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68dc0745 | 1 | =head1 NAME |
2 | ||
109f0441 | 3 | perlfaq7 - General Perl Language Issues |
68dc0745 | 4 | |
5 | =head1 DESCRIPTION | |
6 | ||
7 | This section deals with general Perl language issues that don't | |
8 | clearly fit into any of the other sections. | |
9 | ||
10 | =head2 Can I get a BNF/yacc/RE for the Perl language? | |
11 | ||
c8db1d39 TC |
12 | There is no BNF, but you can paw your way through the yacc grammar in |
13 | perly.y in the source distribution if you're particularly brave. The | |
14 | grammar relies on very smart tokenizing code, so be prepared to | |
15 | venture into toke.c as well. | |
16 | ||
17 | In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF. | |
18 | The work of parsing perl is distributed between yacc, the lexer, smoke | |
19 | and mirrors." | |
68dc0745 | 20 | |
d92eb7b0 | 21 | =head2 What are all these $@%&* punctuation signs, and how do I know when to use them? |
68dc0745 | 22 | |
23 | They are type specifiers, as detailed in L<perldata>: | |
24 | ||
ac003c96 RGS |
25 | $ for scalar values (number, string or reference) |
26 | @ for arrays | |
27 | % for hashes (associative arrays) | |
28 | & for subroutines (aka functions, procedures, methods) | |
29 | * for all types of that symbol name. In version 4 you used them like | |
30 | pointers, but in modern perls you can just use references. | |
68dc0745 | 31 | |
4c7f4b6b FC |
32 | There are a couple of other symbols that |
33 | you're likely to encounter that aren't | |
a6dd486b | 34 | really type specifiers: |
68dc0745 | 35 | |
ac003c96 RGS |
36 | <> are used for inputting a record from a filehandle. |
37 | \ takes a reference to something. | |
68dc0745 | 38 | |
c47ff5f1 GS |
39 | Note that <FILE> is I<neither> the type specifier for files |
40 | nor the name of the handle. It is the C<< <> >> operator applied | |
a6dd486b | 41 | to the handle FILE. It reads one line (well, record--see |
197aec24 | 42 | L<perlvar/$E<sol>>) from the handle FILE in scalar context, or I<all> lines |
68dc0745 | 43 | in list context. When performing open, close, or any other operation |
a6dd486b | 44 | besides C<< <> >> on files, or even when talking about the handle, do |
68dc0745 | 45 | I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0, |
46 | 2)> and "copying from STDIN to FILE". | |
47 | ||
48 | =head2 Do I always/never have to quote my strings or use semicolons and commas? | |
49 | ||
50 | Normally, a bareword doesn't need to be quoted, but in most cases | |
51 | probably should be (and must be under C<use strict>). But a hash key | |
b056a7aa FC |
52 | consisting of a simple word and the left-hand |
53 | operand to the C<< => >> operator both | |
68dc0745 | 54 | count as though they were quoted: |
55 | ||
ac003c96 RGS |
56 | This is like this |
57 | ------------ --------------- | |
58 | $foo{line} $foo{'line'} | |
59 | bar => stuff 'bar' => stuff | |
68dc0745 | 60 | |
61 | The final semicolon in a block is optional, as is the final comma in a | |
62 | list. Good style (see L<perlstyle>) says to put them in except for | |
63 | one-liners: | |
64 | ||
ac003c96 RGS |
65 | if ($whoops) { exit 1 } |
66 | @nums = (1, 2, 3); | |
109f0441 | 67 | |
ac003c96 RGS |
68 | if ($whoops) { |
69 | exit 1; | |
70 | } | |
68dc0745 | 71 | |
ac003c96 | 72 | @lines = ( |
68dc0745 | 73 | "There Beren came from mountains cold", |
74 | "And lost he wandered under leaves", | |
ac003c96 | 75 | ); |
68dc0745 | 76 | |
77 | =head2 How do I skip some return values? | |
78 | ||
79 | One way is to treat the return values as a list and index into it: | |
80 | ||
ac003c96 | 81 | $dir = (getpwnam($user))[7]; |
68dc0745 | 82 | |
83 | Another way is to use undef as an element on the left-hand-side: | |
84 | ||
ac003c96 | 85 | ($dev, $ino, undef, undef, $uid, $gid) = stat($file); |
197aec24 | 86 | |
49d635f9 RGS |
87 | You can also use a list slice to select only the elements that |
88 | you need: | |
89 | ||
90 | ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5]; | |
68dc0745 | 91 | |
92 | =head2 How do I temporarily block warnings? | |
93 | ||
9f1b1f2d GS |
94 | If you are running Perl 5.6.0 or better, the C<use warnings> pragma |
95 | allows fine control of what warning are produced. | |
96 | See L<perllexwarn> for more details. | |
97 | ||
ac003c96 | 98 | { |
9f1b1f2d GS |
99 | no warnings; # temporarily turn off warnings |
100 | $a = $b + $c; # I know these might be undef | |
ac003c96 | 101 | } |
6670e5e7 | 102 | |
28b41a80 RGS |
103 | Additionally, you can enable and disable categories of warnings. |
104 | You turn off the categories you want to ignore and you can still | |
105 | get other categories of warnings. See L<perllexwarn> for the | |
106 | complete details, including the category names and hierarchy. | |
107 | ||
108 | { | |
109 | no warnings 'uninitialized'; | |
110 | $a = $b + $c; | |
111 | } | |
9f1b1f2d GS |
112 | |
113 | If you have an older version of Perl, the C<$^W> variable (documented | |
114 | in L<perlvar>) controls runtime warnings for a block: | |
68dc0745 | 115 | |
ac003c96 | 116 | { |
68dc0745 | 117 | local $^W = 0; # temporarily turn off warnings |
118 | $a = $b + $c; # I know these might be undef | |
ac003c96 | 119 | } |
68dc0745 | 120 | |
121 | Note that like all the punctuation variables, you cannot currently | |
122 | use my() on C<$^W>, only local(). | |
123 | ||
68dc0745 | 124 | =head2 What's an extension? |
125 | ||
a6dd486b JB |
126 | An extension is a way of calling compiled C code from Perl. Reading |
127 | L<perlxstut> is a good place to learn more about extensions. | |
68dc0745 | 128 | |
129 | =head2 Why do Perl operators have different precedence than C operators? | |
130 | ||
131 | Actually, they don't. All C operators that Perl copies have the same | |
132 | precedence in Perl as they do in C. The problem is with operators that C | |
133 | doesn't have, especially functions that give a list context to everything | |
a6dd486b | 134 | on their right, eg. print, chmod, exec, and so on. Such functions are |
68dc0745 | 135 | called "list operators" and appear as such in the precedence table in |
