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1 | =head1 NAME |
2 | |
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3 | perlfaq7 - Perl Language Issues ($Revision: 1.1 $, $Date: 2001/09/20 03:03:00 $) |
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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/$/>) from the handle FILE in scalar context, or I<all> lines |
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 | ------------ --------------- |
57 | $foo{line} $foo{"line"} |
58 | bar => stuff "bar" => stuff |
59 | |
60 | The final semicolon in a block is optional, as is the final comma in a |
61 | list. Good style (see L<perlstyle>) says to put them in except for |
62 | one-liners: |
63 | |
64 | if ($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); |
84 | |
85 | =head2 How do I temporarily block warnings? |
86 | |
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87 | If you are running Perl 5.6.0 or better, the C<use warnings> pragma |
88 | allows fine control of what warning are produced. |
89 | See L<perllexwarn> for more details. |
90 | |
91 | { |
92 | no warnings; # temporarily turn off warnings |
93 | $a = $b + $c; # I know these might be undef |
94 | } |
95 | |
96 | If you have an older version of Perl, the C<$^W> variable (documented |
97 | in L<perlvar>) controls runtime warnings for a block: |
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98 | |
99 | { |
100 | local $^W = 0; # temporarily turn off warnings |
101 | $a = $b + $c; # I know these might be undef |
102 | } |
103 | |
104 | Note that like all the punctuation variables, you cannot currently |
105 | use my() on C<$^W>, only local(). |
106 | |
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107 | =head2 What's an extension? |
108 | |
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109 | An extension is a way of calling compiled C code from Perl. Reading |
110 | L<perlxstut> is a good place to learn more about extensions. |
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111 | |
112 | =head2 Why do Perl operators have different precedence than C operators? |
113 | |
114 | Actually, they don't. All C operators that Perl copies have the same |
115 | precedence in Perl as they do in C. The problem is with operators that C |
116 | doesn't have, especially functions that give a list context to everything |
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117 | on their right, eg. print, chmod, exec, and so on. Such functions are |
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118 | called "list operators" and appear as such in the precedence table in |
119 | L<perlop>. |
120 | |
121 | A common mistake is to write: |
122 | |
123 | unlink $file || die "snafu"; |
124 | |
125 | This gets interpreted as: |
126 | |
127 | unlink ($file || die "snafu"); |
128 | |
129 | To avoid this problem, either put in extra parentheses or use the |
130 | super low precedence C<or> operator: |
131 | |
132 | (unlink $file) || die "snafu"; |
133 | unlink $file or die "snafu"; |
134 | |
135 | The "English" operators (C<and>, C<or>, C<xor>, and C<not>) |
136 | deliberately have precedence lower than that of list operators for |
137 | just such situations as the one above. |
138 | |
139 | Another operator with surprising precedence is exponentiation. It |
140 | binds more tightly even than unary minus, making C<-2**2> product a |
141 | negative not a positive four. It is also right-associating, meaning |
142 | that C<2**3**2> is two raised to the ninth power, not eight squared. |
143 | |
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144 | Although it has the same precedence as in C, Perl's C<?:> operator |
145 | produces an lvalue. This assigns $x to either $a or $b, depending |
146 | on the trueness of $maybe: |
147 | |
148 | ($maybe ? $a : $b) = $x; |
149 | |
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150 | =head2 How do I declare/create a structure? |
151 | |
152 | In general, you don't "declare" a structure. Just use a (probably |
153 | anonymous) hash reference. See L<perlref> and L<perldsc> for details. |
154 | Here's an example: |
155 | |
156 | $person = {}; # new anonymous hash |
157 | $person->{AGE} = 24; # set field AGE to 24 |
158 | $person->{NAME} = "Nat"; # set field NAME to "Nat" |
159 | |
160 | If you're looking for something a bit more rigorous, try L<perltoot>. |
161 | |
162 | =head2 How do I create a module? |
163 | |
164 | A module is a package that lives in a file of the same name. For |
165 | example, the Hello::There module would live in Hello/There.pm. For |
166 | details, read L<perlmod>. You'll also find L<Exporter> helpful. If |
167 | you're writing a C or mixed-language module with both C and Perl, then |
168 | you should study L<perlxstut>. |
169 | |
170 | Here's a convenient template you might wish you use when starting your |
171 | own module. Make sure to change the names appropriately. |
172 | |
173 | package Some::Module; # assumes Some/Module.pm |
174 | |
175 | use strict; |
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176 | use warnings; |
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177 | |
