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PERLMOD(1)
NAME
perlmod - Perl modules (packages and symbol tables)
DESCRIPTION
Packages
Perl provides a mechanism for alternative namespaces to protect packages
from stomping on each other's variables. In fact, 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, "eval", or file, whichever comes first (the same scope as the my()
and local() operators). Unqualified dynamic identifiers will be in this
namespace, except for those few identifiers that if unqualified, default to
the main package instead of the current one as described below. A package
statement affects only dynamic variables--including those you've used
local() on--but not lexical variables created with my(). Typically it
would be the first declaration in a file included by the "do", "require",
or "use" operators. 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: "$Package::Variable". If the package name is null, the "main"
package is assumed. That is, "$::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--as opposed to using the
single quote as separator, which was there to make Ada programmers feel
like they knew what's going on. Because the old-fashioned syntax is still
supported for backwards compatibility, if you try to use a string like
""This is $owner's house"", you'll be accessing "$owner::s"; that is, the
$s variable in package "owner", which is probably not what you meant. Use
braces to disambiguate, as in ""This is ${owner}'s house"".
Packages may themselves contain package separators, as in
"$OUTER::INNER::var". This implies nothing about the order of name
lookups, however. There are no relative packages: 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 punctuation variables, like $_. In addition, when
unqualified, 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. If you have a package called "m", "s",
or "y", then you can't use the qualified form of an identifier because it
would be instead interpreted as a pattern match, a substitution, or a
transliteration.
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. $_ is
still global though. See also the Technical Note on the Syntax of Variable
Names entry in the perlvar manpage.
"eval"ed strings are compiled in the package in which the eval() was
compiled. (Assignments to "$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 program 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.
The special symbol "__PACKAGE__" contains the current package, but cannot
(easily) be used to construct variables.
See the perlsub manpage for other scoping issues related to my() and
local(), and the perlref manpage regarding closures.
Symbol Tables
The symbol table for a package happens to be stored in the hash of that
name with two colons appended. The main symbol table's name is thus
"%main::", or "%::" for short. Likewise the symbol table for the nested
package mentioned earlier is named "%OUTER::INNER::".
The value in each entry of the hash 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:
local *main::foo = *main::bar;
local $main::{foo} = $main::{bar};
(Be sure to note the vast difference between the second line above and
"local $main::foo = $main::bar". The former is accessing the hash
"%main::", which is the symbol table of package "main". The latter is
simply assigning scalar "$bar" in package "main" to scalar "$foo" of the
same package.)
You can use this to print out all the variables in a package, for instance.
The standard but antiquated dumpvar.pl library and the CPAN module
Devel::Symdump make use of this.
Assignment to a typeglob performs an aliasing operation, i.e.,
*dick = *richard;
causes variables, subroutines, formats, and file and directory handles
accessible via the identifier "richard" also to be accessible via the
identifier "dick". If you want to alias only a particular variable or
subroutine, assign a reference instead:
*dick = \$richard;
Which makes $richard and $dick the same variable, but leaves @richard and
@dick as separate arrays. Tricky, eh?
This mechanism may be used to pass and return cheap references into or from
subroutines if you don't want to copy the whole thing. It only works when
assigning to dynamic variables, not lexicals.
%some_hash = (); # can't be my()
*some_hash = fn( \%another_hash );
sub fn {
local *hashsym = shift;
# now use %hashsym normally, and you
# will affect the caller's %another_hash
my %nhash = (); # do what you want
return \%nhash;
}
On return, the reference will overwrite the hash slot in the symbol table
specified by the *some_hash typeglob. This is a somewhat tricky way of
passing around references cheaply when you don't want to have to remember
to dereference variables explicitly.
Another use of symbol tables is for making "constant" scalars.
*PI = \3.14159265358979;
Now you cannot alter "$PI", which is probably a good thing all in all.