136 | L<perlop>. | |
137 | ||
138 | A common mistake is to write: | |
139 | ||
ac003c96 | 140 | unlink $file || die "snafu"; |
68dc0745 | 141 | |
142 | This gets interpreted as: | |
143 | ||
ac003c96 | 144 | unlink ($file || die "snafu"); |
68dc0745 | 145 | |
146 | To avoid this problem, either put in extra parentheses or use the | |
147 | super low precedence C<or> operator: | |
148 | ||
ac003c96 RGS |
149 | (unlink $file) || die "snafu"; |
150 | unlink $file or die "snafu"; | |
68dc0745 | 151 | |
152 | The "English" operators (C<and>, C<or>, C<xor>, and C<not>) | |
153 | deliberately have precedence lower than that of list operators for | |
154 | just such situations as the one above. | |
155 | ||
156 | Another operator with surprising precedence is exponentiation. It | |
109f0441 | 157 | binds more tightly even than unary minus, making C<-2**2> produce a |
68dc0745 | 158 | negative not a positive four. It is also right-associating, meaning |
159 | that C<2**3**2> is two raised to the ninth power, not eight squared. | |
160 | ||
c8db1d39 TC |
161 | Although it has the same precedence as in C, Perl's C<?:> operator |
162 | produces an lvalue. This assigns $x to either $a or $b, depending | |
163 | on the trueness of $maybe: | |
164 | ||
ac003c96 | 165 | ($maybe ? $a : $b) = $x; |
c8db1d39 | 166 | |
68dc0745 | 167 | =head2 How do I declare/create a structure? |
168 | ||
169 | In general, you don't "declare" a structure. Just use a (probably | |
170 | anonymous) hash reference. See L<perlref> and L<perldsc> for details. | |
171 | Here's an example: | |
172 | ||
ac003c96 RGS |
173 | $person = {}; # new anonymous hash |
174 | $person->{AGE} = 24; # set field AGE to 24 | |
175 | $person->{NAME} = "Nat"; # set field NAME to "Nat" | |
109f0441 | 176 | |
68dc0745 | 177 | If you're looking for something a bit more rigorous, try L<perltoot>. |
178 | ||
179 | =head2 How do I create a module? | |
180 | ||
7678cced | 181 | (contributed by brian d foy) |
68dc0745 | 182 | |
7678cced | 183 | L<perlmod>, L<perlmodlib>, L<perlmodstyle> explain modules |
cf525c36 | 184 | in all the gory details. L<perlnewmod> gives a brief |
7678cced RGS |
185 | overview of the process along with a couple of suggestions |
186 | about style. | |
65acb1b1 | 187 | |
7678cced RGS |
188 | If you need to include C code or C library interfaces in |
189 | your module, you'll need h2xs. h2xs will create the module | |
190 | distribution structure and the initial interface files | |
191 | you'll need. L<perlxs> and L<perlxstut> explain the details. | |
7207e29d | 192 | |
7678cced RGS |
193 | If you don't need to use C code, other tools such as |
194 | ExtUtils::ModuleMaker and Module::Starter, can help you | |
195 | create a skeleton module distribution. | |
196 | ||
197 | You may also want to see Sam Tregar's "Writing Perl Modules | |
198 | for CPAN" ( http://apress.com/book/bookDisplay.html?bID=14 ) | |
199 | which is the best hands-on guide to creating module | |
200 | distributions. | |
65acb1b1 | 201 | |
c195e131 RGS |
202 | =head2 How do I adopt or take over a module already on CPAN? |
203 | ||
204 | (contributed by brian d foy) | |
205 | ||
206 | The easiest way to take over a module is to have the current | |
207 | module maintainer either make you a co-maintainer or transfer | |
208 | the module to you. | |
209 | ||
210 | If you can't reach the author for some reason (e.g. email bounces), | |
211 | the PAUSE admins at modules@perl.org can help. The PAUSE admins | |
212 | treat each case individually. | |
213 | ||
214 | =over 4 | |
215 | ||
1b279fcb | 216 | =item * |
c195e131 RGS |
217 | |
218 | Get a login for the Perl Authors Upload Server (PAUSE) if you don't | |
219 | already have one: http://pause.perl.org | |
220 | ||
1b279fcb | 221 | =item * |
c195e131 RGS |
222 | |
223 | Write to modules@perl.org explaining what you did to contact the | |
224 | current maintainer. The PAUSE admins will also try to reach the | |
225 | maintainer. | |
226 | ||
1b279fcb | 227 | =item * |
c195e131 RGS |
228 | |
229 | Post a public message in a heavily trafficked site announcing your | |
230 | intention to take over the module. | |
231 | ||
1b279fcb | 232 | =item * |
c195e131 RGS |
233 | |
234 | Wait a bit. The PAUSE admins don't want to act too quickly in case | |
109f0441 | 235 | the current maintainer is on holiday. If there's no response to |
c195e131 RGS |
236 | private communication or the public post, a PAUSE admin can transfer |
237 | it to you. | |
238 | ||
239 | =back | |
240 | ||
68dc0745 | 241 | =head2 How do I create a class? |
109f0441 | 242 | X<class, creation> X<package> |
68dc0745 | 243 | |
109f0441 S |
244 | (contributed by brian d foy) |
245 | ||
246 | In Perl, a class is just a package, and methods are just subroutines. | |
247 | Perl doesn't get more formal than that and lets you set up the package | |
248 | just the way that you like it (that is, it doesn't set up anything for | |
249 | you). | |
250 | ||
251 | The Perl documentation has several tutorials that cover class | |
252 | creation, including L<perlboot> (Barnyard Object Oriented Tutorial), | |
253 | L<perltoot> (Tom's Object Oriented Tutorial), L<perlbot> (Bag o' | |
254 | Object Tricks), and L<perlobj>. | |
68dc0745 | 255 | |
256 | =head2 How can I tell if a variable is tainted? | |
257 | ||
213329dd JH |
258 | You can use the tainted() function of the Scalar::Util module, available |
259 | from CPAN (or included with Perl since release 5.8.0). | |
260 | See also L<perlsec/"Laundering and Detecting Tainted Data">. | |
68dc0745 | 261 | |
262 | =head2 What's a closure? | |
263 | ||
264 | Closures are documented in L<perlref>. | |
265 | ||
266 | I<Closure> is a computer science term with a precise but | |
322be77c RGS |
267 | hard-to-explain meaning. Usually, closures are implemented in Perl as |
268 | anonymous subroutines with lasting references to lexical variables | |
269 | outside their own scopes. These lexicals magically refer to the | |
109f0441 | 270 | variables that were around when the subroutine was defined (deep |
322be77c RGS |
271 | binding). |
272 | ||
