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1 | =head1 NAME |
2 | |
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3 | perlfaq7 - General Perl Language Issues ($Revision: 9309 $) |
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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 | |
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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." |
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20 | |
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21 | =head2 What are all these $@%&* punctuation signs, and how do I know when to use them? |
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22 | |
23 | They are type specifiers, as detailed in L<perldata>: |
24 | |
25 | $ for scalar values (number, string or reference) |
26 | @ for arrays |
27 | % for hashes (associative arrays) |
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28 | & for subroutines (aka functions, procedures, methods) |
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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. |
31 | |
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32 | There are couple of other symbols that you're likely to encounter that aren't |
33 | really type specifiers: |
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34 | |
35 | <> are used for inputting a record from a filehandle. |
36 | \ takes a reference to something. |
37 | |
c47ff5f1 |
38 | Note that <FILE> is I<neither> the type specifier for files |
39 | nor the name of the handle. It is the C<< <> >> operator applied |
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40 | to the handle FILE. It reads one line (well, record--see |
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41 | L<perlvar/$E<sol>>) from the handle FILE in scalar context, or I<all> lines |
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42 | in list context. When performing open, close, or any other operation |
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43 | besides C<< <> >> on files, or even when talking about the handle, do |
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44 | I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0, |
45 | 2)> and "copying from STDIN to FILE". |
46 | |
47 | =head2 Do I always/never have to quote my strings or use semicolons and commas? |
48 | |
49 | Normally, a bareword doesn't need to be quoted, but in most cases |
50 | probably should be (and must be under C<use strict>). But a hash key |
51 | consisting of a simple word (that isn't the name of a defined |
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52 | subroutine) and the left-hand operand to the C<< => >> operator both |
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53 | count as though they were quoted: |
54 | |
55 | This is like this |
56 | ------------ --------------- |
571e049f |
57 | $foo{line} $foo{'line'} |
58 | bar => stuff 'bar' => stuff |
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59 | |
60 | The final semicolon in a block is optional, as is the final comma in a |
61 | list. Good style (see L<perlstyle>) says to put them in except for |
62 | one-liners: |
63 | |
64 | if ($whoops) { exit 1 } |
65 | @nums = (1, 2, 3); |
66 | |
67 | if ($whoops) { |
68 | exit 1; |
69 | } |
70 | @lines = ( |
71 | "There Beren came from mountains cold", |
72 | "And lost he wandered under leaves", |
73 | ); |
74 | |
75 | =head2 How do I skip some return values? |
76 | |
77 | One way is to treat the return values as a list and index into it: |
78 | |
79 | $dir = (getpwnam($user))[7]; |
80 | |
81 | Another way is to use undef as an element on the left-hand-side: |
82 | |
83 | ($dev, $ino, undef, undef, $uid, $gid) = stat($file); |
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84 | |
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85 | You can also use a list slice to select only the elements that |
86 | you need: |
87 | |
88 | ($dev, $ino, $uid, $gid) = ( stat($file) )[0,1,4,5]; |
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89 | |
90 | =head2 How do I temporarily block warnings? |
91 | |
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92 | If you are running Perl 5.6.0 or better, the C<use warnings> pragma |
93 | allows fine control of what warning are produced. |
94 | See L<perllexwarn> for more details. |
95 | |
96 | { |
97 | no warnings; # temporarily turn off warnings |
98 | $a = $b + $c; # I know these might be undef |
99 | } |
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100 | |
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101 | Additionally, you can enable and disable categories of warnings. |
102 | You turn off the categories you want to ignore and you can still |
103 | get other categories of warnings. See L<perllexwarn> for the |
104 | complete details, including the category names and hierarchy. |
105 | |
106 | { |
107 | no warnings 'uninitialized'; |
108 | $a = $b + $c; |
109 | } |
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110 | |
111 | If you have an older version of Perl, the C<$^W> variable (documented |
112 | in L<perlvar>) controls runtime warnings for a block: |
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113 | |
114 | { |
115 | local $^W = 0; # temporarily turn off warnings |
116 | $a = $b + $c; # I know these might be undef |
117 | } |
118 | |
119 | Note that like all the punctuation variables, you cannot currently |
120 | use my() on C<$^W>, only local(). |
121 | |
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122 | =head2 What's an extension? |
123 | |
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124 | An extension is a way of calling compiled C code from Perl. Reading |
125 | L<perlxstut> is a good place to learn more about extensions. |
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126 | |
127 | =head2 Why do Perl operators have different precedence than C operators? |
128 | |
129 | Actually, they don't. All C operators that Perl copies have the same |