178 | BEGIN { |
179 | use Exporter (); |
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180 | our ($VERSION, @ISA, @EXPORT, @EXPORT_OK, %EXPORT_TAGS); |
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181 | |
182 | ## set the version for version checking; uncomment to use |
183 | ## $VERSION = 1.00; |
184 | |
185 | # if using RCS/CVS, this next line may be preferred, |
186 | # but beware two-digit versions. |
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187 | $VERSION = do{my@r=q$Revision: 1.1 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r}; |
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188 | |
189 | @ISA = qw(Exporter); |
190 | @EXPORT = qw(&func1 &func2 &func3); |
191 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], |
192 | |
193 | # your exported package globals go here, |
194 | # as well as any optionally exported functions |
195 | @EXPORT_OK = qw($Var1 %Hashit); |
196 | } |
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197 | our @EXPORT_OK; |
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198 | |
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199 | # exported package globals go here |
200 | our $Var1; |
201 | our %Hashit; |
202 | |
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203 | # non-exported package globals go here |
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204 | our @more; |
205 | our $stuff; |
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206 | |
207 | # initialize package globals, first exported ones |
208 | $Var1 = ''; |
209 | %Hashit = (); |
210 | |
211 | # then the others (which are still accessible as $Some::Module::stuff) |
212 | $stuff = ''; |
213 | @more = (); |
214 | |
215 | # all file-scoped lexicals must be created before |
216 | # the functions below that use them. |
217 | |
218 | # file-private lexicals go here |
219 | my $priv_var = ''; |
220 | my %secret_hash = (); |
221 | |
222 | # here's a file-private function as a closure, |
223 | # callable as &$priv_func; it cannot be prototyped. |
224 | my $priv_func = sub { |
225 | # stuff goes here. |
226 | }; |
227 | |
228 | # make all your functions, whether exported or not; |
229 | # remember to put something interesting in the {} stubs |
230 | sub func1 {} # no prototype |
231 | sub func2() {} # proto'd void |
232 | sub func3($$) {} # proto'd to 2 scalars |
233 | |
234 | # this one isn't exported, but could be called! |
235 | sub func4(\%) {} # proto'd to 1 hash ref |
236 | |
237 | END { } # module clean-up code here (global destructor) |
238 | |
239 | 1; # modules must return true |
240 | |
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241 | The h2xs program will create stubs for all the important stuff for you: |
242 | |
243 | % h2xs -XA -n My::Module |
244 | |
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245 | =head2 How do I create a class? |
246 | |
247 | See L<perltoot> for an introduction to classes and objects, as well as |
248 | L<perlobj> and L<perlbot>. |
249 | |
250 | =head2 How can I tell if a variable is tainted? |
251 | |
252 | See L<perlsec/"Laundering and Detecting Tainted Data">. Here's an |
253 | example (which doesn't use any system calls, because the kill() |
254 | is given no processes to signal): |
255 | |
256 | sub is_tainted { |
257 | return ! eval { join('',@_), kill 0; 1; }; |
258 | } |
259 | |
260 | This is not C<-w> clean, however. There is no C<-w> clean way to |
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261 | detect taintedness--take this as a hint that you should untaint |
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262 | all possibly-tainted data. |
263 | |
264 | =head2 What's a closure? |
265 | |
266 | Closures are documented in L<perlref>. |
267 | |
268 | I<Closure> is a computer science term with a precise but |
269 | hard-to-explain meaning. Closures are implemented in Perl as anonymous |
270 | subroutines with lasting references to lexical variables outside their |
271 | own scopes. These lexicals magically refer to the variables that were |
272 | around when the subroutine was defined (deep binding). |
273 | |
274 | Closures make sense in any programming language where you can have the |
275 | return value of a function be itself a function, as you can in Perl. |
276 | Note that some languages provide anonymous functions but are not |
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277 | capable of providing proper closures: the Python language, for |
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278 | example. For more information on closures, check out any textbook on |
279 | functional programming. Scheme is a language that not only supports |
280 | but encourages closures. |
281 | |
282 | Here's a classic function-generating function: |
283 | |
284 | sub add_function_generator { |
285 | return sub { shift + shift }; |
286 | } |
287 | |
288 | $add_sub = add_function_generator(); |
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289 | $sum = $add_sub->(4,5); # $sum is 9 now. |
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290 | |
291 | The closure works as a I<function template> with some customization |
