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1 | =head1 NAME |
2 | |
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3 | perlfaq7 - Perl Language Issues ($Revision: 1.3 $, $Date: 2001/10/19 14:39:24 $) |
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4 | |
5 | =head1 DESCRIPTION |
6 | |
7 | This section deals with general Perl language issues that don't |
8 | clearly fit into any of the other sections. |
9 | |
10 | =head2 Can I get a BNF/yacc/RE for the Perl language? |
11 | |
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12 | There is no BNF, but you can paw your way through the yacc grammar in |
13 | perly.y in the source distribution if you're particularly brave. The |
14 | grammar relies on very smart tokenizing code, so be prepared to |
15 | venture into toke.c as well. |
16 | |
17 | In the words of Chaim Frenkel: "Perl's grammar can not be reduced to BNF. |
18 | The work of parsing perl is distributed between yacc, the lexer, smoke |
19 | and mirrors." |
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20 | |
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21 | =head2 What are all these $@%&* punctuation signs, and how do I know when to use them? |
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22 | |
23 | They are type specifiers, as detailed in L<perldata>: |
24 | |
25 | $ for scalar values (number, string or reference) |
26 | @ for arrays |
27 | % for hashes (associative arrays) |
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28 | & for subroutines (aka functions, procedures, methods) |
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29 | * for all types of that symbol name. In version 4 you used them like |
30 | pointers, but in modern perls you can just use references. |
31 | |
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32 | There are couple of other symbols that you're likely to encounter that aren't |
33 | really type specifiers: |
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34 | |
35 | <> are used for inputting a record from a filehandle. |
36 | \ takes a reference to something. |
37 | |
c47ff5f1 |
38 | Note that <FILE> is I<neither> the type specifier for files |
39 | nor the name of the handle. It is the C<< <> >> operator applied |
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40 | to the handle FILE. It reads one line (well, record--see |
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41 | L<perlvar/$/>) 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 | |
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170 | The C<h2xs> program will create stubs for all the important stuff for you: |
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171 | |
172 | % h2xs -XA -n My::Module |
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173 | |
174 | The C<-X> switch tells C<h2xs> that you are not using C<XS> extension |
175 | code. The C<-A> switch tells C<h2xs> that you are not using the |
176 | AutoLoader, and the C<-n> switch specifies the name of the module. |
177 | See L<h2xs> for more details. |
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178 | |
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179 | =head2 How do I create a class? |
180 | |
181 | See L<perltoot> for an introduction to classes and objects, as well as |
182 | L<perlobj> and L<perlbot>. |
183 | |
184 | =head2 How can I tell if a variable is tainted? |
185 | |
186 | See L<perlsec/"Laundering and Detecting Tainted Data">. Here's an |
187 | example (which doesn't use any system calls, because the kill() |
188 | is given no processes to signal): |
189 | |
190 | sub is_tainted { |
191 | return ! eval { join('',@_), kill 0; 1; }; |
192 | } |
193 | |
194 | This is not C<-w> clean, however. There is no C<-w> clean way to |
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195 | detect taintedness--take this as a hint that you should untaint |
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196 | all possibly-tainted data. |
197 | |
198 | =head2 What's a closure? |
199 | |
200 | Closures are documented in L<perlref>. |
201 | |
202 | I<Closure> is a computer science term with a precise but |
203 | hard-to-explain meaning. Closures are implemented in Perl as anonymous |
204 | subroutines with lasting references to lexical variables outside their |
205 | own scopes. These lexicals magically refer to the variables that were |
206 | around when the subroutine was defined (deep binding). |
207 | |
208 | Closures make sense in any programming language where you can have the |
209 | return value of a function be itself a function, as you can in Perl. |
