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a0d0e21e LW |
1 | =head1 NAME |
2 | ||
f102b883 | 3 | perlmod - Perl modules (packages and symbol tables) |
a0d0e21e LW |
4 | |
5 | =head1 DESCRIPTION | |
6 | ||
7 | =head2 Packages | |
8 | ||
19799a22 GS |
9 | Perl provides a mechanism for alternative namespaces to protect |
10 | packages from stomping on each other's variables. In fact, there's | |
11 | really no such thing as a global variable in Perl . The package | |
12 | statement declares the compilation unit as being in the given | |
13 | namespace. The scope of the package declaration is from the | |
14 | declaration itself through the end of the enclosing block, C<eval>, | |
15 | or file, whichever comes first (the same scope as the my() and | |
16 | local() operators). Unqualified dynamic identifiers will be in | |
17 | this namespace, except for those few identifiers that if unqualified, | |
18 | default to the main package instead of the current one as described | |
19 | below. A package statement affects only dynamic variables--including | |
20 | those you've used local() on--but I<not> lexical variables created | |
21 | with my(). Typically it would be the first declaration in a file | |
22 | included by the C<do>, C<require>, or C<use> operators. You can | |
23 | switch into a package in more than one place; it merely influences | |
24 | which symbol table is used by the compiler for the rest of that | |
25 | block. You can refer to variables and filehandles in other packages | |
26 | by prefixing the identifier with the package name and a double | |
27 | colon: C<$Package::Variable>. If the package name is null, the | |
28 | C<main> package is assumed. That is, C<$::sail> is equivalent to | |
29 | C<$main::sail>. | |
a0d0e21e | 30 | |
d3ebb66b GS |
31 | The old package delimiter was a single quote, but double colon is now the |
32 | preferred delimiter, in part because it's more readable to humans, and | |
33 | in part because it's more readable to B<emacs> macros. It also makes C++ | |
34 | programmers feel like they know what's going on--as opposed to using the | |
35 | single quote as separator, which was there to make Ada programmers feel | |
36 | like they knew what's going on. Because the old-fashioned syntax is still | |
37 | supported for backwards compatibility, if you try to use a string like | |
38 | C<"This is $owner's house">, you'll be accessing C<$owner::s>; that is, | |
39 | the $s variable in package C<owner>, which is probably not what you meant. | |
40 | Use braces to disambiguate, as in C<"This is ${owner}'s house">. | |
a0d0e21e | 41 | |
19799a22 GS |
42 | Packages may themselves contain package separators, as in |
43 | C<$OUTER::INNER::var>. This implies nothing about the order of | |
44 | name lookups, however. There are no relative packages: all symbols | |
a0d0e21e LW |
45 | are either local to the current package, or must be fully qualified |
46 | from the outer package name down. For instance, there is nowhere | |
19799a22 GS |
47 | within package C<OUTER> that C<$INNER::var> refers to |
48 | C<$OUTER::INNER::var>. It would treat package C<INNER> as a totally | |
49 | separate global package. | |
50 | ||
51 | Only identifiers starting with letters (or underscore) are stored | |
52 | in a package's symbol table. All other symbols are kept in package | |
53 | C<main>, including all punctuation variables, like $_. In addition, | |
54 | when unqualified, the identifiers STDIN, STDOUT, STDERR, ARGV, | |
55 | ARGVOUT, ENV, INC, and SIG are forced to be in package C<main>, | |
56 | even when used for other purposes than their built-in one. If you | |
57 | have a package called C<m>, C<s>, or C<y>, then you can't use the | |
58 | qualified form of an identifier because it would be instead interpreted | |
59 | as a pattern match, a substitution, or a transliteration. | |
60 | ||
61 | Variables beginning with underscore used to be forced into package | |
a0d0e21e | 62 | main, but we decided it was more useful for package writers to be able |
cb1a09d0 | 63 | to use leading underscore to indicate private variables and method names. |
19799a22 GS |
64 | $_ is still global though. See also L<perlvar/"Technical Note on the |
65 | Syntax of Variable Names">. | |
a0d0e21e | 66 | |
19799a22 | 67 | C<eval>ed strings are compiled in the package in which the eval() was |
a0d0e21e | 68 | compiled. (Assignments to C<$SIG{}>, however, assume the signal |
