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1 | =head1 NAME |
2 | |
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3 | perlfaq7 - General Perl Language Issues ($Revision: 1.27 $, $Date: 2005/10/28 17:38:32 $) |
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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 | |
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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 | ------------ --------------- |
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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 | |
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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 |
216 | hard-to-explain meaning. Closures are implemented in Perl as anonymous |
217 | subroutines with lasting references to lexical variables outside their |
218 | own scopes. These lexicals magically refer to the variables that were |
219 | around when the subroutine was defined (deep binding). |
220 | |
221 | Closures make sense in any programming language where you can have the |
222 | return value of a function be itself a function, as you can in Perl. |
223 | Note that some languages provide anonymous functions but are not |
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224 | capable of providing proper closures: the Python language, for |
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225 | example. For more information on closures, check out any textbook on |
226 | functional programming. Scheme is a language that not only supports |
227 | but encourages closures. |
228 | |
229 | Here's a classic function-generating function: |
230 | |
231 | sub add_function_generator { |
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232 | return sub { shift() + shift() }; |
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233 | } |
234 | |
235 | $add_sub = add_function_generator(); |
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236 | $sum = $add_sub->(4,5); # $sum is 9 now. |
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237 | |
238 | The closure works as a I<function template> with some customization |
239 | slots left out to be filled later. The anonymous subroutine returned |
240 | by add_function_generator() isn't technically a closure because it |
241 | refers to no lexicals outside its own scope. |
242 | |
243 | Contrast this with the following make_adder() function, in which the |
244 | returned anonymous function contains a reference to a lexical variable |
245 | outside the scope of that function itself. Such a reference requires |
246 | that Perl return a proper closure, thus locking in for all time the |
247 | value that the lexical had when the function was created. |
248 | |
249 | sub make_adder { |
250 | my $addpiece = shift; |
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251 | return sub { shift() + $addpiece }; |
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252 | } |
253 | |
254 | $f1 = make_adder(20); |
255 | $f2 = make_adder(555); |
256 | |
257 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas |
258 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece |
259 | in the closure sticks around. |
260 | |
261 | Closures are often used for less esoteric purposes. For example, when |
262 | you want to pass in a bit of code into a function: |
263 | |
264 | my $line; |
265 | timeout( 30, sub { $line = <STDIN> } ); |
266 | |
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267 | If the code to execute had been passed in as a string, |
268 | C<< '$line = <STDIN>' >>, there would have been no way for the |
269 | hypothetical timeout() function to access the lexical variable |
270 | $line back in its caller's scope. |
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271 | |
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272 | =head2 What is variable suicide and how can I prevent it? |
273 | |
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274 | This problem was fixed in perl 5.004_05, so preventing it means upgrading |
275 | your version of perl. ;) |
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276 | |
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277 | Variable suicide is when you (temporarily or permanently) lose the value |
278 | of a variable. It is caused by scoping through my() and local() |
279 | interacting with either closures or aliased foreach() iterator variables |
280 | and subroutine arguments. It used to be easy to inadvertently lose a |
281 | variable's value this way, but now it's much harder. Take this code: |
282 | |
