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1 | =head1 NAME |
2 | |
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3 | perlfaq7 - Perl Language Issues ($Revision: 1.28 $, $Date: 1999/05/23 20:36:18 $) |
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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 | A couple of others that you're likely to encounter that aren't |
33 | really type specifiers are: |
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 |
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 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 | |
109 | A way of calling compiled C code from Perl. Reading L<perlxstut> |
110 | is a good place to learn more about extensions. |
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 |
117 | on their right, eg print, chmod, exec, and so on. Such functions are |
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: 1.28 $=~/\d+/g;sprintf '%d.'.'%02d'x$#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 | |
199 | # non-exported package globals go here |
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200 | our @more; |
201 | our $stuff; |
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202 | |
203 | # initialize package globals, first exported ones |
204 | $Var1 = ''; |
205 | %Hashit = (); |
206 | |
207 | # then the others (which are still accessible as $Some::Module::stuff) |
208 | $stuff = ''; |
209 | @more = (); |
210 | |
211 | # all file-scoped lexicals must be created before |
212 | # the functions below that use them. |
213 | |
214 | # file-private lexicals go here |
215 | my $priv_var = ''; |
216 | my %secret_hash = (); |
217 | |
218 | # here's a file-private function as a closure, |
219 | # callable as &$priv_func; it cannot be prototyped. |
220 | my $priv_func = sub { |
221 | # stuff goes here. |
222 | }; |
223 | |
224 | # make all your functions, whether exported or not; |
225 | # remember to put something interesting in the {} stubs |
226 | sub func1 {} # no prototype |
227 | sub func2() {} # proto'd void |
228 | sub func3($$) {} # proto'd to 2 scalars |
229 | |
230 | # this one isn't exported, but could be called! |
231 | sub func4(\%) {} # proto'd to 1 hash ref |
232 | |
233 | END { } # module clean-up code here (global destructor) |
234 | |
235 | 1; # modules must return true |
236 | |
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237 | The h2xs program will create stubs for all the important stuff for you: |
238 | |
239 | % h2xs -XA -n My::Module |
240 | |
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241 | =head2 How do I create a class? |
242 | |
243 | See L<perltoot> for an introduction to classes and objects, as well as |
244 | L<perlobj> and L<perlbot>. |
245 | |
246 | =head2 How can I tell if a variable is tainted? |
247 | |
248 | See L<perlsec/"Laundering and Detecting Tainted Data">. Here's an |
249 | example (which doesn't use any system calls, because the kill() |
250 | is given no processes to signal): |
251 | |
252 | sub is_tainted { |
253 | return ! eval { join('',@_), kill 0; 1; }; |
254 | } |
255 | |
256 | This is not C<-w> clean, however. There is no C<-w> clean way to |
257 | detect taintedness - take this as a hint that you should untaint |
258 | all possibly-tainted data. |
259 | |
260 | =head2 What's a closure? |
261 | |
262 | Closures are documented in L<perlref>. |
263 | |
264 | I<Closure> is a computer science term with a precise but |
265 | hard-to-explain meaning. Closures are implemented in Perl as anonymous |
266 | subroutines with lasting references to lexical variables outside their |
267 | own scopes. These lexicals magically refer to the variables that were |
268 | around when the subroutine was defined (deep binding). |
269 | |
270 | Closures make sense in any programming language where you can have the |
271 | return value of a function be itself a function, as you can in Perl. |
272 | Note that some languages provide anonymous functions but are not |
273 | capable of providing proper closures; the Python language, for |
274 | example. For more information on closures, check out any textbook on |
275 | functional programming. Scheme is a language that not only supports |
276 | but encourages closures. |
277 | |
278 | Here's a classic function-generating function: |
279 | |