210 | Note that some languages provide anonymous functions but are not |
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211 | capable of providing proper closures: the Python language, for |
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212 | example. For more information on closures, check out any textbook on |
213 | functional programming. Scheme is a language that not only supports |
214 | but encourages closures. |
215 | |
216 | Here's a classic function-generating function: |
217 | |
218 | sub add_function_generator { |
219 | return sub { shift + shift }; |
220 | } |
221 | |
222 | $add_sub = add_function_generator(); |
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223 | $sum = $add_sub->(4,5); # $sum is 9 now. |
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224 | |
225 | The closure works as a I<function template> with some customization |
226 | slots left out to be filled later. The anonymous subroutine returned |
227 | by add_function_generator() isn't technically a closure because it |
228 | refers to no lexicals outside its own scope. |
229 | |
230 | Contrast this with the following make_adder() function, in which the |
231 | returned anonymous function contains a reference to a lexical variable |
232 | outside the scope of that function itself. Such a reference requires |
233 | that Perl return a proper closure, thus locking in for all time the |
234 | value that the lexical had when the function was created. |
235 | |
236 | sub make_adder { |
237 | my $addpiece = shift; |
238 | return sub { shift + $addpiece }; |
239 | } |
240 | |
241 | $f1 = make_adder(20); |
242 | $f2 = make_adder(555); |
243 | |
244 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas |
245 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece |
246 | in the closure sticks around. |
247 | |
248 | Closures are often used for less esoteric purposes. For example, when |
249 | you want to pass in a bit of code into a function: |
250 | |
251 | my $line; |
252 | timeout( 30, sub { $line = <STDIN> } ); |
253 | |
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254 | If the code to execute had been passed in as a string, |
255 | C<< '$line = <STDIN>' >>, there would have been no way for the |
256 | hypothetical timeout() function to access the lexical variable |
257 | $line back in its caller's scope. |
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258 | |
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259 | =head2 What is variable suicide and how can I prevent it? |
260 | |
261 | Variable suicide is when you (temporarily or permanently) lose the |
262 | value of a variable. It is caused by scoping through my() and local() |
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263 | interacting with either closures or aliased foreach() iterator |
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264 | variables and subroutine arguments. It used to be easy to |
265 | inadvertently lose a variable's value this way, but now it's much |
266 | harder. Take this code: |
267 | |
268 | my $f = "foo"; |
269 | sub T { |
270 | while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } |
271 | } |
272 | T; |
273 | print "Finally $f\n"; |
274 | |
275 | The $f that has "bar" added to it three times should be a new C<$f> |
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276 | (C<my $f> should create a new local variable each time through the loop). |
277 | It isn't, however. This was a bug, now fixed in the latest releases |
278 | (tested against 5.004_05, 5.005_03, and 5.005_56). |
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279 | |
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280 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? |
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281 | |
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282 | With the exception of regexes, you need to pass references to these |
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283 | objects. See L<perlsub/"Pass by Reference"> for this particular |
284 | question, and L<perlref> for information on references. |
285 | |
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286 | See ``Passing Regexes'', below, for information on passing regular |
287 | expressions. |
288 | |
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289 | =over 4 |
290 | |
291 | =item Passing Variables and Functions |
292 | |
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293 | Regular variables and functions are quite easy to pass: just pass in a |