283 | my $f = 'foo'; |
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284 | sub T { |
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285 | while ($i++ < 3) { my $f = $f; $f .= $i; print $f, "\n" } |
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286 | } |
287 | T; |
288 | print "Finally $f\n"; |
289 | |
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290 | If you are experiencing variable suicide, that C<my $f> in the subroutine |
291 | doesn't pick up a fresh copy of the C<$f> whose value is <foo>. The output |
292 | shows that inside the subroutine the value of C<$f> leaks through when it |
293 | shouldn't, as in this output: |
294 | |
295 | foobar |
296 | foobarbar |
297 | foobarbarbar |
298 | Finally foo |
299 | |
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300 | The $f that has "bar" added to it three times should be a new C<$f> |
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301 | C<my $f> should create a new lexical variable each time through the loop. |
302 | The expected output is: |
303 | |
304 | foobar |
305 | foobar |
306 | foobar |
307 | Finally foo |
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308 | |
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309 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? |
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310 | |
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311 | With the exception of regexes, you need to pass references to these |
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312 | objects. See L<perlsub/"Pass by Reference"> for this particular |
313 | question, and L<perlref> for information on references. |
314 | |
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315 | See "Passing Regexes", below, for information on passing regular |
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316 | expressions. |
317 | |
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318 | =over 4 |
319 | |
320 | =item Passing Variables and Functions |
321 | |
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322 | Regular variables and functions are quite easy to pass: just pass in a |
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323 | reference to an existing or anonymous variable or function: |
324 | |
325 | func( \$some_scalar ); |
326 | |
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327 | func( \@some_array ); |
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328 | func( [ 1 .. 10 ] ); |
329 | |
330 | func( \%some_hash ); |
331 | func( { this => 10, that => 20 } ); |
332 | |
333 | func( \&some_func ); |
334 | func( sub { $_[0] ** $_[1] } ); |
335 | |
336 | =item Passing Filehandles |
337 | |
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338 | As of Perl 5.6, you can represent filehandles with scalar variables |
339 | which you treat as any other scalar. |
340 | |
341 | open my $fh, $filename or die "Cannot open $filename! $!"; |
342 | func( $fh ); |
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343 | |
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344 | sub func { |
345 | my $passed_fh = shift; |
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346 | |
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347 | my $line = <$fh>; |
348 | } |
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349 | |
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350 | Before Perl 5.6, you had to use the C<*FH> or C<\*FH> notations. |
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351 | These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles"> |
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352 | and especially L<perlsub/"Pass by Reference"> for more information. |
353 | |
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354 | =item Passing Regexes |
355 | |
356 | To pass regexes around, you'll need to be using a release of Perl |
357 | sufficiently recent as to support the C<qr//> construct, pass around |
358 | strings and use an exception-trapping eval, or else be very, very clever. |
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359 | |
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360 | Here's an example of how to pass in a string to be regex compared |
361 | using C<qr//>: |
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362 | |
363 | sub compare($$) { |
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364 | my ($val1, $regex) = @_; |
365 | my $retval = $val1 =~ /$regex/; |
366 | return $retval; |
367 | } |
368 | $match = compare("old McDonald", qr/d.*D/i); |
369 | |
370 | Notice how C<qr//> allows flags at the end. That pattern was compiled |