748a9306 | 69 | handler specified is in the C<main> package. Qualify the signal handler |
a0d0e21e LW |
70 | name if you wish to have a signal handler in a package.) For an |
71 | example, examine F<perldb.pl> in the Perl library. It initially switches | |
72 | to the C<DB> package so that the debugger doesn't interfere with variables | |
19799a22 | 73 | in the program you are trying to debug. At various points, however, it |
a0d0e21e LW |
74 | temporarily switches back to the C<main> package to evaluate various |
75 | expressions in the context of the C<main> package (or wherever you came | |
76 | from). See L<perldebug>. | |
77 | ||
f102b883 TC |
78 | The special symbol C<__PACKAGE__> contains the current package, but cannot |
79 | (easily) be used to construct variables. | |
80 | ||
5f05dabc | 81 | See L<perlsub> for other scoping issues related to my() and local(), |
f102b883 | 82 | and L<perlref> regarding closures. |
cb1a09d0 | 83 | |
a0d0e21e LW |
84 | =head2 Symbol Tables |
85 | ||
aa689395 | 86 | The symbol table for a package happens to be stored in the hash of that |
87 | name with two colons appended. The main symbol table's name is thus | |
88 | C<%main::>, or C<%::> for short. Likewise symbol table for the nested | |
89 | package mentioned earlier is named C<%OUTER::INNER::>. | |
90 | ||
91 | The value in each entry of the hash is what you are referring to when you | |
92 | use the C<*name> typeglob notation. In fact, the following have the same | |
93 | effect, though the first is more efficient because it does the symbol | |
94 | table lookups at compile time: | |
a0d0e21e | 95 | |
f102b883 TC |
96 | local *main::foo = *main::bar; |
97 | local $main::{foo} = $main::{bar}; | |
a0d0e21e LW |
98 | |
99 | You can use this to print out all the variables in a package, for | |
19799a22 GS |
100 | instance. The standard but antequated F<dumpvar.pl> library and |
101 | the CPAN module Devel::Symdump make use of this. | |
a0d0e21e | 102 | |
cb1a09d0 | 103 | Assignment to a typeglob performs an aliasing operation, i.e., |
a0d0e21e LW |
104 | |
105 | *dick = *richard; | |
106 | ||
5a964f20 TC |
107 | causes variables, subroutines, formats, and file and directory handles |
108 | accessible via the identifier C<richard> also to be accessible via the | |
109 | identifier C<dick>. If you want to alias only a particular variable or | |
19799a22 | 110 | subroutine, assign a reference instead: |
a0d0e21e LW |
111 | |
112 | *dick = \$richard; | |
113 | ||
5a964f20 | 114 | Which makes $richard and $dick the same variable, but leaves |
a0d0e21e LW |
115 | @richard and @dick as separate arrays. Tricky, eh? |
116 | ||
cb1a09d0 AD |
117 | This mechanism may be used to pass and return cheap references |
118 | into or from subroutines if you won't want to copy the whole | |
5a964f20 TC |
119 | thing. It only works when assigning to dynamic variables, not |
120 | lexicals. | |
cb1a09d0 | 121 | |
5a964f20 | 122 | %some_hash = (); # can't be my() |
cb1a09d0 AD |
123 | *some_hash = fn( \%another_hash ); |
124 | sub fn { | |
125 | local *hashsym = shift; | |
126 | # now use %hashsym normally, and you | |
127 | # will affect the caller's %another_hash | |
128 | my %nhash = (); # do what you want | |
5f05dabc | 129 | return \%nhash; |
cb1a09d0 AD |
130 | } |
131 | ||
5f05dabc | 132 | On return, the reference will overwrite the hash slot in the |
cb1a09d0 | 133 | symbol table specified by the *some_hash typeglob. This |
c36e9b62 | 134 | is a somewhat tricky way of passing around references cheaply |
cb1a09d0 AD |
135 | when you won't want to have to remember to dereference variables |
136 | explicitly. | |
137 | ||
19799a22 | 138 | Another use of symbol tables is for making "constant" scalars. |
cb1a09d0 AD |
139 | |
140 | *PI = \3.14159265358979; | |
141 | ||
142 | Now you cannot alter $PI, which is probably a good thing all in all. | |
5a964f20 TC |
143 | This isn't the same as a constant subroutine, which is subject to |
144 | optimization at compile-time. This isn't. A constant subroutine is one | |
145 | prototyped to take no arguments and to return a constant expression. | |
146 | See L<perlsub> for details on these. The C<use constant> pragma is a | |
147 | convenient shorthand for these. | |
cb1a09d0 | 148 | |
55497cff | 149 | You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and |
150 | package the *foo symbol table entry comes from. This may be useful | |
5a964f20 | 151 | in a subroutine that gets passed typeglobs as arguments: |
