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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 | |
bc8df162 | 11 | really no such thing as a global variable in Perl. The package |
19799a22 GS |
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 | |
5803be0d | 88 | C<%main::>, or C<%::> for short. Likewise the symbol table for the nested |
aa689395 | 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 | 98 | |
bc8df162 DC |
99 | (Be sure to note the B<vast> difference between the second line above |
100 | and C<local $main::foo = $main::bar>. The former is accessing the hash | |
101 | C<%main::>, which is the symbol table of package C<main>. The latter is | |
102 | simply assigning scalar C<$bar> in package C<main> to scalar C<$foo> of | |
103 | the same package.) | |
104 | ||
a0d0e21e | 105 | You can use this to print out all the variables in a package, for |
4375e838 | 106 | instance. The standard but antiquated F<dumpvar.pl> library and |
19799a22 | 107 | the CPAN module Devel::Symdump make use of this. |
a0d0e21e | 108 | |
cb1a09d0 | 109 | Assignment to a typeglob performs an aliasing operation, i.e., |
a0d0e21e LW |
110 | |
111 | *dick = *richard; | |
112 | ||
5a964f20 TC |
113 | causes variables, subroutines, formats, and file and directory handles |
114 | accessible via the identifier C<richard> also to be accessible via the | |
115 | identifier C<dick>. If you want to alias only a particular variable or | |
19799a22 | 116 | subroutine, assign a reference instead: |
a0d0e21e LW |
117 | |
118 | *dick = \$richard; | |
119 | ||
5a964f20 | 120 | Which makes $richard and $dick the same variable, but leaves |
a0d0e21e LW |
121 | @richard and @dick as separate arrays. Tricky, eh? |
122 | ||
cb1a09d0 | 123 | This mechanism may be used to pass and return cheap references |
5803be0d | 124 | into or from subroutines if you don't want to copy the whole |
5a964f20 TC |
125 | thing. It only works when assigning to dynamic variables, not |
126 | lexicals. | |
cb1a09d0 | 127 | |
5a964f20 | 128 | %some_hash = (); # can't be my() |
cb1a09d0 AD |
129 | *some_hash = fn( \%another_hash ); |
130 | sub fn { | |
131 | local *hashsym = shift; | |
132 | # now use %hashsym normally, and you | |
133 | # will affect the caller's %another_hash | |
134 | my %nhash = (); # do what you want | |
5f05dabc | 135 | return \%nhash; |
cb1a09d0 AD |
136 | } |
137 | ||
5f05dabc | 138 | On return, the reference will overwrite the hash slot in the |
cb1a09d0 | 139 | symbol table specified by the *some_hash typeglob. This |
c36e9b62 | 140 | is a somewhat tricky way of passing around references cheaply |
5803be0d | 141 | when you don't want to have to remember to dereference variables |
cb1a09d0 AD |
142 | explicitly. |
143 | ||
19799a22 | 144 | Another use of symbol tables is for making "constant" scalars. |
cb1a09d0 AD |
145 | |
146 | *PI = \3.14159265358979; | |
147 | ||
bc8df162 | 148 | Now you cannot alter C<$PI>, which is probably a good thing all in all. |
5a964f20 | 149 | This isn't the same as a constant subroutine, which is subject to |
5803be0d GS |
150 | optimization at compile-time. A constant subroutine is one prototyped |
151 | to take no arguments and to return a constant expression. See | |
152 | L<perlsub> for details on these. The C<use constant> pragma is a | |
5a964f20 | 153 | convenient shorthand for these. |
cb1a09d0 | 154 | |
55497cff | 155 | You can say C<*foo{PACKAGE}> and C<*foo{NAME}> to find out what name and |
156 | package the *foo symbol table entry comes from. This may be useful | |
5a964f20 | 157 | in a subroutine that gets passed typeglobs as arguments: |
55497cff | 158 | |
159 | sub identify_typeglob { | |
160 | my $glob = shift; | |
161 | print 'You gave me ', *{$glob}{PACKAGE}, '::', *{$glob}{NAME}, "\n"; | |
162 | } | |
163 | identify_typeglob *foo; | |
164 | identify_typeglob *bar::baz; | |
165 | ||
166 | This prints | |
167 | ||
168 | You gave me main::foo | |
169 | You gave me bar::baz | |
170 | ||
19799a22 | 171 | The C<*foo{THING}> notation can also be used to obtain references to the |
5803be0d | 172 | individual elements of *foo. See L<perlref>. |
55497cff | 173 | |
9263d47b GS |
174 | Subroutine definitions (and declarations, for that matter) need |
175 | not necessarily be situated in the package whose symbol table they | |
176 | occupy. You can define a subroutine outside its package by | |
177 | explicitly qualifying the name of the subroutine: | |
