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NAME ^

perlmod - 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, 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, eval, 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 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 to be included by the require or use operator. You can switch into a package in more than one place; it influences merely 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.)

Packages may be nested inside other packages: $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 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 of the punctuation variables like $_. In addition, 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 builtin one. Note also that, if you have a package called m, s, or 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 $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 script 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 variables.

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 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. 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 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 dumpvar, the name of the subroutine is qualified so that its name is inserted into package main. While popular many years ago, this is now considered very poor style; in general, you should be writing modules and using the normal export mechanism instead of hammering someone else's namespace, even main's.

Assignment to a typeglob performs an aliasing operation, i.e.,

    *dick = *richard;

causes variables, subroutines, and file handles accessible via the identifier richard to also be accessible via the identifier dick. If you want to alias only 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 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 (one prototyped to take no arguments and to return a constant expression), which is subject to optimization at compile-time. This isn't. See perlsub for details on 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 which is 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.

Package Constructors and Destructors

There are two special subroutine definitions that function as package constructors and destructors. These are the BEGIN 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 multiple 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 undefined 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, 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 END blocks within a file--they will execute in reverse order of definition; that is: last in, first out (LIFO).

Inside an END subroutine $? contains the value that the script is going to pass to exit(). You can modify $? to change the exit value of the script. Beware of changing $? by accident (e.g. by running something via system).

Note that when you use the -n and -p switches to Perl, BEGIN and END work just as they do in awk, as a degenerate case.

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 perltoot and perlobj.

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 definition and make its semantics available implicitly through method calls on the class and its objects, without explicit exportation of any symbols. Or it can do a little of both.

For example, to start a normal 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;

    BEGIN {
        use Exporter   ();
        use vars       qw($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);
    }
    use vars      @EXPORT_OK;

    # non-exported package globals go here
    use vars      qw(@more $stuff);

    # initalize 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)

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"; import Module; }

or

    BEGIN { require "Module.pm"; import Module LIST; }

As a special case

    use Module ();

is exactly equivalent to

    BEGIN { require "Module.pm"; }

All Perl module files have the extension .pm. use assumes this so that 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).

Because the use statement implies a BEGIN block, the importation of semantics happens at the moment 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 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;.

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 makes 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 or autoloaded subroutine definitions 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. The POSIX module happens to do both dynamic loading and autoloading, but the user can say just use POSIX to get it all.

For more information on writing extension modules, see perlxstut and perlguts.

SEE ALSO ^

See perlmodlib for general style issues related to building Perl modules and classes as well as descriptions of the standard library and CPAN, Exporter for how Perl's standard import/export mechanism works, perltoot for an in-depth tutorial on creating classes, perlobj for a hard-core reference document on objects, and perlsub for an explanation of functions and scoping.

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