273 | Closures are most often used in programming languages where you can | |
274 | have the return value of a function be itself a function, as you can | |
275 | in Perl. Note that some languages provide anonymous functions but are | |
276 | not capable of providing proper closures: the Python language, for | |
68dc0745 | 277 | example. For more information on closures, check out any textbook on |
278 | functional programming. Scheme is a language that not only supports | |
279 | but encourages closures. | |
280 | ||
322be77c | 281 | Here's a classic non-closure function-generating function: |
68dc0745 | 282 | |
ac003c96 RGS |
283 | sub add_function_generator { |
284 | return sub { shift() + shift() }; | |
285 | } | |
68dc0745 | 286 | |
ac003c96 RGS |
287 | $add_sub = add_function_generator(); |
288 | $sum = $add_sub->(4,5); # $sum is 9 now. | |
68dc0745 | 289 | |
322be77c RGS |
290 | The anonymous subroutine returned by add_function_generator() isn't |
291 | technically a closure because it refers to no lexicals outside its own | |
292 | scope. Using a closure gives you a I<function template> with some | |
293 | customization slots left out to be filled later. | |
68dc0745 | 294 | |
295 | Contrast this with the following make_adder() function, in which the | |
296 | returned anonymous function contains a reference to a lexical variable | |
297 | outside the scope of that function itself. Such a reference requires | |
298 | that Perl return a proper closure, thus locking in for all time the | |
299 | value that the lexical had when the function was created. | |
300 | ||
ac003c96 RGS |
301 | sub make_adder { |
302 | my $addpiece = shift; | |
303 | return sub { shift() + $addpiece }; | |
304 | } | |
109f0441 | 305 | |
ac003c96 RGS |
306 | $f1 = make_adder(20); |
307 | $f2 = make_adder(555); | |
68dc0745 | 308 | |
309 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas | |
310 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece | |
311 | in the closure sticks around. | |
312 | ||
313 | Closures are often used for less esoteric purposes. For example, when | |
314 | you want to pass in a bit of code into a function: | |
315 | ||
ac003c96 RGS |
316 | my $line; |
317 | timeout( 30, sub { $line = <STDIN> } ); | |
68dc0745 | 318 | |
c47ff5f1 GS |
319 | If the code to execute had been passed in as a string, |
320 | C<< '$line = <STDIN>' >>, there would have been no way for the | |
321 | hypothetical timeout() function to access the lexical variable | |
322 | $line back in its caller's scope. | |
68dc0745 | 323 | |
322be77c RGS |
324 | Another use for a closure is to make a variable I<private> to a |
325 | named subroutine, e.g. a counter that gets initialized at creation | |
326 | time of the sub and can only be modified from within the sub. | |
327 | This is sometimes used with a BEGIN block in package files to make | |
328 | sure a variable doesn't get meddled with during the lifetime of the | |
329 | package: | |
330 | ||
ac003c96 RGS |
331 | BEGIN { |
332 | my $id = 0; | |
333 | sub next_id { ++$id } | |
334 | } | |
322be77c | 335 | |
4c7f4b6b | 336 | This is discussed in more detail in L<perlsub>; see the entry on |
322be77c RGS |
337 | I<Persistent Private Variables>. |
338 | ||
46fc3d4c | 339 | =head2 What is variable suicide and how can I prevent it? |
340 | ||
9e72e4c6 RGS |
341 | This problem was fixed in perl 5.004_05, so preventing it means upgrading |
342 | your version of perl. ;) | |
46fc3d4c | 343 | |
9e72e4c6 RGS |
344 | Variable suicide is when you (temporarily or permanently) lose the value |
345 | of a variable. It is caused by scoping through my() and local() | |
346 | interacting with either closures or aliased foreach() iterator variables | |
347 | and subroutine arguments. It used to be easy to inadvertently lose a | |
348 | variable's value this way, but now it's much harder. Take this code: | |
349 | ||
ac003c96 RGS |
350 | my $f = 'foo'; |
351 | sub T { | |
352 | while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } | |
353 | } | |
354 | ||
355 | T; | |
356 | print "Finally $f\n"; | |
46fc3d4c | 357 | |
9e72e4c6 RGS |
358 | If you are experiencing variable suicide, that C<my $f> in the subroutine |
359 | doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output | |
360 | shows that inside the subroutine the value of C<$f> leaks through when it | |
361 | shouldn't, as in this output: | |
362 | ||
363 | foobar | |
364 | foobarbar | |
365 | foobarbarbar | |
366 | Finally foo | |
367 | ||
46fc3d4c | 368 | The $f that has "bar" added to it three times should be a new C<$f> |
9e72e4c6 RGS |
369 | C<my $f> should create a new lexical variable each time through the loop. |
370 | The expected output is: | |
371 | ||
372 | foobar | |
373 | foobar | |
374 | foobar | |
375 | Finally foo | |
46fc3d4c | 376 | |
d92eb7b0 | 377 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? |
68dc0745 | 378 | |
d12d61cf | 379 | You need to pass references to these objects. See L<perlsub/"Pass by |
380 | Reference"> for this particular question, and L<perlref> for | |
381 | information on references. | |
a6dd486b | 382 | |
68dc0745 | 383 | =over 4 |
384 | ||
385 | =item Passing Variables and Functions | |
386 | ||
a6dd486b | 387 | Regular variables and functions are quite easy to pass: just pass in a |
68dc0745 | 388 | reference to an existing or anonymous variable or function: |
389 | ||
ac003c96 | 390 | func( \$some_scalar ); |
68dc0745 | 391 | |
ac003c96 RGS |
392 | func( \@some_array ); |
393 | func( [ 1 .. 10 ] ); | |
68dc0745 | 394 | |
ac003c96 RGS |
395 | func( \%some_hash ); |
396 | func( { this => 10, that => 20 } ); | |
68dc0745 | 397 | |
ac003c96 RGS |
398 | func( \&some_func ); |
399 | func( sub { $_[0] ** $_[1] } ); | |
68dc0745 | 400 | |
401 | =item Passing Filehandles | |
402 | ||
49d635f9 RGS |
403 | As of Perl 5.6, you can represent filehandles with scalar variables |
404 | which you treat as any other scalar. | |
405 | ||
406 | open my $fh, $filename or die "Cannot open $filename! $!"; | |
407 | func( $fh ); | |
197aec24 | 408 | |
49d635f9 RGS |
409 | sub func { |
410 | my $passed_fh = shift; | |
197aec24 | 411 | |
ac003c96 | 412 | my $line = <$passed_fh>; |