55497cff | 152 | |
153 | sub identify_typeglob { | |
154 | my $glob = shift; | |
155 | print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n"; | |
156 | } | |
157 | identify_typeglob *foo; | |
158 | identify_typeglob *bar::baz; | |
159 | ||
160 | This prints | |
161 | ||
162 | You gave me main::foo | |
163 | You gave me bar::baz | |
164 | ||
19799a22 | 165 | The C<*foo{THING}> notation can also be used to obtain references to the |
55497cff | 166 | individual elements of *foo, see L<perlref>. |
167 | ||
9263d47b GS |
168 | Subroutine definitions (and declarations, for that matter) need |
169 | not necessarily be situated in the package whose symbol table they | |
170 | occupy. You can define a subroutine outside its package by | |
171 | explicitly qualifying the name of the subroutine: | |
172 | ||
173 | package main; | |
174 | sub Some_package::foo { ... } # &foo defined in Some_package | |
175 | ||
176 | This is just a shorthand for a typeglob assignment at compile time: | |
177 | ||
178 | BEGIN { *Some_package::foo = sub { ... } } | |
179 | ||
180 | and is I<not> the same as writing: | |
181 | ||
182 | { | |
183 | package Some_package; | |
184 | sub foo { ... } | |
185 | } | |
186 | ||
187 | In the first two versions, the body of the subroutine is | |
188 | lexically in the main package, I<not> in Some_package. So | |
189 | something like this: | |
190 | ||
191 | package main; | |
192 | ||
193 | $Some_package::name = "fred"; | |
194 | $main::name = "barney"; | |
195 | ||
196 | sub Some_package::foo { | |
197 | print "in ", __PACKAGE__, ": \$name is '$name'\n"; | |
198 | } | |
199 | ||
200 | Some_package::foo(); | |
201 | ||
202 | prints: | |
203 | ||
204 | in main: $name is 'barney' | |
205 | ||
206 | rather than: | |
207 | ||
208 | in Some_package: $name is 'fred' | |
209 | ||
210 | This also has implications for the use of the SUPER:: qualifier | |
211 | (see L<perlobj>). | |
212 | ||
a0d0e21e LW |
213 | =head2 Package Constructors and Destructors |
214 | ||
14218588 | 215 | Three special subroutines act as package |
19799a22 GS |
216 | constructors and destructors. These are the C<BEGIN>, C<INIT>, and |
217 | C<END> routines. The C<sub> is optional for these routines. | |
a0d0e21e | 218 | |
f102b883 TC |
219 | A C<BEGIN> subroutine is executed as soon as possible, that is, the moment |
220 | it is completely defined, even before the rest of the containing file | |
221 | is parsed. You may have multiple C<BEGIN> blocks within a file--they | |
222 | will execute in order of definition. Because a C<BEGIN> block executes | |
223 | immediately, it can pull in definitions of subroutines and such from other | |
224 | files in time to be visible to the rest of the file. Once a C<BEGIN> | |
225 | has run, it is immediately undefined and any code it used is returned to | |
226 | Perl's memory pool. This means you can't ever explicitly call a C<BEGIN>. | |
a0d0e21e | 227 | |
14218588 GS |
228 | Similar to C<BEGIN> blocks, C<INIT> blocks are run just before the |
229 | Perl runtime begins execution. For example, the code generators | |
230 | documented in L<perlcc> make use of C<INIT> blocks to initialize | |
231 | and resolve pointers to XSUBs. | |
232 | ||
5a964f20 TC |
233 | An C<END> subroutine is executed as late as possible, that is, when |
234 | the interpreter is being exited, even if it is exiting as a result of | |
235 | a die() function. (But not if it's polymorphing into another program | |
236 | via C<exec>, or being blown out of the water by a signal--you have to | |
237 | trap that yourself (if you can).) You may have multiple C<END> blocks | |
238 | within a file--they will execute in reverse order of definition; that is: | |
239 | last in, first out (LIFO). | |
a0d0e21e | 240 | |
19799a22 | 241 | Inside an C<END> subroutine, C<$?> contains the value that the program is |
c36e9b62 | 242 | going to pass to C<exit()>. You can modify C<$?> to change the exit |
19799a22 | 243 | value of the program. Beware of changing C<$?> by accident (e.g. by |
c36e9b62 | 244 | running something via C<system>). |
245 | ||
19799a22 | 246 | When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and |
5a964f20 TC |
247 | C<END> work just as they do in B<awk>, as a degenerate case. As currently |
248 | implemented (and subject to change, since its inconvenient at best), | |
19799a22 | 249 | both C<BEGIN> and<END> blocks are run when you use the B<-c> switch |
