perlmod.pod

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=head1 NAME

perlmod - Perl modules (packages and symbol tables)

=head1 DESCRIPTION

=head2 Packages
X<package> X<namespace> X<variable, global> X<global variable> X<global>

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, C<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 I<not> lexical variables created
with my().  Typically it would be the first declaration in a file
included by the C<do>, C<require>, or C<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: 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 was 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">.
X<::> X<'>

Packages may themselves contain package separators, as in
C<$OUTER::INNER::var>.  This implies nothing about the order of
name lookups, however.  There are no relative packages: all symbols
are either local to the current package, or must be fully qualified
from the outer package name down.  For instance, there is nowhere
within package C<OUTER> that C<$INNER::var> refers to
C<$OUTER::INNER::var>.  C<INNER> refers to 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 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 built-in ones.  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 would be instead interpreted
as a pattern match, a substitution, or a transliteration.
X<variable, punctuation> 

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.
However, variables and functions named with a single C<_>, such as
$_ and C<sub _>, are still forced into the package C<main>.  See also
L<perlvar/"Technical Note on the Syntax of Variable Names">.

C<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 program 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 variable names.

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

=head2 Symbol Tables
X<symbol table> X<stash> X<%::> X<%main::> X<typeglob> X<glob> X<alias>

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 the 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};

(Be sure to note the B<vast> difference between the second line above
and C<local $main::foo = $main::bar>. The former is accessing the hash
C<%main::>, which is the symbol table of package C<main>. The latter is
simply assigning scalar C<$bar> in package C<main> to scalar C<$foo> of
the same package.)

You can use this to print out all the variables in a package, for
instance.  The standard but antiquated 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, assign a reference instead:

    *dick = \$richard;

Which makes $richard and $dick the same variable, but leaves
@richard and @dick as separate arrays.  Tricky, eh?

There is one subtle difference between the following statements:

    *foo = *bar;
    *foo = \$bar;

C<*foo = *bar> makes the typeglobs themselves synonymous while
C<*foo = \$bar> makes the SCALAR portions of two distinct typeglobs
refer to the same scalar value. This means that the following code:

    $bar = 1;
    *foo = \$bar;       # Make $foo an alias for $bar

    {
        local $bar = 2; # Restrict changes to block
        print $foo;     # Prints '1'!
    }

Would print '1', because C<$foo> holds a reference to the I<original>
C<$bar> -- the one that was stuffed away by C<local()> and which will be
restored when the block ends. Because variables are accessed through the
typeglob, you can use C<*foo = *bar> to create an alias which can be
localized. (But be aware that this means you can't have a separate
C<@foo> and C<@bar>, etc.)

What makes all of this important is that the Exporter module uses glob
aliasing as the import/export mechanism. Whether or not you can properly
localize a variable that has been exported from a module depends on how
it was exported:

    @EXPORT = qw($FOO); # Usual form, can't be localized
    @EXPORT = qw(*FOO); # Can be localized

You can work around the first case by using the fully qualified name
(C<$Package::FOO>) where you need a local value, or by overriding it
by saying C<*FOO = *Package::FOO> in your script.

The C<*x = \$y> 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.
X<constant> X<scalar, constant>

    *PI = \3.14159265358979;

Now you cannot alter C<$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
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 C<*foo{THING}> notation can also be used to obtain references to the
individual elements of *foo.  See L<perlref>.

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 I<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, I<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 L<perlobj>).

=head2 BEGIN, UNITCHECK, CHECK, INIT and END
X<BEGIN> X<UNITCHECK> X<CHECK> X<INIT> X<END>

Five specially named code blocks are executed at the beginning and at
the end of a running Perl program.  These are the C<BEGIN>,
C<UNITCHECK>, C<CHECK>, C<INIT>, and C<END> blocks.

These code blocks can be prefixed with C<sub> to give the appearance of a
subroutine (although this is not considered good style).  One should note
that these code blocks don't really exist as named subroutines (despite
their appearance). The thing that gives this away is the fact that you can
have B<more than one> of these code blocks in a program, and they will get
B<all> executed at the appropriate moment.  So you can't execute any of
these code blocks by name.

A C<BEGIN> code block is executed as soon as possible, that is, the moment
it is completely defined, even before the rest of the containing file (or
string) is parsed.  You may have multiple C<BEGIN> blocks within a file (or
eval'ed string) -- they will execute in order of definition.  Because a C<BEGIN>
code 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 compile
and run time.  Once a C<BEGIN> has run, it is immediately undefined and any
code it used is returned to Perl's memory pool.

It should be noted that C<BEGIN> and C<UNITCHECK> code blocks B<are>
executed inside string C<eval()>'s.  The C<CHECK> and C<INIT> code
blocks are B<not> executed inside a string eval, which e.g. can be a
problem in a mod_perl environment.

An C<END> code block 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 morphing 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).  C<END> blocks are not executed when you run perl with the
C<-c> switch, or if compilation fails.

Note that C<END> code blocks are B<not> executed at the end of a string
C<eval()>: if any C<END> code blocks are created in a string C<eval()>,
they will be executed just as any other C<END> code block of that package
in LIFO order just before the interpreter is being exited.