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294 | reference to an existing or anonymous variable or function: |
295 | |
296 | func( \$some_scalar ); |
297 | |
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298 | func( \@some_array ); |
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299 | func( [ 1 .. 10 ] ); |
300 | |
301 | func( \%some_hash ); |
302 | func( { this => 10, that => 20 } ); |
303 | |
304 | func( \&some_func ); |
305 | func( sub { $_[0] ** $_[1] } ); |
306 | |
307 | =item Passing Filehandles |
308 | |
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309 | To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations. |
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310 | These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles"> |
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311 | and especially L<perlsub/"Pass by Reference"> for more information. |
312 | |
313 | Here's an excerpt: |
314 | |
315 | If you're passing around filehandles, you could usually just use the bare |
316 | typeglob, like *STDOUT, but typeglobs references would be better because |
317 | they'll still work properly under C<use strict 'refs'>. For example: |
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318 | |
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319 | splutter(\*STDOUT); |
320 | sub splutter { |
321 | my $fh = shift; |
322 | print $fh "her um well a hmmm\n"; |
323 | } |
324 | |
325 | $rec = get_rec(\*STDIN); |
326 | sub get_rec { |
327 | my $fh = shift; |
328 | return scalar <$fh>; |
329 | } |
330 | |
331 | If you're planning on generating new filehandles, you could do this: |
332 | |
333 | sub openit { |
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334 | my $path = shift; |
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335 | local *FH; |
336 | return open (FH, $path) ? *FH : undef; |
337 | } |
338 | $fh = openit('< /etc/motd'); |
339 | print <$fh>; |
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340 | |
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341 | =item Passing Regexes |
342 | |
343 | To pass regexes around, you'll need to be using a release of Perl |
344 | sufficiently recent as to support the C<qr//> construct, pass around |
345 | strings and use an exception-trapping eval, or else be very, very clever. |
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346 | |
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347 | Here's an example of how to pass in a string to be regex compared |
348 | using C<qr//>: |
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349 | |
350 | sub compare($$) { |
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351 | my ($val1, $regex) = @_; |
352 | my $retval = $val1 =~ /$regex/; |
353 | return $retval; |
354 | } |
355 | $match = compare("old McDonald", qr/d.*D/i); |
356 | |
357 | Notice how C<qr//> allows flags at the end. That pattern was compiled |
358 | at compile time, although it was executed later. The nifty C<qr//> |
359 | notation wasn't introduced until the 5.005 release. Before that, you |
360 | had to approach this problem much less intuitively. For example, here |
361 | it is again if you don't have C<qr//>: |
362 | |
363 | sub compare($$) { |
364 | my ($val1, $regex) = @_; |
365 | my $retval = eval { $val1 =~ /$regex/ }; |
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366 | die if $@; |
367 | return $retval; |
368 | } |
369 | |
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370 | $match = compare("old McDonald", q/($?i)d.*D/); |
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371 | |
372 | Make sure you never say something like this: |
373 | |
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374 | return eval "\$val =~ /$regex/"; # WRONG |
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375 | |
d92eb7b0 |
376 | or someone can sneak shell escapes into the regex due to the double |
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377 | interpolation of the eval and the double-quoted string. For example: |
378 | |
379 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; |
380 | |
381 | eval "\$string =~ /$pattern_of_evil/"; |
382 | |
383 | Those preferring to be very, very clever might see the O'Reilly book, |
384 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's |
385 | Build_MatchMany_Function() is particularly interesting. A complete |
386 | citation of this book is given in L<perlfaq2>. |
387 | |
388 | =item Passing Methods |
389 | |
390 | To pass an object method into a subroutine, you can do this: |
391 | |
392 | call_a_lot(10, $some_obj, "methname") |