371 | at compile time, although it was executed later. The nifty C<qr//> |
372 | notation wasn't introduced until the 5.005 release. Before that, you |
373 | had to approach this problem much less intuitively. For example, here |
374 | it is again if you don't have C<qr//>: |
375 | |
376 | sub compare($$) { |
377 | my ($val1, $regex) = @_; |
378 | my $retval = eval { $val1 =~ /$regex/ }; |
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379 | die if $@; |
380 | return $retval; |
381 | } |
382 | |
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383 | $match = compare("old McDonald", q/($?i)d.*D/); |
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384 | |
385 | Make sure you never say something like this: |
386 | |
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387 | return eval "\$val =~ /$regex/"; # WRONG |
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388 | |
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389 | or someone can sneak shell escapes into the regex due to the double |
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390 | interpolation of the eval and the double-quoted string. For example: |
391 | |
392 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; |
393 | |
394 | eval "\$string =~ /$pattern_of_evil/"; |
395 | |
396 | Those preferring to be very, very clever might see the O'Reilly book, |
397 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's |
398 | Build_MatchMany_Function() is particularly interesting. A complete |
399 | citation of this book is given in L<perlfaq2>. |
400 | |
401 | =item Passing Methods |
402 | |
403 | To pass an object method into a subroutine, you can do this: |
404 | |
405 | call_a_lot(10, $some_obj, "methname") |
406 | sub call_a_lot { |
407 | my ($count, $widget, $trick) = @_; |
408 | for (my $i = 0; $i < $count; $i++) { |
409 | $widget->$trick(); |
410 | } |
411 | } |
412 | |
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413 | Or, you can use a closure to bundle up the object, its |
414 | method call, and arguments: |
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415 | |
416 | my $whatnot = sub { $some_obj->obfuscate(@args) }; |
417 | func($whatnot); |
418 | sub func { |
419 | my $code = shift; |
420 | &$code(); |
421 | } |
422 | |
423 | You could also investigate the can() method in the UNIVERSAL class |
424 | (part of the standard perl distribution). |
425 | |
426 | =back |
427 | |
428 | =head2 How do I create a static variable? |
429 | |
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430 | (contributed by brian d foy) |
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431 | |
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432 | Perl doesn't have "static" variables, which can only be accessed from |
433 | the function in which they are declared. You can get the same effect |
434 | with lexical variables, though. |
435 | |
436 | You can fake a static variable by using a lexical variable which goes |
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437 | out of scope. In this example, you define the subroutine C<counter>, and |
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438 | it uses the lexical variable C<$count>. Since you wrap this in a BEGIN |
439 | block, C<$count> is defined at compile-time, but also goes out of |
440 | scope at the end of the BEGIN block. The BEGIN block also ensures that |
441 | the subroutine and the value it uses is defined at compile-time so the |
442 | subroutine is ready to use just like any other subroutine, and you can |
443 | put this code in the same place as other subroutines in the program |
444 | text (i.e. at the end of the code, typically). The subroutine |
445 | C<counter> still has a reference to the data, and is the only way you |
446 | can access the value (and each time you do, you increment the value). |
447 | The data in chunk of memory defined by C<$count> is private to |
448 | C<counter>. |
449 | |
3a205795 |
450 | BEGIN { |
451 | my $count = 1; |
452 | sub counter { $count++ } |
453 | } |
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454 | |
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455 | my $start = count(); |
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456 | |
3a205795 |
457 | .... # code that calls count(); |
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458 | |
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459 | my $end = count(); |
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460 | |
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461 | In the previous example, you created a function-private variable |