280 | sub add_function_generator { |
281 | return sub { shift + shift }; |
282 | } |
283 | |
284 | $add_sub = add_function_generator(); |
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285 | $sum = $add_sub->(4,5); # $sum is 9 now. |
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286 | |
287 | The closure works as a I<function template> with some customization |
288 | slots left out to be filled later. The anonymous subroutine returned |
289 | by add_function_generator() isn't technically a closure because it |
290 | refers to no lexicals outside its own scope. |
291 | |
292 | Contrast this with the following make_adder() function, in which the |
293 | returned anonymous function contains a reference to a lexical variable |
294 | outside the scope of that function itself. Such a reference requires |
295 | that Perl return a proper closure, thus locking in for all time the |
296 | value that the lexical had when the function was created. |
297 | |
298 | sub make_adder { |
299 | my $addpiece = shift; |
300 | return sub { shift + $addpiece }; |
301 | } |
302 | |
303 | $f1 = make_adder(20); |
304 | $f2 = make_adder(555); |
305 | |
306 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas |
307 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece |
308 | in the closure sticks around. |
309 | |
310 | Closures are often used for less esoteric purposes. For example, when |
311 | you want to pass in a bit of code into a function: |
312 | |
313 | my $line; |
314 | timeout( 30, sub { $line = <STDIN> } ); |
315 | |
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316 | If the code to execute had been passed in as a string, |
317 | C<< '$line = <STDIN>' >>, there would have been no way for the |
318 | hypothetical timeout() function to access the lexical variable |
319 | $line back in its caller's scope. |
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320 | |
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321 | =head2 What is variable suicide and how can I prevent it? |
322 | |
323 | Variable suicide is when you (temporarily or permanently) lose the |
324 | value of a variable. It is caused by scoping through my() and local() |
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325 | interacting with either closures or aliased foreach() iterator |
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326 | variables and subroutine arguments. It used to be easy to |
327 | inadvertently lose a variable's value this way, but now it's much |
328 | harder. Take this code: |
329 | |
330 | my $f = "foo"; |
331 | sub T { |
332 | while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } |
333 | } |
334 | T; |
335 | print "Finally $f\n"; |
336 | |
337 | The $f that has "bar" added to it three times should be a new C<$f> |
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338 | (C<my $f> should create a new local variable each time through the loop). |
339 | It isn't, however. This was a bug, now fixed in the latest releases |
340 | (tested against 5.004_05, 5.005_03, and 5.005_56). |
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341 | |
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342 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? |
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343 | |
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344 | With the exception of regexes, you need to pass references to these |
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345 | objects. See L<perlsub/"Pass by Reference"> for this particular |
346 | question, and L<perlref> for information on references. |
347 | |
348 | =over 4 |
349 | |
350 | =item Passing Variables and Functions |
351 | |
352 | Regular variables and functions are quite easy: just pass in a |
353 | reference to an existing or anonymous variable or function: |
354 | |
355 | func( \$some_scalar ); |
356 | |
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357 | func( \@some_array ); |
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358 | func( [ 1 .. 10 ] ); |
359 | |
360 | func( \%some_hash ); |
361 | func( { this => 10, that => 20 } ); |
362 | |
363 | func( \&some_func ); |
364 | func( sub { $_[0] ** $_[1] } ); |
365 | |
366 | =item Passing Filehandles |
367 | |
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368 | To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations. |