393 | sub call_a_lot { |
394 | my ($count, $widget, $trick) = @_; |
395 | for (my $i = 0; $i < $count; $i++) { |
396 | $widget->$trick(); |
397 | } |
398 | } |
399 | |
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400 | Or, you can use a closure to bundle up the object, its |
401 | method call, and arguments: |
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402 | |
403 | my $whatnot = sub { $some_obj->obfuscate(@args) }; |
404 | func($whatnot); |
405 | sub func { |
406 | my $code = shift; |
407 | &$code(); |
408 | } |
409 | |
410 | You could also investigate the can() method in the UNIVERSAL class |
411 | (part of the standard perl distribution). |
412 | |
413 | =back |
414 | |
415 | =head2 How do I create a static variable? |
416 | |
417 | As with most things in Perl, TMTOWTDI. What is a "static variable" in |
418 | other languages could be either a function-private variable (visible |
419 | only within a single function, retaining its value between calls to |
420 | that function), or a file-private variable (visible only to functions |
421 | within the file it was declared in) in Perl. |
422 | |
423 | Here's code to implement a function-private variable: |
424 | |
425 | BEGIN { |
426 | my $counter = 42; |
427 | sub prev_counter { return --$counter } |
428 | sub next_counter { return $counter++ } |
429 | } |
430 | |
431 | Now prev_counter() and next_counter() share a private variable $counter |
432 | that was initialized at compile time. |
433 | |
434 | To declare a file-private variable, you'll still use a my(), putting |
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435 | the declaration at the outer scope level at the top of the file. |
436 | Assume this is in file Pax.pm: |
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437 | |
438 | package Pax; |
439 | my $started = scalar(localtime(time())); |
440 | |
441 | sub begun { return $started } |
442 | |
443 | When C<use Pax> or C<require Pax> loads this module, the variable will |
444 | be initialized. It won't get garbage-collected the way most variables |
445 | going out of scope do, because the begun() function cares about it, |
446 | but no one else can get it. It is not called $Pax::started because |
447 | its scope is unrelated to the package. It's scoped to the file. You |
448 | could conceivably have several packages in that same file all |
449 | accessing the same private variable, but another file with the same |
450 | package couldn't get to it. |
451 | |
c2611fb3 |
452 | See L<perlsub/"Persistent Private Variables"> for details. |
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453 | |
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454 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
455 | |
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456 | C<local($x)> saves away the old value of the global variable C<$x> |
457 | and assigns a new value for the duration of the subroutine I<which is |
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458 | visible in other functions called from that subroutine>. This is done |
459 | at run-time, so is called dynamic scoping. local() always affects global |
460 | variables, also called package variables or dynamic variables. |
461 | |
462 | C<my($x)> creates a new variable that is only visible in the current |
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463 | subroutine. This is done at compile-time, so it is called lexical or |
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464 | static scoping. my() always affects private variables, also called |
465 | lexical variables or (improperly) static(ly scoped) variables. |
466 | |
467 | For instance: |
468 | |
469 | sub visible { |
470 | print "var has value $var\n"; |
471 | } |
472 | |
473 | sub dynamic { |
474 | local $var = 'local'; # new temporary value for the still-global |
475 | visible(); # variable called $var |
476 | } |
477 | |
478 | sub lexical { |
479 | my $var = 'private'; # new private variable, $var |
480 | visible(); # (invisible outside of sub scope) |
481 | } |
482 | |
483 | $var = 'global'; |
484 | |
485 | visible(); # prints global |
486 | dynamic(); # prints local |
487 | lexical(); # prints global |
488 | |
489 | Notice how at no point does the value "private" get printed. That's |
490 | because $var only has that value within the block of the lexical() |
491 | function, and it is hidden from called subroutine. |
492 | |
493 | In summary, local() doesn't make what you think of as private, local |
494 | variables. It gives a global variable a temporary value. my() is |