49d635f9 | 413 | } |
197aec24 | 414 | |
49d635f9 | 415 | Before Perl 5.6, you had to use the C<*FH> or C<\*FH> notations. |
a6dd486b | 416 | These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles"> |
c8db1d39 TC |
417 | and especially L<perlsub/"Pass by Reference"> for more information. |
418 | ||
d92eb7b0 GS |
419 | =item Passing Regexes |
420 | ||
d12d61cf | 421 | Here's an example of how to pass in a string and a regular expression |
422 | for it to match against. You construct the pattern with the C<qr//> | |
423 | operator: | |
68dc0745 | 424 | |
ac003c96 RGS |
425 | sub compare($$) { |
426 | my ($val1, $regex) = @_; | |
427 | my $retval = $val1 =~ /$regex/; | |
d92eb7b0 | 428 | return $retval; |
ac003c96 RGS |
429 | } |
430 | $match = compare("old McDonald", qr/d.*D/i); | |
d92eb7b0 | 431 | |
68dc0745 | 432 | =item Passing Methods |
433 | ||
434 | To pass an object method into a subroutine, you can do this: | |
435 | ||
ac003c96 RGS |
436 | call_a_lot(10, $some_obj, "methname") |
437 | sub call_a_lot { | |
438 | my ($count, $widget, $trick) = @_; | |
439 | for (my $i = 0; $i < $count; $i++) { | |
440 | $widget->$trick(); | |
441 | } | |
442 | } | |
68dc0745 | 443 | |
a6dd486b JB |
444 | Or, you can use a closure to bundle up the object, its |
445 | method call, and arguments: | |
68dc0745 | 446 | |
ac003c96 RGS |
447 | my $whatnot = sub { $some_obj->obfuscate(@args) }; |
448 | func($whatnot); | |
449 | sub func { | |
450 | my $code = shift; | |
451 | &$code(); | |
452 | } | |
68dc0745 | 453 | |
454 | You could also investigate the can() method in the UNIVERSAL class | |
455 | (part of the standard perl distribution). | |
456 | ||
457 | =back | |
458 | ||
459 | =head2 How do I create a static variable? | |
460 | ||
6670e5e7 | 461 | (contributed by brian d foy) |
68dc0745 | 462 | |
109f0441 S |
463 | In Perl 5.10, declare the variable with C<state>. The C<state> |
464 | declaration creates the lexical variable that persists between calls | |
465 | to the subroutine: | |
466 | ||
467 | sub counter { state $count = 1; $counter++ } | |
6670e5e7 RGS |
468 | |
469 | You can fake a static variable by using a lexical variable which goes | |
a05e4845 | 470 | out of scope. In this example, you define the subroutine C<counter>, and |
6670e5e7 RGS |
471 | it uses the lexical variable C<$count>. Since you wrap this in a BEGIN |
472 | block, C<$count> is defined at compile-time, but also goes out of | |
473 | scope at the end of the BEGIN block. The BEGIN block also ensures that | |
474 | the subroutine and the value it uses is defined at compile-time so the | |
475 | subroutine is ready to use just like any other subroutine, and you can | |
476 | put this code in the same place as other subroutines in the program | |
477 | text (i.e. at the end of the code, typically). The subroutine | |
478 | C<counter> still has a reference to the data, and is the only way you | |
479 | can access the value (and each time you do, you increment the value). | |
480 | The data in chunk of memory defined by C<$count> is private to | |
481 | C<counter>. | |
482 | ||
ac003c96 RGS |
483 | BEGIN { |
484 | my $count = 1; | |
485 | sub counter { $count++ } | |
486 | } | |
109f0441 | 487 | |
ac003c96 | 488 | my $start = counter(); |
109f0441 | 489 | |
ac003c96 | 490 | .... # code that calls counter(); |
109f0441 | 491 | |
ac003c96 | 492 | my $end = counter(); |
68dc0745 | 493 | |
6670e5e7 RGS |
494 | In the previous example, you created a function-private variable |
495 | because only one function remembered its reference. You could define | |
496 | multiple functions while the variable is in scope, and each function | |
497 | can share the "private" variable. It's not really "static" because you | |
498 | can access it outside the function while the lexical variable is in | |
499 | scope, and even create references to it. In this example, | |
500 | C<increment_count> and C<return_count> share the variable. One | |
501 | function adds to the value and the other simply returns the value. | |
502 | They can both access C<$count>, and since it has gone out of scope, | |
503 | there is no other way to access it. | |
68dc0745 | 504 | |
ac003c96 RGS |
505 | BEGIN { |
506 | my $count = 1; | |
507 | sub increment_count { $count++ } | |
508 | sub return_count { $count } | |
509 | } | |
68dc0745 | 510 | |
6670e5e7 RGS |
511 | To declare a file-private variable, you still use a lexical variable. |
512 | A file is also a scope, so a lexical variable defined in the file | |
513 | cannot be seen from any other file. | |
68dc0745 | 514 | |
6670e5e7 RGS |
515 | See L<perlsub/"Persistent Private Variables"> for more information. |
516 | The discussion of closures in L<perlref> may help you even though we | |
517 | did not use anonymous subroutines in this answer. See | |
518 | L<perlsub/"Persistent Private Variables"> for details. | |
c8db1d39 | 519 | |
68dc0745 | 520 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
521 | ||
a6dd486b JB |
522 | C<local($x)> saves away the old value of the global variable C<$x> |
523 | and assigns a new value for the duration of the subroutine I<which is | |
68dc0745 | 524 | visible in other functions called from that subroutine>. This is done |
525 | at run-time, so is called dynamic scoping. local() always affects global | |
526 | variables, also called package variables or dynamic variables. | |
527 | ||
528 | C<my($x)> creates a new variable that is only visible in the current | |
a6dd486b | 529 | subroutine. This is done at compile-time, so it is called lexical or |
68dc0745 | 530 | static scoping. my() always affects private variables, also called |
531 | lexical variables or (improperly) static(ly scoped) variables. | |
532 | ||
533 | For instance: | |
534 | ||
ac003c96 RGS |
535 | sub visible { |
536 | print "var has value $var\n"; | |
537 | } | |
109f0441 | 538 | |
ac003c96 RGS |
539 | sub dynamic { |
540 | local $var = 'local'; # new temporary value for the still-global | |
541 | visible(); # variable called $var | |
542 | } | |
109f0441 | 543 | |
ac003c96 RGS |
544 | sub lexical { |