292 | slots left out to be filled later. The anonymous subroutine returned |
293 | by add_function_generator() isn't technically a closure because it |
294 | refers to no lexicals outside its own scope. |
295 | |
296 | Contrast this with the following make_adder() function, in which the |
297 | returned anonymous function contains a reference to a lexical variable |
298 | outside the scope of that function itself. Such a reference requires |
299 | that Perl return a proper closure, thus locking in for all time the |
300 | value that the lexical had when the function was created. |
301 | |
302 | sub make_adder { |
303 | my $addpiece = shift; |
304 | return sub { shift + $addpiece }; |
305 | } |
306 | |
307 | $f1 = make_adder(20); |
308 | $f2 = make_adder(555); |
309 | |
310 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas |
311 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece |
312 | in the closure sticks around. |
313 | |
314 | Closures are often used for less esoteric purposes. For example, when |
315 | you want to pass in a bit of code into a function: |
316 | |
317 | my $line; |
318 | timeout( 30, sub { $line = <STDIN> } ); |
319 | |
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320 | If the code to execute had been passed in as a string, |
321 | C<< '$line = <STDIN>' >>, there would have been no way for the |
322 | hypothetical timeout() function to access the lexical variable |
323 | $line back in its caller's scope. |
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324 | |
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325 | =head2 What is variable suicide and how can I prevent it? |
326 | |
327 | Variable suicide is when you (temporarily or permanently) lose the |
328 | value of a variable. It is caused by scoping through my() and local() |
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329 | interacting with either closures or aliased foreach() iterator |
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330 | variables and subroutine arguments. It used to be easy to |
331 | inadvertently lose a variable's value this way, but now it's much |
332 | harder. Take this code: |
333 | |
334 | my $f = "foo"; |
335 | sub T { |
336 | while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } |
337 | } |
338 | T; |
339 | print "Finally $f\n"; |
340 | |
341 | The $f that has "bar" added to it three times should be a new C<$f> |
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342 | (C<my $f> should create a new local variable each time through the loop). |
343 | It isn't, however. This was a bug, now fixed in the latest releases |
344 | (tested against 5.004_05, 5.005_03, and 5.005_56). |
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345 | |
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346 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? |
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347 | |
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348 | With the exception of regexes, you need to pass references to these |
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349 | objects. See L<perlsub/"Pass by Reference"> for this particular |
350 | question, and L<perlref> for information on references. |
351 | |
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352 | See ``Passing Regexes'', below, for information on passing regular |
353 | expressions. |
354 | |
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355 | =over 4 |
356 | |
357 | =item Passing Variables and Functions |
358 | |
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359 | Regular variables and functions are quite easy to pass: just pass in a |
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360 | reference to an existing or anonymous variable or function: |
361 | |
362 | func( \$some_scalar ); |
363 | |
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364 | func( \@some_array ); |
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365 | func( [ 1 .. 10 ] ); |
366 | |
367 | func( \%some_hash ); |
368 | func( { this => 10, that => 20 } ); |
369 | |
370 | func( \&some_func ); |
371 | func( sub { $_[0] ** $_[1] } ); |
372 | |
373 | =item Passing Filehandles |
374 | |
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375 | To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations. |
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376 | These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles"> |
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377 | and especially L<perlsub/"Pass by Reference"> for more information. |
378 | |
379 | Here's an excerpt: |
380 | |
381 | If you're passing around filehandles, you could usually just use the bare |
382 | typeglob, like *STDOUT, but typeglobs references would be better because |
383 | they'll still work properly under C<use strict 'refs'>. For example: |
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384 | |
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385 | splutter(\*STDOUT); |
386 | sub splutter { |
387 | my $fh = shift; |
388 | print $fh "her um well a hmmm\n"; |
389 | } |
390 | |
391 | $rec = get_rec(\*STDIN); |