This isn't the same as a constant subroutine, which is subject to
optimization at compile-time. A constant subroutine is one prototyped to
take no arguments and to return a constant expression. See the perlsub
manpage for details on these. The "use constant" pragma is a convenient
shorthand for these.
You can say "*foo{PACKAGE}" and "*foo{NAME}" to find out what name and
package the *foo symbol table entry comes from. This may be useful in a
subroutine that gets passed typeglobs as arguments:
sub identify_typeglob {
my $glob = shift;
print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n";
}
identify_typeglob *foo;
identify_typeglob *bar::baz;
This prints
You gave me main::foo
You gave me bar::baz
The "*foo{THING}" notation can also be used to obtain references to the
individual elements of *foo. See the perlref manpage.
Subroutine definitions (and declarations, for that matter) need not
necessarily be situated in the package whose symbol table they occupy. You
can define a subroutine outside its package by explicitly qualifying the
name of the subroutine:
package main;
sub Some_package::foo { ... } # &foo defined in Some_package
This is just a shorthand for a typeglob assignment at compile time:
BEGIN { *Some_package::foo = sub { ... } }
and is not the same as writing:
{
package Some_package;
sub foo { ... }
}
In the first two versions, the body of the subroutine is lexically in the
main package, not in Some_package. So something like this:
package main;
$Some_package::name = "fred";
$main::name = "barney";
sub Some_package::foo {
print "in ", __PACKAGE__, ": \$name is '$name'\n";
}
Some_package::foo();
prints:
in main: $name is 'barney'
rather than:
in Some_package: $name is 'fred'
This also has implications for the use of the SUPER:: qualifier (see the
perlobj manpage).
Package Constructors and Destructors
Four special subroutines act as package constructors and destructors.
These are the "BEGIN", "CHECK", "INIT", and "END" routines. The "sub" is
optional for these routines.
A "BEGIN" subroutine is executed as soon as possible, that is, the moment
it is completely defined, even before the rest of the containing file is
parsed. You may have multiple "BEGIN" blocks within a file--they will
execute in order of definition. Because a "BEGIN" block executes
immediately, it can pull in definitions of subroutines and such from other
files in time to be visible to the rest of the file. Once a "BEGIN" has
run, it is immediately undefined and any code it used is returned to Perl's
memory pool. This means you can't ever explicitly call a "BEGIN".
An "END" subroutine is executed as late as possible, that is, after perl
has finished running the program and just before the interpreter is being
exited, even if it is exiting as a result of a die() function. (But not if
it's polymorphing into another program via "exec", or being blown out of
the water by a signal--you have to trap that yourself (if you can).) You
may have multiple "END" blocks within a file--they will execute in reverse
order of definition; that is: last in, first out (LIFO). "END" blocks are
not executed when you run perl with the "-c" switch, or if compilation
fails.
Inside an "END" subroutine, "$?" contains the value that the program is
going to pass to "exit()". You can modify "$?" to change the exit value of
the program. Beware of changing "$?" by accident (e.g. by running
something via "system").
Similar to "BEGIN" blocks, "INIT" blocks are run just before the Perl
runtime begins execution, in "first in, first out" (FIFO) order. For
example, the code generators documented in the perlcc manpage make use of
"INIT" blocks to initialize and resolve pointers to XSUBs.
Similar to "END" blocks, "CHECK" blocks are run just after the Perl compile
phase ends and before the run time begins, in LIFO order. "CHECK" blocks
are again useful in the Perl compiler suite to save the compiled state of
the program.
When you use the -n and -p switches to Perl, "BEGIN" and "END" work just as
they do in awk, as a degenerate case. Both "BEGIN" and "CHECK" blocks are
run when you use the -c switch for a compile-only syntax check, although
your main code is not.
Perl Classes
There is no special class syntax in Perl, but a package may act as a class
if it provides subroutines to act as methods. Such a package may also
derive some of its methods from another class (package) by listing the
other package name(s) in its global @ISA array (which must be a package
global, not a lexical).
For more on this, see the perltoot manpage and the perlobj manpage.
