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Diffstat (limited to 'pod/perlmod.pod')
| -rw-r--r-- | pod/perlmod.pod | 661 |
1 files changed, 620 insertions, 41 deletions
diff --git a/pod/perlmod.pod b/pod/perlmod.pod index c5ab08a07c..3ada156f6e 100644 --- a/pod/perlmod.pod +++ b/pod/perlmod.pod @@ -7,14 +7,16 @@ perlmod - Perl modules (packages) =head2 Packages Perl provides a mechanism for alternative namespaces to protect packages -from stomping on each others variables. In fact, apart from certain magical -variables, there's really no such thing as a global variable in Perl. -By default, a Perl script starts -compiling into the package known as C<main>. You can switch namespaces -using the C<package> declaration. The scope of the package declaration is -from the declaration itself to the end of the enclosing block (the same -scope as the local() operator). Typically it would be the first -declaration in a file to be included by the C<require> operator. You can +from stomping on each others variables. In fact, apart from certain +magical variables, there's really no such thing as a global variable in +Perl. The package statement declares the compilation unit as being in the +given namespace. The scope of the package declaration is from the +declaration itself through the end of the enclosing block (the same scope +as the local() operator). All further unqualified dynamic identifiers +will be in this namespace. A package statement only affects dynamic +variables--including those you've used local() on--but I<not> lexical +variables created with my(). Typically it would be the first declaration +in a file to be included by the C<require> or C<use> operator. You can switch into a package in more than one place; it merely influences which symbol table is used by the compiler for the rest of that block. You can refer to variables and filehandles in other packages by prefixing the @@ -35,17 +37,19 @@ within package C<OUTER> that C<$INNER::var> refers to C<$OUTER::INNER::var>. It would treat package C<INNER> as a totally separate global package. Only identifiers starting with letters (or underscore) are stored in a -package's symbol table. All other symbols are kept in package C<main>. -In addition, the identifiers STDIN, STDOUT, STDERR, ARGV, -ARGVOUT, ENV, INC and SIG are forced to be in package C<main>, -even when used for other purposes than their built-in one. Note also -that, if you have a package called C<m>, C<s> or C<y>, then you can't use -the qualified form of an identifier because it will be interpreted instead -as a pattern match, a substitution, or a translation. +package's symbol table. All other symbols are kept in package C<main>, +including all of the punctuation variables like $_. In addition, the +identifiers STDIN, STDOUT, STDERR, ARGV, ARGVOUT, ENV, INC and SIG are +forced to be in package C<main>, even when used for other purposes than +their built-in one. Note also that, if you have a package called C<m>, +C<s> or C<y>, then you can't use the qualified form of an identifier +because it will be interpreted instead as a pattern match, a substitution, +or a translation. (Variables beginning with underscore used to be forced into package main, but we decided it was more useful for package writers to be able -to use leading underscore to indicate private variables and method names.) +to use leading underscore to indicate private variables and method names. +$_ is still global though.) Eval()ed strings are compiled in the package in which the eval() was compiled. (Assignments to C<$SIG{}>, however, assume the signal @@ -58,6 +62,9 @@ temporarily switches back to the C<main> package to evaluate various expressions in the context of the C<main> package (or wherever you came from). See L<perldebug>. +See L<perlsub> for other scoping issues related to my() and local(), +or L<perlref> regarding closures. + =head2 Symbol Tables The symbol table for a package happens to be stored in the associative @@ -65,10 +72,10 @@ array of that name appended with two colons. The main symbol table's name is thus C<%main::>, or C<%::> for short. Likewise the nested package mentioned earlier is named C<%OUTER::INNER::>. -The value in each entry of the associative array is what you are -referring to when you use the C<*name> typeglob notation. In fact, the following -have the same effect, though the first is more efficient because it -does the symbol table lookups at compile time: +The value in each entry of the associative array is what you are referring +to when you use the C<*name> typeglob notation. In fact, the following +have the same effect, though the first is more efficient because it does +the symbol table lookups at compile time: local(*main::foo) = *main::bar; local($main::{'foo'}) = $main::{'bar'}; @@ -108,8 +115,7 @@ Note that even though the subroutine is compiled in package C<dumpvar>, the name of the subroutine is qualified so that its name is inserted into package C<main>. -Assignment to a typeglob performs an aliasing operation, -i.e., +Assignment to a typeglob performs an aliasing operation, i.e., *dick = *richard; @@ -123,6 +129,32 @@ assign a reference instead: makes $richard and $dick the same variable, but leaves @richard and @dick as separate arrays. Tricky, eh? +This mechanism may be used to pass and return cheap