495 | what you're looking for if you want private variables. |
496 | |
13a2d996 |
497 | See L<perlsub/"Private Variables via my()"> and |
498 | L<perlsub/"Temporary Values via local()"> for excruciating details. |
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499 | |
500 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? |
501 | |
502 | You can do this via symbolic references, provided you haven't set |
503 | C<use strict "refs">. So instead of $var, use C<${'var'}>. |
504 | |
505 | local $var = "global"; |
506 | my $var = "lexical"; |
507 | |
508 | print "lexical is $var\n"; |
509 | |
510 | no strict 'refs'; |
511 | print "global is ${'var'}\n"; |
512 | |
513 | If you know your package, you can just mention it explicitly, as in |
514 | $Some_Pack::var. Note that the notation $::var is I<not> the dynamic |
515 | $var in the current package, but rather the one in the C<main> |
516 | package, as though you had written $main::var. Specifying the package |
517 | directly makes you hard-code its name, but it executes faster and |
518 | avoids running afoul of C<use strict "refs">. |
519 | |
520 | =head2 What's the difference between deep and shallow binding? |
521 | |
522 | In deep binding, lexical variables mentioned in anonymous subroutines |
523 | are the same ones that were in scope when the subroutine was created. |
524 | In shallow binding, they are whichever variables with the same names |
525 | happen to be in scope when the subroutine is called. Perl always uses |
526 | deep binding of lexical variables (i.e., those created with my()). |
527 | However, dynamic variables (aka global, local, or package variables) |
528 | are effectively shallowly bound. Consider this just one more reason |
529 | not to use them. See the answer to L<"What's a closure?">. |
530 | |
c47ff5f1 |
531 | =head2 Why doesn't "my($foo) = <FILE>;" work right? |
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532 | |
c8db1d39 |
533 | C<my()> and C<local()> give list context to the right hand side |
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534 | of C<=>. The <FH> read operation, like so many of Perl's |
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535 | functions and operators, can tell which context it was called in and |
536 | behaves appropriately. In general, the scalar() function can help. |
537 | This function does nothing to the data itself (contrary to popular myth) |
538 | but rather tells its argument to behave in whatever its scalar fashion is. |
539 | If that function doesn't have a defined scalar behavior, this of course |
540 | doesn't help you (such as with sort()). |
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541 | |
542 | To enforce scalar context in this particular case, however, you need |
543 | merely omit the parentheses: |
544 | |
545 | local($foo) = <FILE>; # WRONG |
546 | local($foo) = scalar(<FILE>); # ok |
547 | local $foo = <FILE>; # right |
548 | |
549 | You should probably be using lexical variables anyway, although the |
550 | issue is the same here: |
551 | |
552 | my($foo) = <FILE>; # WRONG |
553 | my $foo = <FILE>; # right |
554 | |
54310121 |
555 | =head2 How do I redefine a builtin function, operator, or method? |
68dc0745 |
556 | |
557 | Why do you want to do that? :-) |
558 | |
559 | If you want to override a predefined function, such as open(), |
560 | then you'll have to import the new definition from a different |
4a4eefd0 |
561 | module. See L<perlsub/"Overriding Built-in Functions">. There's |
65acb1b1 |
562 | also an example in L<perltoot/"Class::Template">. |
68dc0745 |
563 | |
564 | If you want to overload a Perl operator, such as C<+> or C<**>, |
565 | then you'll want to use the C<use overload> pragma, documented |
566 | in L<overload>. |
567 | |
568 | If you're talking about obscuring method calls in parent classes, |
569 | see L<perltoot/"Overridden Methods">. |
570 | |
571 | =head2 What's the difference between calling a function as &foo and foo()? |
572 | |
573 | When you call a function as C<&foo>, you allow that function access to |
a6dd486b |
574 | your current @_ values, and you bypass prototypes. |
575 | The function doesn't get an empty @_--it gets yours! While not |
68dc0745 |
576 | strictly speaking a bug (it's documented that way in L<perlsub>), it |
577 | would be hard to consider this a feature in most cases. |
578 | |
c8db1d39 |
579 | When you call your function as C<&foo()>, then you I<do> get a new @_, |
68dc0745 |
580 | but prototyping is still circumvented. |
581 | |
582 | Normally, you want to call a function using C<foo()>. You may only |