369 | These are "typeglobs" - see L<perldata/"Typeglobs and Filehandles"> |
370 | and especially L<perlsub/"Pass by Reference"> for more information. |
371 | |
372 | Here's an excerpt: |
373 | |
374 | If you're passing around filehandles, you could usually just use the bare |
375 | typeglob, like *STDOUT, but typeglobs references would be better because |
376 | they'll still work properly under C<use strict 'refs'>. For example: |
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377 | |
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378 | splutter(\*STDOUT); |
379 | sub splutter { |
380 | my $fh = shift; |
381 | print $fh "her um well a hmmm\n"; |
382 | } |
383 | |
384 | $rec = get_rec(\*STDIN); |
385 | sub get_rec { |
386 | my $fh = shift; |
387 | return scalar <$fh>; |
388 | } |
389 | |
390 | If you're planning on generating new filehandles, you could do this: |
391 | |
392 | sub openit { |
393 | my $name = shift; |
394 | local *FH; |
395 | return open (FH, $path) ? *FH : undef; |
396 | } |
397 | $fh = openit('< /etc/motd'); |
398 | print <$fh>; |
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399 | |
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400 | =item Passing Regexes |
401 | |
402 | To pass regexes around, you'll need to be using a release of Perl |
403 | sufficiently recent as to support the C<qr//> construct, pass around |
404 | strings and use an exception-trapping eval, or else be very, very clever. |
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405 | |
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406 | Here's an example of how to pass in a string to be regex compared |
407 | using C<qr//>: |
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408 | |
409 | sub compare($$) { |
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410 | my ($val1, $regex) = @_; |
411 | my $retval = $val1 =~ /$regex/; |
412 | return $retval; |
413 | } |
414 | $match = compare("old McDonald", qr/d.*D/i); |
415 | |
416 | Notice how C<qr//> allows flags at the end. That pattern was compiled |
417 | at compile time, although it was executed later. The nifty C<qr//> |
418 | notation wasn't introduced until the 5.005 release. Before that, you |
419 | had to approach this problem much less intuitively. For example, here |
420 | it is again if you don't have C<qr//>: |
421 | |
422 | sub compare($$) { |
423 | my ($val1, $regex) = @_; |
424 | my $retval = eval { $val1 =~ /$regex/ }; |
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425 | die if $@; |
426 | return $retval; |
427 | } |
428 | |
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429 | $match = compare("old McDonald", q/($?i)d.*D/); |
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430 | |
431 | Make sure you never say something like this: |
432 | |
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433 | return eval "\$val =~ /$regex/"; # WRONG |
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434 | |
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435 | or someone can sneak shell escapes into the regex due to the double |
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436 | interpolation of the eval and the double-quoted string. For example: |
437 | |
438 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; |
439 | |
440 | eval "\$string =~ /$pattern_of_evil/"; |
441 | |
442 | Those preferring to be very, very clever might see the O'Reilly book, |
443 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's |
444 | Build_MatchMany_Function() is particularly interesting. A complete |
445 | citation of this book is given in L<perlfaq2>. |
446 | |
447 | =item Passing Methods |
448 | |
449 | To pass an object method into a subroutine, you can do this: |
450 | |
451 | call_a_lot(10, $some_obj, "methname") |
452 | sub call_a_lot { |
453 | my ($count, $widget, $trick) = @_; |
454 | for (my $i = 0; $i < $count; $i++) { |
455 | $widget->$trick(); |
456 | } |
457 | } |
458 | |
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459 | Or you can use a closure to bundle up the object and its method call |
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460 | and arguments: |
461 | |
462 | my $whatnot = sub { $some_obj->obfuscate(@args) }; |
463 | func($whatnot); |