462 | because only one function remembered its reference. You could define |
463 | multiple functions while the variable is in scope, and each function |
464 | can share the "private" variable. It's not really "static" because you |
465 | can access it outside the function while the lexical variable is in |
466 | scope, and even create references to it. In this example, |
467 | C<increment_count> and C<return_count> share the variable. One |
468 | function adds to the value and the other simply returns the value. |
469 | They can both access C<$count>, and since it has gone out of scope, |
470 | there is no other way to access it. |
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471 | |
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472 | BEGIN { |
473 | my $count = 1; |
474 | sub increment_count { $count++ } |
475 | sub return_count { $count } |
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476 | } |
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477 | |
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478 | To declare a file-private variable, you still use a lexical variable. |
479 | A file is also a scope, so a lexical variable defined in the file |
480 | cannot be seen from any other file. |
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481 | |
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482 | See L<perlsub/"Persistent Private Variables"> for more information. |
483 | The discussion of closures in L<perlref> may help you even though we |
484 | did not use anonymous subroutines in this answer. See |
485 | L<perlsub/"Persistent Private Variables"> for details. |
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486 | |
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487 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
488 | |
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489 | C<local($x)> saves away the old value of the global variable C<$x> |
490 | and assigns a new value for the duration of the subroutine I<which is |
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491 | visible in other functions called from that subroutine>. This is done |
492 | at run-time, so is called dynamic scoping. local() always affects global |
493 | variables, also called package variables or dynamic variables. |
494 | |
495 | C<my($x)> creates a new variable that is only visible in the current |
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496 | subroutine. This is done at compile-time, so it is called lexical or |
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497 | static scoping. my() always affects private variables, also called |
498 | lexical variables or (improperly) static(ly scoped) variables. |
499 | |
500 | For instance: |
501 | |
502 | sub visible { |
503 | print "var has value $var\n"; |
504 | } |
505 | |
506 | sub dynamic { |
507 | local $var = 'local'; # new temporary value for the still-global |
508 | visible(); # variable called $var |
509 | } |
510 | |
511 | sub lexical { |
512 | my $var = 'private'; # new private variable, $var |
513 | visible(); # (invisible outside of sub scope) |
514 | } |
515 | |
516 | $var = 'global'; |
517 | |
518 | visible(); # prints global |
519 | dynamic(); # prints local |
520 | lexical(); # prints global |
521 | |
522 | Notice how at no point does the value "private" get printed. That's |
523 | because $var only has that value within the block of the lexical() |
524 | function, and it is hidden from called subroutine. |
525 | |
526 | In summary, local() doesn't make what you think of as private, local |
527 | variables. It gives a global variable a temporary value. my() is |
528 | what you're looking for if you want private variables. |
529 | |
197aec24 |
530 | See L<perlsub/"Private Variables via my()"> and |
13a2d996 |
531 | L<perlsub/"Temporary Values via local()"> for excruciating details. |
68dc0745 |
532 | |
533 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? |
534 | |
49d635f9 |
535 | If you know your package, you can just mention it explicitly, as in |
536 | $Some_Pack::var. Note that the notation $::var is B<not> the dynamic $var |
537 | in the current package, but rather the one in the "main" package, as |
538 | though you had written $main::var. |
539 | |
540 | use vars '$var'; |
541 | local $var = "global"; |
542 | my $var = "lexical"; |
68dc0745 |
543 | |
49d635f9 |
544 | print "lexical is $var\n"; |
545 | print "global is $main::var\n"; |
68dc0745 |
546 | |
49d635f9 |
547 | Alternatively you can use the compiler directive our() to bring a |
