IRIX Base Documentation 1998 November/IRIX 6.5.2 Base Documentation November 1998.img/usr/share/catman/u_man/cat1/perlmod.z/perlmod

Text File | 1998-10-30 | 20KB | 529 lines

PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))                                                          PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))



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     perlmod - Perl modules (packages and symbol tables)

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     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 _m_y() and
     _l_o_c_a_l() operators).  All further unqualified dynamic identifiers will be
     in this namespace.  A package statement affects only dynamic variables--
     including those you've used _l_o_c_a_l() on--but _n_o_t lexical variables created
     with _m_y().  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 eeeemmmmaaaaccccssss 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.)




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PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))                                                          PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))



     _E_v_a_l()ed strings are compiled in the package in which the _e_v_a_l() 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
     _p_e_r_l_d_b._p_l 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 the _p_e_r_l_d_e_b_u_g
     manpage.

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

     See the _p_e_r_l_s_u_b manpage for other scoping issues related to _m_y() and
     _l_o_c_a_l(), and the _p_e_r_l_r_e_f manpage regarding closures.

     SSSSyyyymmmmbbbboooollll TTTTaaaabbbblllleeeessss

     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 _d_u_m_p_v_a_r._p_l 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";
                }



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PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))                                                          PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))



                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.





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PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))                                                          PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))



         *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 the _p_e_r_l_s_u_b manpage 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 the _p_e_r_l_r_e_f manpage.

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     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 _d_i_e()
     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



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PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))                                                          PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))



     system).

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

     PPPPeeeerrrrllll CCCCllllaaaasssssssseeeessss

     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 the _p_e_r_l_t_o_o_t manpage and the _p_e_r_l_o_b_j manpage.

     PPPPeeeerrrrllll MMMMoooodddduuuulllleeeessss

     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);



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PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))                                                          PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))



         # 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 the _E_x_p_o_r_t_e_r manpage and the the _p_e_r_l_m_o_d_l_i_b 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.pm"; import Module; }

     or






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PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))                                                          PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))



         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 ._p_m.  use assumes this so that
     you don't have to spell out "_M_o_d_u_l_e._p_m" in quotes.  This also helps to
     differentiate new modules from old ._p_l and ._p_h 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 _T_e_x_t/_S_o_u_n_d_e_x._p_m.

     Perl modules always have a ._p_m 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 ._p_m 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.






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PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))                                                          PPPPEEEERRRRLLLLMMMMOOOODDDD((((1111))))



     For more information on writing extension modules, see the _p_e_r_l_x_s_t_u_t
     manpage and the _p_e_r_l_g_u_t_s manpage.

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












































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