464 | sub func { |
465 | my $code = shift; |
466 | &$code(); |
467 | } |
468 | |
469 | You could also investigate the can() method in the UNIVERSAL class |
470 | (part of the standard perl distribution). |
471 | |
472 | =back |
473 | |
474 | =head2 How do I create a static variable? |
475 | |
476 | As with most things in Perl, TMTOWTDI. What is a "static variable" in |
477 | other languages could be either a function-private variable (visible |
478 | only within a single function, retaining its value between calls to |
479 | that function), or a file-private variable (visible only to functions |
480 | within the file it was declared in) in Perl. |
481 | |
482 | Here's code to implement a function-private variable: |
483 | |
484 | BEGIN { |
485 | my $counter = 42; |
486 | sub prev_counter { return --$counter } |
487 | sub next_counter { return $counter++ } |
488 | } |
489 | |
490 | Now prev_counter() and next_counter() share a private variable $counter |
491 | that was initialized at compile time. |
492 | |
493 | To declare a file-private variable, you'll still use a my(), putting |
494 | it at the outer scope level at the top of the file. Assume this is in |
495 | file Pax.pm: |
496 | |
497 | package Pax; |
498 | my $started = scalar(localtime(time())); |
499 | |
500 | sub begun { return $started } |
501 | |
502 | When C<use Pax> or C<require Pax> loads this module, the variable will |
503 | be initialized. It won't get garbage-collected the way most variables |
504 | going out of scope do, because the begun() function cares about it, |
505 | but no one else can get it. It is not called $Pax::started because |
506 | its scope is unrelated to the package. It's scoped to the file. You |
507 | could conceivably have several packages in that same file all |
508 | accessing the same private variable, but another file with the same |
509 | package couldn't get to it. |
510 | |
c2611fb3 |
511 | See L<perlsub/"Persistent Private Variables"> for details. |
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512 | |
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513 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
514 | |
515 | C<local($x)> saves away the old value of the global variable C<$x>, |
516 | and assigns a new value for the duration of the subroutine, I<which is |
517 | visible in other functions called from that subroutine>. This is done |
518 | at run-time, so is called dynamic scoping. local() always affects global |
519 | variables, also called package variables or dynamic variables. |
520 | |
521 | C<my($x)> creates a new variable that is only visible in the current |
522 | subroutine. This is done at compile-time, so is called lexical or |
523 | static scoping. my() always affects private variables, also called |
524 | lexical variables or (improperly) static(ly scoped) variables. |
525 | |
526 | For instance: |
527 | |
528 | sub visible { |
529 | print "var has value $var\n"; |
530 | } |
531 | |
532 | sub dynamic { |
533 | local $var = 'local'; # new temporary value for the still-global |
534 | visible(); # variable called $var |
535 | } |
536 | |
537 | sub lexical { |
538 | my $var = 'private'; # new private variable, $var |
539 | visible(); # (invisible outside of sub scope) |
540 | } |
541 | |
542 | $var = 'global'; |
543 | |
544 | visible(); # prints global |
545 | dynamic(); # prints local |
546 | lexical(); # prints global |
547 | |
548 | Notice how at no point does the value "private" get printed. That's |
549 | because $var only has that value within the block of the lexical() |
550 | function, and it is hidden from called subroutine. |
551 | |
552 | In summary, local() doesn't make what you think of as private, local |
553 | variables. It gives a global variable a temporary value. my() is |
554 | what you're looking for if you want private variables. |
555 | |
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556 | See L<perlsub/"Private Variables via my()"> and L<perlsub/"Temporary |
557 | Values via local()"> for excruciating details. |
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558 | |
559 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? |
560 | |
561 | You can do this via symbolic references, provided you haven't set |
562 | C<use strict "refs">. So instead of $var, use C<${'var'}>. |
