba ftp://theory.uwinnipeg.ca/pub/CPAN/
1018 California ftp://ftp.digital.com/pub/plan/perl/CPAN/
1019 ftp://ftp.cdrom.com/pub/perl/CPAN/
1020 Colorado ftp://ftp.cs.colorado.edu/pub/perl/CPAN/
1021 Florida ftp://ftp.cis.ufl.edu/pub/perl/CPAN/
1022 Illinois ftp://uiarchive.uiuc.edu/pub/lang/perl/CPAN/
1023 Massachusetts ftp://ftp.iguide.com/pub/mirrors/packages/perl/CPAN/
1024 New York ftp://ftp.rge.com/pub/languages/perl/
1025 North Carolina ftp://ftp.duke.edu/pub/perl/
1026 Oklahoma ftp://ftp.ou.edu/mirrors/CPAN/
1027 Oregon http://www.perl.org/CPAN/
1028 ftp://ftp.orst.edu/pub/packages/CPAN/
1029 Pennsylvania ftp://ftp.epix.net/pub/languages/perl/
1030 Texas ftp://ftp.sedl.org/pub/mirrors/CPAN/
1031 ftp://ftp.metronet.com/pub/perl/
1036 Chile ftp://sunsite.dcc.uchile.cl/pub/Lang/perl/CPAN/
1040 For an up-to-date listing of CPAN sites,
1041 see F<http://www.perl.com/perl/CPAN> or F<ftp://ftp.perl.com/perl/>.
1043 =head1 Modules: Creation, Use, and Abuse
1045 (The following section is borrowed directly from Tim Bunce's modules
1046 file, available at your nearest CPAN site.)
1048 Perl implements a class using a package, but the presence of a
1049 package doesn't imply the presence of a class. A package is just a
1050 namespace. A class is a package that provides subroutines that can be
1051 used as methods. A method is just a subroutine that expects, as its
1052 first argument, either the name of a package (for "static" methods),
1053 or a reference to something (for "virtual" methods).
1055 A module is a file that (by convention) provides a class of the same
1056 name (sans the .pm), plus an import method in that class that can be
1057 called to fetch exported symbols. This module may implement some of
1058 its methods by loading dynamic C or C++ objects, but that should be
1059 totally transparent to the user of the module. Likewise, the module
1060 might set up an AUTOLOAD function to slurp in subroutine definitions on
1061 demand, but this is also transparent. Only the .pm file is required to
1064 =head2 Guidelines for Module Creation
1068 =item Do similar modules already exist in some form?
1070 If so, please try to reuse the existing modules either in whole or
1071 by inheriting useful features into a new class. If this is not
1072 practical try to get together with the module authors to work on
1073 extending or enhancing the functionality of the existing modules.
1074 A perfect example is the plethora of packages in perl4 for dealing
1075 with command line options.
1077 If you are writing a module to expand an already existing set of
1078 modules, please coordinate with the author of the package. It
1079 helps if you follow the same naming scheme and module interaction
1080 scheme as the original author.
1082 =item Try to design the new module to be easy to extend and reuse.
1084 Use blessed references. Use the two argument form of bless to bless
1085 into the class name given as the first parameter of the constructor,
1090 return bless {}, $class;
1093 or even this if you'd like it to be used as either a static
1094 or a virtual method.
1098 my $class = ref($self) || $self;
1099 return bless {}, $class;
1102 Pass arrays as references so more parameters can be added later
1103 (it's also faster). Convert functions into methods where
1104 appropriate. Split large methods into smaller more flexible ones.
1105 Inherit methods from other modules if appropriate.
1107 Avoid class name tests like: C<die "Invalid" unless ref $ref eq 'FOO'>.
1108 Generally you can delete the "C<eq 'FOO'>" part with no harm at all.
1109 Let the objects look after themselves! Generally, avoid hard-wired
1110 class names as far as possible.
1112 Avoid C<$r-E<gt>Class::func()> where using C<@ISA=qw(... Class ...)> and
1113 C<$r-E<gt>func()> would work (see L<perlbot> for more details).
1115 Use autosplit so little used or newly added functions won't be a
1116 burden to programs which don't use them. Add test functions to
1117 the module after __END__ either using AutoSplit or by saying:
1119 eval join('',<main::DATA>) || die $@ unless caller();
1121 Does your module pass the 'empty subclass' test? If you say
1122 "C<@SUBCLASS::ISA = qw(YOURCLASS);>" your applications should be able
1123 to use SUBCLASS in exactly the same way as YOURCLASS. For example,
1124 does your application still work if you change: C<$obj = new YOURCLASS;>
1125 into: C<$obj = new SUBCLASS;> ?
1127 Avoid keeping any state information in your packages. It makes it
1128 difficult for multiple other packages to use yours. Keep state
1129 information in objects.
1131 Always use B<-w>. Try to C<use strict;> (or C<use strict qw(...);>).
1132 Remember that you can add C<no strict qw(...);> to individual blocks
1133 of code which need less strictness. Always use B<-w>. Always use B<-w>!