references +into or from subroutines if you won't want to copy the whole +thing. + + %some_hash = (); + *some_hash = fn( \%another_hash ); + sub fn { + local *hashsym = shift; + # now use %hashsym normally, and you + # will affect the caller's %another_hash + my %nhash = (); # do what you want + return \%nhash; + } + +On return, the reference wil overwrite the hash slot in the +symbol table specified by the *some_hash typeglob. This +is a somewhat tricky way of passing around refernces cheaply +when you won't want to have to remember to dereference variables +explicitly. + +Another use of symbol tables is for making "constant" scalars. + + *PI = \3.14159265358979; + +Now you cannot alter $PI, which is probably a good thing all in all. + =head2 Package Constructors and Destructors There are two special subroutine definitions that function as package @@ -196,6 +228,14 @@ or BEGIN { require "Module.pm"; import Module LIST; } +As a special case + + use Module (); + +is exactly equivalent to + + BEGIN { require "Module.pm"; } + All Perl module files have the extension F<.pm>. C<use> assumes this so that you don't have to spell out "F<Module.pm>" in quotes. This also helps to differentiate new modules from old F<.pl> and F<.ph> files. @@ -209,9 +249,7 @@ before the rest of the file is compiled. This is how it is able to function as a pragma mechanism, and also how modules are able to declare subroutines that are then visible as list operators for the rest of the current file. This will not work if you use C<require> -instead of C<use>. Therefore, if you're planning on the module altering -your namespace, use C<use>; otherwise, use C<require>. Otherwise you -can get into this problem: +instead of C<use>. With require you can get into this problem: require Cwd; # make Cwd:: accessible $here = Cwd::getcwd(); @@ -222,6 +260,8 @@ can get into this problem: require Cwd; # make Cwd:: accessible $here = getcwd(); # oops! no main::getcwd() +In general C<use Module ();> is recommended over C<require Module;>. + Perl packages may be nested inside other package names, so we can have package names containing C<::>. But if we used that package name directly as a filename it would makes for unwieldy or impossible @@ -283,27 +323,31 @@ The following programs are defined (and have their own documentation). =over 12 -=item C<diagnostics> +=item diagnostics Pragma to produce enhanced diagnostics -=item C<integer> +=item integer Pragma to compute arithmetic in integer instead of double -=item C<less> +=item less Pragma to request less of something from the compiler -=item C<sigtrap> +=item overload + +Pragma for overloading operators + +=item sigtrap Pragma to enable stack backtrace on unexpected signals -=item C<strict> +=item strict Pragma to restrict unsafe constructs -=item C<subs> +=item subs Pragma to predeclare sub names @@ -315,7 +359,152 @@ Standard, bundled modules are all expected to behave in a well-defined manner with respect to namespace pollution because they use the Exporter module. See their own documentation for details. -To find out all the modules installed on your system, do this: +=over 12 + +=item AnyDBM_File + +provide framework for multiple DBMs + +=item AutoLoader + +load functions only on demand + +=item AutoSplit + +split a package for autoloading + +=item Benchmark + +benchmark running times of code + +=item Carp + +warn of errors (from perspective of caller) + +=item Config + +access Perl configuration option + +=item Cwd + +get pathname of current working directory + +=item DB_File + +Perl access to Berkeley DB + +=item Devel::SelfStubber + +generate stubs for a SelfLoading module + +=item DynaLoader + +Dynamically load C libraries into Perl code + +=item English + +use nice English (or awk) names for ugly punctuation variables + +=item Env + +perl module that imports environment variables + +=item Exporter + +provide inport/export controls for Perl modules + +=item ExtUtils::Liblist + +determine libraries to use and how to use them + +=item ExtUtils::MakeMaker + +create an extension Makefile + +=item ExtUtils::Manifest + +utilities to write and check a MANIFEST file + +=item ExtUtils::Mkbootstrap + +make a bootstrap file for use by DynaLoader + +=item ExtUtils::Miniperl + +!!!GOOD QUESTION!!! + +=item Fcntl + +load the C Fcntl.h defines + +=item File::Basename + +parse file specifications + +=item File::CheckTree + +run many filetest checks on a tree + +=item File::Find + +traverse a file tree + +=item FileHandle + +supply object methods for filehandles + +=item File::Path + +create or remove a series of directories + +=item Getopt::Long + +extended getopt processing + +=item Getopt::Std + +Process single-character switches with switch clustering + +=item I18N::Collate + +compare 8-bit scalar data according to the current locale + +=item IPC::Open2 + +a process for both reading and writing + +=item IPC::Open3 + +open a process for reading, writing, and error handling + +=item Net::Ping + +check a host for upness + +=item POSIX + +Perl interface to IEEE Std 1003.1 + +=item SelfLoader + +load functions only on demand + +=item Socket + +load the C socket.h defines and structure manipulators + +=item Test::Harness + +run perl standard test scripts with statistics + +=item Text::Abbrev + +rceate an abbreviation table from a list + +=back + +To