392 | sub get_rec { |
393 | my $fh = shift; |
394 | return scalar <$fh>; |
395 | } |
396 | |
397 | If you're planning on generating new filehandles, you could do this: |
398 | |
399 | sub openit { |
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400 | my $path = shift; |
c8db1d39 |
401 | local *FH; |
402 | return open (FH, $path) ? *FH : undef; |
403 | } |
404 | $fh = openit('< /etc/motd'); |
405 | print <$fh>; |
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406 | |
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407 | =item Passing Regexes |
408 | |
409 | To pass regexes around, you'll need to be using a release of Perl |
410 | sufficiently recent as to support the C<qr//> construct, pass around |
411 | strings and use an exception-trapping eval, or else be very, very clever. |
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412 | |
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413 | Here's an example of how to pass in a string to be regex compared |
414 | using C<qr//>: |
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415 | |
416 | sub compare($$) { |
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417 | my ($val1, $regex) = @_; |
418 | my $retval = $val1 =~ /$regex/; |
419 | return $retval; |
420 | } |
421 | $match = compare("old McDonald", qr/d.*D/i); |
422 | |
423 | Notice how C<qr//> allows flags at the end. That pattern was compiled |
424 | at compile time, although it was executed later. The nifty C<qr//> |
425 | notation wasn't introduced until the 5.005 release. Before that, you |
426 | had to approach this problem much less intuitively. For example, here |
427 | it is again if you don't have C<qr//>: |
428 | |
429 | sub compare($$) { |
430 | my ($val1, $regex) = @_; |
431 | my $retval = eval { $val1 =~ /$regex/ }; |
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432 | die if $@; |
433 | return $retval; |
434 | } |
435 | |
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436 | $match = compare("old McDonald", q/($?i)d.*D/); |
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437 | |
438 | Make sure you never say something like this: |
439 | |
d92eb7b0 |
440 | return eval "\$val =~ /$regex/"; # WRONG |
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441 | |
d92eb7b0 |
442 | or someone can sneak shell escapes into the regex due to the double |
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443 | interpolation of the eval and the double-quoted string. For example: |
444 | |
445 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; |
446 | |
447 | eval "\$string =~ /$pattern_of_evil/"; |
448 | |
449 | Those preferring to be very, very clever might see the O'Reilly book, |
450 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's |
451 | Build_MatchMany_Function() is particularly interesting. A complete |
452 | citation of this book is given in L<perlfaq2>. |
453 | |
454 | =item Passing Methods |
455 | |
456 | To pass an object method into a subroutine, you can do this: |
457 | |
458 | call_a_lot(10, $some_obj, "methname") |
459 | sub call_a_lot { |
460 | my ($count, $widget, $trick) = @_; |
461 | for (my $i = 0; $i < $count; $i++) { |
462 | $widget->$trick(); |
463 | } |
464 | } |
465 | |
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466 | Or, you can use a closure to bundle up the object, its |
467 | method call, and arguments: |
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468 | |
469 | my $whatnot = sub { $some_obj->obfuscate(@args) }; |
470 | func($whatnot); |
471 | sub func { |
472 | my $code = shift; |
473 | &$code(); |
474 | } |
475 | |
476 | You could also investigate the can() method in the UNIVERSAL class |
477 | (part of the standard perl distribution). |
478 | |
479 | =back |
480 | |
481 | =head2 How do I create a static variable? |
482 | |
483 | As with most things in Perl, TMTOWTDI. What is a "static variable" in |
484 | other languages could be either a function-private variable (visible |
485 | only within a single function, retaining its value between calls to |
486 | that function), or a file-private variable (visible only to functions |
487 | within the file it was declared in) in Perl. |
488 | |
489 | Here's code to implement a function-private variable: |
490 | |
491 | BEGIN { |
492 | my $counter = 42; |
493 | sub prev_counter { return --$counter } |
494 | sub next_counter { return $counter++ } |
495 | } |
496 | |
497 | Now prev_counter() and next_counter() share a private variable $counter |
498 | that was initialized at compile time. |
499 | |
500 | To declare a file-private variable, you'll still use a my(), putting |
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501 | the declaration at the outer scope level at the top of the file. |
502 | Assume this is in file Pax.pm: |
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503 | |
504 | package Pax; |
505 | my $started = scalar(localtime(time())); |
506 | |
507 | sub begun { return $started } |
508 | |
509 | When C<use Pax> or C<require Pax> loads this module, the variable will |
510 | be initialized. It won't get garbage-collected the way most variables |
511 | going out of scope do, because the begun() function cares about it, |