545 | my $var = 'private'; # new private variable, $var | |
546 | visible(); # (invisible outside of sub scope) | |
547 | } | |
109f0441 | 548 | |
ac003c96 | 549 | $var = 'global'; |
109f0441 | 550 | |
ac003c96 RGS |
551 | visible(); # prints global |
552 | dynamic(); # prints local | |
553 | lexical(); # prints global | |
68dc0745 | 554 | |
555 | Notice how at no point does the value "private" get printed. That's | |
556 | because $var only has that value within the block of the lexical() | |
4c7f4b6b | 557 | function, and it is hidden from the called subroutine. |
68dc0745 | 558 | |
559 | In summary, local() doesn't make what you think of as private, local | |
560 | variables. It gives a global variable a temporary value. my() is | |
561 | what you're looking for if you want private variables. | |
562 | ||
197aec24 | 563 | See L<perlsub/"Private Variables via my()"> and |
13a2d996 | 564 | L<perlsub/"Temporary Values via local()"> for excruciating details. |
68dc0745 | 565 | |
566 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? | |
567 | ||
49d635f9 RGS |
568 | If you know your package, you can just mention it explicitly, as in |
569 | $Some_Pack::var. Note that the notation $::var is B<not> the dynamic $var | |
570 | in the current package, but rather the one in the "main" package, as | |
571 | though you had written $main::var. | |
572 | ||
573 | use vars '$var'; | |
574 | local $var = "global"; | |
575 | my $var = "lexical"; | |
68dc0745 | 576 | |
49d635f9 RGS |
577 | print "lexical is $var\n"; |
578 | print "global is $main::var\n"; | |
68dc0745 | 579 | |
49d635f9 RGS |
580 | Alternatively you can use the compiler directive our() to bring a |
581 | dynamic variable into the current lexical scope. | |
68dc0745 | 582 | |
49d635f9 RGS |
583 | require 5.006; # our() did not exist before 5.6 |
584 | use vars '$var'; | |
68dc0745 | 585 | |
49d635f9 RGS |
586 | local $var = "global"; |
587 | my $var = "lexical"; | |
588 | ||
589 | print "lexical is $var\n"; | |
590 | ||
591 | { | |
ac003c96 RGS |
592 | our $var; |
593 | print "global is $var\n"; | |
49d635f9 | 594 | } |
68dc0745 | 595 | |
596 | =head2 What's the difference between deep and shallow binding? | |
597 | ||
598 | In deep binding, lexical variables mentioned in anonymous subroutines | |
599 | are the same ones that were in scope when the subroutine was created. | |
600 | In shallow binding, they are whichever variables with the same names | |
601 | happen to be in scope when the subroutine is called. Perl always uses | |
602 | deep binding of lexical variables (i.e., those created with my()). | |
603 | However, dynamic variables (aka global, local, or package variables) | |
604 | are effectively shallowly bound. Consider this just one more reason | |
605 | not to use them. See the answer to L<"What's a closure?">. | |
606 | ||
04d666b1 | 607 | =head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right? |
68dc0745 | 608 | |
c8db1d39 | 609 | C<my()> and C<local()> give list context to the right hand side |
c47ff5f1 | 610 | of C<=>. The <FH> read operation, like so many of Perl's |
c8db1d39 TC |
611 | functions and operators, can tell which context it was called in and |
612 | behaves appropriately. In general, the scalar() function can help. | |
613 | This function does nothing to the data itself (contrary to popular myth) | |
614 | but rather tells its argument to behave in whatever its scalar fashion is. | |
615 | If that function doesn't have a defined scalar behavior, this of course | |
616 | doesn't help you (such as with sort()). | |
68dc0745 | 617 | |
618 | To enforce scalar context in this particular case, however, you need | |
619 | merely omit the parentheses: | |
620 | ||
ac003c96 RGS |
621 | local($foo) = <FILE>; # WRONG |
622 | local($foo) = scalar(<FILE>); # ok | |
623 | local $foo = <FILE>; # right | |
68dc0745 | 624 | |
625 | You should probably be using lexical variables anyway, although the | |
626 | issue is the same here: | |
627 | ||
ac003c96 RGS |
628 | my($foo) = <FILE>; # WRONG |
629 | my $foo = <FILE>; # right | |
68dc0745 | 630 | |
54310121 | 631 | =head2 How do I redefine a builtin function, operator, or method? |
68dc0745 | 632 | |
633 | Why do you want to do that? :-) | |
634 | ||
635 | If you want to override a predefined function, such as open(), | |
636 | then you'll have to import the new definition from a different | |
96090e4f | 637 | module. See L<perlsub/"Overriding Built-in Functions">. |
68dc0745 | 638 | |
639 | If you want to overload a Perl operator, such as C<+> or C<**>, | |
640 | then you'll want to use the C<use overload> pragma, documented | |
641 | in L<overload>. | |
642 | ||
643 | If you're talking about obscuring method calls in parent classes, | |
644 | see L<perltoot/"Overridden Methods">. | |
645 | ||
646 | =head2 What's the difference between calling a function as &foo and foo()? | |
647 | ||
109f0441 S |
648 | (contributed by brian d foy) |
649 | ||
650 | Calling a subroutine as C<&foo> with no trailing parentheses ignores | |
589a5df2 | 651 | the prototype of C<foo> and passes it the current value of the argument |
109f0441 | 652 | list, C<@_>. Here's an example; the C<bar> subroutine calls C<&foo>, |
23bec515 | 653 | which prints its arguments list: |
109f0441 S |
654 | |
655 | sub bar { &foo } | |
656 | ||
657 | sub foo { print "Args in foo are: @_\n" } | |
658 | ||
659 | bar( qw( a b c ) ); | |
660 | ||
661 | When you call C<bar> with arguments, you see that C<foo> got the same C<@_>: | |
68dc0745 | 662 | |
109f0441 | 663 | Args in foo are: a b c |
68dc0745 | 664 | |
109f0441 S |
665 | Calling the subroutine with trailing parentheses, with or without arguments, |
666 | does not use the current C<@_> and respects the subroutine prototype. Changing | |
667 | the example to put parentheses after the call to C<foo> changes the program: | |
668 | ||
669 | sub bar { &foo() } | |
670 | ||
671 | sub foo { print "Args in foo are: @_\n" } | |
672 | ||
673 | bar( qw( a b c ) ); | |
674 | ||
675 | Now the output shows that C<foo> doesn't get the C<@_> from its caller. | |
676 | ||
677 | Args in foo are: | |
678 | ||