548 | dynamic variable into the current lexical scope. |
68dc0745 |
549 | |
49d635f9 |
550 | require 5.006; # our() did not exist before 5.6 |
551 | use vars '$var'; |
68dc0745 |
552 | |
49d635f9 |
553 | local $var = "global"; |
554 | my $var = "lexical"; |
555 | |
556 | print "lexical is $var\n"; |
557 | |
558 | { |
559 | our $var; |
560 | print "global is $var\n"; |
561 | } |
68dc0745 |
562 | |
563 | =head2 What's the difference between deep and shallow binding? |
564 | |
565 | In deep binding, lexical variables mentioned in anonymous subroutines |
566 | are the same ones that were in scope when the subroutine was created. |
567 | In shallow binding, they are whichever variables with the same names |
568 | happen to be in scope when the subroutine is called. Perl always uses |
569 | deep binding of lexical variables (i.e., those created with my()). |
570 | However, dynamic variables (aka global, local, or package variables) |
571 | are effectively shallowly bound. Consider this just one more reason |
572 | not to use them. See the answer to L<"What's a closure?">. |
573 | |
04d666b1 |
574 | =head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right? |
68dc0745 |
575 | |
c8db1d39 |
576 | C<my()> and C<local()> give list context to the right hand side |
c47ff5f1 |
577 | of C<=>. The <FH> read operation, like so many of Perl's |
c8db1d39 |
578 | functions and operators, can tell which context it was called in and |
579 | behaves appropriately. In general, the scalar() function can help. |
580 | This function does nothing to the data itself (contrary to popular myth) |
581 | but rather tells its argument to behave in whatever its scalar fashion is. |
582 | If that function doesn't have a defined scalar behavior, this of course |
583 | doesn't help you (such as with sort()). |
68dc0745 |
584 | |
585 | To enforce scalar context in this particular case, however, you need |
586 | merely omit the parentheses: |
587 | |
588 | local($foo) = <FILE>; # WRONG |
589 | local($foo) = scalar(<FILE>); # ok |
590 | local $foo = <FILE>; # right |
591 | |
592 | You should probably be using lexical variables anyway, although the |
593 | issue is the same here: |
594 | |
595 | my($foo) = <FILE>; # WRONG |
596 | my $foo = <FILE>; # right |
597 | |
54310121 |
598 | =head2 How do I redefine a builtin function, operator, or method? |
68dc0745 |
599 | |
600 | Why do you want to do that? :-) |
601 | |
602 | If you want to override a predefined function, such as open(), |
603 | then you'll have to import the new definition from a different |
4a4eefd0 |
604 | module. See L<perlsub/"Overriding Built-in Functions">. There's |
65acb1b1 |
605 | also an example in L<perltoot/"Class::Template">. |
68dc0745 |
606 | |
607 | If you want to overload a Perl operator, such as C<+> or C<**>, |
608 | then you'll want to use the C<use overload> pragma, documented |
609 | in L<overload>. |
610 | |
611 | If you're talking about obscuring method calls in parent classes, |
612 | see L<perltoot/"Overridden Methods">. |
613 | |
614 | =head2 What's the difference between calling a function as &foo and foo()? |
615 | |
616 | When you call a function as C<&foo>, you allow that function access to |
a6dd486b |
617 | your current @_ values, and you bypass prototypes. |
618 | The function doesn't get an empty @_--it gets yours! While not |
68dc0745 |
619 | strictly speaking a bug (it's documented that way in L<perlsub>), it |
620 | would be hard to consider this a feature in most cases. |
621 | |
c8db1d39 |
622 | When you call your function as C<&foo()>, then you I<do> get a new @_, |
68dc0745 |
623 | but prototyping is still circumvented. |
624 | |
625 | Normally, you want to call a function using C<foo()>. You may only |
626 | omit the parentheses if the function is already known to the compiler |
627 | because it already saw the definition (C<use> but not C<require>), |
628 | or via a forward reference or C<use subs> declaration. Even in this |
629 | case, you get a clean @_ without any of the old values leaking through |
630 | where they don't belong. |
631 | |
632 | =head2 How do I create a switch or case statement? |
633 | |
634 | This is explained in more depth in the L<perlsyn>. Briefly, there's |
635 | no official case statement, because of the variety of tests possible |
636 | in Perl (numeric comparison, string comparison, glob comparison, |
83df6a1d |
637 | regex matching, overloaded comparisons, ...). |
638 | Larry couldn't decide how best to do this, so he left it out, even |
639 | though it's been on the wish list since perl1. |
68dc0745 |
640 | |
83df6a1d |