find out I<all> the modules installed on your system, including +those without documentation or outside the standard release, do this: find `perl -e 'print "@INC"'` -name '*.pm' -print @@ -328,15 +517,15 @@ Extension modules are written in C (or a mix of Perl and C) and get dynamically loaded into Perl if and when you need them. Supported extension modules include the Socket, Fcntl, and POSIX modules. -Many popular C extension modules -do not come bundled (at least, not completely) -due to their size, volatility, or simply lack of time for adequate testing -and configuration across the multitude of platforms on which Perl was -beta-tested. You are encouraged to look for them in archie(1L), the Perl -FAQ or Meta-FAQ, the WWW page, and even with their authors before randomly -posting asking for their present condition and disposition. +Many popular C extension modules do not come bundled (at least, not +completely) due to their size, volatility, or simply lack of time for +adequate testing and configuration across the multitude of platforms on +which Perl was beta-tested. You are encouraged to look for them in +archie(1L), the Perl FAQ or Meta-FAQ, the WWW page, and even with their +authors before randomly posting asking for their present condition and +disposition. -=head2 CPAN +=head1 CPAN CPAN stands for the Comprehensive Perl Archive Network. This is a globally replicated collection of all known Perl materials, including hundreds @@ -483,4 +672,394 @@ ftp://ftp.is.co.za/programming/perl/CPAN/ =back For an up-to-date listing of CPAN sites, -see http://www.perl.com/perl/ or ftp://ftp.perl.com/perl/. +see http://www.perl.com/perl/ or ftp://ftp.perl.com/perl/ . + +=head1 Modules: Creation, Use and Abuse + +(The following section is borrowed directly from Tim Bunce's modules +file, available at your nearest CPAN site.) + +Perl 5 implements a class using a package, but the presence of a +package doesn't imply the presence of a class. A package is just a +namespace. A class is a package that provides subroutines that can be +used as methods. A method is just a subroutine that expects, as its +first argument, either the name of a package (for "static" methods), +or a reference to something (for "virtual" methods). + +A module is a file that (by convention) provides a class of the same +name (sans the .pm), plus an import method in that class that can be +called to fetch exported symbols. This module may implement some of +its methods by loading dynamic C or C++ objects, but that should be +totally transparent to the user of the module. Likewise, the module +might set up an AUTOLOAD function to slurp in subroutine definitions on +demand, but this is also transparent. Only the .pm file is required to +exist. + +=head2 Guidelines for Module Creation + +=over 4 + +=item Do similar modules already exist in some form? + +If so, please try to reuse the existing modules either in whole or +by inheriting useful features into a new class. If this is not +practical try to get together with the module authors to work on +extending or enhancing the functionality of the existing modules. +A perfect example is the plethora of packages in perl4 for dealing +with command line options. + +If you are writing a module to expand an already existing set of +modules, please coordinate with the author of the package. It +helps if you follow the same naming scheme and module interaction +scheme as the original author. + +=item Try to design the new module to be easy to extend and reuse. + +Use blessed references. Use the two argument form of bless to bless +into the class name given as the first parameter of the constructor, +e.g.: + + sub new { + my $class = shift; + return bless {}, $class; + } + +or even this if you'd like it to be used as either a static +or a virtual method. + + sub new { + my $self = shift; + my $class = ref($self) || $self; + return bless {}, $class; + } + +Pass arrays as references so more parameters can be added later +(it's also faster). Convert functions into methods where +appropriate. Split large methods into smaller more flexible ones. +Inherit methods from other modules if appropriate. + +Avoid class name tests like: die "Invalid" unless ref $ref eq 'FOO'. +Generally you can delete the "eq 'FOO'" part with no harm at all. +Let the objects look after themselves! Generally, avoid hardwired +class names as far as possible. + +Avoid $r->Class::func() where using @ISA=qw(... Class ...) and +$r->func() would work (see perlbot man page for more details). + +Use autosplit so little used or newly added functions won't be a +burden to programs which don't use them. Add test functions to +the module after __END__ either using AutoSplit or by saying: + + eval join('',<main::DATA>) || die $@ unless caller(); + +Does your module pass the 'empty sub-class' 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: $obj = new SUBCLASS; ? + +Avoid keeping any state information in your packages. It makes it +difficult for multiple other packages to use yours. Keep state +information in objects. + +Always use C<-w>. Try to C<use strict;> (or C<use strict qw(...);>). +Remember that you can add C<no strict qw(...);> to individual blocks +of code which need less strictness. Always use C<-w>. Always use C<-w>! +Follow the guidelines in the perlstyle(1) manual. + +=item