Perl Modules
A module is just a set of related functions in a library file, i.e., a Perl
package with the same name as the file. It is specifically designed to be
reusable by other modules or programs. 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 explicitly exporting anything. Or it can do a little of
both.
For example, to start a traditional, non-OO module called Some::Module,
create a file called Some/Module.pm and start with this template:
package Some::Module; # assumes Some/Module.pm
use strict;
use warnings;
BEGIN {
use Exporter ();
our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS);
# set the version for version checking
$VERSION = 1.00;
# if using RCS/CVS, this may be preferred
$VERSION = do { my @r = (q$Revision: 2.21 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r }; # must be all one line, for MakeMaker
@ISA = qw(Exporter);
@EXPORT = qw(&func1 &func2 &func4);
%EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ],
# your exported package globals go here,
# as well as any optionally exported functions
@EXPORT_OK = qw($Var1 %Hashit &func3);
}
our @EXPORT_OK;
# exported package globals go here
our $Var1;
our %Hashit;
# non-exported package globals go here
our @more;
our $stuff;
# initialize package globals, first exported ones
$Var1 = '';
%Hashit = ();
# then the others (which are still accessible as $Some::Module::stuff)
$stuff = '';
@more = ();
# all file-scoped lexicals must be created before
# the functions below that use them.
# file-private lexicals go here
my $priv_var = '';
my %secret_hash = ();
# here's a file-private function as a closure,
# callable as &$priv_func; it cannot be prototyped.
my $priv_func = sub {
# stuff goes here.
};
# make all your functions, whether exported or not;
# remember to put something interesting in the {} stubs
sub func1 {} # no prototype
sub func2() {} # proto'd void
sub func3($$) {} # proto'd to 2 scalars
# this one isn't exported, but could be called!
sub func4(\%) {} # proto'd to 1 hash ref
END { } # module clean-up code here (global destructor)
## YOUR CODE GOES HERE
1; # don't forget to return a true value from the file
Then go on to declare and use your variables in functions without any
qualifications. See the Exporter manpage and the the perlmodlib manpage
for details on mechanics and style issues in module creation.
Perl modules are included into your program by saying
use Module;
or
use Module LIST;
This is exactly equivalent to
BEGIN { require Module; import Module; }
or
BEGIN { require Module; import Module LIST; }
As a special case
use Module ();
is exactly equivalent to
BEGIN { require Module; }
All Perl module files have the extension .pm. The "use" operator assumes
this so 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).
The two statements:
require SomeModule;
require "SomeModule.pm";
differ from each other in two ways. In the first case, any double colons
in the module name, such as "Some::Module", are translated into your
system's directory separator, usually "/". The second case does not, and
would have to be specified literally. The other difference is that seeing
the first "require" clues in the compiler that uses of indirect object
notation involving "SomeModule", as in "$ob = purge SomeModule", are method
calls, not function calls. (Yes, this really can make a difference.)
Because the "use" statement implies a "BEGIN" block, the importing of
semantics happens as soon as 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 or unary 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:
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()
In general, "use Module ()" is recommended over "require Module", because
it determines module availability at compile time, not in the middle of
your program's execution. An exception would be if two modules each tried
to "use" each other, and each also called a function from that other
module. In that case, it's easy to use "require"s instead.
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 make 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 (often ending in .so) or autoloaded subroutine
definitions (often ending in .al) 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. For example, although the POSIX module happens
to do both dynamic loading and autoloading, the user can say just "use
POSIX" to get it all.
SEE ALSO
See the perlmodlib manpage for general style issues related to building
Perl modules and classes, as well as descriptions of the standard library
and CPAN, the Exporter manpage for how Perl's standard import/export
mechanism works, the perltoot manpage and the perltootc manpage for an in-
depth tutorial on creating classes, the perlobj manpage for a hard-core
reference document on objects, the perlsub manpage for an explanation of
functions and scoping, and the perlxstut manpage and the perlguts manpage
for more information on writing extension modules.
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