5a964f20 | 250 | for a compile-only syntax check, although your main code is not. |
a0d0e21e LW |
251 | |
252 | =head2 Perl Classes | |
253 | ||
19799a22 | 254 | There is no special class syntax in Perl, but a package may act |
5a964f20 TC |
255 | as a class if it provides subroutines to act as methods. Such a |
256 | package may also derive some of its methods from another class (package) | |
19799a22 | 257 | by listing the other package name(s) in its global @ISA array (which |
5a964f20 | 258 | must be a package global, not a lexical). |
4633a7c4 | 259 | |
f102b883 | 260 | For more on this, see L<perltoot> and L<perlobj>. |
a0d0e21e LW |
261 | |
262 | =head2 Perl Modules | |
263 | ||
19799a22 GS |
264 | A module is just a set of related function in a library file a Perl |
265 | package with the same name as the file. It is specifically designed | |
266 | to be reusable by other modules or programs. It may do this by | |
267 | providing a mechanism for exporting some of its symbols into the | |
268 | symbol table of any package using it. Or it may function as a class | |
269 | definition and make its semantics available implicitly through | |
270 | method calls on the class and its objects, without explicitly | |
271 | exportating anything. Or it can do a little of both. | |
a0d0e21e | 272 | |
19799a22 GS |
273 | For example, to start a traditional, non-OO module called Some::Module, |
274 | create a file called F<Some/Module.pm> and start with this template: | |
9607fc9c | 275 | |
276 | package Some::Module; # assumes Some/Module.pm | |
277 | ||
278 | use strict; | |
279 | ||
280 | BEGIN { | |
281 | use Exporter (); | |
282 | use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS); | |
283 | ||
284 | # set the version for version checking | |
285 | $VERSION = 1.00; | |
286 | # if using RCS/CVS, this may be preferred | |
287 | $VERSION = do { my @r = (q$Revision: 2.21 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r }; # must be all one line, for MakeMaker | |
288 | ||
289 | @ISA = qw(Exporter); | |
290 | @EXPORT = qw(&func1 &func2 &func4); | |
291 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], | |
292 | ||
293 | # your exported package globals go here, | |
294 | # as well as any optionally exported functions | |
295 | @EXPORT_OK = qw($Var1 %Hashit &func3); | |
296 | } | |
297 | use vars @EXPORT_OK; | |
298 | ||
299 | # non-exported package globals go here | |
300 | use vars qw(@more $stuff); | |
301 | ||
c2611fb3 | 302 | # initialize package globals, first exported ones |
9607fc9c | 303 | $Var1 = ''; |
304 | %Hashit = (); | |
305 | ||
306 | # then the others (which are still accessible as $Some::Module::stuff) | |
307 | $stuff = ''; | |
308 | @more = (); | |
309 | ||
310 | # all file-scoped lexicals must be created before | |
311 | # the functions below that use them. | |
312 | ||
313 | # file-private lexicals go here | |
314 | my $priv_var = ''; | |
315 | my %secret_hash = (); | |
316 | ||
317 | # here's a file-private function as a closure, | |
318 | # callable as &$priv_func; it cannot be prototyped. | |
319 | my $priv_func = sub { | |
320 | # stuff goes here. | |
321 | }; | |
322 | ||
323 | # make all your functions, whether exported or not; | |
324 | # remember to put something interesting in the {} stubs | |
325 | sub func1 {} # no prototype | |
326 | sub func2() {} # proto'd void | |
327 | sub func3($$) {} # proto'd to 2 scalars | |
328 | ||
329 | # this one isn't exported, but could be called! | |
330 | sub func4(\%) {} # proto'd to 1 hash ref | |
331 | ||
332 | END { } # module clean-up code here (global destructor) | |
4633a7c4 | 333 | |
19799a22 GS |
334 | ## YOUR CODE GOES HERE |
335 | ||
336 | 1; # don't forget to return a true value from the file | |
337 | ||
338 | Then go on to declare and use your variables in functions without | |
339 | any qualifications. See L<Exporter> and the L<perlmodlib> for | |
340 | details on mechanics and style issues in module creation. | |
4633a7c4 LW |
341 | |
342 | Perl modules are included into your program by saying | |
a0d0e21e LW |
343 | |
344 | use Module; | |
345 | ||
346 | or | |
347 | ||
348 | use Module LIST; | |
349 | ||
350 | This is exactly equivalent to | |
351 | ||
5a964f20 | 352 | BEGIN { require Module; import Module; } |
a0d0e21e LW |
353 | |
354 | or | |
355 | ||
5a964f20 | 356 | BEGIN { require Module; import Module LIST; } |
a0d0e21e | 357 | |
cb1a09d0 AD |
358 | As a special case |
359 | ||
360 | use Module (); | |