641 | Starting from Perl 5.8 to get switch and case one can use the |
642 | Switch extension and say: |
643 | |
644 | use Switch; |
645 | |
646 | after which one has switch and case. It is not as fast as it could be |
647 | because it's not really part of the language (it's done using source |
648 | filters) but it is available, and it's very flexible. |
649 | |
650 | But if one wants to use pure Perl, the general answer is to write a |
651 | 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 | |
c8db1d39 |
660 | Here's a simple example of a switch based on pattern matching, this |
661 | time 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 | |
197aec24 |
695 | See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other |
c8db1d39 |
696 | examples in this style. |
697 | |
698 | Sometimes you should change the positions of the constant and the variable. |
699 | For example, let's say you wanted to test which of many answers you were |
700 | given, but in a case-insensitive way that also allows abbreviations. |
701 | You can use the following technique if the strings all start with |
a6dd486b |
702 | different characters or if you want to arrange the matches so that |
c8db1d39 |
703 | one takes precedence over another, as C<"SEND"> has precedence over |
704 | C<"STOP"> here: |
705 | |
706 | chomp($answer = <>); |
707 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } |
708 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } |
709 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } |
710 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } |
711 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } |
712 | |
197aec24 |
713 | A totally different approach is to create a hash of function references. |
c8db1d39 |
714 | |
715 | my %commands = ( |
716 | "happy" => \&joy, |
717 | "sad", => \&sullen, |
718 | "done" => sub { die "See ya!" }, |
719 | "mad" => \&angry, |
720 | ); |
721 | |
722 | print "How are you? "; |
723 | chomp($string = <STDIN>); |
724 | if ($commands{$string}) { |
725 | $commands{$string}->(); |
726 | } else { |
727 | print "No such command: $string\n"; |
197aec24 |
728 | } |
c8db1d39 |
729 | |
49d635f9 |
730 | =head2 How can I catch accesses to undefined variables, functions, or methods? |
68dc0745 |
731 | |
732 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and |
733 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to |
734 | undefined functions and methods. |
735 | |
736 | When it comes to undefined variables that would trigger a warning |
49d635f9 |
737 | under C<use warnings>, you can promote the warning to an error. |
68dc0745 |
738 | |
49d635f9 |
739 | use warnings FATAL => qw(uninitialized); |
68dc0745 |
740 | |
741 | =head2 Why can't a method included in this same file be found? |
742 | |
743 | Some possible reasons: your inheritance is getting confused, you've |
744 | misspelled the method name, or the object is of the wrong type. Check |
a6dd486b |
745 | out L<perltoot> for details about any of the above cases. You may |
746 | also use C<print ref($object)> to find out the class C<$object> was |
747 | blessed into. |
68dc0745 |
748 | |
749 | Another possible reason for problems is because you've used the |
750 | indirect object syntax (eg, C<find Guru "Samy">) on a class name |
751 | before Perl has seen that such a package exists. It's wisest to make |
752 | sure your packages are all defined before you start using them, which |
753 | will be taken care of if you use the C<use> statement instead of |
a6dd486b |
754 | C<require>. If not, make sure to use arrow notation (eg., |
c47ff5f1 |
755 | C<< Guru->find("Samy") >>) instead. Object notation is explained in |
68dc0745 |
756 | L<perlobj>. |
757 | |
c8db1d39 |
758 | Make sure to read about creating modules in L<perlmod> and |
ae93639c |
759 | the perils of indirect objects in L<perlobj/"Method Invocation">. |
c8db1d39 |
760 | |
68dc0745 |
761 | =head2 How can I find out my current package? |
762 | |
763 | If you're just a random program, you can do this to find |
764 | out what the currently compiled package is: |
765 | |
c8db1d39 |
766 | my $packname = __PACKAGE__; |
68dc0745 |
767 | |
a6dd486b |
768 | But, if you're a method and you want to print an error message |
68dc0745 |
769 | that includes the kind of object you were called on (which is |
770 | not necessarily the same as the one in which you were compiled): |
771 | |
772 | sub amethod { |
92c2ed05 |
773 | my $self = shift; |
68dc0745 |
774 | my $class = ref($self) || $self; |
775 | warn "called me from a $class object"; |
776 | } |
777 | |
46fc3d4c |
