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1 | =head1 NAME |
2 | |
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3 | perlmod - Perl modules (packages and symbol tables) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | =head2 Packages |
8 | |
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9 | Perl provides a mechanism for alternative namespaces to protect |
10 | packages from stomping on each other's variables. In fact, there's |
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11 | really no such thing as a global variable in Perl. The package |
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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>. |
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30 | |
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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 ${owner}'s house">. |
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41 | |
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42 | Packages may themselves contain package separators, as in |
43 | C<$OUTER::INNER::var>. This implies nothing about the order of |
44 | name lookups, however. There are no relative packages: all symbols |
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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 |
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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 |
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62 | main, but we decided it was more useful for package writers to be able |
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63 | to use leading underscore to indicate private variables and method names. |
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64 | $_ is still global though. See also |
65 | L<perlvar/"Technical Note on the Syntax of Variable Names">. |
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66 | |
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67 | C<eval>ed strings are compiled in the package in which the eval() was |
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68 | compiled. (Assignments to C<$SIG{}>, however, assume the signal |
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69 | handler specified is in the C<main> package. Qualify the signal handler |
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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 |
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73 | in the program you are trying to debug. At various points, however, it |
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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 | |
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78 | The special symbol C<__PACKAGE__> contains the current package, but cannot |
79 | (easily) be used to construct variables. |
80 | |
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81 | See L<perlsub> for other scoping issues related to my() and local(), |
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82 | and L<perlref> regarding closures. |
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83 | |
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84 | =head2 Symbol Tables |
85 | |
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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 |
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88 | C<%main::>, or C<%::> for short. Likewise the symbol table for the nested |
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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: |
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95 | |
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96 | local *main::foo = *main::bar; |
97 | local $main::{foo} = $main::{bar}; |
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98 | |
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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 | |
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105 | You can use this to print out all the variables in a package, for |
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106 | instance. The standard but antiquated F<dumpvar.pl> library and |
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107 | the CPAN module Devel::Symdump make use of this. |
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108 | |
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109 | Assignment to a typeglob performs an aliasing operation, i.e., |
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110 | |
111 | *dick = *richard; |
112 | |
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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 |
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116 | subroutine, assign a reference instead: |
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117 | |
118 | *dick = \$richard; |
119 | |
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120 | Which makes $richard and $dick the same variable, but leaves |
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121 | @richard and @dick as separate arrays. Tricky, eh? |
122 | |
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123 | This mechanism may be used to pass and return cheap references |
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124 | into or from subroutines if you don't want to copy the whole |
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125 | thing. It only works when assigning to dynamic variables, not |
126 | lexicals. |
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127 | |
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128 | %some_hash = (); # can't be my() |
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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 |
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135 | return \%nhash; |
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136 | } |
137 | |
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138 | On return, the reference will overwrite the hash slot in the |
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139 | symbol table specified by the *some_hash typeglob. This |
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140 | is a somewhat tricky way of passing around references cheaply |
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141 | when you don't want to have to remember to dereference variables |
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142 | explicitly. |
143 | |
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144 | Another use of symbol tables is for making "constant" scalars. |
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145 | |
146 | *PI = \3.14159265358979; |
147 | |
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148 | Now you cannot alter C<$PI>, which is probably a good thing all in all. |
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149 | This isn't the same as a constant subroutine, which is subject to |
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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 |
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153 | convenient shorthand for these. |
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154 | |
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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 |
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157 | in a subroutine that gets passed typeglobs as arguments: |
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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 | |
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171 | The C<*foo{THING}> notation can also be used to obtain references to the |
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172 | individual elements of *foo. See L<perlref>. |
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173 | |
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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 | |
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219 | =head2 Package Constructors and Destructors |
220 | |
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221 | Four special subroutines act as package constructors and destructors. |
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222 | These are the C<BEGIN>, C<CHECK>, C<INIT>, and C<END> routines. The |
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223 | C<sub> is optional for these routines. |
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224 | |
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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>. |
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233 | |
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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 |
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242 | C<-c> switch, or if compilation fails. |
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243 | |
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244 | Inside an C<END> subroutine, C<$?> contains the value that the program is |
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245 | going to pass to C<exit()>. You can modify C<$?> to change the exit |
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246 | value of the program. Beware of changing C<$?> by accident (e.g. by |
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247 | running something via C<system>). |
248 | |
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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 | |
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254 | Similar to C<END> blocks, C<CHECK> blocks are run just after the |
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255 | Perl compile phase ends and before the run time begins, in |
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256 | LIFO order. C<CHECK> blocks are again useful in the Perl compiler |
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257 | suite to save the compiled state of the program. |
258 | |
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259 | When you use the B<-n> and B<-p> switches to Perl, C<BEGIN> and |
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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. |
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264 | |
265 | =head2 Perl Classes |
266 | |
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267 | There is no special class syntax in Perl, but a package may act |
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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) |
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270 | by listing the other package name(s) in its global @ISA array (which |
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271 | must be a package global, not a lexical). |
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272 | |
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273 | For more on this, see L<perltoot> and L<perlobj>. |
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274 | |
275 | =head2 Perl Modules |
276 | |
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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 |
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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 |
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284 | exporting anything. Or it can do a little of both. |
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285 | |
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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: |
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288 | |
289 | package Some::Module; # assumes Some/Module.pm |
290 | |
291 | use strict; |
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292 | use warnings; |
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293 | |
294 | BEGIN { |
295 | use Exporter (); |
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296 | our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); |
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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 | } |
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311 | our @EXPORT_OK; |
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312 | |
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313 | # exported package globals go here |
314 | our $Var1; |
315 | our %Hashit; |
316 | |
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317 | # non-exported package globals go here |
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318 | our @more; |
319 | our $stuff; |
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320 | |
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321 | # initialize package globals, first exported ones |
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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) |
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352 | |
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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. |
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360 | |
361 | Perl modules are included into your program by saying |
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362 | |
363 | use Module; |
364 | |
365 | or |
366 | |
367 | use Module LIST; |
368 | |
369 | This is exactly equivalent to |
370 | |
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371 | BEGIN { require Module; import Module; } |
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372 | |
373 | or |
374 | |
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375 | BEGIN { require Module; import Module LIST; } |
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376 | |
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377 | As a special case |
378 | |
379 | use Module (); |
380 | |
381 | is exactly equivalent to |
382 | |
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383 | BEGIN { require Module; } |
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384 | |
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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). |
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392 | |
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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 |
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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, |
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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 |
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411 | declare subroutines that are then visible as list or unary operators for |
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412 | the rest of the current file. This will not work if you use C<require> |
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413 | instead of C<use>. With C<require> you can get into this problem: |
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414 | |
415 | require Cwd; # make Cwd:: accessible |
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416 | $here = Cwd::getcwd(); |
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417 | |
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418 | use Cwd; # import names from Cwd:: |
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419 | $here = getcwd(); |
420 | |
421 | require Cwd; # make Cwd:: accessible |
422 | $here = getcwd(); # oops! no main::getcwd() |
423 | |
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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. |
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429 | |
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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 |
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432 | directly as a filename it would make for unwieldy or impossible |
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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 | |
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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 |
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439 | subroutine definitions (often ending in F<.al>) associated with the |
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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 |
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444 | autoloading, the user can say just C<use POSIX> to get it all. |
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445 | |
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446 | =head1 SEE ALSO |
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447 | |
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448 | See L<perlmodlib> for general style issues related to building Perl |
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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. |