512 | but no one else can get it. It is not called $Pax::started because |
513 | its scope is unrelated to the package. It's scoped to the file. You |
514 | could conceivably have several packages in that same file all |
515 | accessing the same private variable, but another file with the same |
516 | package couldn't get to it. |
517 | |
c2611fb3 |
518 | See L<perlsub/"Persistent Private Variables"> for details. |
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519 | |
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520 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
521 | |
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522 | C<local($x)> saves away the old value of the global variable C<$x> |
523 | and assigns a new value for the duration of the subroutine I<which is |
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524 | visible in other functions called from that subroutine>. This is done |
525 | at run-time, so is called dynamic scoping. local() always affects global |
526 | variables, also called package variables or dynamic variables. |
527 | |
528 | C<my($x)> creates a new variable that is only visible in the current |
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529 | subroutine. This is done at compile-time, so it is called lexical or |
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530 | static scoping. my() always affects private variables, also called |
531 | lexical variables or (improperly) static(ly scoped) variables. |
532 | |
533 | For instance: |
534 | |
535 | sub visible { |
536 | print "var has value $var\n"; |
537 | } |
538 | |
539 | sub dynamic { |
540 | local $var = 'local'; # new temporary value for the still-global |
541 | visible(); # variable called $var |
542 | } |
543 | |
544 | sub lexical { |
545 | my $var = 'private'; # new private variable, $var |
546 | visible(); # (invisible outside of sub scope) |
547 | } |
548 | |
549 | $var = 'global'; |
550 | |
551 | visible(); # prints global |
552 | dynamic(); # prints local |
553 | lexical(); # prints global |
554 | |
555 | Notice how at no point does the value "private" get printed. That's |
556 | because $var only has that value within the block of the lexical() |
557 | function, and it is hidden from called subroutine. |
558 | |
559 | In summary, local() doesn't make what you think of as private, local |
560 | variables. It gives a global variable a temporary value. my() is |
561 | what you're looking for if you want private variables. |
562 | |
13a2d996 |
563 | See L<perlsub/"Private Variables via my()"> and |
564 | L<perlsub/"Temporary Values via local()"> for excruciating details. |
68dc0745 |
565 | |
566 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? |
567 | |
568 | You can do this via symbolic references, provided you haven't set |
569 | C<use strict "refs">. So instead of $var, use C<${'var'}>. |
570 | |
571 | local $var = "global"; |
572 | my $var = "lexical"; |
573 | |
574 | print "lexical is $var\n"; |
575 | |
576 | no strict 'refs'; |
577 | print "global is ${'var'}\n"; |
578 | |
579 | If you know your package, you can just mention it explicitly, as in |
580 | $Some_Pack::var. Note that the notation $::var is I<not> the dynamic |
581 | $var in the current package, but rather the one in the C<main> |
582 | package, as though you had written $main::var. Specifying the package |
583 | directly makes you hard-code its name, but it executes faster and |
584 | avoids running afoul of C<use strict "refs">. |
585 | |
586 | =head2 What's the difference between deep and shallow binding? |
587 | |
588 | In deep binding, lexical variables mentioned in anonymous subroutines |
589 | are the same ones that were in scope when the subroutine was created. |
590 | In shallow binding, they are whichever variables with the same names |
591 | happen to be in scope when the subroutine is called. Perl always uses |
592 | deep binding of lexical variables (i.e., those created with my()). |
593 | However, dynamic variables (aka global, local, or package variables) |
594 | are effectively shallowly bound. Consider this just one more reason |
595 | not to use them. See the answer to L<"What's a closure?">. |
596 | |
c47ff5f1 |
597 | =head2 Why doesn't "my($foo) = <FILE>;" work right? |
68dc0745 |
598 | |
c8db1d39 |
599 | C<my()> and C<local()> give list context to the right hand side |
c47ff5f1 |
600 | of C<=>. The <FH> read operation, like so many of Perl's |
c8db1d39 |
601 | functions and operators, can tell which context it was called in and |
602 | behaves appropriately. In general, the scalar() function can help. |
603 | This function does nothing to the data itself (contrary to popular myth) |
604 | but rather tells its argument to behave in whatever its scalar fashion is. |
605 | If that function doesn't have a defined scalar behavior, this of course |
606 | doesn't help you (such as with sort()). |
68dc0745 |
607 | |
608 | To enforce scalar context in this particular case, however, you need |
609 | merely omit the parentheses: |
610 | |
611 | local($foo) = <FILE>; # WRONG |
612 | local($foo) = scalar(<FILE>); # ok |