583 | omit the parentheses if the function is already known to the compiler |
584 | because it already saw the definition (C<use> but not C<require>), |
585 | or via a forward reference or C<use subs> declaration. Even in this |
586 | case, you get a clean @_ without any of the old values leaking through |
587 | where they don't belong. |
588 | |
589 | =head2 How do I create a switch or case statement? |
590 | |
591 | This is explained in more depth in the L<perlsyn>. Briefly, there's |
592 | no official case statement, because of the variety of tests possible |
593 | in Perl (numeric comparison, string comparison, glob comparison, |
83df6a1d |
594 | regex matching, overloaded comparisons, ...). |
595 | Larry couldn't decide how best to do this, so he left it out, even |
596 | though it's been on the wish list since perl1. |
68dc0745 |
597 | |
83df6a1d |
598 | Starting from Perl 5.8 to get switch and case one can use the |
599 | Switch extension and say: |
600 | |
601 | use Switch; |
602 | |
603 | after which one has switch and case. It is not as fast as it could be |
604 | because it's not really part of the language (it's done using source |
605 | filters) but it is available, and it's very flexible. |
606 | |
607 | But if one wants to use pure Perl, the general answer is to write a |
608 | construct like this: |
c8db1d39 |
609 | |
610 | for ($variable_to_test) { |
611 | if (/pat1/) { } # do something |
612 | elsif (/pat2/) { } # do something else |
613 | elsif (/pat3/) { } # do something else |
614 | else { } # default |
615 | } |
68dc0745 |
616 | |
c8db1d39 |
617 | Here's a simple example of a switch based on pattern matching, this |
618 | time lined up in a way to make it look more like a switch statement. |
8305e449 |
619 | We'll do a multiway conditional based on the type of reference stored |
c8db1d39 |
620 | in $whatchamacallit: |
621 | |
622 | SWITCH: for (ref $whatchamacallit) { |
68dc0745 |
623 | |
624 | /^$/ && die "not a reference"; |
625 | |
626 | /SCALAR/ && do { |
627 | print_scalar($$ref); |
628 | last SWITCH; |
629 | }; |
630 | |
631 | /ARRAY/ && do { |
632 | print_array(@$ref); |
633 | last SWITCH; |
634 | }; |
635 | |
636 | /HASH/ && do { |
637 | print_hash(%$ref); |
638 | last SWITCH; |
639 | }; |
640 | |
641 | /CODE/ && do { |
642 | warn "can't print function ref"; |
643 | last SWITCH; |
644 | }; |
645 | |
646 | # DEFAULT |
647 | |
648 | warn "User defined type skipped"; |
649 | |
650 | } |
651 | |
c8db1d39 |
652 | See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other |
653 | examples in this style. |
654 | |
655 | Sometimes you should change the positions of the constant and the variable. |
656 | For example, let's say you wanted to test which of many answers you were |
657 | given, but in a case-insensitive way that also allows abbreviations. |
658 | You can use the following technique if the strings all start with |
a6dd486b |
659 | different characters or if you want to arrange the matches so that |
c8db1d39 |
660 | one takes precedence over another, as C<"SEND"> has precedence over |
661 | C<"STOP"> here: |
662 | |
663 | chomp($answer = <>); |
664 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } |
665 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } |
666 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } |
667 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } |
668 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } |
669 | |
670 | A totally different approach is to create a hash of function references. |
671 | |
672 | my %commands = ( |
673 | "happy" => \&joy, |
674 | "sad", => \&sullen, |
675 | "done" => sub { die "See ya!" }, |
676 | "mad" => \&angry, |
677 | ); |
678 | |
679 | print "How are you? "; |
680 | chomp($string = <STDIN>); |
681 | if ($commands{$string}) { |
682 | $commands{$string}->(); |
683 | } else { |
684 | print "No such command: $string\n"; |
685 | } |
686 | |
68dc0745 |
687 | =head2 How can I catch accesses to undefined variables/functions/methods? |
688 | |
689 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and |
690 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to |
691 | undefined functions and methods. |
692 | |
693 | When it comes to undefined variables that would trigger a warning |
694 | under C<-w>, you can use a handler to trap the pseudo-signal |
695 | C<__WARN__> like this: |
696 | |
697 | $SIG{__WARN__} = sub { |
698 | |
c8db1d39 |
699 | for ( $_[0] ) { # voici un switch statement |
68dc0745 |
700 | |
701 | /Use of uninitialized value/ && do { |
702 | # promote warning to a fatal |
703 | die $_; |
