PERLMOD(1)PERLMOD(1)NAME
perlmod - Perl modules (packages)
DESCRIPTION
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 main. You can switch namespaces using the 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 require 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 identifier
with the package name and a double colon: $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.
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.)
Eval()ed strings are compiled in the package in which the eval() was
compiled. (Assignments to $SIG{}, however, assume the signal handler
specified is in the main package. Qualify the signal handler name if
you wish to have a signal handler in a package.) For an example,
examine perldb.pl in the Perl library. It initially switches to the DB
package so that the debugger doesn't interfere with variables in the
script you are trying to debug. At various points, however, it
temporarily switches back to the main package to evaluate various
expressions in the context of the main package (or wherever you came
from). See the perldebug manpage.
Symbol Tables
The symbol table for a package happens to be stored in the associative
array of that name appended with two colons. The main symbol table's
name is thus %main::, or %:: for short. Likewise the nested package
mentioned earlier is named %OUTER::INNER::.
The value in each entry of the associative array is what you are
referring to when you use the *name notation. In fact, the following
have the same effect, though the first is more efficient because it
does the symbol table lookups at compile time:
local(*main::foo) = *main::bar; local($main::{'foo'}) =
$main::{'bar'};
You can use this to print out all the variables in a package, for
instance. Here is dumpvar.pl from the Perl library:
package dumpvar;
sub main::dumpvar {
($package) = @_;
local(*stab) = eval("*${package}::");
while (($key,$val) = each(%stab)) {
local(*entry) = $val;
if (defined $entry) {
print "\$$key = '$entry'\n";
}
if (defined @entry) {
print "\@$key = (\n";
foreach $num ($[ .. $#entry) {
print " $num\t'",$entry[$num],"'\n";
}
print ")\n";
}
if ($key ne "${package}::" && defined %entry) {
print "\%$key = (\n";
foreach $key (sort keys(%entry)) {
print " $key\t'",$entry{$key},"'\n";
}
print ")\n";
}
}
}
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 symbol table entry performs an aliasing operation,
i.e.,
*dick = *richard;
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:
*dick = \$richard;
makes $richard and $dick the same variable, but leaves @richard and
@dick as separate arrays. Tricky, eh?
Package Constructors and Destructors
There are two special subroutine definitions that function as package
constructors and destructors. These are the 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.
Perl Classes
There is no special class syntax in Perl 5, but a package may function
as a class if it provides subroutines that function as methods. Such a
package may also derive some of its methods from another class package
by listing the other package name in its @ISA array. For more on this,
see the perlobj manpage.
Perl Modules
In Perl 5, the notion of packages has been extended into the notion of
modules. A module is a package that is defined in a library file of
the same name, and is designed to be reusable. It may do this by
providing a mechanism for exporting some of its symbols into the symbol
table of any package using it. Or it may function as a class
definition and make its semantics available implicitly through method
calls on the class and its objects, without explicit exportation of any
symbols. Or it can do a little of both.
Perl modules are included by saying
use Module;
or
use Module LIST;
This is exactly equivalent to
BEGIN { require "Module.pm"; import Module; }
or
BEGIN { require "Module.pm"; import Module LIST; }
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.
Therefore, if you're planning on the module altering your namespace,
use use; otherwise, use require. Otherwise you can get into this
problem:
require Cwd; # make Cwd:: accessible
$here = Cwd::getcwd();
use Cwd; # import names from Cwd::
$here = getcwd();
require Cwd; # make Cwd:: accessible
$here = getcwd(); # oops! no main::getcwd()
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 perlapi
manpage and the perlguts manpage.
NOTE
Perl does not enforce private and public parts of its modules as you
may have been used to in other languages like C++, Ada, or Modula-17.
Perl doesn't have an infatuation with enforced privacy. It would
prefer that you stayed out of its living room because you weren't
invited, not because it has a shotgun.
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.