130 | precedence in Perl as they do in C. The problem is with operators that C |
131 | doesn't have, especially functions that give a list context to everything |
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132 | on their right, eg. print, chmod, exec, and so on. Such functions are |
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133 | called "list operators" and appear as such in the precedence table in |
134 | L<perlop>. |
135 | |
136 | A common mistake is to write: |
137 | |
138 | unlink $file || die "snafu"; |
139 | |
140 | This gets interpreted as: |
141 | |
142 | unlink ($file || die "snafu"); |
143 | |
144 | To avoid this problem, either put in extra parentheses or use the |
145 | super low precedence C<or> operator: |
146 | |
147 | (unlink $file) || die "snafu"; |
148 | unlink $file or die "snafu"; |
149 | |
150 | The "English" operators (C<and>, C<or>, C<xor>, and C<not>) |
151 | deliberately have precedence lower than that of list operators for |
152 | just such situations as the one above. |
153 | |
154 | Another operator with surprising precedence is exponentiation. It |
155 | binds more tightly even than unary minus, making C<-2**2> product a |
156 | negative not a positive four. It is also right-associating, meaning |
157 | that C<2**3**2> is two raised to the ninth power, not eight squared. |
158 | |
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159 | Although it has the same precedence as in C, Perl's C<?:> operator |
160 | produces an lvalue. This assigns $x to either $a or $b, depending |
161 | on the trueness of $maybe: |
162 | |
163 | ($maybe ? $a : $b) = $x; |
164 | |
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165 | =head2 How do I declare/create a structure? |
166 | |
167 | In general, you don't "declare" a structure. Just use a (probably |
168 | anonymous) hash reference. See L<perlref> and L<perldsc> for details. |
169 | Here's an example: |
170 | |
171 | $person = {}; # new anonymous hash |
172 | $person->{AGE} = 24; # set field AGE to 24 |
173 | $person->{NAME} = "Nat"; # set field NAME to "Nat" |
174 | |
175 | If you're looking for something a bit more rigorous, try L<perltoot>. |
176 | |
177 | =head2 How do I create a module? |
178 | |
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179 | (contributed by brian d foy) |
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180 | |
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181 | L<perlmod>, L<perlmodlib>, L<perlmodstyle> explain modules |
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182 | in all the gory details. L<perlnewmod> gives a brief |
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183 | overview of the process along with a couple of suggestions |
184 | about style. |
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185 | |
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186 | If you need to include C code or C library interfaces in |
187 | your module, you'll need h2xs. h2xs will create the module |
188 | distribution structure and the initial interface files |
189 | you'll need. L<perlxs> and L<perlxstut> explain the details. |
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190 | |
7678cced |
191 | If you don't need to use C code, other tools such as |
192 | ExtUtils::ModuleMaker and Module::Starter, can help you |
193 | create a skeleton module distribution. |
194 | |
195 | You may also want to see Sam Tregar's "Writing Perl Modules |
196 | for CPAN" ( http://apress.com/book/bookDisplay.html?bID=14 ) |
197 | which is the best hands-on guide to creating module |
198 | distributions. |
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199 | |
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200 | =head2 How do I create a class? |
201 | |
202 | See L<perltoot> for an introduction to classes and objects, as well as |
203 | L<perlobj> and L<perlbot>. |
204 | |
205 | =head2 How can I tell if a variable is tainted? |
206 | |
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207 | You can use the tainted() function of the Scalar::Util module, available |
208 | from CPAN (or included with Perl since release 5.8.0). |
209 | See also L<perlsec/"Laundering and Detecting Tainted Data">. |
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210 | |
211 | =head2 What's a closure? |
212 | |
213 | Closures are documented in L<perlref>. |
214 | |
215 | I<Closure> is a computer science term with a precise but |
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216 | hard-to-explain meaning. Usually, closures are implemented in Perl as |
217 | anonymous subroutines with lasting references to lexical variables |
218 | outside their own scopes. These lexicals magically refer to the |
219 | variables that were around when the subroutine was defined (deep |
220 | binding). |
221 | |
222 | Closures are most often used in programming languages where you can |
223 | have the return value of a function be itself a function, as you can |
224 | in Perl. Note that some languages provide anonymous functions but are |
225 | not capable of providing proper closures: the Python language, for |
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226 | example. For more information on closures, check out any textbook on |
227 | functional programming. Scheme is a language that not only supports |
228 | but encourages closures. |
229 | |
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230 | Here's a classic non-closure function-generating function: |
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231 | |
232 | sub add_function_generator { |
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233 | return sub { shift() + shift() }; |
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234 | } |
235 | |
236 | $add_sub = add_function_generator(); |
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237 | $sum = $add_sub->(4,5); # $sum is 9 now. |
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238 | |