704 | }; |
705 | |
706 | # other warning cases to catch could go here; |
707 | |
708 | warn $_; |
709 | } |
710 | |
711 | }; |
712 | |
713 | =head2 Why can't a method included in this same file be found? |
714 | |
715 | Some possible reasons: your inheritance is getting confused, you've |
716 | misspelled the method name, or the object is of the wrong type. Check |
a6dd486b |
717 | out L<perltoot> for details about any of the above cases. You may |
718 | also use C<print ref($object)> to find out the class C<$object> was |
719 | blessed into. |
68dc0745 |
720 | |
721 | Another possible reason for problems is because you've used the |
722 | indirect object syntax (eg, C<find Guru "Samy">) on a class name |
723 | before Perl has seen that such a package exists. It's wisest to make |
724 | sure your packages are all defined before you start using them, which |
725 | will be taken care of if you use the C<use> statement instead of |
a6dd486b |
726 | C<require>. If not, make sure to use arrow notation (eg., |
c47ff5f1 |
727 | C<< Guru->find("Samy") >>) instead. Object notation is explained in |
68dc0745 |
728 | L<perlobj>. |
729 | |
c8db1d39 |
730 | Make sure to read about creating modules in L<perlmod> and |
ae93639c |
731 | the perils of indirect objects in L<perlobj/"Method Invocation">. |
c8db1d39 |
732 | |
68dc0745 |
733 | =head2 How can I find out my current package? |
734 | |
735 | If you're just a random program, you can do this to find |
736 | out what the currently compiled package is: |
737 | |
c8db1d39 |
738 | my $packname = __PACKAGE__; |
68dc0745 |
739 | |
a6dd486b |
740 | But, if you're a method and you want to print an error message |
68dc0745 |
741 | that includes the kind of object you were called on (which is |
742 | not necessarily the same as the one in which you were compiled): |
743 | |
744 | sub amethod { |
92c2ed05 |
745 | my $self = shift; |
68dc0745 |
746 | my $class = ref($self) || $self; |
747 | warn "called me from a $class object"; |
748 | } |
749 | |
46fc3d4c |
750 | =head2 How can I comment out a large block of perl code? |
751 | |
752 | Use embedded POD to discard it: |
753 | |
754 | # program is here |
755 | |
756 | =for nobody |
757 | This paragraph is commented out |
758 | |
759 | # program continues |
760 | |
761 | =begin comment text |
762 | |
763 | all of this stuff |
764 | |
765 | here will be ignored |
766 | by everyone |
767 | |
768 | =end comment text |
769 | |
fc36a67e |
770 | =cut |
771 | |
c8db1d39 |
772 | This can't go just anywhere. You have to put a pod directive where |
773 | the parser is expecting a new statement, not just in the middle |
774 | of an expression or some other arbitrary yacc grammar production. |
775 | |
65acb1b1 |
776 | =head2 How do I clear a package? |
777 | |
778 | Use this code, provided by Mark-Jason Dominus: |
779 | |
780 | sub scrub_package { |
781 | no strict 'refs'; |
782 | my $pack = shift; |
783 | die "Shouldn't delete main package" |
784 | if $pack eq "" || $pack eq "main"; |
785 | my $stash = *{$pack . '::'}{HASH}; |
786 | my $name; |
787 | foreach $name (keys %$stash) { |
788 | my $fullname = $pack . '::' . $name; |
789 | # Get rid of everything with that name. |
790 | undef $$fullname; |
791 | undef @$fullname; |
792 | undef %$fullname; |
793 | undef &$fullname; |
794 | undef *$fullname; |
795 | } |
796 | } |
797 | |
798 | Or, if you're using a recent release of Perl, you can |
799 | just use the Symbol::delete_package() function instead. |
800 | |
d92eb7b0 |
801 | =head2 How can I use a variable as a variable name? |
802 | |
803 | Beginners often think they want to have a variable contain the name |
804 | of a variable. |
805 | |
806 | $fred = 23; |
807 | $varname = "fred"; |
808 | ++$$varname; # $fred now 24 |
809 | |
810 | This works I<sometimes>, but it is a very bad idea for two reasons. |
811 | |
a6dd486b |
812 | The first reason is that this technique I<only works on global |
813 | variables>. That means that if $fred is a lexical variable created |
814 | with my() in the above example, the code wouldn't work at all: you'd |
815 | accidentally access the global and skip right over the private lexical |
816 | altogether. Global variables are bad because they can easily collide |
817 | accidentally and in general make for non-scalable and confusing code. |
d92eb7b0 |
818 | |
819 | Symbolic references are forbidden under the C<use strict> pragma. |
820 | They are not true references and consequently are not reference counted |
821 | or garbage collected. |
822 | |
823 | The other reason why using a variable to hold the name of another |
