$Package::Variable. If the package name is null, the main package
as assumed. That is, $::sail is equivalent to $main::sail.
(The old package delimiter was a single quote, but double colon is now the preferred delimiter, in part because it's more readable to humans, and in part because it's more readable to emacs macros. It also makes C++ programmers feel like they know what's going on.)
Packages may be nested inside other packages: $OUTER::INNER::var. This
implies nothing about the order of name lookups, however. All symbols
are either local to the current package, or must be fully qualified
from the outer package name down. For instance, there is nowhere
within package OUTER that $INNER::var refers to $OUTER::INNER::var.
It would treat package 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 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 main, even when used for other purposes than
their built-in one. Note also that, if you have a package called m,
s or y, then you can't use the qualified form of an identifier
because it will be interpreted instead as a pattern match, a substitution,
or a translation.
(Variables beginning with underscore used to be forced into package main, but we decided it was more useful for package writers to be able to use leading underscore to indicate private variables and method names. $_ is still global though.)
Eval()ed strings are compiled in the package in which the
eval()
was
compiled. (Assignments to $SIG{}, however, assume the signal
handler specified is in the main package. Qualify the signal handler
name if you wish to have a signal handler in a package.) For an
example, examine perldb.pl in the Perl library. It initially switches
to the DB package so that the debugger doesn't interfere with variables
in the script you are trying to debug. At various points, however, it
temporarily switches back to the main package to evaluate various
expressions in the context of the main package (or wherever you came
from). See
the perldebug manpage
.
See the perlsub manpage for other scoping issues related to my() and local() , or the perlref manpage regarding closures.
%main::, or %:: for short. Likewise the nested package
mentioned earlier is named %OUTER::INNER::.
The value in each entry of the associative array is what you are referring
to when you use the *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:
You can use this to print out all the variables in a package, for instance. Here is dumpvar.pl from the Perl library:
Note that even though the subroutine is compiled in package dumpvar,
the name of the subroutine is qualified so that its name is inserted
into package main.
Assignment to a typeglob performs an aliasing operation, i.e.,
causes variables, subroutines and file handles accessible via the
identifier richard to also be accessible via the symbol dick. If
you only want to alias a particular variable or subroutine, you can
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.
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.
Now you cannot alter $PI, which is probably a good thing all in all.
BEGIN and END
routines. The
sub
is optional for these routines.
A BEGIN subroutine is executed as soon as possible, that is, the
moment it is completely defined, even before the rest of the containing
file is parsed. You may have multiple BEGIN blocks within a
file--they will execute in order of definition. Because a BEGIN
block executes immediately, it can pull in definitions of subroutines
and such from other files in time to be visible to the rest of the
file.
An END subroutine is executed as late as possible, that is, when the
interpreter is being exited, even if it is exiting as a result of a
die()
function. (But not if it's is being blown out of the water by a
signal--you have to trap that yourself (if you can).) You may have
multiple END blocks within a file--they will execute in reverse
order of definition; that is: last in, first out (LIFO).
Note that when you use the
-n
and
-p
switches to Perl, BEGIN
and END work just as they do in awk, as a degenerate case.
For more on this, see the perlobj manpage .
For example, to start a normal module called Fred, create a file called Fred.pm and put this at the start of it:
Then go on to declare and use your variables in functions without any qualifications. See Exporter and the Perl Modules File for details on mechanics and style issues in module creation.
Perl modules are included into your program by saying
or
This is exactly equivalent to
or
As a special case
is exactly equivalent to
All Perl module files have the extension .pm. use assumes this so that you don't have to spell out ``Module.pm'' in quotes. This also helps to differentiate new modules from old .pl and .ph files. Module names are also capitalized unless they're functioning as pragmas, ``Pragmas'' are in effect compiler directives, and are sometimes called ``pragmatic modules'' (or even ``pragmata'' if you're a classicist).
Because the
use
statement implies a BEGIN block, the importation
of semantics happens at the moment the
use
statement is compiled,
before the rest of the file is compiled. This is how it is able
to function as a pragma mechanism, and also how modules are able to
declare subroutines that are then visible as list operators for
the rest of the current file. This will not work if you use
require
instead of
use
. With require you can get into this problem:
In general use Module (); is recommended over require Module; .
Perl packages may be nested inside other package names, so we can have
package names containing ::. But if we used that package name
directly as a filename it would makes for unwieldy or impossible
filenames on some systems. Therefore, if a module's name is, say,
Text::Soundex, then its definition is actually found in the library
file Text/Soundex.pm.
Perl modules always have a .pm file, but there may also be dynamically linked executables or autoloaded subroutine definitions associated with the module. If so, these will be entirely transparent to the user of the module. It is the responsibility of the .pm file to load (or arrange to autoload) any additional functionality. The POSIX module happens to do both dynamic loading and autoloading, but the user can just say use POSIX to get it all.
For more information on writing extension modules, see the perlxs manpage and the perlguts manpage .
The module and its user have a contract, part of which is common law, and part of which is ``written''. Part of the common law contract is that a module doesn't pollute any namespace it wasn't asked to. The written contract for the module (AKA documentation) may make other provisions. But then you know when you use RedefineTheWorld that you're redefining the world and willing to take the consequences.
which lasts until the end of that BLOCK.
The following programs are defined (and have their own documentation).
They should all have their own documentation installed and accessible via your system man(1) command. If that fails, try the perldoc program.
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.
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.
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.
or even this if you'd like it to be used as either a static or a virtual method.
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:
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 -w . Try to use strict; (or use strict qw(...); ). Remember that you can add no strict qw(...); to individual blocks of code which need less strictness. Always use -w . Always use -w ! Follow the guidelines in the perlstyle(1) manual.
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:
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.
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.
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.
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!
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:
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.
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.
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
or upload to one of these sites:
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!
.
.