THE PERL MODULE LIBRARY
A number of modules are included the the Perl distribution. These are
described below, and all end in .pm. You may also discover files in
the library directory that end in either .pl or .ph. These are old
libraries supplied so that old programs that use them still run. The
.pl files will all eventually be converted into standard modules, and
the .ph files made by h2ph will probably end up as extension modules
made by h2xs. (Some .ph values may already be available through the
POSIX module.) The pl2pm file in the distribution may help in your
conversion, but it's just a mechanical process, so is far from bullet
proof.
Pragmatic Modules
They work somewhat like pragmas in that they tend to affect the
compilation of your program, and thus will usually only work well when
used within a use, or no. These are locally scoped, so an inner BLOCK
may countermand any of these by saying
no integer;
no strict 'refs';
which lasts until the end of that BLOCK.
The following programs are defined (and have their own documentation).
integer Perl pragma to compute arithmetic in integer instead of
double
less Perl pragma to request less of something from the compiler
sigtrap Perl pragma to enable stack backtrace on unexpected signals
strict Perl pragma to restrict unsafe constructs
subs Perl pragma to predeclare sub names
Standard Modules
The following 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.
Abbrev create an abbreviation table from a list
AnyDBM_File provide framework for multiple DBMs
AutoLoader load functions only on demand
AutoSplit split a package for autoloading
Basename parse file name and path from a specification
Benchmark benchmark running times of code
Carp warn or die of errors (from perspective of caller)
CheckTree run many filetest checks on a tree
Collate compare 8-bit scalar data according to the current locale
Config access Perl configuration option
Cwd get pathname of current working directory
DynaLoader Dynamically load C libraries into Perl code
English use nice English (or awk) names for ugly punctuation
variables
Env Perl module that imports environment variables
Exporter module to control namespace manipulations
Fcntl load the C Fcntl.h defines
FileHandle supply object methods for filehandles
Find traverse a file tree
Finddepth traverse a directory structure depth-first
Getopt basic and extended getopt(3) processing
MakeMaker generate a Makefile for Perl extension
Open2 open a process for both reading and writing
Open3 open a process for reading, writing, and error handling
POSIX Perl interface to IEEE 1003.1 namespace
Ping check a host for upness
Socket load the C socket.h defines
Extension Modules
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.
The following are popular C extension modules, which while available at
Perl 5.0 release time, 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. There's no guarantee that the names or addresses below
have not changed since printing, and in fact, they probably have!
Curses Written by William Setzer <William_Setzer@ncsu.edu>, while
not included with the standard distribution, this extension
module ports to most systems. FTP from your nearest Perl
archive site, or try
ftp://ftp.ncsu.edu/pub/math/wsetzer/cursperl5??.tar.gz
It is currently in alpha test, so the name and ftp location
may change.
DBI This is the portable database interface written by
<Tim.Bunce@ig.co.uk>. This supersedes the many perl4 ports
for database extensions. The official archive for DBperl
extensions is ftp.demon.co.uk:/pub/perl/db. This archive
contains copies of perl4 ports for Ingres, Oracle, Sybase,
Informix, Unify, Postgres, and Interbase, as well as rdb
and shql and other non-SQL systems.
DB_File Fastest and most restriction-free of the DBM bindings, this
extension module uses the popular Berkeley DB to tie() into
your hashes. This has a standardly-distributed man page
and dynamic loading extension module, but you'll have to
fetch the Berkeley code yourself. See the DB_File manpage
for where.
Sx This extension module is a front to the Athena and Xlib
libraries for Perl GUI programming, originally written by
by Dominic Giampaolo <dbg@sgi.com>, then and rewritten for
Sx by Frederic Chauveau <fmc@pasteur.fr>. It's available
for FTP from
ftp.pasteur.fr:/pub/Perl/Sx.tar.gz
Tk This extension module is an object-oriented Perl5 binding
to the popular tcl/tk X11 package. However, you need know
no TCL to use it! It was written by Malcolm Beattie
<mbeattie@sable.ox.ac.uk>. If you are unable to locate it
using archie(1L) or a similar tool, you may try retrieving
it from /private/Tk-october.tar.gz from Malcolm's machine
listed above.
3rd Berkeley DistributionPERLMOD(1)