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perlmod - Perl modules (packages and symbol tables)

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This doc
Perl's packages, namespaces, and some info on classes.

perlnewmod
Tutorial on making a new module.

perlmodstyle
Best practices for making a new module.

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() 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 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 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 was 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. "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 "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 "main", even when used for other
purposes than their built-in ones. 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 package writers to be able to use leading underscore to indicate
private variables and method names. However, variables and functions named with a single
"_", such as $_ and "sub _", are still forced into the package "main". See also "The
Syntax of Variable Names" in perlvar.

"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 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 perldebug.

The special symbol "__PACKAGE__" contains the current package, but cannot (easily) be used
to construct variable names.

See perlsub for other scoping issues related to my() and local(), and perlref 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 symbol 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 referring to when you use the *name
typeglob notation.

local *main::foo = *main::bar;

You can use this to print out all the variables in a package, for instance. The standard
but antiquated dumpvar.pl library and the CPAN module Devel::Symdump make use of this.

The results of creating new symbol table entries directly or modifying any entries that
are not already typeglobs are undefined and subject to change between releases of perl.

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?

There is one subtle difference between the following statements:

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

"*foo = *bar" makes the typeglobs themselves synonymous while "*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 $foo holds a reference to the original $bar. The one that was
stuffed away by "local()" and which will be restored when the block ends. Because
variables are accessed through the typeglob, you can use "*foo = *bar" to create an alias
which can be localized. (But be aware that this means you can't have a separate @foo and
@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 ($Package::FOO) where
you need a local value, or by overriding it by saying "*FOO = *Package::FOO" in your
script.

The "*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.

*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 constant
subroutine is one prototyped to take no arguments and to return a constant expression.
See perlsub 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 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 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 perlobj).

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

These code blocks can be prefixed with "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 more than one of these code blocks in a program,
and they will get all executed at the appropriate moment. So you can't execute any of
these code blocks by name.

A "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 "BEGIN" blocks within a file (or eval'ed string); they will execute in order of
definition. Because a "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 "BEGIN" has run, it is immediately undefined and any code it used is
returned to Perl's memory pool.

An "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
"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.

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

Inside an "END" code block, $? 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").

Inside of a "END" block, the value of "${^GLOBAL_PHASE}" will be "END".

"UNITCHECK", "CHECK" and "INIT" code blocks are useful to catch the transition between the
compilation phase and the execution phase of the main program.

"UNITCHECK" blocks are run just after the unit which defined them has been compiled. The
main program file and each module it loads are compilation units, as are string "eval"s,
run-time code compiled using the "(?{ })" construct in a regex, calls to "do FILE",
"require FILE", and code after the "-e" switch on the command line.

"BEGIN" and "UNITCHECK" blocks are not directly related to the phase of the interpreter.
They can be created and executed during any phase.

"CHECK" code blocks are run just after the initial Perl compile phase ends and before the
run time begins, in LIFO order. "CHECK" code blocks are used in the Perl compiler suite
to save the compiled state of the program.

Inside of a "CHECK" block, the value of "${^GLOBAL_PHASE}" will be "CHECK".

"INIT" blocks are run just before the Perl runtime begins execution, in "first in, first
out" (FIFO) order.

Inside of an "INIT" block, the value of "${^GLOBAL_PHASE}" will be "INIT".

The "CHECK" and "INIT" blocks in code compiled by "require", string "do", or string "eval"
will not be executed if they occur after the end of the main compilation phase; that can
be a problem in mod_perl and other persistent environments which use those functions to
load code at runtime.

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.