613 | local $foo = <FILE>; # right |
614 | |
615 | You should probably be using lexical variables anyway, although the |
616 | issue is the same here: |
617 | |
618 | my($foo) = <FILE>; # WRONG |
619 | my $foo = <FILE>; # right |
620 | |
54310121 |
621 | =head2 How do I redefine a builtin function, operator, or method? |
68dc0745 |
622 | |
623 | Why do you want to do that? :-) |
624 | |
625 | If you want to override a predefined function, such as open(), |
626 | then you'll have to import the new definition from a different |
4a4eefd0 |
627 | module. See L<perlsub/"Overriding Built-in Functions">. There's |
65acb1b1 |
628 | also an example in L<perltoot/"Class::Template">. |
68dc0745 |
629 | |
630 | If you want to overload a Perl operator, such as C<+> or C<**>, |
631 | then you'll want to use the C<use overload> pragma, documented |
632 | in L<overload>. |
633 | |
634 | If you're talking about obscuring method calls in parent classes, |
635 | see L<perltoot/"Overridden Methods">. |
636 | |
637 | =head2 What's the difference between calling a function as &foo and foo()? |
638 | |
639 | When you call a function as C<&foo>, you allow that function access to |
a6dd486b |
640 | your current @_ values, and you bypass prototypes. |
641 | The function doesn't get an empty @_--it gets yours! While not |
68dc0745 |
642 | strictly speaking a bug (it's documented that way in L<perlsub>), it |
643 | would be hard to consider this a feature in most cases. |
644 | |
c8db1d39 |
645 | When you call your function as C<&foo()>, then you I<do> get a new @_, |
68dc0745 |
646 | but prototyping is still circumvented. |
647 | |
648 | Normally, you want to call a function using C<foo()>. You may only |
649 | omit the parentheses if the function is already known to the compiler |
650 | because it already saw the definition (C<use> but not C<require>), |
651 | or via a forward reference or C<use subs> declaration. Even in this |
652 | case, you get a clean @_ without any of the old values leaking through |
653 | where they don't belong. |
654 | |
655 | =head2 How do I create a switch or case statement? |
656 | |
657 | This is explained in more depth in the L<perlsyn>. Briefly, there's |
658 | no official case statement, because of the variety of tests possible |
659 | in Perl (numeric comparison, string comparison, glob comparison, |
83df6a1d |
660 | regex matching, overloaded comparisons, ...). |
661 | Larry couldn't decide how best to do this, so he left it out, even |
662 | though it's been on the wish list since perl1. |
68dc0745 |
663 | |
83df6a1d |
664 | Starting from Perl 5.8 to get switch and case one can use the |
665 | Switch extension and say: |
666 | |
667 | use Switch; |
668 | |
669 | after which one has switch and case. It is not as fast as it could be |
670 | because it's not really part of the language (it's done using source |
671 | filters) but it is available, and it's very flexible. |
672 | |
673 | But if one wants to use pure Perl, the general answer is to write a |
674 | construct like this: |
c8db1d39 |
675 | |
676 | for ($variable_to_test) { |
677 | if (/pat1/) { } # do something |
678 | elsif (/pat2/) { } # do something else |
679 | elsif (/pat3/) { } # do something else |
680 | else { } # default |
681 | } |
68dc0745 |
682 | |
c8db1d39 |
683 | Here's a simple example of a switch based on pattern matching, this |
684 | time lined up in a way to make it look more like a switch statement. |
685 | We'll do a multi-way conditional based on the type of reference stored |
686 | in $whatchamacallit: |
687 | |
688 | SWITCH: for (ref $whatchamacallit) { |
68dc0745 |
689 | |
690 | /^$/ && die "not a reference"; |
691 | |
692 | /SCALAR/ && do { |
693 | print_scalar($$ref); |
694 | last SWITCH; |
695 | }; |
696 | |
697 | /ARRAY/ && do { |
698 | print_array(@$ref); |
699 | last SWITCH; |
700 | }; |
701 | |
702 | /HASH/ && do { |
703 | print_hash(%$ref); |
704 | last SWITCH; |
705 | }; |
706 | |
707 | /CODE/ && do { |
708 | warn "can't print function ref"; |
709 | last SWITCH; |
710 | }; |
711 | |
712 | # DEFAULT |
713 | |
714 | warn "User defined type skipped"; |
715 | |
716 | } |
717 | |
c8db1d39 |
718 | See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other |
719 | examples in this style. |
720 | |
721 | Sometimes you should change the positions of the constant and the variable. |
722 | For example, let's say you wanted to test which of many answers you were |
723 | given, but in a case-insensitive way that also allows abbreviations. |
724 | You can use the following technique if the strings all start with |
a6dd486b |
725 | different characters or if you want to arrange the matches so that |
c8db1d39 |
726 | one takes precedence over another, as C<"SEND"> has precedence over |
727 | C<"STOP"> here: |
728 | |
729 | chomp($answer = <>); |
730 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } |
731 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } |
732 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } |
733 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } |
734 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } |
735 | |