Some simple style guidelines + +The perlstyle manual supplied with perl has many helpful points. + +Coding style is a matter of personal taste. Many people evolve their +style over several years as they learn what helps them write and +maintain good code. Here's one set of assorted suggestions that +seem to be widely used by experienced developers: + +Use underscores to separate words. It is generally easier to read +$var_names_like_this than $VarNamesLikeThis, especially for +non-native speakers of English. It's also a simple rule that works +consistently with VAR_NAMES_LIKE_THIS. + +Package/Module names are an exception to this rule. Perl informally +reserves lowercase module names for 'pragma' modules like integer +and strict. Other modules normally begin with a capital letter and +use mixed case with no underscores (need to be short and portable). + +You may find it helpful to use letter case to indicate the scope +or nature of a variable. For example: + + $ALL_CAPS_HERE constants only (beware clashes with perl vars) + $Some_Caps_Here package-wide global/static + $no_caps_here function scope my() or local() variables + +Function and method names seem to work best as all lowercase. +E.g., $obj->as_string(). + +You can use a leading underscore to indicate that a variable or +function should not be used outside the package that defined it. + +=item Select what to export. + +Do NOT export method names! + +Do NOT export anything else by default without a good reason! + +Exports pollute the namespace of the module user. If you must +export try to use @EXPORT_OK in preference to @EXPORT and avoid +short or common names to reduce the risk of name clashes. + +Generally anything not exported is still accessible from outside the +module using the ModuleName::item_name (or $blessed_ref->method) +syntax. By convention you can use a leading underscore on names to +informally indicate that they are 'internal' and not for public use. + +(It is actually possible to get private functions by saying: +my $subref = sub { ... }; &$subref; But there's no way to call that +directly as a method, since a method must have a name in the symbol +table.) + +As a general rule, if the module is trying to be object oriented +then export nothing. If it's just a collection of functions then +@EXPORT_OK anything but use @EXPORT with caution. + +=item Select a name for the module. + +This name should be as descriptive, accurate and complete as +possible. Avoid any risk of ambiguity. Always try to use two or +more whole words. Generally the name should reflect what is special +about what the module does rather than how it does it. Please use +nested module names to informally group or categorise a module. +A module should have a very good reason not to have a nested name. +Module names should begin with a capital letter. + +Having 57 modules all called Sort will not make life easy for anyone +(though having 23 called Sort::Quick is only marginally better :-). +Imagine someone trying to install your module alongside many others. +If in any doubt ask for suggestions in comp.lang.perl.misc. + +If you are developing a suite of related modules/classes it's good +practice to use nested classes with a common prefix as this will +avoid namespace clashes. For example: Xyz::Control, Xyz::View, +Xyz::Model etc. Use the modules in this list as a naming guide. + +If adding a new module to a set, follow the original author's +standards for naming modules and the interface to methods in +those modules. + +To be portable each component of a module name should be limited to +11 characters. If it might be used on DOS then try to ensure each is +unique in the first 8 characters. Nested modules make this easier. + +=item Have you got it right? + +How do you know that you've made the right decisions? Have you +picked an interface design that will cause problems later? Have +you picked the most appropriate name? Do you have any questions? + +The best way to know for sure, and pick up many helpful suggestions, +is to ask someone who knows. Comp.lang.perl.misc is read by just about +all the people who develop modules and it's the best place to ask. + +All you need to do is post a short summary of the module, its +purpose and interfaces. A few lines on each of the main methods is +probably enough. (If you post the whole module it might be ignored +by busy people - generally the very people you want to read it!) + +Don't worry about posting if you can't say when the module will be +ready - just say so in the message. It might be worth inviting +others to help you, they may be able to complete it for you! + +=item README and other Additional Files. + +It's well known that software developers usually fully document the +software they write. If, however, the world is in urgent need of +your software and there is not enough time to write the full +documentation please at least provide a README file containing: + +=over 10 + +=item * +A description of the module/package/extension etc. + +=item * +A copyright notice - see below. + +=item * +Prerequisites - what else you may need to have. + +=item * +How to build it - possible changes to Makefile.PL etc. + +=item * +How to install it. + +=item * +Recent changes in this release, especially incompatibilities + +=item * +Changes / enhancements you plan