178 | ||
179 | package main; | |
180 | sub Some_package::foo { ... } # &foo defined in Some_package | |
181 | ||
182 | This is just a shorthand for a typeglob assignment at compile time: | |
183 | ||
184 | BEGIN { *Some_package::foo = sub { ... } } | |
185 | ||
186 | and is I<not> the same as writing: | |
187 | ||
188 | { | |
189 | package Some_package; | |
190 | sub foo { ... } | |
191 | } | |
192 | ||
193 | In the first two versions, the body of the subroutine is | |
194 | lexically in the main package, I<not> in Some_package. So | |
195 | something like this: | |
196 | ||
197 | package main; | |
198 | ||
199 | $Some_package::name = "fred"; | |
200 | $main::name = "barney"; | |
201 | ||
202 | sub Some_package::foo { | |
203 | print "in ", __PACKAGE__, ": \$name is '$name'\n"; | |
204 | } | |
205 | ||
206 | Some_package::foo(); | |
207 | ||
208 | prints: | |
209 | ||
210 | in main: $name is 'barney' | |
211 | ||
212 | rather than: | |
213 | ||
214 | in Some_package: $name is 'fred' | |
215 | ||
216 | This also has implications for the use of the SUPER:: qualifier | |
217 | (see L<perlobj>). | |
218 | ||
a0d0e21e LW |
219 | =head2 Package Constructors and Destructors |
220 | ||
7d981616 | 221 | Four special subroutines act as package constructors and destructors. |
7d30b5c4 | 222 | These are the C<BEGIN>, C<CHECK>, C<INIT>, and C<END> routines. The |
7d981616 | 223 | C<sub> is optional for these routines. |
a0d0e21e | 224 | |
f102b883 TC |
225 | A C<BEGIN> subroutine is executed as soon as possible, that is, the moment |
226 | it is completely defined, even before the rest of the containing file | |
227 | is parsed. You may have multiple C<BEGIN> blocks within a file--they | |
228 | will execute in order of definition. Because a C<BEGIN> block executes | |
229 | immediately, it can pull in definitions of subroutines and such from other | |
230 | files in time to be visible to the rest of the file. Once a C<BEGIN> | |
231 | has run, it is immediately undefined and any code it used is returned to | |
232 | Perl's memory pool. This means you can't ever explicitly call a C<BEGIN>. | |
a0d0e21e | 233 | |
4f25aa18 GS |
234 | An C<END> subroutine is executed as late as possible, that is, after |
235 | perl has finished running the program and just before the interpreter | |
236 | is being exited, even if it is exiting as a result of a die() function. | |
237 | (But not if it's polymorphing into another program via C<exec>, or | |
238 | being blown out of the water by a signal--you have to trap that yourself | |
239 | (if you can).) You may have multiple C<END> blocks within a file--they | |
240 | will execute in reverse order of definition; that is: last in, first | |
241 | out (LIFO). C<END> blocks are not executed when you run perl with the | |
db517d64 | 242 | C<-c> switch, or if compilation fails. |
a0d0e21e | 243 | |
19799a22 | 244 | Inside an C<END> subroutine, C<$?> contains the value that the program is |
c36e9b62 | 245 | going to pass to C<exit()>. You can modify C<$?> to change the exit |
19799a22 | 246 | value of the program. Beware of changing C<$?> by accident (e.g. by |
c36e9b62 | 247 | running something via C<system>). |
248 | ||
4f25aa18 GS |
249 | Similar to C<BEGIN> blocks, C<INIT> blocks are run just before the |
250 | Perl runtime begins execution, in "first in, first out" (FIFO) order. | |
251 | For example, the code generators documented in L<perlcc> make use of | |
252 | C<INIT> blocks to initialize and resolve pointers to XSUBs. | |
253 | ||
7d30b5c4 | 254 | Similar to C<END> blocks, C<CHECK> blocks are run just after the |
4f25aa18 | 255 | Perl compile phase ends and before the run time begins, in |
7d30b5c4 | 256 | LIFO order. C<CHECK> blocks are again useful in the Perl compiler |
4f25aa18 GS |
257 | suite to save the compiled state of the program. |
258 | ||
19799a22 | 259 | When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and |
4375e838 GS |
260 | C<END> work just as they do in B<awk>, as a degenerate case. |
261 | Both C<BEGIN> and C<CHECK> blocks are run when you use the B<-c> | |
262 | switch for a compile-only syntax check, although your main code | |
263 | is not. | |
a0d0e21e LW |
264 | |
265 | =head2 Perl Classes | |
266 | ||
19799a22 | 267 | There is no special class syntax in Perl, but a package may act |
5a964f20 TC |
268 | as a class if it provides subroutines to act as methods. Such a |