1134 Follow the guidelines in the perlstyle(1) manual.
1136 =item Some simple style guidelines
1138 The perlstyle manual supplied with perl has many helpful points.
1140 Coding style is a matter of personal taste. Many people evolve their
1141 style over several years as they learn what helps them write and
1142 maintain good code. Here's one set of assorted suggestions that
1143 seem to be widely used by experienced developers:
1145 Use underscores to separate words. It is generally easier to read
1146 $var_names_like_this than $VarNamesLikeThis, especially for
1147 non-native speakers of English. It's also a simple rule that works
1148 consistently with VAR_NAMES_LIKE_THIS.
1150 Package/Module names are an exception to this rule. Perl informally
1151 reserves lowercase module names for 'pragma' modules like integer
1152 and strict. Other modules normally begin with a capital letter and
1153 use mixed case with no underscores (need to be short and portable).
1155 You may find it helpful to use letter case to indicate the scope
1156 or nature of a variable. For example:
1158 $ALL_CAPS_HERE constants only (beware clashes with perl vars)
1159 $Some_Caps_Here package-wide global/static
1160 $no_caps_here function scope my() or local() variables
1162 Function and method names seem to work best as all lowercase.
1163 e.g.,, C<$obj-E<gt>as_string()>.
1165 You can use a leading underscore to indicate that a variable or
1166 function should not be used outside the package that defined it.
1168 =item Select what to export.
1170 Do NOT export method names!
1172 Do NOT export anything else by default without a good reason!
1174 Exports pollute the namespace of the module user. If you must
1175 export try to use @EXPORT_OK in preference to @EXPORT and avoid
1176 short or common names to reduce the risk of name clashes.
1178 Generally anything not exported is still accessible from outside the
1179 module using the ModuleName::item_name (or C<$blessed_ref-E<gt>method>)
1180 syntax. By convention you can use a leading underscore on names to
1181 indicate informally that they are 'internal' and not for public use.
1183 (It is actually possible to get private functions by saying:
1184 C<my $subref = sub { ... }; &$subref;>. But there's no way to call that
1185 directly as a method, because a method must have a name in the symbol
1188 As a general rule, if the module is trying to be object oriented
1189 then export nothing. If it's just a collection of functions then
1190 @EXPORT_OK anything but use @EXPORT with caution.
1192 =item Select a name for the module.
1194 This name should be as descriptive, accurate, and complete as
1195 possible. Avoid any risk of ambiguity. Always try to use two or
1196 more whole words. Generally the name should reflect what is special
1197 about what the module does rather than how it does it. Please use
1198 nested module names to group informally or categorize a module.
1199 There should be a very good reason for a module not to have a nested name.
1200 Module names should begin with a capital letter.
1202 Having 57 modules all called Sort will not make life easy for anyone
1203 (though having 23 called Sort::Quick is only marginally better :-).
1204 Imagine someone trying to install your module alongside many others.
1205 If in any doubt ask for suggestions in comp.lang.perl.misc.
1207 If you are developing a suite of related modules/classes it's good
1208 practice to use nested classes with a common prefix as this will
1209 avoid namespace clashes. For example: Xyz::Control, Xyz::View,
1210 Xyz::Model etc. Use the modules in this list as a naming guide.
1212 If adding a new module to a set, follow the original author's
1213 standards for naming modules and the interface to methods in
1216 To be portable each component of a module name should be limited to
1217 11 characters. If it might be used on MS-DOS then try to ensure each is
1218 unique in the first 8 characters. Nested modules make this easier.
1220 =item Have you got it right?
1222 How do you know that you've made the right decisions? Have you
1223 picked an interface design that will cause problems later? Have
1224 you picked the most appropriate name? Do you have any questions?
1226 The best way to know for sure, and pick up many helpful suggestions,
1227 is to ask someone who knows. Comp.lang.perl.misc is read by just about
1228 all the people who develop modules and it's the best place to ask.
1230 All you need to do is post a short summary of the module, its
1231 purpose and interfaces. A few lines on each of the main methods is
1232 probably enough. (If you post the whole module it might be ignored
1233 by busy people - generally the very people you want to read it!)
1235 Don't worry about posting if you can't say when the module will be
1236 ready - just say so in the message. It might be worth inviting
1237 others to help you, they may be able to complete it for you!
1239 =item README and other Additional Files.
1241 It's well known that software developers usually fully document the
1242 software they write. If, however, the world is in urgent need of
1243 your software and there is not enough time to write the full
1244 documentation please at least provide a README file containing:
1249 A description of the module/package/extension etc.
1252 A copyright notice - see below.
1255 Prerequisites - what else you may need to have.
1258 How to build it - possible changes to Makefile.PL etc.
1264 Recent changes in this release, especially incompatibilities
1267 Changes / enhancements you plan to make in the future.
1271 If the README file seems to be getting too large you may wish to
1272 split out some of the sections into separate files: INSTALL,
1277 =item Adding a Copyright Notice.