679 | The main use of the C<@_> pass-through feature is to write subroutines | |
680 | whose main job it is to call other subroutines for you. For further | |
681 | details, see L<perlsub>. | |
68dc0745 | 682 | |
683 | =head2 How do I create a switch or case statement? | |
684 | ||
109f0441 S |
685 | In Perl 5.10, use the C<given-when> construct described in L<perlsyn>: |
686 | ||
687 | use 5.010; | |
688 | ||
689 | given ( $string ) { | |
690 | when( 'Fred' ) { say "I found Fred!" } | |
691 | when( 'Barney' ) { say "I found Barney!" } | |
692 | when( /Bamm-?Bamm/ ) { say "I found Bamm-Bamm!" } | |
693 | default { say "I don't recognize the name!" } | |
694 | }; | |
695 | ||
f449fe8a | 696 | If one wants to use pure Perl and to be compatible with Perl versions |
109f0441 | 697 | prior to 5.10, the general answer is to use C<if-elsif-else>: |
c8db1d39 | 698 | |
ac003c96 RGS |
699 | for ($variable_to_test) { |
700 | if (/pat1/) { } # do something | |
701 | elsif (/pat2/) { } # do something else | |
702 | elsif (/pat3/) { } # do something else | |
703 | else { } # default | |
704 | } | |
68dc0745 | 705 | |
f449fe8a RGS |
706 | Here's a simple example of a switch based on pattern matching, |
707 | lined up in a way to make it look more like a switch statement. | |
8305e449 | 708 | We'll do a multiway conditional based on the type of reference stored |
c8db1d39 TC |
709 | in $whatchamacallit: |
710 | ||
711 | SWITCH: for (ref $whatchamacallit) { | |
68dc0745 | 712 | |
713 | /^$/ && die "not a reference"; | |
714 | ||
715 | /SCALAR/ && do { | |
716 | print_scalar($$ref); | |
717 | last SWITCH; | |
718 | }; | |
719 | ||
720 | /ARRAY/ && do { | |
721 | print_array(@$ref); | |
722 | last SWITCH; | |
723 | }; | |
724 | ||
725 | /HASH/ && do { | |
726 | print_hash(%$ref); | |
727 | last SWITCH; | |
728 | }; | |
729 | ||
730 | /CODE/ && do { | |
731 | warn "can't print function ref"; | |
732 | last SWITCH; | |
733 | }; | |
734 | ||
735 | # DEFAULT | |
736 | ||
737 | warn "User defined type skipped"; | |
738 | ||
739 | } | |
740 | ||
f449fe8a | 741 | See L<perlsyn> for other examples in this style. |
c8db1d39 TC |
742 | |
743 | Sometimes you should change the positions of the constant and the variable. | |
744 | For example, let's say you wanted to test which of many answers you were | |
745 | given, but in a case-insensitive way that also allows abbreviations. | |
746 | You can use the following technique if the strings all start with | |
a6dd486b | 747 | different characters or if you want to arrange the matches so that |
c8db1d39 TC |
748 | one takes precedence over another, as C<"SEND"> has precedence over |
749 | C<"STOP"> here: | |
750 | ||
ac003c96 RGS |
751 | chomp($answer = <>); |
752 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } | |
753 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } | |
754 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } | |
755 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } | |
756 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } | |
c8db1d39 | 757 | |
197aec24 | 758 | A totally different approach is to create a hash of function references. |
c8db1d39 | 759 | |
ac003c96 RGS |
760 | my %commands = ( |
761 | "happy" => \&joy, | |
762 | "sad", => \&sullen, | |
763 | "done" => sub { die "See ya!" }, | |
764 | "mad" => \&angry, | |
765 | ); | |
109f0441 | 766 | |
ac003c96 RGS |
767 | print "How are you? "; |
768 | chomp($string = <STDIN>); | |
769 | if ($commands{$string}) { | |
770 | $commands{$string}->(); | |
771 | } else { | |
772 | print "No such command: $string\n"; | |
773 | } | |
c8db1d39 | 774 | |
f449fe8a RGS |
775 | Starting from Perl 5.8, a source filter module, C<Switch>, can also be |
776 | used to get switch and case. Its use is now discouraged, because it's | |
777 | not fully compatible with the native switch of Perl 5.10, and because, | |
778 | as it's implemented as a source filter, it doesn't always work as intended | |
779 | when complex syntax is involved. | |
780 | ||
49d635f9 | 781 | =head2 How can I catch accesses to undefined variables, functions, or methods? |
68dc0745 | 782 | |
783 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and | |
784 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to | |
785 | undefined functions and methods. | |
786 | ||
787 | When it comes to undefined variables that would trigger a warning | |
49d635f9 | 788 | under C<use warnings>, you can promote the warning to an error. |
68dc0745 | 789 | |
49d635f9 | 790 | use warnings FATAL => qw(uninitialized); |
68dc0745 | 791 | |
792 | =head2 Why can't a method included in this same file be found? | |
793 | ||
794 | Some possible reasons: your inheritance is getting confused, you've | |
795 | misspelled the method name, or the object is of the wrong type. Check | |
a6dd486b JB |
796 | out L<perltoot> for details about any of the above cases. You may |
797 | also use C<print ref($object)> to find out the class C<$object> was | |
798 | blessed into. | |
68dc0745 | 799 | |
4c7f4b6b | 800 | Another possible reason for problems is that you've used the |
68dc0745 | 801 | indirect object syntax (eg, C<find Guru "Samy">) on a class name |
802 | before Perl has seen that such a package exists. It's wisest to make | |
803 | sure your packages are all defined before you start using them, which | |
804 | will be taken care of if you use the C<use> statement instead of | |
a6dd486b | 805 | C<require>. If not, make sure to use arrow notation (eg., |
c47ff5f1 | 806 | C<< Guru->find("Samy") >>) instead. Object notation is explained in |
68dc0745 | 807 | L<perlobj>. |
808 | ||
c8db1d39 | 809 | Make sure to read about creating modules in L<perlmod> and |
ae93639c | 810 | the perils of indirect objects in L<perlobj/"Method Invocation">. |
c8db1d39 | 811 | |
109f0441 | 812 | =head2 How can I find out my current or calling package? |
68dc0745 | 813 | |
109f0441 | 814 | (contributed by brian d foy) |
68dc0745 | 815 | |
109f0441 S |
816 | To find the package you are currently in, use the special literal |
817 | C<__PACKAGE__>, as documented in L<perldata>. You can only use the | |
818 | special literals as separate tokens, so you can't interpolate them | |