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239 | The anonymous subroutine returned by add_function_generator() isn't |
240 | technically a closure because it refers to no lexicals outside its own |
241 | scope. Using a closure gives you a I<function template> with some |
242 | customization slots left out to be filled later. |
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243 | |
244 | Contrast this with the following make_adder() function, in which the |
245 | returned anonymous function contains a reference to a lexical variable |
246 | outside the scope of that function itself. Such a reference requires |
247 | that Perl return a proper closure, thus locking in for all time the |
248 | value that the lexical had when the function was created. |
249 | |
250 | sub make_adder { |
251 | my $addpiece = shift; |
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252 | return sub { shift() + $addpiece }; |
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253 | } |
254 | |
255 | $f1 = make_adder(20); |
256 | $f2 = make_adder(555); |
257 | |
258 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas |
259 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece |
260 | in the closure sticks around. |
261 | |
262 | Closures are often used for less esoteric purposes. For example, when |
263 | you want to pass in a bit of code into a function: |
264 | |
265 | my $line; |
266 | timeout( 30, sub { $line = <STDIN> } ); |
267 | |
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268 | If the code to execute had been passed in as a string, |
269 | C<< '$line = <STDIN>' >>, there would have been no way for the |
270 | hypothetical timeout() function to access the lexical variable |
271 | $line back in its caller's scope. |
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272 | |
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273 | Another use for a closure is to make a variable I<private> to a |
274 | named subroutine, e.g. a counter that gets initialized at creation |
275 | time of the sub and can only be modified from within the sub. |
276 | This is sometimes used with a BEGIN block in package files to make |
277 | sure a variable doesn't get meddled with during the lifetime of the |
278 | package: |
279 | |
280 | BEGIN { |
281 | my $id = 0; |
282 | sub next_id { ++$id } |
283 | } |
284 | |
285 | This is discussed in more detail in L<perlsub>, see the entry on |
286 | I<Persistent Private Variables>. |
287 | |
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288 | =head2 What is variable suicide and how can I prevent it? |
289 | |
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290 | This problem was fixed in perl 5.004_05, so preventing it means upgrading |
291 | your version of perl. ;) |
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292 | |
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293 | Variable suicide is when you (temporarily or permanently) lose the value |
294 | of a variable. It is caused by scoping through my() and local() |
295 | interacting with either closures or aliased foreach() iterator variables |
296 | and subroutine arguments. It used to be easy to inadvertently lose a |
297 | variable's value this way, but now it's much harder. Take this code: |
298 | |
299 | my $f = 'foo'; |
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300 | sub T { |
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301 | while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } |
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302 | } |
303 | T; |
304 | print "Finally $f\n"; |
305 | |
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306 | If you are experiencing variable suicide, that C<my $f> in the subroutine |
307 | doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output |
308 | shows that inside the subroutine the value of C<$f> leaks through when it |
309 | shouldn't, as in this output: |
310 | |
311 | foobar |
312 | foobarbar |
313 | foobarbarbar |
314 | Finally foo |
315 | |
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316 | The $f that has "bar" added to it three times should be a new C<$f> |
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317 | C<my $f> should create a new lexical variable each time through the loop. |
318 | The expected output is: |
319 | |
320 | foobar |
321 | foobar |
322 | foobar |
323 | Finally foo |
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324 | |
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325 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? |
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326 | |
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327 | With the exception of regexes, you need to pass references to these |
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328 | objects. See L<perlsub/"Pass by Reference"> for this particular |
329 | question, and L<perlref> for information on references. |
330 | |
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331 | See "Passing Regexes", later in L<perlfaq7>, for information on |
332 | passing regular expressions. |
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333 | |
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334 | =over 4 |
335 | |
336 | =item Passing Variables and Functions |
337 | |
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338 | Regular variables and functions are quite easy to pass: just pass in a |
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339 | reference to an existing or anonymous variable or function: |
340 | |
341 | func( \$some_scalar ); |
342 | |
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343 | func( \@some_array ); |
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344 | func( [ 1 .. 10 ] ); |
345 | |
346 | func( \%some_hash ); |
347 | func( { this => 10, that => 20 } ); |
348 | |
349 | func( \&some_func ); |
350 | func( sub { $_[0] ** $_[1] } ); |
351 | |
352 | =item Passing Filehandles |
353 | |
