perlmod - Perl modules (packages and symbol tables)
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 (although some identifiers default to
the main package instead of the current one). 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, sub, or end of file, whichever comes first (the same scope as the
my() and
local() operators). 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
not lexical variables created with
my(). Typically it would be the
first declaration in a file to be included by the require or
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 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 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, 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 builtin 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
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.)
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.
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.
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 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};
You can use this to print out all the variables in a package, for instance. The standard 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, you can
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 won'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 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. This isn't the same as a constant subroutine, which is subject to optimization at compile-time. This isn't.
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.
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. 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, when 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).
Inside an END subroutine, $? contains the value that the script is going to pass to exit(). You can modify $? to change the exit value of the script. Beware of changing $? by accident (e.g. by running something via system).
Note that when you use the -n and -p switches to Perl, BEGIN and
END work just as they do in awk, as a degenerate case. As currently implemented (and subject to change,
since its inconvenient at best), both BEGIN and END blocks are run when you use the -c switch for a compile-only syntax check, although your main code is not.
There is no special class syntax in Perl, but a package may function as a
class if it provides subroutines to act as methods. Such a package may also
derive some of its methods from another class (package) by listing the
other package name in its global @ISA array (which must be a
package global, not a lexical).
For more on this, see the perltoot manpage and the perlobj manpage.
A module is just a package that is defined in a library file of the same name, and is designed to be reusable. It may do this by providing a mechanism for exporting some of its symbols into the symbol table of any package using it. Or it may function as a class definition and make its semantics available implicitly through method calls on the class and its objects, without explicit exportation of any symbols. Or it can do a little of both.
For example, to start a normal module called Some::Module, create a file called Some/Module.pm and start with this template:
package Some::Module; # assumes Some/Module.pm
use strict;
BEGIN {
use Exporter ();
use vars qw($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);
}
use vars @EXPORT_OK;
# non-exported package globals go here
use vars qw(@more $stuff);
# initalize 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)
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. 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).
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 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:
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 requires 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 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 say just use POSIX to get it all.
For more information on writing extension modules, see the perlxstut manpage and the perlguts manpage.
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 for an in-depth tutorial on creating classes, the perlobj manpage for a hard-core reference document on objects, and the perlsub manpage for an explanation of functions and scoping.
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.