778 | =head2 How can I comment out a large block of perl code? |
779 | |
659cfd94 |
780 | You can use embedded POD to discard it. Enclose the blocks you want |
7678cced |
781 | to comment out in POD markers. The <=begin> directive marks a section |
782 | for a specific formatter. Use the C<comment> format, which no formatter |
783 | should claim to understand (by policy). Mark the end of the block |
784 | with <=end>. |
46fc3d4c |
785 | |
786 | # program is here |
787 | |
7678cced |
788 | =begin comment |
46fc3d4c |
789 | |
790 | all of this stuff |
791 | |
792 | here will be ignored |
793 | by everyone |
794 | |
7678cced |
795 | =end comment |
6670e5e7 |
796 | |
659cfd94 |
797 | =cut |
798 | |
799 | # program continues |
46fc3d4c |
800 | |
f05bbc40 |
801 | The pod directives cannot go just anywhere. You must put a |
802 | pod directive where the parser is expecting a new statement, |
803 | not just in the middle of an expression or some other |
659cfd94 |
804 | arbitrary grammar production. |
fc36a67e |
805 | |
f05bbc40 |
806 | See L<perlpod> for more details. |
c8db1d39 |
807 | |
65acb1b1 |
808 | =head2 How do I clear a package? |
809 | |
810 | Use this code, provided by Mark-Jason Dominus: |
811 | |
812 | sub scrub_package { |
813 | no strict 'refs'; |
814 | my $pack = shift; |
197aec24 |
815 | die "Shouldn't delete main package" |
65acb1b1 |
816 | if $pack eq "" || $pack eq "main"; |
817 | my $stash = *{$pack . '::'}{HASH}; |
818 | my $name; |
819 | foreach $name (keys %$stash) { |
820 | my $fullname = $pack . '::' . $name; |
821 | # Get rid of everything with that name. |
822 | undef $$fullname; |
823 | undef @$fullname; |
824 | undef %$fullname; |
825 | undef &$fullname; |
826 | undef *$fullname; |
827 | } |
828 | } |
829 | |
197aec24 |
830 | Or, if you're using a recent release of Perl, you can |
65acb1b1 |
831 | just use the Symbol::delete_package() function instead. |
832 | |
d92eb7b0 |
833 | =head2 How can I use a variable as a variable name? |
834 | |
835 | Beginners often think they want to have a variable contain the name |
836 | of a variable. |
837 | |
838 | $fred = 23; |
839 | $varname = "fred"; |
840 | ++$$varname; # $fred now 24 |
841 | |
842 | This works I<sometimes>, but it is a very bad idea for two reasons. |
843 | |
a6dd486b |
844 | The first reason is that this technique I<only works on global |
845 | variables>. That means that if $fred is a lexical variable created |
846 | with my() in the above example, the code wouldn't work at all: you'd |
847 | accidentally access the global and skip right over the private lexical |
848 | altogether. Global variables are bad because they can easily collide |
849 | accidentally and in general make for non-scalable and confusing code. |
d92eb7b0 |
850 | |
851 | Symbolic references are forbidden under the C<use strict> pragma. |
852 | They are not true references and consequently are not reference counted |
853 | or garbage collected. |
854 | |
855 | The other reason why using a variable to hold the name of another |
a6dd486b |
856 | variable is a bad idea is that the question often stems from a lack of |
d92eb7b0 |
857 | understanding of Perl data structures, particularly hashes. By using |
858 | symbolic references, you are just using the package's symbol-table hash |
859 | (like C<%main::>) instead of a user-defined hash. The solution is to |
860 | use your own hash or a real reference instead. |
861 | |
369b44b4 |
862 | $USER_VARS{"fred"} = 23; |
d92eb7b0 |
863 | $varname = "fred"; |
864 | $USER_VARS{$varname}++; # not $$varname++ |
865 | |
866 | There we're using the %USER_VARS hash instead of symbolic references. |
867 | Sometimes this comes up in reading strings from the user with variable |
868 | references and wanting to expand them to the values of your perl |
869 | program's variables. This is also a bad idea because it conflates the |
870 | program-addressable namespace and the user-addressable one. Instead of |
871 | reading a string and expanding it to the actual contents of your program's |
872 | own variables: |
873 | |
874 | $str = 'this has a $fred and $barney in it'; |
875 | $str =~ s/(\$\w+)/$1/eeg; # need double eval |
876 | |
a6dd486b |
877 | it would be better to keep a hash around like %USER_VARS and have |
d92eb7b0 |
878 | variable references actually refer to entries in that hash: |
879 | |
880 | $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all |
881 | |
882 | That's faster, cleaner, and safer than the previous approach. Of course, |
883 | you don't need to use a dollar sign. You could use your own scheme to |