361 | ||
362 | is exactly equivalent to | |
363 | ||
5a964f20 | 364 | BEGIN { require Module; } |
cb1a09d0 | 365 | |
19799a22 GS |
366 | All Perl module files have the extension F<.pm>. The C<use> operator |
367 | assumes this so you don't have to spell out "F<Module.pm>" in quotes. | |
368 | This also helps to differentiate new modules from old F<.pl> and | |
369 | F<.ph> files. Module names are also capitalized unless they're | |
370 | functioning as pragmas; pragmas are in effect compiler directives, | |
371 | and are sometimes called "pragmatic modules" (or even "pragmata" | |
372 | if you're a classicist). | |
a0d0e21e | 373 | |
5a964f20 TC |
374 | The two statements: |
375 | ||
376 | require SomeModule; | |
377 | require "SomeModule.pm"; | |
378 | ||
379 | differ from each other in two ways. In the first case, any double | |
380 | colons in the module name, such as C<Some::Module>, are translated | |
381 | into your system's directory separator, usually "/". The second | |
19799a22 GS |
382 | case does not, and would have to be specified literally. The other |
383 | difference is that seeing the first C<require> clues in the compiler | |
384 | that uses of indirect object notation involving "SomeModule", as | |
385 | in C<$ob = purge SomeModule>, are method calls, not function calls. | |
386 | (Yes, this really can make a difference.) | |
387 | ||
388 | Because the C<use> statement implies a C<BEGIN> block, the importing | |
389 | of semantics happens as soon as the C<use> statement is compiled, | |
a0d0e21e LW |
390 | before the rest of the file is compiled. This is how it is able |
391 | to function as a pragma mechanism, and also how modules are able to | |
19799a22 | 392 | declare subroutines that are then visible as list or unary operators for |
a0d0e21e | 393 | the rest of the current file. This will not work if you use C<require> |
19799a22 | 394 | instead of C<use>. With C<require> you can get into this problem: |
a0d0e21e LW |
395 | |
396 | require Cwd; # make Cwd:: accessible | |
54310121 | 397 | $here = Cwd::getcwd(); |
a0d0e21e | 398 | |
5f05dabc | 399 | use Cwd; # import names from Cwd:: |
a0d0e21e LW |
400 | $here = getcwd(); |
401 | ||
402 | require Cwd; # make Cwd:: accessible | |
403 | $here = getcwd(); # oops! no main::getcwd() | |
404 | ||
5a964f20 TC |
405 | In general, C<use Module ()> is recommended over C<require Module>, |
406 | because it determines module availability at compile time, not in the | |
407 | middle of your program's execution. An exception would be if two modules | |
408 | each tried to C<use> each other, and each also called a function from | |
409 | that other module. In that case, it's easy to use C<require>s instead. | |
cb1a09d0 | 410 | |
a0d0e21e LW |
411 | Perl packages may be nested inside other package names, so we can have |
412 | package names containing C<::>. But if we used that package name | |
413 | directly as a filename it would makes for unwieldy or impossible | |
414 | filenames on some systems. Therefore, if a module's name is, say, | |
415 | C<Text::Soundex>, then its definition is actually found in the library | |
416 | file F<Text/Soundex.pm>. | |
417 | ||
19799a22 GS |
418 | Perl modules always have a F<.pm> file, but there may also be |
419 | dynamically linked executables (often ending in F<.so>) or autoloaded | |
420 | subroutine definitions (often ending in F<.al> associated with the | |
421 | module. If so, these will be entirely transparent to the user of | |
422 | the module. It is the responsibility of the F<.pm> file to load | |
423 | (or arrange to autoload) any additional functionality. For example, | |
424 | although the POSIX module happens to do both dynamic loading and | |
425 | autoloading, but the user can say just C<use POSIX> to get it all. | |
a0d0e21e | 426 | |
f102b883 | 427 | =head1 SEE ALSO |
cb1a09d0 | 428 | |
f102b883 | 429 | See L<perlmodlib> for general style issues related to building Perl |
19799a22 GS |
430 | modules and classes, as well as descriptions of the standard library |
431 | and CPAN, L<Exporter> for how Perl's standard import/export mechanism | |
432 | works, L<perltoot> and L<perltootc> for an in-depth tutorial on | |
433 | creating classes, L<perlobj> for a hard-core reference document on | |
434 | objects, L<perlsub> for an explanation of functions and scoping, | |
435 | and L<perlxstut> and L<perlguts> for more information on writing | |
436 | extension modules. |