1279 How you choose to license your work is a personal decision.
1280 The general mechanism is to assert your Copyright and then make
1281 a declaration of how others may copy/use/modify your work.
1283 Perl, for example, is supplied with two types of licence: The GNU
1284 GPL and The Artistic Licence (see the files README, Copying, and
1285 Artistic). Larry has good reasons for NOT just using the GNU GPL.
1287 My personal recommendation, out of respect for Larry, Perl, and the
1288 perl community at large is to state something simply like:
1290 Copyright (c) 1995 Your Name. All rights reserved.
1291 This program is free software; you can redistribute it and/or
1292 modify it under the same terms as Perl itself.
1294 This statement should at least appear in the README file. You may
1295 also wish to include it in a Copying file and your source files.
1296 Remember to include the other words in addition to the Copyright.
1298 =item Give the module a version/issue/release number.
1300 To be fully compatible with the Exporter and MakeMaker modules you
1301 should store your module's version number in a non-my package
1302 variable called $VERSION. This should be a floating point
1303 number with at least two digits after the decimal (i.e., hundredths,
1304 e.g, C<$VERSION = "0.01">). Don't use a "1.3.2" style version.
1305 See Exporter.pm in Perl5.001m or later for details.
1307 It may be handy to add a function or method to retrieve the number.
1308 Use the number in announcements and archive file names when
1309 releasing the module (ModuleName-1.02.tar.Z).
1310 See perldoc ExtUtils::MakeMaker.pm for details.
1312 =item How to release and distribute a module.
1314 It's good idea to post an announcement of the availability of your
1315 module (or the module itself if small) to the comp.lang.perl.announce
1316 Usenet newsgroup. This will at least ensure very wide once-off
1319 If possible you should place the module into a major ftp archive and
1320 include details of its location in your announcement.
1322 Some notes about ftp archives: Please use a long descriptive file
1323 name which includes the version number. Most incoming directories
1324 will not be readable/listable, i.e., you won't be able to see your
1325 file after uploading it. Remember to send your email notification
1326 message as soon as possible after uploading else your file may get
1327 deleted automatically. Allow time for the file to be processed
1328 and/or check the file has been processed before announcing its
1331 FTP Archives for Perl Modules:
1333 Follow the instructions and links on
1335 http://franz.ww.tu-berlin.de/modulelist
1337 or upload to one of these sites:
1339 ftp://franz.ww.tu-berlin.de/incoming
1340 ftp://ftp.cis.ufl.edu/incoming
1342 and notify <F<upload@franz.ww.tu-berlin.de>>.
1344 By using the WWW interface you can ask the Upload Server to mirror
1345 your modules from your ftp or WWW site into your own directory on
1348 Please remember to send me an updated entry for the Module list!
1350 =item Take care when changing a released module.
1352 Always strive to remain compatible with previous released versions
1353 (see 2.2 above) Otherwise try to add a mechanism to revert to the
1354 old behaviour if people rely on it. Document incompatible changes.
1360 =head2 Guidelines for Converting Perl 4 Library Scripts into Modules
1364 =item There is no requirement to convert anything.
1366 If it ain't broke, don't fix it! Perl 4 library scripts should
1367 continue to work with no problems. You may need to make some minor
1368 changes (like escaping non-array @'s in double quoted strings) but
1369 there is no need to convert a .pl file into a Module for just that.
1371 =item Consider the implications.
1373 All the perl applications which make use of the script will need to
1374 be changed (slightly) if the script is converted into a module. Is
1375 it worth it unless you plan to make other changes at the same time?
1377 =item Make the most of the opportunity.
1379 If you are going to convert the script to a module you can use the
1380 opportunity to redesign the interface. The 'Guidelines for Module
1381 Creation' above include many of the issues you should consider.
1383 =item The pl2pm utility will get you started.
1385 This utility will read *.pl files (given as parameters) and write
1386 corresponding *.pm files. The pl2pm utilities does the following:
1391 Adds the standard Module prologue lines
1394 Converts package specifiers from ' to ::
1397 Converts die(...) to croak(...)
1400 Several other minor changes
1404 Being a mechanical process pl2pm is not bullet proof. The converted
1405 code will need careful checking, especially any package statements.
1406 Don't delete the original .pl file till the new .pm one works!
1410 =head2 Guidelines for Reusing Application Code
1414 =item Complete applications rarely belong in the Perl Module Library.
1416 =item Many applications contain some perl code which could be reused.
1418 Help save the world! Share your code in a form that makes it easy
1421 =item Break-out the reusable code into one or more separate module files.
1423 =item Take the opportunity to reconsider and redesign the interfaces.
1425 =item In some cases the 'application' can then be reduced to a small
1427 fragment of code built on top of the reusable modules. In these cases
1428 the application could invoked as:
1430 perl -e 'use Module::Name; method(@ARGV)' ...
1432 perl -mModule::Name ... (in perl5.002 or higher)