a6dd486b |
824 | variable is a bad idea is that the question often stems from a lack of |
d92eb7b0 |
825 | understanding of Perl data structures, particularly hashes. By using |
826 | symbolic references, you are just using the package's symbol-table hash |
827 | (like C<%main::>) instead of a user-defined hash. The solution is to |
828 | use your own hash or a real reference instead. |
829 | |
830 | $fred = 23; |
831 | $varname = "fred"; |
832 | $USER_VARS{$varname}++; # not $$varname++ |
833 | |
834 | There we're using the %USER_VARS hash instead of symbolic references. |
835 | Sometimes this comes up in reading strings from the user with variable |
836 | references and wanting to expand them to the values of your perl |
837 | program's variables. This is also a bad idea because it conflates the |
838 | program-addressable namespace and the user-addressable one. Instead of |
839 | reading a string and expanding it to the actual contents of your program's |
840 | own variables: |
841 | |
842 | $str = 'this has a $fred and $barney in it'; |
843 | $str =~ s/(\$\w+)/$1/eeg; # need double eval |
844 | |
a6dd486b |
845 | it would be better to keep a hash around like %USER_VARS and have |
d92eb7b0 |
846 | variable references actually refer to entries in that hash: |
847 | |
848 | $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all |
849 | |
850 | That's faster, cleaner, and safer than the previous approach. Of course, |
851 | you don't need to use a dollar sign. You could use your own scheme to |
852 | make it less confusing, like bracketed percent symbols, etc. |
853 | |
854 | $str = 'this has a %fred% and %barney% in it'; |
855 | $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all |
856 | |
a6dd486b |
857 | Another reason that folks sometimes think they want a variable to |
858 | contain the name of a variable is because they don't know how to build |
859 | proper data structures using hashes. For example, let's say they |
860 | wanted two hashes in their program: %fred and %barney, and that they |
861 | wanted to use another scalar variable to refer to those by name. |
d92eb7b0 |
862 | |
863 | $name = "fred"; |
864 | $$name{WIFE} = "wilma"; # set %fred |
865 | |
866 | $name = "barney"; |
867 | $$name{WIFE} = "betty"; # set %barney |
868 | |
869 | This is still a symbolic reference, and is still saddled with the |
870 | problems enumerated above. It would be far better to write: |
871 | |
872 | $folks{"fred"}{WIFE} = "wilma"; |
873 | $folks{"barney"}{WIFE} = "betty"; |
874 | |
875 | And just use a multilevel hash to start with. |
876 | |
877 | The only times that you absolutely I<must> use symbolic references are |
878 | when you really must refer to the symbol table. This may be because it's |
879 | something that can't take a real reference to, such as a format name. |
880 | Doing so may also be important for method calls, since these always go |
881 | through the symbol table for resolution. |
882 | |
883 | In those cases, you would turn off C<strict 'refs'> temporarily so you |
884 | can play around with the symbol table. For example: |
885 | |
886 | @colors = qw(red blue green yellow orange purple violet); |
887 | for my $name (@colors) { |
888 | no strict 'refs'; # renege for the block |
889 | *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; |
890 | } |
891 | |
892 | All those functions (red(), blue(), green(), etc.) appear to be separate, |
893 | but the real code in the closure actually was compiled only once. |
894 | |
895 | So, sometimes you might want to use symbolic references to directly |
896 | manipulate the symbol table. This doesn't matter for formats, handles, and |
a6dd486b |
897 | subroutines, because they are always global--you can't use my() on them. |
898 | For scalars, arrays, and hashes, though--and usually for subroutines-- |
899 | you probably only want to use hard references. |
d92eb7b0 |
900 | |
68dc0745 |
901 | =head1 AUTHOR AND COPYRIGHT |
902 | |
65acb1b1 |
903 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 |
904 | All rights reserved. |
905 | |
5a7beb56 |
906 | This documentation is free; you can redistribute it and/or modify it |
907 | under the same terms as Perl itself. |
5a964f20 |
908 | |
909 | Irrespective of its distribution, all code examples in this file |
910 | are hereby placed into the public domain. You are permitted and |
911 | encouraged to use this code in your own programs for fun |
912 | or for profit as you see fit. A simple comment in the code giving |
913 | credit would be courteous but is not required. |
a6dd486b |
914 | |