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354 | As of Perl 5.6, you can represent filehandles with scalar variables |
355 | which you treat as any other scalar. |
356 | |
357 | open my $fh, $filename or die "Cannot open $filename! $!"; |
358 | func( $fh ); |
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359 | |
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360 | sub func { |
361 | my $passed_fh = shift; |
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362 | |
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363 | my $line = <$fh>; |
364 | } |
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365 | |
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366 | Before Perl 5.6, you had to use the C<*FH> or C<\*FH> notations. |
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367 | These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles"> |
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368 | and especially L<perlsub/"Pass by Reference"> for more information. |
369 | |
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370 | =item Passing Regexes |
371 | |
372 | To pass regexes around, you'll need to be using a release of Perl |
373 | sufficiently recent as to support the C<qr//> construct, pass around |
374 | strings and use an exception-trapping eval, or else be very, very clever. |
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375 | |
d92eb7b0 |
376 | Here's an example of how to pass in a string to be regex compared |
377 | using C<qr//>: |
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378 | |
379 | sub compare($$) { |
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380 | my ($val1, $regex) = @_; |
381 | my $retval = $val1 =~ /$regex/; |
382 | return $retval; |
383 | } |
384 | $match = compare("old McDonald", qr/d.*D/i); |
385 | |
386 | Notice how C<qr//> allows flags at the end. That pattern was compiled |
387 | at compile time, although it was executed later. The nifty C<qr//> |
388 | notation wasn't introduced until the 5.005 release. Before that, you |
389 | had to approach this problem much less intuitively. For example, here |
390 | it is again if you don't have C<qr//>: |
391 | |
392 | sub compare($$) { |
393 | my ($val1, $regex) = @_; |
394 | my $retval = eval { $val1 =~ /$regex/ }; |
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395 | die if $@; |
396 | return $retval; |
397 | } |
398 | |
d92eb7b0 |
399 | $match = compare("old McDonald", q/($?i)d.*D/); |
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400 | |
401 | Make sure you never say something like this: |
402 | |
d92eb7b0 |
403 | return eval "\$val =~ /$regex/"; # WRONG |
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404 | |
d92eb7b0 |
405 | or someone can sneak shell escapes into the regex due to the double |
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406 | interpolation of the eval and the double-quoted string. For example: |
407 | |
408 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; |
409 | |
410 | eval "\$string =~ /$pattern_of_evil/"; |
411 | |
412 | Those preferring to be very, very clever might see the O'Reilly book, |
413 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's |
414 | Build_MatchMany_Function() is particularly interesting. A complete |
415 | citation of this book is given in L<perlfaq2>. |
416 | |
417 | =item Passing Methods |
418 | |
419 | To pass an object method into a subroutine, you can do this: |
420 | |
421 | call_a_lot(10, $some_obj, "methname") |
422 | sub call_a_lot { |
423 | my ($count, $widget, $trick) = @_; |
424 | for (my $i = 0; $i < $count; $i++) { |
425 | $widget->$trick(); |
426 | } |
427 | } |
428 | |
a6dd486b |
429 | Or, you can use a closure to bundle up the object, its |
430 | method call, and arguments: |
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431 | |
432 | my $whatnot = sub { $some_obj->obfuscate(@args) }; |
433 | func($whatnot); |
434 | sub func { |
435 | my $code = shift; |
436 | &$code(); |
437 | } |
438 | |
439 | You could also investigate the can() method in the UNIVERSAL class |
440 | (part of the standard perl distribution). |
441 | |
442 | =back |
443 | |
444 | =head2 How do I create a static variable? |
445 | |
6670e5e7 |
446 | (contributed by brian d foy) |
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447 | |
6670e5e7 |
448 | Perl doesn't have "static" variables, which can only be accessed from |
449 | the function in which they are declared. You can get the same effect |
450 | with lexical variables, though. |
451 | |
452 | You can fake a static variable by using a lexical variable which goes |
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453 | out of scope. In this example, you define the subroutine C<counter>, and |
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454 | it uses the lexical variable C<$count>. Since you wrap this in a BEGIN |
455 | block, C<$count> is defined at compile-time, but also goes out of |
456 | scope at the end of the BEGIN block. The BEGIN block also ensures that |
457 | the subroutine and the value it uses is defined at compile-time so the |
458 | subroutine is ready to use just like any other subroutine, and you can |
459 | put this code in the same place as other subroutines in the program |
460 | text (i.e. at the end of the code, typically). The subroutine |
461 | C<counter> still has a reference to the data, and is the only way you |
462 | can access the value (and each time you do, you increment the value). |
463 | The data in chunk of memory defined by C<$count> is private to |
464 | C<counter>. |
465 | |
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466 | BEGIN { |
467 | my $count = 1; |
468 | sub counter { $count++ } |
469 | } |
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470 | |
ee891a00 |
471 | my $start = counter(); |
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472 | |
ee891a00 |
473 | .... # code that calls counter(); |
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474 | |