563 | |
564 | local $var = "global"; |
565 | my $var = "lexical"; |
566 | |
567 | print "lexical is $var\n"; |
568 | |
569 | no strict 'refs'; |
570 | print "global is ${'var'}\n"; |
571 | |
572 | If you know your package, you can just mention it explicitly, as in |
573 | $Some_Pack::var. Note that the notation $::var is I<not> the dynamic |
574 | $var in the current package, but rather the one in the C<main> |
575 | package, as though you had written $main::var. Specifying the package |
576 | directly makes you hard-code its name, but it executes faster and |
577 | avoids running afoul of C<use strict "refs">. |
578 | |
579 | =head2 What's the difference between deep and shallow binding? |
580 | |
581 | In deep binding, lexical variables mentioned in anonymous subroutines |
582 | are the same ones that were in scope when the subroutine was created. |
583 | In shallow binding, they are whichever variables with the same names |
584 | happen to be in scope when the subroutine is called. Perl always uses |
585 | deep binding of lexical variables (i.e., those created with my()). |
586 | However, dynamic variables (aka global, local, or package variables) |
587 | are effectively shallowly bound. Consider this just one more reason |
588 | not to use them. See the answer to L<"What's a closure?">. |
589 | |
c47ff5f1 |
590 | =head2 Why doesn't "my($foo) = <FILE>;" work right? |
68dc0745 |
591 | |
c8db1d39 |
592 | C<my()> and C<local()> give list context to the right hand side |
c47ff5f1 |
593 | of C<=>. The <FH> read operation, like so many of Perl's |
c8db1d39 |
594 | functions and operators, can tell which context it was called in and |
595 | behaves appropriately. In general, the scalar() function can help. |
596 | This function does nothing to the data itself (contrary to popular myth) |
597 | but rather tells its argument to behave in whatever its scalar fashion is. |
598 | If that function doesn't have a defined scalar behavior, this of course |
599 | doesn't help you (such as with sort()). |
68dc0745 |
600 | |
601 | To enforce scalar context in this particular case, however, you need |
602 | merely omit the parentheses: |
603 | |
604 | local($foo) = <FILE>; # WRONG |
605 | local($foo) = scalar(<FILE>); # ok |
606 | local $foo = <FILE>; # right |
607 | |
608 | You should probably be using lexical variables anyway, although the |
609 | issue is the same here: |
610 | |
611 | my($foo) = <FILE>; # WRONG |
612 | my $foo = <FILE>; # right |
613 | |
54310121 |
614 | =head2 How do I redefine a builtin function, operator, or method? |
68dc0745 |
615 | |
616 | Why do you want to do that? :-) |
617 | |
618 | If you want to override a predefined function, such as open(), |
619 | then you'll have to import the new definition from a different |
4a4eefd0 |
620 | module. See L<perlsub/"Overriding Built-in Functions">. There's |
65acb1b1 |
621 | also an example in L<perltoot/"Class::Template">. |
68dc0745 |
622 | |
623 | If you want to overload a Perl operator, such as C<+> or C<**>, |
624 | then you'll want to use the C<use overload> pragma, documented |
625 | in L<overload>. |
626 | |
627 | If you're talking about obscuring method calls in parent classes, |
628 | see L<perltoot/"Overridden Methods">. |
629 | |
630 | =head2 What's the difference between calling a function as &foo and foo()? |
631 | |
632 | When you call a function as C<&foo>, you allow that function access to |
633 | your current @_ values, and you by-pass prototypes. That means that |
634 | the function doesn't get an empty @_, it gets yours! While not |
635 | strictly speaking a bug (it's documented that way in L<perlsub>), it |
636 | would be hard to consider this a feature in most cases. |
637 | |
c8db1d39 |
638 | When you call your function as C<&foo()>, then you I<do> get a new @_, |
68dc0745 |
639 | but prototyping is still circumvented. |
640 | |
641 | Normally, you want to call a function using C<foo()>. You may only |
642 | omit the parentheses if the function is already known to the compiler |
643 | because it already saw the definition (C<use> but not C<require>), |
644 | or via a forward reference or C<use subs> declaration. Even in this |
645 | case, you get a clean @_ without any of the old values leaking through |
646 | where they don't belong. |
647 | |