736 | A totally different approach is to create a hash of function references. |
737 | |
738 | my %commands = ( |
739 | "happy" => \&joy, |
740 | "sad", => \&sullen, |
741 | "done" => sub { die "See ya!" }, |
742 | "mad" => \&angry, |
743 | ); |
744 | |
745 | print "How are you? "; |
746 | chomp($string = <STDIN>); |
747 | if ($commands{$string}) { |
748 | $commands{$string}->(); |
749 | } else { |
750 | print "No such command: $string\n"; |
751 | } |
752 | |
68dc0745 |
753 | =head2 How can I catch accesses to undefined variables/functions/methods? |
754 | |
755 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and |
756 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to |
757 | undefined functions and methods. |
758 | |
759 | When it comes to undefined variables that would trigger a warning |
760 | under C<-w>, you can use a handler to trap the pseudo-signal |
761 | C<__WARN__> like this: |
762 | |
763 | $SIG{__WARN__} = sub { |
764 | |
c8db1d39 |
765 | for ( $_[0] ) { # voici un switch statement |
68dc0745 |
766 | |
767 | /Use of uninitialized value/ && do { |
768 | # promote warning to a fatal |
769 | die $_; |
770 | }; |
771 | |
772 | # other warning cases to catch could go here; |
773 | |
774 | warn $_; |
775 | } |
776 | |
777 | }; |
778 | |
779 | =head2 Why can't a method included in this same file be found? |
780 | |
781 | Some possible reasons: your inheritance is getting confused, you've |
782 | misspelled the method name, or the object is of the wrong type. Check |
a6dd486b |
783 | out L<perltoot> for details about any of the above cases. You may |
784 | also use C<print ref($object)> to find out the class C<$object> was |
785 | blessed into. |
68dc0745 |
786 | |
787 | Another possible reason for problems is because you've used the |
788 | indirect object syntax (eg, C<find Guru "Samy">) on a class name |
789 | before Perl has seen that such a package exists. It's wisest to make |
790 | sure your packages are all defined before you start using them, which |
791 | will be taken care of if you use the C<use> statement instead of |
a6dd486b |
792 | C<require>. If not, make sure to use arrow notation (eg., |
c47ff5f1 |
793 | C<< Guru->find("Samy") >>) instead. Object notation is explained in |
68dc0745 |
794 | L<perlobj>. |
795 | |
c8db1d39 |
796 | Make sure to read about creating modules in L<perlmod> and |
ae93639c |
797 | the perils of indirect objects in L<perlobj/"Method Invocation">. |
c8db1d39 |
798 | |
68dc0745 |
799 | =head2 How can I find out my current package? |
800 | |
801 | If you're just a random program, you can do this to find |
802 | out what the currently compiled package is: |
803 | |
c8db1d39 |
804 | my $packname = __PACKAGE__; |
68dc0745 |
805 | |
a6dd486b |
806 | But, if you're a method and you want to print an error message |
68dc0745 |
807 | that includes the kind of object you were called on (which is |
808 | not necessarily the same as the one in which you were compiled): |
809 | |
810 | sub amethod { |
92c2ed05 |
811 | my $self = shift; |
68dc0745 |
812 | my $class = ref($self) || $self; |
813 | warn "called me from a $class object"; |
814 | } |
815 | |
46fc3d4c |
816 | =head2 How can I comment out a large block of perl code? |
817 | |
818 | Use embedded POD to discard it: |
819 | |
820 | # program is here |
821 | |
822 | =for nobody |
823 | This paragraph is commented out |
824 | |
825 | # program continues |
826 | |
827 | =begin comment text |
828 | |
829 | all of this stuff |
830 | |
831 | here will be ignored |
832 | by everyone |
833 | |
834 | =end comment text |
835 | |
fc36a67e |
836 | =cut |
837 | |
c8db1d39 |
838 | This can't go just anywhere. You have to put a pod directive where |
839 | the parser is expecting a new statement, not just in the middle |
840 | of an expression or some other arbitrary yacc grammar production. |
841 | |
65acb1b1 |
842 | =head2 How do I clear a package? |
843 | |
844 | Use this code, provided by Mark-Jason Dominus: |
845 | |
846 | sub scrub_package { |
847 | no strict 'refs'; |
848 | my $pack = shift; |
849 | die "Shouldn't delete main package" |
850 | if $pack eq "" || $pack eq "main"; |
851 | my $stash = *{$pack . '::'}{HASH}; |
852 | my $name; |
853 | foreach $name (keys %$stash) { |
854 | my $fullname = $pack . '::' . $name; |
855 | # Get rid of everything with that name. |
856 | undef $$fullname; |
857 | undef @$fullname; |
858 | undef %$fullname; |
859 | undef &$fullname; |
860 | undef *$fullname; |
861 | } |
862 | } |
863 | |
864 | Or, if you're using a recent release of Perl, you can |
865 | just use the Symbol::delete_package() function instead. |
866 | |
d92eb7b0 |
867 | =head2 How can I use a variable as a variable name? |
868 | |
869 | Beginners often think they want to have a variable contain the name |
870 | of a variable. |
871 | |
872 | $fred = 23; |
873 | $varname = "fred"; |
874 | ++$$varname; # $fred now 24 |
875 | |
876 | This works I<sometimes>, but it is a very bad idea for two reasons. |
877 | |
a6dd486b |
878 | The first reason is that this technique I<only works on global |
879 | variables>. That means that if $fred is a lexical variable created |