884 | make it less confusing, like bracketed percent symbols, etc. |
885 | |
886 | $str = 'this has a %fred% and %barney% in it'; |
887 | $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all |
888 | |
a6dd486b |
889 | Another reason that folks sometimes think they want a variable to |
890 | contain the name of a variable is because they don't know how to build |
891 | proper data structures using hashes. For example, let's say they |
892 | wanted two hashes in their program: %fred and %barney, and that they |
893 | wanted to use another scalar variable to refer to those by name. |
d92eb7b0 |
894 | |
895 | $name = "fred"; |
896 | $$name{WIFE} = "wilma"; # set %fred |
897 | |
197aec24 |
898 | $name = "barney"; |
d92eb7b0 |
899 | $$name{WIFE} = "betty"; # set %barney |
900 | |
901 | This is still a symbolic reference, and is still saddled with the |
902 | problems enumerated above. It would be far better to write: |
903 | |
904 | $folks{"fred"}{WIFE} = "wilma"; |
905 | $folks{"barney"}{WIFE} = "betty"; |
906 | |
907 | And just use a multilevel hash to start with. |
908 | |
909 | The only times that you absolutely I<must> use symbolic references are |
910 | when you really must refer to the symbol table. This may be because it's |
911 | something that can't take a real reference to, such as a format name. |
912 | Doing so may also be important for method calls, since these always go |
913 | through the symbol table for resolution. |
914 | |
915 | In those cases, you would turn off C<strict 'refs'> temporarily so you |
916 | can play around with the symbol table. For example: |
917 | |
918 | @colors = qw(red blue green yellow orange purple violet); |
919 | for my $name (@colors) { |
920 | no strict 'refs'; # renege for the block |
921 | *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; |
197aec24 |
922 | } |
d92eb7b0 |
923 | |
924 | All those functions (red(), blue(), green(), etc.) appear to be separate, |
925 | but the real code in the closure actually was compiled only once. |
926 | |
927 | So, sometimes you might want to use symbolic references to directly |
928 | manipulate the symbol table. This doesn't matter for formats, handles, and |
a6dd486b |
929 | subroutines, because they are always global--you can't use my() on them. |
930 | For scalars, arrays, and hashes, though--and usually for subroutines-- |
931 | you probably only want to use hard references. |
d92eb7b0 |
932 | |
5cd0b561 |
933 | =head2 What does "bad interpreter" mean? |
934 | |
571e049f |
935 | (contributed by brian d foy) |
936 | |
5cd0b561 |
937 | The "bad interpreter" message comes from the shell, not perl. The |
938 | actual message may vary depending on your platform, shell, and locale |
939 | settings. |
940 | |
941 | If you see "bad interpreter - no such file or directory", the first |
942 | line in your perl script (the "shebang" line) does not contain the |
6670e5e7 |
943 | right path to perl (or any other program capable of running scripts). |
5cd0b561 |
944 | Sometimes this happens when you move the script from one machine to |
945 | another and each machine has a different path to perl---/usr/bin/perl |
571e049f |
946 | versus /usr/local/bin/perl for instance. It may also indicate |
6670e5e7 |
947 | that the source machine has CRLF line terminators and the |
948 | destination machine has LF only: the shell tries to find |
571e049f |
949 | /usr/bin/perl<CR>, but can't. |
5cd0b561 |
950 | |
951 | If you see "bad interpreter: Permission denied", you need to make your |
952 | script executable. |
953 | |
954 | In either case, you should still be able to run the scripts with perl |
955 | explicitly: |
956 | |
957 | % perl script.pl |
958 | |
959 | If you get a message like "perl: command not found", perl is not in |
960 | your PATH, which might also mean that the location of perl is not |
961 | where you expect it so you need to adjust your shebang line. |
962 | |
68dc0745 |
963 | =head1 AUTHOR AND COPYRIGHT |
964 | |
7678cced |
965 | Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and |
966 | other authors as noted. All rights reserved. |
5a964f20 |
967 | |
5a7beb56 |
968 | This documentation is free; you can redistribute it and/or modify it |
969 | under the same terms as Perl itself. |
5a964f20 |
970 | |
971 | Irrespective of its distribution, all code examples in this file |
972 | are hereby placed into the public domain. You are permitted and |
973 | encouraged to use this code in your own programs for fun |
974 | or for profit as you see fit. A simple comment in the code giving |
975 | credit would be courteous but is not required. |
a6dd486b |
976 | |