648 | =head2 How do I create a switch or case statement? |
649 | |
650 | This is explained in more depth in the L<perlsyn>. Briefly, there's |
651 | no official case statement, because of the variety of tests possible |
652 | in Perl (numeric comparison, string comparison, glob comparison, |
d92eb7b0 |
653 | regex matching, overloaded comparisons, ...). Larry couldn't decide |
68dc0745 |
654 | how best to do this, so he left it out, even though it's been on the |
655 | wish list since perl1. |
656 | |
c8db1d39 |
657 | The general answer is to write a construct like this: |
658 | |
659 | for ($variable_to_test) { |
660 | if (/pat1/) { } # do something |
661 | elsif (/pat2/) { } # do something else |
662 | elsif (/pat3/) { } # do something else |
663 | else { } # default |
664 | } |
68dc0745 |
665 | |
c8db1d39 |
666 | Here's a simple example of a switch based on pattern matching, this |
667 | time lined up in a way to make it look more like a switch statement. |
668 | We'll do a multi-way conditional based on the type of reference stored |
669 | in $whatchamacallit: |
670 | |
671 | SWITCH: for (ref $whatchamacallit) { |
68dc0745 |
672 | |
673 | /^$/ && die "not a reference"; |
674 | |
675 | /SCALAR/ && do { |
676 | print_scalar($$ref); |
677 | last SWITCH; |
678 | }; |
679 | |
680 | /ARRAY/ && do { |
681 | print_array(@$ref); |
682 | last SWITCH; |
683 | }; |
684 | |
685 | /HASH/ && do { |
686 | print_hash(%$ref); |
687 | last SWITCH; |
688 | }; |
689 | |
690 | /CODE/ && do { |
691 | warn "can't print function ref"; |
692 | last SWITCH; |
693 | }; |
694 | |
695 | # DEFAULT |
696 | |
697 | warn "User defined type skipped"; |
698 | |
699 | } |
700 | |
c8db1d39 |
701 | See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other |
702 | examples in this style. |
703 | |
704 | Sometimes you should change the positions of the constant and the variable. |
705 | For example, let's say you wanted to test which of many answers you were |
706 | given, but in a case-insensitive way that also allows abbreviations. |
707 | You can use the following technique if the strings all start with |
708 | different characters, or if you want to arrange the matches so that |
709 | one takes precedence over another, as C<"SEND"> has precedence over |
710 | C<"STOP"> here: |
711 | |
712 | chomp($answer = <>); |
713 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } |
714 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } |
715 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } |
716 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } |
717 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } |
718 | |
719 | A totally different approach is to create a hash of function references. |
720 | |
721 | my %commands = ( |
722 | "happy" => \&joy, |
723 | "sad", => \&sullen, |
724 | "done" => sub { die "See ya!" }, |
725 | "mad" => \&angry, |
726 | ); |
727 | |
728 | print "How are you? "; |
729 | chomp($string = <STDIN>); |
730 | if ($commands{$string}) { |
731 | $commands{$string}->(); |
732 | } else { |
733 | print "No such command: $string\n"; |
734 | } |
735 | |
68dc0745 |
736 | =head2 How can I catch accesses to undefined variables/functions/methods? |
737 | |
738 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and |
739 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to |
740 | undefined functions and methods. |
741 | |
742 | When it comes to undefined variables that would trigger a warning |
743 | under C<-w>, you can use a handler to trap the pseudo-signal |
744 | C<__WARN__> like this: |
745 | |
746 | $SIG{__WARN__} = sub { |
747 | |
c8db1d39 |
748 | for ( $_[0] ) { # voici un switch statement |
68dc0745 |
749 | |
750 | /Use of uninitialized value/ && do { |
751 | # promote warning to a fatal |
752 | die $_; |
753 | }; |
754 | |
755 | # other warning cases to catch could go here; |
756 | |
757 | warn $_; |
758 | } |
759 | |
760 | }; |
761 | |
762 | =head2 Why can't a method included in this same file be found? |
763 | |
764 | Some possible reasons: your inheritance is getting confused, you've |
765 | misspelled the method name, or the object is of the wrong type. Check |
766 | out L<perltoot> for details on these. You may also use C<print |
767 | ref($object)> to find out the class C<$object> was blessed into. |
768 | |