880 | with my() in the above example, the code wouldn't work at all: you'd |
881 | accidentally access the global and skip right over the private lexical |
882 | altogether. Global variables are bad because they can easily collide |
883 | accidentally and in general make for non-scalable and confusing code. |
d92eb7b0 |
884 | |
885 | Symbolic references are forbidden under the C<use strict> pragma. |
886 | They are not true references and consequently are not reference counted |
887 | or garbage collected. |
888 | |
889 | The other reason why using a variable to hold the name of another |
a6dd486b |
890 | variable is a bad idea is that the question often stems from a lack of |
d92eb7b0 |
891 | understanding of Perl data structures, particularly hashes. By using |
892 | symbolic references, you are just using the package's symbol-table hash |
893 | (like C<%main::>) instead of a user-defined hash. The solution is to |
894 | use your own hash or a real reference instead. |
895 | |
896 | $fred = 23; |
897 | $varname = "fred"; |
898 | $USER_VARS{$varname}++; # not $$varname++ |
899 | |
900 | There we're using the %USER_VARS hash instead of symbolic references. |
901 | Sometimes this comes up in reading strings from the user with variable |
902 | references and wanting to expand them to the values of your perl |
903 | program's variables. This is also a bad idea because it conflates the |
904 | program-addressable namespace and the user-addressable one. Instead of |
905 | reading a string and expanding it to the actual contents of your program's |
906 | own variables: |
907 | |
908 | $str = 'this has a $fred and $barney in it'; |
909 | $str =~ s/(\$\w+)/$1/eeg; # need double eval |
910 | |
a6dd486b |
911 | it would be better to keep a hash around like %USER_VARS and have |
d92eb7b0 |
912 | variable references actually refer to entries in that hash: |
913 | |
914 | $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all |
915 | |
916 | That's faster, cleaner, and safer than the previous approach. Of course, |
917 | you don't need to use a dollar sign. You could use your own scheme to |
918 | make it less confusing, like bracketed percent symbols, etc. |
919 | |
920 | $str = 'this has a %fred% and %barney% in it'; |
921 | $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all |
922 | |
a6dd486b |
923 | Another reason that folks sometimes think they want a variable to |
924 | contain the name of a variable is because they don't know how to build |
925 | proper data structures using hashes. For example, let's say they |
926 | wanted two hashes in their program: %fred and %barney, and that they |
927 | wanted to use another scalar variable to refer to those by name. |
d92eb7b0 |
928 | |
929 | $name = "fred"; |
930 | $$name{WIFE} = "wilma"; # set %fred |
931 | |
932 | $name = "barney"; |
933 | $$name{WIFE} = "betty"; # set %barney |
934 | |
935 | This is still a symbolic reference, and is still saddled with the |
936 | problems enumerated above. It would be far better to write: |
937 | |
938 | $folks{"fred"}{WIFE} = "wilma"; |
939 | $folks{"barney"}{WIFE} = "betty"; |
940 | |
941 | And just use a multilevel hash to start with. |
942 | |
943 | The only times that you absolutely I<must> use symbolic references are |
944 | when you really must refer to the symbol table. This may be because it's |
945 | something that can't take a real reference to, such as a format name. |
946 | Doing so may also be important for method calls, since these always go |
947 | through the symbol table for resolution. |
948 | |
949 | In those cases, you would turn off C<strict 'refs'> temporarily so you |
950 | can play around with the symbol table. For example: |
951 | |
952 | @colors = qw(red blue green yellow orange purple violet); |
953 | for my $name (@colors) { |
954 | no strict 'refs'; # renege for the block |
955 | *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; |
956 | } |
957 | |
958 | All those functions (red(), blue(), green(), etc.) appear to be separate, |
959 | but the real code in the closure actually was compiled only once. |
960 | |
961 | So, sometimes you might want to use symbolic references to directly |
962 | manipulate the symbol table. This doesn't matter for formats, handles, and |
a6dd486b |
963 | subroutines, because they are always global--you can't use my() on them. |
964 | For scalars, arrays, and hashes, though--and usually for subroutines-- |
965 | you probably only want to use hard references. |
d92eb7b0 |
966 | |
68dc0745 |
967 | =head1 AUTHOR AND COPYRIGHT |
968 | |
65acb1b1 |
969 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 |
970 | All rights reserved. |
971 | |
5a7beb56 |
972 | This documentation is free; you can redistribute it and/or modify it |
973 | under the same terms as Perl itself. |
5a964f20 |
974 | |
975 | Irrespective of its distribution, all code examples in this file |
976 | are hereby placed into the public domain. You are permitted and |
977 | encouraged to use this code in your own programs for fun |
978 | or for profit as you see fit. A simple comment in the code giving |
979 | credit would be courteous but is not required. |
a6dd486b |
980 | |