269 | package may also derive some of its methods from another class (package) | |
19799a22 | 270 | by listing the other package name(s) in its global @ISA array (which |
5a964f20 | 271 | must be a package global, not a lexical). |
4633a7c4 | 272 | |
f102b883 | 273 | For more on this, see L<perltoot> and L<perlobj>. |
a0d0e21e LW |
274 | |
275 | =head2 Perl Modules | |
276 | ||
5803be0d GS |
277 | A module is just a set of related functions in a library file, i.e., |
278 | a Perl package with the same name as the file. It is specifically | |
279 | designed to be reusable by other modules or programs. It may do this | |
280 | by providing a mechanism for exporting some of its symbols into the | |
19799a22 GS |
281 | symbol table of any package using it. Or it may function as a class |
282 | definition and make its semantics available implicitly through | |
283 | method calls on the class and its objects, without explicitly | |
4375e838 | 284 | exporting anything. Or it can do a little of both. |
a0d0e21e | 285 | |
19799a22 GS |
286 | For example, to start a traditional, non-OO module called Some::Module, |
287 | create a file called F<Some/Module.pm> and start with this template: | |
9607fc9c | 288 | |
289 | package Some::Module; # assumes Some/Module.pm | |
290 | ||
291 | use strict; | |
9f1b1f2d | 292 | use warnings; |
9607fc9c | 293 | |
294 | BEGIN { | |
295 | use Exporter (); | |
77ca0c92 | 296 | our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); |
9607fc9c | 297 | |
298 | # set the version for version checking | |
299 | $VERSION = 1.00; | |
300 | # if using RCS/CVS, this may be preferred | |
301 | $VERSION = do { my @r = (q$Revision: 2.21 $ =~ /\d+/g); sprintf "%d."."%02d" x $#r, @r }; # must be all one line, for MakeMaker | |
302 | ||
303 | @ISA = qw(Exporter); | |
304 | @EXPORT = qw(&func1 &func2 &func4); | |
305 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], | |
306 | ||
307 | # your exported package globals go here, | |
308 | # as well as any optionally exported functions | |
309 | @EXPORT_OK = qw($Var1 %Hashit &func3); | |
310 | } | |
77ca0c92 | 311 | our @EXPORT_OK; |
9607fc9c | 312 | |
3da4c8f2 DB |
313 | # exported package globals go here |
314 | our $Var1; | |
315 | our %Hashit; | |
316 | ||
9607fc9c | 317 | # non-exported package globals go here |
77ca0c92 LW |
318 | our @more; |
319 | our $stuff; | |
9607fc9c | 320 | |
c2611fb3 | 321 | # initialize package globals, first exported ones |
9607fc9c | 322 | $Var1 = ''; |
323 | %Hashit = (); | |
324 | ||
325 | # then the others (which are still accessible as $Some::Module::stuff) | |
326 | $stuff = ''; | |
327 | @more = (); | |
328 | ||
329 | # all file-scoped lexicals must be created before | |
330 | # the functions below that use them. | |
331 | ||
332 | # file-private lexicals go here | |
333 | my $priv_var = ''; | |
334 | my %secret_hash = (); | |
335 | ||
336 | # here's a file-private function as a closure, | |
337 | # callable as &$priv_func; it cannot be prototyped. | |
338 | my $priv_func = sub { | |
339 | # stuff goes here. | |
340 | }; | |
341 | ||
342 | # make all your functions, whether exported or not; | |
343 | # remember to put something interesting in the {} stubs | |
344 | sub func1 {} # no prototype | |
345 | sub func2() {} # proto'd void | |
346 | sub func3($$) {} # proto'd to 2 scalars | |
347 | ||
348 | # this one isn't exported, but could be called! | |
349 | sub func4(\%) {} # proto'd to 1 hash ref | |
350 | ||
351 | END { } # module clean-up code here (global destructor) | |
4633a7c4 | 352 | |
19799a22 GS |
353 | ## YOUR CODE GOES HERE |
354 | ||
355 | 1; # don't forget to return a true value from the file | |
356 | ||
357 | Then go on to declare and use your variables in functions without | |
358 | any qualifications. See L<Exporter> and the L<perlmodlib> for | |
359 | details on mechanics and style issues in module creation. | |
4633a7c4 LW |
360 | |
361 | Perl modules are included into your program by saying | |
a0d0e21e LW |
362 | |
363 | use Module; | |
364 | ||
365 | or | |
366 | ||
367 | use Module LIST; | |
368 | ||
369 | This is exactly equivalent to | |
370 | ||
5a964f20 | 371 | BEGIN { require Module; import Module; } |
a0d0e21e LW |
372 | |
373 | or | |
374 | ||
5a964f20 | 375 | BEGIN { require Module; import Module LIST; } |
a0d0e21e | 376 | |
cb1a09d0 AD |
377 | As a special case |
378 | ||
379 | use Module (); | |
380 | ||
381 | is exactly equivalent to | |
382 | ||