ee891a00 |
475 | my $end = counter(); |
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476 | |
6670e5e7 |
477 | In the previous example, you created a function-private variable |
478 | because only one function remembered its reference. You could define |
479 | multiple functions while the variable is in scope, and each function |
480 | can share the "private" variable. It's not really "static" because you |
481 | can access it outside the function while the lexical variable is in |
482 | scope, and even create references to it. In this example, |
483 | C<increment_count> and C<return_count> share the variable. One |
484 | function adds to the value and the other simply returns the value. |
485 | They can both access C<$count>, and since it has gone out of scope, |
486 | there is no other way to access it. |
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487 | |
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488 | BEGIN { |
489 | my $count = 1; |
490 | sub increment_count { $count++ } |
491 | sub return_count { $count } |
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492 | } |
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493 | |
6670e5e7 |
494 | To declare a file-private variable, you still use a lexical variable. |
495 | A file is also a scope, so a lexical variable defined in the file |
496 | cannot be seen from any other file. |
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497 | |
6670e5e7 |
498 | See L<perlsub/"Persistent Private Variables"> for more information. |
499 | The discussion of closures in L<perlref> may help you even though we |
500 | did not use anonymous subroutines in this answer. See |
501 | L<perlsub/"Persistent Private Variables"> for details. |
c8db1d39 |
502 | |
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503 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
504 | |
a6dd486b |
505 | C<local($x)> saves away the old value of the global variable C<$x> |
506 | and assigns a new value for the duration of the subroutine I<which is |
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507 | visible in other functions called from that subroutine>. This is done |
508 | at run-time, so is called dynamic scoping. local() always affects global |
509 | variables, also called package variables or dynamic variables. |
510 | |
511 | C<my($x)> creates a new variable that is only visible in the current |
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512 | subroutine. This is done at compile-time, so it is called lexical or |
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513 | static scoping. my() always affects private variables, also called |
514 | lexical variables or (improperly) static(ly scoped) variables. |
515 | |
516 | For instance: |
517 | |
518 | sub visible { |
519 | print "var has value $var\n"; |
520 | } |
521 | |
522 | sub dynamic { |
523 | local $var = 'local'; # new temporary value for the still-global |
524 | visible(); # variable called $var |
525 | } |
526 | |
527 | sub lexical { |
528 | my $var = 'private'; # new private variable, $var |
529 | visible(); # (invisible outside of sub scope) |
530 | } |
531 | |
532 | $var = 'global'; |
533 | |
534 | visible(); # prints global |
535 | dynamic(); # prints local |
536 | lexical(); # prints global |
537 | |
538 | Notice how at no point does the value "private" get printed. That's |
539 | because $var only has that value within the block of the lexical() |
540 | function, and it is hidden from called subroutine. |
541 | |
542 | In summary, local() doesn't make what you think of as private, local |
543 | variables. It gives a global variable a temporary value. my() is |
544 | what you're looking for if you want private variables. |
545 | |
197aec24 |
546 | See L<perlsub/"Private Variables via my()"> and |
13a2d996 |
547 | L<perlsub/"Temporary Values via local()"> for excruciating details. |
68dc0745 |
548 | |
549 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? |
550 | |
49d635f9 |
551 | If you know your package, you can just mention it explicitly, as in |
552 | $Some_Pack::var. Note that the notation $::var is B<not> the dynamic $var |
553 | in the current package, but rather the one in the "main" package, as |
554 | though you had written $main::var. |
555 | |
556 | use vars '$var'; |
557 | local $var = "global"; |
558 | my $var = "lexical"; |
68dc0745 |
559 | |
49d635f9 |
560 | print "lexical is $var\n"; |
561 | print "global is $main::var\n"; |
68dc0745 |
562 | |
49d635f9 |
563 | Alternatively you can use the compiler directive our() to bring a |
564 | dynamic variable into the current lexical scope. |
68dc0745 |
565 | |
49d635f9 |
566 | require 5.006; # our() did not exist before 5.6 |
567 | use vars '$var'; |
68dc0745 |
568 | |
49d635f9 |
569 | local $var = "global"; |
570 | my $var = "lexical"; |
571 | |
572 | print "lexical is $var\n"; |
573 | |
574 | { |
575 | our $var; |
576 | print "global is $var\n"; |
577 | } |
68dc0745 |
578 | |
579 | =head2 What's the difference between deep and shallow binding? |
580 | |
581 | In deep binding, lexical variables mentioned in anonymous subroutines |
582 | are the same ones that were in scope when the subroutine was created. |
583 | In shallow binding, they are whichever variables with the same names |
584 | happen to be in scope when the subroutine is called. Perl always uses |
585 | deep binding of lexical variables (i.e., those created with my()). |
586 | However, dynamic variables (aka global, local, or package variables) |
587 | are effectively shallowly bound. Consider this just one more reason |
588 | not to use them. See the answer to L<"What's a closure?">. |
589 | |
04d666b1 |
590 | =head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right? |
68dc0745 |
591 | |
c8db1d39 |
592 | C<my()> and C<local()> give list context to the right hand side |
c47ff5f1 |
593 | of C<=>. The <FH> read operation, like so many of Perl's |
c8db1d39 |