819 | into strings like you can with variables: | |
68dc0745 | 820 | |
109f0441 S |
821 | my $current_package = __PACKAGE__; |
822 | print "I am in package $current_package\n"; | |
68dc0745 | 823 | |
109f0441 S |
824 | If you want to find the package calling your code, perhaps to give better |
825 | diagnostics as C<Carp> does, use the C<caller> built-in: | |
826 | ||
827 | sub foo { | |
828 | my @args = ...; | |
829 | my( $package, $filename, $line ) = caller; | |
830 | ||
831 | print "I was called from package $package\n"; | |
832 | ); | |
833 | ||
9b03c0a2 FC |
834 | By default, your program starts in package C<main>, so you will |
835 | always be in some package. | |
109f0441 | 836 | |
d12d61cf | 837 | This is different from finding out the package an object is blessed |
838 | into, which might not be the current package. For that, use C<blessed> | |
839 | from C<Scalar::Util>, part of the Standard Library since Perl 5.8: | |
840 | ||
841 | use Scalar::Util qw(blessed); | |
842 | my $object_package = blessed( $object ); | |
843 | ||
844 | Most of the time, you shouldn't care what package an object is blessed | |
845 | into, however, as long as it claims to inherit from that class: | |
846 | ||
847 | my $is_right_class = eval { $object->isa( $package ) }; # true or false | |
848 | ||
849 | And, with Perl 5.10 and later, you don't have to check for an | |
850 | inheritance to see if the object can handle a role. For that, you can | |
851 | use C<DOES>, which comes from C<UNIVERSAL>: | |
852 | ||
853 | my $class_does_it = eval { $object->DOES( $role ) }; # true or false | |
854 | ||
855 | You can safely replace C<isa> with C<DOES> (although the converse is not true). | |
856 | ||
109f0441 S |
857 | =head2 How can I comment out a large block of Perl code? |
858 | ||
859 | (contributed by brian d foy) | |
46fc3d4c | 860 | |
109f0441 S |
861 | The quick-and-dirty way to comment out more than one line of Perl is |
862 | to surround those lines with Pod directives. You have to put these | |
863 | directives at the beginning of the line and somewhere where Perl | |
864 | expects a new statement (so not in the middle of statements like the # | |
865 | comments). You end the comment with C<=cut>, ending the Pod section: | |
866 | ||
867 | =pod | |
868 | ||
869 | my $object = NotGonnaHappen->new(); | |
870 | ||
871 | ignored_sub(); | |
872 | ||
873 | $wont_be_assigned = 37; | |
874 | ||
875 | =cut | |
876 | ||
d12d61cf | 877 | The quick-and-dirty method only works well when you don't plan to |
109f0441 S |
878 | leave the commented code in the source. If a Pod parser comes along, |
879 | you're multiline comment is going to show up in the Pod translation. | |
d12d61cf | 880 | A better way hides it from Pod parsers as well. |
109f0441 S |
881 | |
882 | The C<=begin> directive can mark a section for a particular purpose. | |
883 | If the Pod parser doesn't want to handle it, it just ignores it. Label | |
884 | the comments with C<comment>. End the comment using C<=end> with the | |
885 | same label. You still need the C<=cut> to go back to Perl code from | |
886 | the Pod comment: | |
46fc3d4c | 887 | |
ac003c96 | 888 | =begin comment |
109f0441 S |
889 | |
890 | my $object = NotGonnaHappen->new(); | |
891 | ||
892 | ignored_sub(); | |
893 | ||
894 | $wont_be_assigned = 37; | |
895 | ||
7678cced | 896 | =end comment |
46fc3d4c | 897 | |
109f0441 | 898 | =cut |
fc36a67e | 899 | |
109f0441 | 900 | For more information on Pod, check out L<perlpod> and L<perlpodspec>. |
c8db1d39 | 901 | |
65acb1b1 TC |
902 | =head2 How do I clear a package? |
903 | ||
904 | Use this code, provided by Mark-Jason Dominus: | |
905 | ||
ac003c96 RGS |
906 | sub scrub_package { |
907 | no strict 'refs'; | |
908 | my $pack = shift; | |
909 | die "Shouldn't delete main package" | |
910 | if $pack eq "" || $pack eq "main"; | |
911 | my $stash = *{$pack . '::'}{HASH}; | |
912 | my $name; | |
913 | foreach $name (keys %$stash) { | |
914 | my $fullname = $pack . '::' . $name; | |
915 | # Get rid of everything with that name. | |
916 | undef $$fullname; | |
917 | undef @$fullname; | |
918 | undef %$fullname; | |
919 | undef &$fullname; | |
920 | undef *$fullname; | |
921 | } | |
65acb1b1 | 922 | } |
65acb1b1 | 923 | |
197aec24 | 924 | Or, if you're using a recent release of Perl, you can |
65acb1b1 TC |
925 | just use the Symbol::delete_package() function instead. |
926 | ||
d92eb7b0 GS |
927 | =head2 How can I use a variable as a variable name? |
928 | ||
929 | Beginners often think they want to have a variable contain the name | |
930 | of a variable. | |
931 | ||
ac003c96 RGS |
932 | $fred = 23; |
933 | $varname = "fred"; | |
934 | ++$$varname; # $fred now 24 | |
d92eb7b0 GS |
935 | |
936 | This works I<sometimes>, but it is a very bad idea for two reasons. | |
937 | ||
a6dd486b JB |
938 | The first reason is that this technique I<only works on global |
939 | variables>. That means that if $fred is a lexical variable created | |
940 | with my() in the above example, the code wouldn't work at all: you'd | |
941 | accidentally access the global and skip right over the private lexical | |
942 | altogether. Global variables are bad because they can easily collide | |
943 | accidentally and in general make for non-scalable and confusing code. | |
d92eb7b0 GS |
944 | |
945 | Symbolic references are forbidden under the C<use strict> pragma. | |
4c7f4b6b FC |
946 | They are not true references and consequently are not reference-counted |
947 | or garbage-collected. | |
d92eb7b0 GS |
948 | |
949 | The other reason why using a variable to hold the name of another | |
a6dd486b | 950 | variable is a bad idea is that the question often stems from a lack of |
d92eb7b0 GS |
951 | understanding of Perl data structures, particularly hashes. By using |
952 | symbolic references, you are just using the package's symbol-table hash | |
953 | (like C<%main::>) instead of a user-defined hash. The solution is to | |
954 | use your own hash or a real reference instead. | |
955 | ||
ac003c96 RGS |
956 | $USER_VARS{"fred"} = 23; |
957 | $varname = "fred"; | |
958 | $USER_VARS{$varname}++; # not $$varname++ | |