5a964f20 | 383 | BEGIN { require Module; } |
cb1a09d0 | 384 | |
19799a22 GS |
385 | All Perl module files have the extension F<.pm>. The C<use> operator |
386 | assumes this so you don't have to spell out "F<Module.pm>" in quotes. | |
387 | This also helps to differentiate new modules from old F<.pl> and | |
388 | F<.ph> files. Module names are also capitalized unless they're | |
389 | functioning as pragmas; pragmas are in effect compiler directives, | |
390 | and are sometimes called "pragmatic modules" (or even "pragmata" | |
391 | if you're a classicist). | |
a0d0e21e | 392 | |
5a964f20 TC |
393 | The two statements: |
394 | ||
395 | require SomeModule; | |
396 | require "SomeModule.pm"; | |
397 | ||
398 | differ from each other in two ways. In the first case, any double | |
399 | colons in the module name, such as C<Some::Module>, are translated | |
400 | into your system's directory separator, usually "/". The second | |
19799a22 GS |
401 | case does not, and would have to be specified literally. The other |
402 | difference is that seeing the first C<require> clues in the compiler | |
403 | that uses of indirect object notation involving "SomeModule", as | |
404 | in C<$ob = purge SomeModule>, are method calls, not function calls. | |
405 | (Yes, this really can make a difference.) | |
406 | ||
407 | Because the C<use> statement implies a C<BEGIN> block, the importing | |
408 | of semantics happens as soon as the C<use> statement is compiled, | |
a0d0e21e LW |
409 | before the rest of the file is compiled. This is how it is able |
410 | to function as a pragma mechanism, and also how modules are able to | |
19799a22 | 411 | declare subroutines that are then visible as list or unary operators for |
a0d0e21e | 412 | the rest of the current file. This will not work if you use C<require> |
19799a22 | 413 | instead of C<use>. With C<require> you can get into this problem: |
a0d0e21e LW |
414 | |
415 | require Cwd; # make Cwd:: accessible | |
54310121 | 416 | $here = Cwd::getcwd(); |
a0d0e21e | 417 | |
5f05dabc | 418 | use Cwd; # import names from Cwd:: |
a0d0e21e LW |
419 | $here = getcwd(); |
420 | ||
421 | require Cwd; # make Cwd:: accessible | |
422 | $here = getcwd(); # oops! no main::getcwd() | |
423 | ||
5a964f20 TC |
424 | In general, C<use Module ()> is recommended over C<require Module>, |
425 | because it determines module availability at compile time, not in the | |
426 | middle of your program's execution. An exception would be if two modules | |
427 | each tried to C<use> each other, and each also called a function from | |
428 | that other module. In that case, it's easy to use C<require>s instead. | |
cb1a09d0 | 429 | |
a0d0e21e LW |
430 | Perl packages may be nested inside other package names, so we can have |
431 | package names containing C<::>. But if we used that package name | |
5803be0d | 432 | directly as a filename it would make for unwieldy or impossible |
a0d0e21e LW |
433 | filenames on some systems. Therefore, if a module's name is, say, |
434 | C<Text::Soundex>, then its definition is actually found in the library | |
435 | file F<Text/Soundex.pm>. | |
436 | ||
19799a22 GS |
437 | Perl modules always have a F<.pm> file, but there may also be |
438 | dynamically linked executables (often ending in F<.so>) or autoloaded | |
5803be0d | 439 | subroutine definitions (often ending in F<.al>) associated with the |
19799a22 GS |
440 | module. If so, these will be entirely transparent to the user of |
441 | the module. It is the responsibility of the F<.pm> file to load | |
442 | (or arrange to autoload) any additional functionality. For example, | |
443 | although the POSIX module happens to do both dynamic loading and | |
5803be0d | 444 | autoloading, the user can say just C<use POSIX> to get it all. |
a0d0e21e | 445 | |
f102b883 | 446 | =head1 SEE ALSO |
cb1a09d0 | 447 | |
f102b883 | 448 | See L<perlmodlib> for general style issues related to building Perl |
19799a22 GS |
449 | modules and classes, as well as descriptions of the standard library |
450 | and CPAN, L<Exporter> for how Perl's standard import/export mechanism | |
451 | works, L<perltoot> and L<perltootc> for an in-depth tutorial on | |
452 | creating classes, L<perlobj> for a hard-core reference document on | |
453 | objects, L<perlsub> for an explanation of functions and scoping, | |
454 | and L<perlxstut> and L<perlguts> for more information on writing | |
455 | extension modules. |