594 | functions and operators, can tell which context it was called in and |
595 | behaves appropriately. In general, the scalar() function can help. |
596 | This function does nothing to the data itself (contrary to popular myth) |
597 | but rather tells its argument to behave in whatever its scalar fashion is. |
598 | If that function doesn't have a defined scalar behavior, this of course |
599 | doesn't help you (such as with sort()). |
68dc0745 |
600 | |
601 | To enforce scalar context in this particular case, however, you need |
602 | merely omit the parentheses: |
603 | |
604 | local($foo) = <FILE>; # WRONG |
605 | local($foo) = scalar(<FILE>); # ok |
606 | local $foo = <FILE>; # right |
607 | |
608 | You should probably be using lexical variables anyway, although the |
609 | issue is the same here: |
610 | |
611 | my($foo) = <FILE>; # WRONG |
612 | my $foo = <FILE>; # right |
613 | |
54310121 |
614 | =head2 How do I redefine a builtin function, operator, or method? |
68dc0745 |
615 | |
616 | Why do you want to do that? :-) |
617 | |
618 | If you want to override a predefined function, such as open(), |
619 | then you'll have to import the new definition from a different |
4a4eefd0 |
620 | module. See L<perlsub/"Overriding Built-in Functions">. There's |
65acb1b1 |
621 | also an example in L<perltoot/"Class::Template">. |
68dc0745 |
622 | |
623 | If you want to overload a Perl operator, such as C<+> or C<**>, |
624 | then you'll want to use the C<use overload> pragma, documented |
625 | in L<overload>. |
626 | |
627 | If you're talking about obscuring method calls in parent classes, |
628 | see L<perltoot/"Overridden Methods">. |
629 | |
630 | =head2 What's the difference between calling a function as &foo and foo()? |
631 | |
632 | When you call a function as C<&foo>, you allow that function access to |
a6dd486b |
633 | your current @_ values, and you bypass prototypes. |
634 | The function doesn't get an empty @_--it gets yours! While not |
68dc0745 |
635 | strictly speaking a bug (it's documented that way in L<perlsub>), it |
636 | would be hard to consider this a feature in most cases. |
637 | |
c8db1d39 |
638 | When you call your function as C<&foo()>, then you I<do> get a new @_, |
68dc0745 |
639 | but prototyping is still circumvented. |
640 | |
641 | Normally, you want to call a function using C<foo()>. You may only |
642 | omit the parentheses if the function is already known to the compiler |
643 | because it already saw the definition (C<use> but not C<require>), |
644 | or via a forward reference or C<use subs> declaration. Even in this |
645 | case, you get a clean @_ without any of the old values leaking through |
646 | where they don't belong. |
647 | |
648 | =head2 How do I create a switch or case statement? |
649 | |
f449fe8a |
650 | If one wants to use pure Perl and to be compatible with Perl versions |
651 | prior to 5.10, the general answer is to write a construct like this: |
c8db1d39 |
652 | |
653 | for ($variable_to_test) { |
654 | if (/pat1/) { } # do something |
655 | elsif (/pat2/) { } # do something else |
656 | elsif (/pat3/) { } # do something else |
657 | else { } # default |
197aec24 |
658 | } |
68dc0745 |
659 | |
f449fe8a |
660 | Here's a simple example of a switch based on pattern matching, |
661 | lined up in a way to make it look more like a switch statement. |
8305e449 |
662 | We'll do a multiway conditional based on the type of reference stored |
c8db1d39 |
663 | in $whatchamacallit: |
664 | |
665 | SWITCH: for (ref $whatchamacallit) { |
68dc0745 |
666 | |
667 | /^$/ && die "not a reference"; |
668 | |
669 | /SCALAR/ && do { |
670 | print_scalar($$ref); |
671 | last SWITCH; |
672 | }; |
673 | |
674 | /ARRAY/ && do { |
675 | print_array(@$ref); |
676 | last SWITCH; |
677 | }; |
678 | |
679 | /HASH/ && do { |
680 | print_hash(%$ref); |
681 | last SWITCH; |
682 | }; |
683 | |
684 | /CODE/ && do { |
685 | warn "can't print function ref"; |
686 | last SWITCH; |
687 | }; |
688 | |
689 | # DEFAULT |
690 | |
691 | warn "User defined type skipped"; |
692 | |
693 | } |
694 | |
f449fe8a |
695 | See L<perlsyn> for other examples in this style. |
c8db1d39 |
696 | |
697 | Sometimes you should change the positions of the constant and the variable. |
698 | For example, let's say you wanted to test which of many answers you were |
699 | given, but in a case-insensitive way that also allows abbreviations. |
700 | You can use the following technique if the strings all start with |
a6dd486b |
701 | different characters or if you want to arrange the matches so that |
c8db1d39 |
702 | one takes precedence over another, as C<"SEND"> has precedence over |
703 | C<"STOP"> here: |
704 | |
705 | chomp($answer = <>); |
706 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } |
707 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } |
708 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } |
709 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } |
710 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } |
711 | |
197aec24 |
712 | A totally different approach is to create a hash of function references. |
c8db1d39 |
713 | |
714 | my %commands = ( |
715 | "happy" => \&joy, |
716 | "sad", => \&sullen, |
717 | "done" => sub { die "See ya!" }, |
718 | "mad" => \&angry, |
719 | ); |
720 | |
721 | print "How are you? "; |
722 | chomp($string = <STDIN>); |
723 | if ($commands{$string}) { |
724 | $commands{$string}->(); |
725 | } else { |
726 | print "No such command: $string\n"; |
197aec24 |
727 | } |
c8db1d39 |
728 | |
f449fe8a |
729 | Note that starting from version 5.10, Perl has now a native switch |
730 | statement. See L<perlsyn>. |
731 | |
732 | Starting from Perl 5.8, a source filter module, C<Switch>, can also be |
733 | used to get switch and case. Its use is now discouraged, because it's |
734 | not fully compatible with the native switch of Perl 5.10, and because, |
735 | as it's implemented as a source filter, it doesn't always work as intended |
736 | when complex syntax is involved. |