The begincheck program makes it all clear, eventually:

#!/usr/bin/perl

# begincheck

print "10. Ordinary code runs at runtime.\n";

END { print "16. So this is the end of the tale.\n" }
INIT { print " 7. INIT blocks run FIFO just before runtime.\n" }
UNITCHECK {
print " 4. And therefore before any CHECK blocks.\n"
}
CHECK { print " 6. So this is the sixth line.\n" }

print "11. It runs in order, of course.\n";

BEGIN { print " 1. BEGIN blocks run FIFO during compilation.\n" }
END { print "15. Read perlmod for the rest of the story.\n" }
CHECK { print " 5. CHECK blocks run LIFO after all compilation.\n" }
INIT { print " 8. Run this again, using Perl's -c switch.\n" }

print "12. This is anti-obfuscated code.\n";

END { print "14. END blocks run LIFO at quitting time.\n" }
BEGIN { print " 2. So this line comes out second.\n" }
UNITCHECK {
print " 3. UNITCHECK blocks run LIFO after each file is compiled.\n"
}
INIT { print " 9. You'll see the difference right away.\n" }

print "13. It only _looks_ like it should be confusing.\n";

__END__

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 methods 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 perlootut and perlobj.

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 modules 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 definition and
make its semantics available implicitly through method calls on the class and its objects,
without explicitly 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 {
require Exporter;

# set the version for version checking
our $VERSION = 1.00;

# Inherit from Exporter to export functions and variables
our @ISA = qw(Exporter);

# Functions and variables which are exported by default
our @EXPORT = qw(func1 func2);

# Functions and variables which can be optionally exported
our @EXPORT_OK = qw($Var1 %Hashit func3);
}

# exported package globals go here
our $Var1 = '';
our %Hashit = ();

# non-exported package globals go here
# (they are still accessible as $Some::Module::stuff)
our @more = ();
our $stuff = '';

# file-private lexicals go here, before any functions which use them
my $priv_var = '';
my %secret_hash = ();

# here's a file-private function as a closure,
# callable as $priv_func->();
my $priv_func = sub {
...
};

# make all your functions, whether exported or not;
# remember to put something interesting in the {} stubs
sub func1 { ... }
sub func2 { ... }

# this one isn't exported, but could be called directly
# as Some::Module::func3()
sub func3 { ... }

END { ... } # module clean-up code here (global destructor)

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
Exporter and the perlmodlib 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.pm'; 'Module'->import; }

or

BEGIN { require 'Module.pm'; 'Module'->import( LIST ); }

As a special case

use Module ();

is exactly equivalent to

BEGIN { require 'Module.pm'; }

All Perl module files have the extension .pm. The "use" operator assumes this so 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 importing of semantics happens as
soon as 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 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 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 "require" 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. 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
(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 responsibility 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.

Making your module threadsafe
Perl supports a type of threads called interpreter threads (ithreads). These threads can
be used explicitly and implicitly.

Ithreads work by cloning the data tree so that no data is shared between different
threads. These threads can be used by using the "threads" module or by doing fork() on
win32 (fake fork() support). When a thread is cloned all Perl data is cloned, however non-
Perl data cannot be cloned automatically. Perl after 5.8.0 has support for the "CLONE"
special subroutine. In "CLONE" you can do whatever you need to do, like for example
handle the cloning of non-Perl data, if necessary. "CLONE" will be called once as a class
method for every package that has it defined (or inherits it). It will be called in the
context of the new thread, so all modifications are made in the new area. Currently CLONE
is called with no parameters other than the invocant package name, but code should not
assume that this will remain unchanged, as it is likely that in future extra parameters
will be passed in to give more information about the state of cloning.

If you want to CLONE all objects you will need to keep track of them per package. This is
simply done using a hash and Scalar::Util::weaken().

Perl after 5.8.7 has support for the "CLONE_SKIP" special subroutine. Like "CLONE",
"CLONE_SKIP" is called once per package; however, it is called just before cloning starts,
and in the context of the parent thread. If it returns a true value, then no objects of
that class will be cloned; or rather, they will be copied as unblessed, undef values. For
example: if in the parent there are two references to a single blessed hash, then in the
child there will be two references to a single undefined scalar value instead. This
provides a simple mechanism for making a module threadsafe; just add "sub CLONE_SKIP { 1
}" at the top of the class, and "DESTROY()" will now only be called once per object. Of
course, if the child thread needs to make use of the objects, then a more sophisticated
approach is needed.

Like "CLONE", "CLONE_SKIP" is currently called with no parameters other than the invocant
package name, although that may change. Similarly, to allow for future expansion, the
return value should be a single 0 or 1 value.

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