to make in the future. + +=back + +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. + +=item Adding a Copyright Notice. + +How you choose to licence 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 License (see the files README, Copying and +Artistic). Larry has good reasons for NOT just using the GNU GPL. + +My personal recommendation, out of respect for Larry, Perl and the +perl community at large is to simply state something like: + + Copyright (c) 1995 Your Name. All rights reserved. + This program is free software; you can redistribute it and/or + modify it under the same terms as Perl itself. + +This statement should at least appear in the README file. You may +also wish to include it in a Copying file and your source files. +Remember to include the other words in addition to the Copyright. + +=item Give the module a version/issue/release number. + +To be fully compatible with the Exporter and MakeMaker modules you +should store your module's version number in a non-my package +variable called $VERSION. This should be a valid floating point +number with at least two digits after the decimal (ie hundredths, +e.g, $VERSION = "0.01"). Don't use a "1.3.2" style version. +See Exporter.pm in Perl5.001m or later for details. + +It may be handy to add a function or method to retrieve the number. +Use the number in announcements and archive file names when +releasing the module (ModuleName-1.02.tar.Z). +See perldoc ExtUtils::MakeMaker.pm for details. + +=item How to release and distribute a module. + +It's good idea to post an announcement of the availability of your +module (or the module itself if small) to the comp.lang.perl.announce +Usenet newsgroup. This will at least ensure very wide once-off +distribution. + +If possible you should place the module into a major ftp archive and +include details of it's location in your announcement. + +Some notes about ftp archives: Please use a long descriptive file +name which includes the version number. Most incoming directories +will not be readable/listable, i.e., you won't be able to see your +file after uploading it. Remember to send your email notification +message as soon as possible after uploading else your file may get +deleted automatically. Allow time for the file to be processed +and/or check the file has been processed before announcing its +location. + +FTP Archives for Perl Modules: + +Follow the instructions and links on + + http://franz.ww.tu-berlin.de/modulelist + +or upload to one of these sites: + + ftp://franz.ww.tu-berlin.de/incoming + ftp://ftp.cis.ufl.edu/incoming + +and notify upload@franz.ww.tu-berlin.de. + +By using the WWW interface you can ask the Upload Server to mirror +your modules from your ftp or WWW site into your own directory on +CPAN! + +Please remember to send me an updated entry for the Module list! + +=item Take care when changing a released module. + +Always strive to remain compatible with previous released versions +(see 2.2 above) Otherwise try to add a mechanism to revert to the +old behaviour if people rely on it. Document incompatible changes. + +=back + +=head2 Guidelines for Converting Perl 4 Library Scripts into Modules + +=over 4 + +=item There is no requirement to convert anything. + +If it ain't broke, don't fix it! Perl 4 library scripts should +continue to work with no problems. You may need to make some minor +changes (like escaping non-array @'s in double quoted strings) but +there is no need to convert a .pl file into a Module for just that. + +=item Consider the implications. + +All the perl applications which make use of the script will need to +be changed (slightly) if the script is converted into a module. Is +it worth it unless you plan to make other changes at the same time? + +=item Make the most of the opportunity. + +If you are going to convert the script to a module you can use the +opportunity to redesign the interface. The 'Guidelines for Module +Creation' above include many of the issues you should consider. + +=item The pl2pm utility will get you started. + +This utility will read *.pl files (given as parameters) and write +corresponding *.pm files. The pl2pm utilities does the following: + +=over 10 + +=item * +Adds the standard Module prologue lines + +=item * +Converts package specifiers from ' to :: + +=item * +Converts die(...) to croak(...) + +=item * +Several other minor changes + +=back + +Being a mechanical process pl2pm is not bullet proof. The converted +code will need careful checking, especially any package statements. +Don't delete the original .pl file till the new .pm one works! + +=back + +=head2 Guidelines for Reusing Application Code + +=over 4 + +=item Complete applications rarely belong in the Perl Module Library. + +=item Many applications contain some perl code which could be reused. + +Help save the world! Share your code in a form that makes it easy +to reuse. + +=item Break-out the reusable code into one or more separate module files. + +=item Take the opportunity to reconsider and redesign the interfaces. + +=item In some cases the 'application' can then be reduced to a small + +fragment of code built on top of the reusable modules. In these cases +the application could invoked as: + + perl -e 'use Module::Name; method(@ARGV)' ... +or + perl -mModule::Name ... (in perl5.002?) + +=back + |