737 | |
49d635f9 |
738 | =head2 How can I catch accesses to undefined variables, functions, or methods? |
68dc0745 |
739 | |
740 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and |
741 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to |
742 | undefined functions and methods. |
743 | |
744 | When it comes to undefined variables that would trigger a warning |
49d635f9 |
745 | under C<use warnings>, you can promote the warning to an error. |
68dc0745 |
746 | |
49d635f9 |
747 | use warnings FATAL => qw(uninitialized); |
68dc0745 |
748 | |
749 | =head2 Why can't a method included in this same file be found? |
750 | |
751 | Some possible reasons: your inheritance is getting confused, you've |
752 | misspelled the method name, or the object is of the wrong type. Check |
a6dd486b |
753 | out L<perltoot> for details about any of the above cases. You may |
754 | also use C<print ref($object)> to find out the class C<$object> was |
755 | blessed into. |
68dc0745 |
756 | |
757 | Another possible reason for problems is because you've used the |
758 | indirect object syntax (eg, C<find Guru "Samy">) on a class name |
759 | before Perl has seen that such a package exists. It's wisest to make |
760 | sure your packages are all defined before you start using them, which |
761 | will be taken care of if you use the C<use> statement instead of |
a6dd486b |
762 | C<require>. If not, make sure to use arrow notation (eg., |
c47ff5f1 |
763 | C<< Guru->find("Samy") >>) instead. Object notation is explained in |
68dc0745 |
764 | L<perlobj>. |
765 | |
c8db1d39 |
766 | Make sure to read about creating modules in L<perlmod> and |
ae93639c |
767 | the perils of indirect objects in L<perlobj/"Method Invocation">. |
c8db1d39 |
768 | |
68dc0745 |
769 | =head2 How can I find out my current package? |
770 | |
771 | If you're just a random program, you can do this to find |
772 | out what the currently compiled package is: |
773 | |
c8db1d39 |
774 | my $packname = __PACKAGE__; |
68dc0745 |
775 | |
a6dd486b |
776 | But, if you're a method and you want to print an error message |
68dc0745 |
777 | that includes the kind of object you were called on (which is |
778 | not necessarily the same as the one in which you were compiled): |
779 | |
780 | sub amethod { |
92c2ed05 |
781 | my $self = shift; |
68dc0745 |
782 | my $class = ref($self) || $self; |
783 | warn "called me from a $class object"; |
784 | } |
785 | |
46fc3d4c |
786 | =head2 How can I comment out a large block of perl code? |
787 | |
659cfd94 |
788 | You can use embedded POD to discard it. Enclose the blocks you want |
7678cced |
789 | to comment out in POD markers. The <=begin> directive marks a section |
790 | for a specific formatter. Use the C<comment> format, which no formatter |
791 | should claim to understand (by policy). Mark the end of the block |
792 | with <=end>. |
46fc3d4c |
793 | |
794 | # program is here |
795 | |
7678cced |
796 | =begin comment |
46fc3d4c |
797 | |
798 | all of this stuff |
799 | |
800 | here will be ignored |
801 | by everyone |
802 | |
7678cced |
803 | =end comment |
6670e5e7 |
804 | |
659cfd94 |
805 | =cut |
806 | |
807 | # program continues |
46fc3d4c |
808 | |
f05bbc40 |
809 | The pod directives cannot go just anywhere. You must put a |
810 | pod directive where the parser is expecting a new statement, |
811 | not just in the middle of an expression or some other |
659cfd94 |
812 | arbitrary grammar production. |
fc36a67e |
813 | |
f05bbc40 |
814 | See L<perlpod> for more details. |
c8db1d39 |
815 | |
65acb1b1 |
816 | =head2 How do I clear a package? |
817 | |
818 | Use this code, provided by Mark-Jason Dominus: |
819 | |
820 | sub scrub_package { |
821 | no strict 'refs'; |
822 | my $pack = shift; |
197aec24 |
823 | die "Shouldn't delete main package" |
65acb1b1 |
824 | if $pack eq "" || $pack eq "main"; |
825 | my $stash = *{$pack . '::'}{HASH}; |
826 | my $name; |
827 | foreach $name (keys %$stash) { |
828 | my $fullname = $pack . '::' . $name; |
829 | # Get rid of everything with that name. |
830 | undef $$fullname; |
831 | undef @$fullname; |
832 | undef %$fullname; |
833 | undef &$fullname; |
834 | undef *$fullname; |
835 | } |
836 | } |
837 | |
197aec24 |
838 | Or, if you're using a recent release of Perl, you can |
65acb1b1 |
839 | just use the Symbol::delete_package() function instead. |
840 | |
d92eb7b0 |
841 | =head2 How can I use a variable as a variable name? |
842 | |
843 | Beginners often think they want to have a variable contain the name |
844 | of a variable. |
845 | |
846 | $fred = 23; |
847 | $varname = "fred"; |
848 | ++$$varname; # $fred now 24 |
849 | |
850 | This works I<sometimes>, but it is a very bad idea for two reasons. |
851 | |
a6dd486b |
852 | The first reason is that this technique I<only works on global |
853 | variables>. That means that if $fred is a lexical variable created |
854 | with my() in the above example, the code wouldn't work at all: you'd |
855 | accidentally access the global and skip right over the private lexical |
856 | altogether. Global variables are bad because they can easily collide |
857 | accidentally and in general make for non-scalable and confusing code. |
d92eb7b0 |
858 | |
859 | Symbolic references are forbidden under the C<use strict> pragma. |
860 | They are not true references and consequently are not reference counted |
861 | or garbage collected. |
862 | |
863 | The other reason why using a variable to hold the name of another |
a6dd486b |
864 | variable is a bad idea is that the question often stems from a lack of |
d92eb7b0 |
865 | understanding of Perl data structures, particularly hashes. By using |
866 | symbolic references, you are just using the package's symbol-table hash |
867 | (like C<%main::>) instead of a user-defined hash. The solution is to |
868 | use your own hash or a real reference instead. |
869 | |
369b44b4 |
870 | $USER_VARS{"fred"} = 23; |
d92eb7b0 |
871 | $varname = "fred"; |
