PERLMOD(1) Perl Programmers Reference Guide PERLMOD(1)NAMEperlmod - 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() opera
tors). 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 com
piler for the rest of that block. You can refer to vari
ables 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 equiva
lent 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 vari
ables, like $_. In addition, when unqualified, the iden
tifiers 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 pack
age writers to be able to use leading underscore to indi
cate 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" pack
age. 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 vari
ables.
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 sym
bol 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 refer
ring 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 pack
age, 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 $dick the same variable, but
leaves @richard and @dick as separate arrays. Tricky, eh?
This mechanism may be used to pass and return cheap refer
ences into or from subroutines if you don't want to copy
the whole thing. It only works when assigning to dynamic
variables, not lexicals.
%some_hash = (); # can't be my()
*some_hash = fn( \%another_hash );
sub fn {
local *hashsym = shift;
# now use %hashsym normally, and you
# will affect the caller's %another_hash
my %nhash = (); # do what you want
return \%nhash;
}
On return, the reference will overwrite the hash slot in
the symbol table specified by the *some_hash typeglob.
This is a somewhat tricky way of passing around references
cheaply when you don't want to have to remember to deref
erence variables explicitly.
Another use of symbol tables is for making "constant"
scalars.
*PI = \3.14159265358979;
Now you cannot alter "$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 con
stant subroutine is one prototyped to take no arguments
and to return a constant expression. See the perlsub man
page 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 ref
erences to the individual elements of *foo. See the perl
ref manpage.
Subroutine definitions (and declarations, for that matter)
need not necessarily be situated in the package whose sym
bol table they occupy. You can define a subroutine out
side 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 com
pile 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: $name is 'fred'
This also has implications for the use of the SUPER::
qualifier (see the perlobj manpage).
Package Constructors and Destructors
Four special subroutines act as package constructors and
destructors. These are the "BEGIN", "CHECK", "INIT", and
"END" routines. The "sub" is optional for these routines.
A "BEGIN" subroutine is executed as soon as possible, that
is, the moment it is completely defined, even before the
rest of the containing file is parsed. You may have mul
tiple "BEGIN" blocks within a file--they will execute in
order of definition. Because a "BEGIN" block executes
immediately, it can pull in definitions of subroutines and
such from other files in time to be visible to the rest of
the file. Once a "BEGIN" has run, it is immediately unde
fined and any code it used is returned to Perl's memory
pool. This means you can't ever explicitly call a
"BEGIN".
An "END" subroutine is executed as late as possible, that
is, after perl has finished running the program and just
before the interpreter is being exited, even if it is
exiting as a result of a die() function. (But not if it's
polymorphing into another program via "exec", or being
blown out of the water by a signal--you have to trap that
yourself (if you can).) You may have multiple "END"
blocks within a file--they will execute in reverse order
of definition; that is: last in, first out (LIFO). "END"
blocks are not executed when you run perl with the "-c"
switch, or if compilation fails.
Inside an "END" subroutine, "$?" 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 meth
ods 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 mod
ules 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 defi
nition and make its semantics available implicitly through
method calls on the class and its objects, without explic
itly 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 "Mod_
ule.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::Mod
ule", are translated into your system's directory separa
tor, 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" state
ment 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 com
pile 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. There
fore, 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 respon
sibility 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 func
tions and scoping, and the perlxstut manpage and the
perlguts manpage for more information on writing extension
modules.
2001-03-18 perl v5.6.1 PERLMOD(1)
