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=head1 NAME
perlmod - Perl modules (packages)
=head1 DESCRIPTION
=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. 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
identifier with the package name and a double colon:
C<$Package::Variable>. If the package name is null, the C<main> package
as assumed. That is, C<$::sail> is equivalent to C<$main::sail>.
(The old package delimiter was a single quote, but double colon
is now the preferred delimiter, in part because it's more readable
to humans, and in part because it's more readable to B<emacs> macros.
It also makes C++ programmers feel like they know what's going on.)
Packages may be nested inside other packages: C<$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 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>,
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.
$_ is still global though.)
Eval()ed strings are compiled in the package in which the eval() was
compiled. (Assignments to C<$SIG{}>, however, assume the signal
handler specified is in the C<main> package. Qualify the signal handler
name if you wish to have a signal handler in a package.) For an
example, examine F<perldb.pl> in the Perl library. It initially switches
to the C<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 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
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:
local(*main::foo) = *main::bar; local($main::{'foo'}) =
$main::{'bar'};
You can use this to print out all the variables in a package, for
instance. Here is F<dumpvar.pl> from the Perl library:
package dumpvar;
sub main::dumpvar {
($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 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.,
*dick = *richard;
causes variables, subroutines and file handles accessible via the
identifier C<richard> to also be accessible via the symbol C<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?
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
constructors and destructors. These are the C<BEGIN> and C<END>
routines. The C<sub> is optional for these routines.
A C<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 C<BEGIN> blocks within a
file--they will execute in order of definition. Because a C<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 C<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 C<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 B<-n> and B<-p> switches to Perl, C<BEGIN>
and C<END> work just as they do in B<awk>, as a degenerate case.
=head2 Perl Classes
There is no special class syntax in Perl, 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 L<perlobj>.
=head2 Perl Modules
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
defin