872 | $USER_VARS{$varname}++; # not $$varname++ |
873 | |
874 | There we're using the %USER_VARS hash instead of symbolic references. |
875 | Sometimes this comes up in reading strings from the user with variable |
876 | references and wanting to expand them to the values of your perl |
877 | program's variables. This is also a bad idea because it conflates the |
878 | program-addressable namespace and the user-addressable one. Instead of |
879 | reading a string and expanding it to the actual contents of your program's |
880 | own variables: |
881 | |
882 | $str = 'this has a $fred and $barney in it'; |
883 | $str =~ s/(\$\w+)/$1/eeg; # need double eval |
884 | |
a6dd486b |
885 | it would be better to keep a hash around like %USER_VARS and have |
d92eb7b0 |
886 | variable references actually refer to entries in that hash: |
887 | |
888 | $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all |
889 | |
890 | That's faster, cleaner, and safer than the previous approach. Of course, |
891 | you don't need to use a dollar sign. You could use your own scheme to |
892 | make it less confusing, like bracketed percent symbols, etc. |
893 | |
894 | $str = 'this has a %fred% and %barney% in it'; |
895 | $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all |
896 | |
a6dd486b |
897 | Another reason that folks sometimes think they want a variable to |
898 | contain the name of a variable is because they don't know how to build |
899 | proper data structures using hashes. For example, let's say they |
900 | wanted two hashes in their program: %fred and %barney, and that they |
901 | wanted to use another scalar variable to refer to those by name. |
d92eb7b0 |
902 | |
903 | $name = "fred"; |
904 | $$name{WIFE} = "wilma"; # set %fred |
905 | |
197aec24 |
906 | $name = "barney"; |
d92eb7b0 |
907 | $$name{WIFE} = "betty"; # set %barney |
908 | |
909 | This is still a symbolic reference, and is still saddled with the |
910 | problems enumerated above. It would be far better to write: |
911 | |
912 | $folks{"fred"}{WIFE} = "wilma"; |
913 | $folks{"barney"}{WIFE} = "betty"; |
914 | |
915 | And just use a multilevel hash to start with. |
916 | |
917 | The only times that you absolutely I<must> use symbolic references are |
918 | when you really must refer to the symbol table. This may be because it's |
919 | something that can't take a real reference to, such as a format name. |
920 | Doing so may also be important for method calls, since these always go |
921 | through the symbol table for resolution. |
922 | |
923 | In those cases, you would turn off C<strict 'refs'> temporarily so you |
924 | can play around with the symbol table. For example: |
925 | |
926 | @colors = qw(red blue green yellow orange purple violet); |
927 | for my $name (@colors) { |
928 | no strict 'refs'; # renege for the block |
929 | *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; |
197aec24 |
930 | } |
d92eb7b0 |
931 | |
932 | All those functions (red(), blue(), green(), etc.) appear to be separate, |
933 | but the real code in the closure actually was compiled only once. |
934 | |
935 | So, sometimes you might want to use symbolic references to directly |
936 | manipulate the symbol table. This doesn't matter for formats, handles, and |
a6dd486b |
937 | subroutines, because they are always global--you can't use my() on them. |
938 | For scalars, arrays, and hashes, though--and usually for subroutines-- |
939 | you probably only want to use hard references. |
d92eb7b0 |
940 | |
5cd0b561 |
941 | =head2 What does "bad interpreter" mean? |
942 | |
571e049f |
943 | (contributed by brian d foy) |
944 | |
5cd0b561 |
945 | The "bad interpreter" message comes from the shell, not perl. The |
946 | actual message may vary depending on your platform, shell, and locale |
947 | settings. |
948 | |
949 | If you see "bad interpreter - no such file or directory", the first |
950 | line in your perl script (the "shebang" line) does not contain the |
6670e5e7 |
951 | right path to perl (or any other program capable of running scripts). |
5cd0b561 |
952 | Sometimes this happens when you move the script from one machine to |
ac9dac7f |
953 | another and each machine has a different path to perl--/usr/bin/perl |
571e049f |
954 | versus /usr/local/bin/perl for instance. It may also indicate |
6670e5e7 |
955 | that the source machine has CRLF line terminators and the |
956 | destination machine has LF only: the shell tries to find |
571e049f |
957 | /usr/bin/perl<CR>, but can't. |
5cd0b561 |
958 | |
959 | If you see "bad interpreter: Permission denied", you need to make your |
960 | script executable. |
961 | |
962 | In either case, you should still be able to run the scripts with perl |
963 | explicitly: |
964 | |
965 | % perl script.pl |
966 | |
967 | If you get a message like "perl: command not found", perl is not in |
968 | your PATH, which might also mean that the location of perl is not |
969 | where you expect it so you need to adjust your shebang line. |
970 | |
500071f4 |
971 | =head1 REVISION |
972 | |
f449fe8a |
973 | Revision: $Revision: 9309 $ |
500071f4 |
974 | |
f449fe8a |
975 | Date: $Date: 2007-03-23 15:28:16 +0100 (Fri, 23 Mar 2007) $ |
500071f4 |
976 | |
977 | See L<perlfaq> for source control details and availability. |
978 | |
68dc0745 |
979 | =head1 AUTHOR AND COPYRIGHT |
980 | |
ee891a00 |
981 | Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and |
7678cced |
982 | other authors as noted. All rights reserved. |
5a964f20 |
983 | |
5a7beb56 |
984 | This documentation is free; you can redistribute it and/or modify it |
985 | under the same terms as Perl itself. |
5a964f20 |
986 | |
987 | Irrespective of its distribution, all code examples in this file |
988 | are hereby placed into the public domain. You are permitted and |
989 | encouraged to use this code in your own programs for fun |
990 | or for profit as you see fit. A simple comment in the code giving |
991 | credit would be courteous but is not required. |
a6dd486b |
992 | |