769 | Another possible reason for problems is because you've used the |
770 | indirect object syntax (eg, C<find Guru "Samy">) on a class name |
771 | before Perl has seen that such a package exists. It's wisest to make |
772 | sure your packages are all defined before you start using them, which |
773 | will be taken care of if you use the C<use> statement instead of |
774 | C<require>. If not, make sure to use arrow notation (eg, |
c47ff5f1 |
775 | C<< Guru->find("Samy") >>) instead. Object notation is explained in |
68dc0745 |
776 | L<perlobj>. |
777 | |
c8db1d39 |
778 | Make sure to read about creating modules in L<perlmod> and |
779 | the perils of indirect objects in L<perlobj/"WARNING">. |
780 | |
68dc0745 |
781 | =head2 How can I find out my current package? |
782 | |
783 | If you're just a random program, you can do this to find |
784 | out what the currently compiled package is: |
785 | |
c8db1d39 |
786 | my $packname = __PACKAGE__; |
68dc0745 |
787 | |
788 | But if you're a method and you want to print an error message |
789 | that includes the kind of object you were called on (which is |
790 | not necessarily the same as the one in which you were compiled): |
791 | |
792 | sub amethod { |
92c2ed05 |
793 | my $self = shift; |
68dc0745 |
794 | my $class = ref($self) || $self; |
795 | warn "called me from a $class object"; |
796 | } |
797 | |
46fc3d4c |
798 | =head2 How can I comment out a large block of perl code? |
799 | |
800 | Use embedded POD to discard it: |
801 | |
802 | # program is here |
803 | |
804 | =for nobody |
805 | This paragraph is commented out |
806 | |
807 | # program continues |
808 | |
809 | =begin comment text |
810 | |
811 | all of this stuff |
812 | |
813 | here will be ignored |
814 | by everyone |
815 | |
816 | =end comment text |
817 | |
fc36a67e |
818 | =cut |
819 | |
c8db1d39 |
820 | This can't go just anywhere. You have to put a pod directive where |
821 | the parser is expecting a new statement, not just in the middle |
822 | of an expression or some other arbitrary yacc grammar production. |
823 | |
65acb1b1 |
824 | =head2 How do I clear a package? |
825 | |
826 | Use this code, provided by Mark-Jason Dominus: |
827 | |
828 | sub scrub_package { |
829 | no strict 'refs'; |
830 | my $pack = shift; |
831 | die "Shouldn't delete main package" |
832 | if $pack eq "" || $pack eq "main"; |
833 | my $stash = *{$pack . '::'}{HASH}; |
834 | my $name; |
835 | foreach $name (keys %$stash) { |
836 | my $fullname = $pack . '::' . $name; |
837 | # Get rid of everything with that name. |
838 | undef $$fullname; |
839 | undef @$fullname; |
840 | undef %$fullname; |
841 | undef &$fullname; |
842 | undef *$fullname; |
843 | } |
844 | } |
845 | |
846 | Or, if you're using a recent release of Perl, you can |
847 | just use the Symbol::delete_package() function instead. |
848 | |
d92eb7b0 |
849 | =head2 How can I use a variable as a variable name? |
850 | |
851 | Beginners often think they want to have a variable contain the name |
852 | of a variable. |
853 | |
854 | $fred = 23; |
855 | $varname = "fred"; |
856 | ++$$varname; # $fred now 24 |
857 | |
858 | This works I<sometimes>, but it is a very bad idea for two reasons. |
859 | |
860 | The first reason is that they I<only work on global variables>. |
861 | That means above that if $fred is a lexical variable created with my(), |
862 | that the code won't work at all: you'll accidentally access the global |
863 | and skip right over the private lexical altogether. Global variables |
864 | are bad because they can easily collide accidentally and in general make |
865 | for non-scalable and confusing code. |
866 | |
867 | Symbolic references are forbidden under the C<use strict> pragma. |
868 | They are not true references and consequently are not reference counted |
869 | or garbage collected. |
870 | |
871 | The other reason why using a variable to hold the name of another |
872 | variable a bad idea is that the question often stems from a lack of |
873 | understanding of Perl data structures, particularly hashes. By using |
874 | symbolic references, you are just using the package's symbol-table hash |
875 | (like C<%main::>) instead of a user-defined hash. The solution is to |
876 | use your own hash or a real reference instead. |
877 | |
878 | $fred = 23; |
879 | $varname = "fred"; |