d92eb7b0 GS |
959 | |
960 | There we're using the %USER_VARS hash instead of symbolic references. | |
961 | Sometimes this comes up in reading strings from the user with variable | |
962 | references and wanting to expand them to the values of your perl | |
963 | program's variables. This is also a bad idea because it conflates the | |
964 | program-addressable namespace and the user-addressable one. Instead of | |
965 | reading a string and expanding it to the actual contents of your program's | |
966 | own variables: | |
967 | ||
ac003c96 RGS |
968 | $str = 'this has a $fred and $barney in it'; |
969 | $str =~ s/(\$\w+)/$1/eeg; # need double eval | |
d92eb7b0 | 970 | |
a6dd486b | 971 | it would be better to keep a hash around like %USER_VARS and have |
d92eb7b0 GS |
972 | variable references actually refer to entries in that hash: |
973 | ||
ac003c96 | 974 | $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all |
d92eb7b0 GS |
975 | |
976 | That's faster, cleaner, and safer than the previous approach. Of course, | |
977 | you don't need to use a dollar sign. You could use your own scheme to | |
978 | make it less confusing, like bracketed percent symbols, etc. | |
979 | ||
ac003c96 RGS |
980 | $str = 'this has a %fred% and %barney% in it'; |
981 | $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all | |
d92eb7b0 | 982 | |
a6dd486b | 983 | Another reason that folks sometimes think they want a variable to |
4c7f4b6b | 984 | contain the name of a variable is that they don't know how to build |
a6dd486b JB |
985 | proper data structures using hashes. For example, let's say they |
986 | wanted two hashes in their program: %fred and %barney, and that they | |
987 | wanted to use another scalar variable to refer to those by name. | |
d92eb7b0 | 988 | |
ac003c96 RGS |
989 | $name = "fred"; |
990 | $$name{WIFE} = "wilma"; # set %fred | |
d92eb7b0 | 991 | |
ac003c96 RGS |
992 | $name = "barney"; |
993 | $$name{WIFE} = "betty"; # set %barney | |
d92eb7b0 GS |
994 | |
995 | This is still a symbolic reference, and is still saddled with the | |
996 | problems enumerated above. It would be far better to write: | |
997 | ||
ac003c96 RGS |
998 | $folks{"fred"}{WIFE} = "wilma"; |
999 | $folks{"barney"}{WIFE} = "betty"; | |
d92eb7b0 GS |
1000 | |
1001 | And just use a multilevel hash to start with. | |
1002 | ||
1003 | The only times that you absolutely I<must> use symbolic references are | |
1004 | when you really must refer to the symbol table. This may be because it's | |
4c7f4b6b | 1005 | something that one can't take a real reference to, such as a format name. |
d92eb7b0 GS |
1006 | Doing so may also be important for method calls, since these always go |
1007 | through the symbol table for resolution. | |
1008 | ||
1009 | In those cases, you would turn off C<strict 'refs'> temporarily so you | |
1010 | can play around with the symbol table. For example: | |
1011 | ||
ac003c96 RGS |
1012 | @colors = qw(red blue green yellow orange purple violet); |
1013 | for my $name (@colors) { | |
1014 | no strict 'refs'; # renege for the block | |
1015 | *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; | |
1016 | } | |
d92eb7b0 GS |
1017 | |
1018 | All those functions (red(), blue(), green(), etc.) appear to be separate, | |
1019 | but the real code in the closure actually was compiled only once. | |
1020 | ||
4c7f4b6b FC |
1021 | So, sometimes you might want to use symbolic references to manipulate |
1022 | the symbol table directly. This doesn't matter for formats, handles, and | |
a6dd486b JB |
1023 | subroutines, because they are always global--you can't use my() on them. |
1024 | For scalars, arrays, and hashes, though--and usually for subroutines-- | |
1025 | you probably only want to use hard references. | |
d92eb7b0 | 1026 | |
5cd0b561 RGS |
1027 | =head2 What does "bad interpreter" mean? |
1028 | ||
571e049f RGS |
1029 | (contributed by brian d foy) |
1030 | ||
5cd0b561 RGS |
1031 | The "bad interpreter" message comes from the shell, not perl. The |
1032 | actual message may vary depending on your platform, shell, and locale | |
1033 | settings. | |
1034 | ||
1035 | If you see "bad interpreter - no such file or directory", the first | |
1036 | line in your perl script (the "shebang" line) does not contain the | |
6670e5e7 | 1037 | right path to perl (or any other program capable of running scripts). |
5cd0b561 | 1038 | Sometimes this happens when you move the script from one machine to |
ac9dac7f | 1039 | another and each machine has a different path to perl--/usr/bin/perl |
571e049f | 1040 | versus /usr/local/bin/perl for instance. It may also indicate |
6670e5e7 RGS |
1041 | that the source machine has CRLF line terminators and the |
1042 | destination machine has LF only: the shell tries to find | |
571e049f | 1043 | /usr/bin/perl<CR>, but can't. |
5cd0b561 RGS |
1044 | |
1045 | If you see "bad interpreter: Permission denied", you need to make your | |
1046 | script executable. | |
1047 | ||
1048 | In either case, you should still be able to run the scripts with perl | |
1049 | explicitly: | |
1050 | ||
1051 | % perl script.pl | |
1052 | ||
1053 | If you get a message like "perl: command not found", perl is not in | |
1054 | your PATH, which might also mean that the location of perl is not | |
1055 | where you expect it so you need to adjust your shebang line. | |
1056 | ||
68dc0745 | 1057 | =head1 AUTHOR AND COPYRIGHT |
1058 | ||
8d2e243f | 1059 | Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and |
7678cced | 1060 | other authors as noted. All rights reserved. |
5a964f20 | 1061 | |
5a7beb56 JH |
1062 | This documentation is free; you can redistribute it and/or modify it |
1063 | under the same terms as Perl itself. | |
5a964f20 TC |
1064 | |
1065 | Irrespective of its distribution, all code examples in this file | |
1066 | are hereby placed into the public domain. You are permitted and | |
1067 | encouraged to use this code in your own programs for fun | |
1068 | or for profit as you see fit. A simple comment in the code giving | |
1069 | credit would be courteous but is not required. | |
a6dd486b | 1070 |