perlmod.pod

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=head1 NAME

perlmod - Perl modules (packages and symbol tables)

=head1 DESCRIPTION

=head2 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 (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,
C<eval>, C<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
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<$Package::Variable>.  If the package name is null, the C<main>
package is assumed.  That is, C<$::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--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
C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is,
the $s variable in package C<owner>, which is probably not what you meant.
Use braces to disambiguate, as in C<"This is ${owner}'s house">.

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, when
unqualified, 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 builtin 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 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 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>.

The special symbol C<__PACKAGE__> contains the current package, but cannot
(easily) be used to construct variables.

See L<perlsub> for other scoping issues related to my() and local(),
and L<perlref> regarding closures.

=head2 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
C<%main::>, or C<%::> for short.  Likewise symbol table for the nested
package mentioned earlier is named C<%OUTER::INNER::>.

The value in each entry of the hash 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.  The standard F<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 C<richard> also to be accessible via the
identifier C<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 L<perlsub> for details on these.  The C<use constant> pragma is a
convenient shorthand for these.

You can say C<*foo{PACKAGE}> and C<*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 L<perlref>.

=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.  Once a C<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 C<BEGIN>.

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 polymorphing into another program
via C<exec>, or 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).

Inside an C<END> subroutine, C<$?> contains the value that the script is
going to pass to C<exit()>.  You can modify C<$?> to change the exit
value of the script.  Beware of changing C<$?> by accident (e.g. by