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perlmod - Perl modules (packages and symbol tables)
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
Technical Note on the Syntax of Variable Names in the perlvar manpage.
evaled 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 the perldebug manpage.
The special symbol __PACKAGE__ contains the current package, but cannot
(easily) be used to construct variable names.
See the perlsub manpage for other scoping issues related to my() and local(),
and the perlref manpage regarding closures.
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. 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 package, 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?
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;
{
local $bar = 2;
print $foo;
}
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);
@EXPORT = qw(*FOO);
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 = ();
*some_hash = fn( \%another_hash );
sub fn {
local *hashsym = shift;
my %nhash = ();
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
the perlsub manpage 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 the perlref manpage.
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 { ... }
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 the perlobj manpage).
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.
It should be noted that BEGIN and UNITCHECK code blocks are
executed inside string eval()'s. The CHECK and INIT code
blocks are not executed inside a string eval, which e.g. can be a
problem in a mod_perl environment.
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).
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 evals, code compiled using the
(?{ }) construct in a regex, calls to do FILE, require FILE,
and code after the -e switch on the command line.
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.
INIT blocks are run just before the Perl runtime begins execution, in
"first in, first out" (FIFO) order.
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:
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 merely _looks_ like it should be confusing.\n";
__END__
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 the perltoot manpage and the perlobj manpage.
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;
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