880 | $USER_VARS{$varname}++; # not $$varname++ |
881 | |
882 | There we're using the %USER_VARS hash instead of symbolic references. |
883 | Sometimes this comes up in reading strings from the user with variable |
884 | references and wanting to expand them to the values of your perl |
885 | program's variables. This is also a bad idea because it conflates the |
886 | program-addressable namespace and the user-addressable one. Instead of |
887 | reading a string and expanding it to the actual contents of your program's |
888 | own variables: |
889 | |
890 | $str = 'this has a $fred and $barney in it'; |
891 | $str =~ s/(\$\w+)/$1/eeg; # need double eval |
892 | |
893 | Instead, it would be better to keep a hash around like %USER_VARS and have |
894 | variable references actually refer to entries in that hash: |
895 | |
896 | $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all |
897 | |
898 | That's faster, cleaner, and safer than the previous approach. Of course, |
899 | you don't need to use a dollar sign. You could use your own scheme to |
900 | make it less confusing, like bracketed percent symbols, etc. |
901 | |
902 | $str = 'this has a %fred% and %barney% in it'; |
903 | $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all |
904 | |
905 | Another reason that folks sometimes think they want a variable to contain |
906 | the name of a variable is because they don't know how to build proper |
907 | data structures using hashes. For example, let's say they wanted two |
908 | hashes in their program: %fred and %barney, and to use another scalar |
909 | variable to refer to those by name. |
910 | |
911 | $name = "fred"; |
912 | $$name{WIFE} = "wilma"; # set %fred |
913 | |
914 | $name = "barney"; |
915 | $$name{WIFE} = "betty"; # set %barney |
916 | |
917 | This is still a symbolic reference, and is still saddled with the |
918 | problems enumerated above. It would be far better to write: |
919 | |
920 | $folks{"fred"}{WIFE} = "wilma"; |
921 | $folks{"barney"}{WIFE} = "betty"; |
922 | |
923 | And just use a multilevel hash to start with. |
924 | |
925 | The only times that you absolutely I<must> use symbolic references are |
926 | when you really must refer to the symbol table. This may be because it's |
927 | something that can't take a real reference to, such as a format name. |
928 | Doing so may also be important for method calls, since these always go |
929 | through the symbol table for resolution. |
930 | |
931 | In those cases, you would turn off C<strict 'refs'> temporarily so you |
932 | can play around with the symbol table. For example: |
933 | |
934 | @colors = qw(red blue green yellow orange purple violet); |
935 | for my $name (@colors) { |
936 | no strict 'refs'; # renege for the block |
937 | *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; |
938 | } |
939 | |
940 | All those functions (red(), blue(), green(), etc.) appear to be separate, |
941 | but the real code in the closure actually was compiled only once. |
942 | |
943 | So, sometimes you might want to use symbolic references to directly |
944 | manipulate the symbol table. This doesn't matter for formats, handles, and |
945 | subroutines, because they are always global -- you can't use my() on them. |
946 | But for scalars, arrays, and hashes -- and usually for subroutines -- |
947 | you probably want to use hard references only. |
948 | |
68dc0745 |
949 | =head1 AUTHOR AND COPYRIGHT |
950 | |
65acb1b1 |
951 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 |
952 | All rights reserved. |
953 | |
954 | When included as part of the Standard Version of Perl, or as part of |
955 | its complete documentation whether printed or otherwise, this work |
d92eb7b0 |
956 | may be distributed only under the terms of Perl's Artistic License. |
5a964f20 |
957 | Any distribution of this file or derivatives thereof I<outside> |
958 | of that package require that special arrangements be made with |
959 | copyright holder. |
960 | |
961 | Irrespective of its distribution, all code examples in this file |
962 | are hereby placed into the public domain. You are permitted and |
963 | encouraged to use this code in your own programs for fun |
964 | or for profit as you see fit. A simple comment in the code giving |
965 | credit would be courteous but is not required. |