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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 | |
38 | Note that E<lt>FILEE<gt> is I<neither> the type specifier for files |
39 | nor the name of the handle. It is the C<E<lt>E<gt>> operator applied |
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 |
43 | besides C<E<lt>E<gt>> on files, or even talking about the handle, do |
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 |
52 | subroutine) and the left-hand operand to the C<=E<gt>> operator both |
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 | |
87 | The C<$^W> variable (documented in L<perlvar>) controls |
88 | runtime warnings for a block: |
89 | |
90 | { |
91 | local $^W = 0; # temporarily turn off warnings |
92 | $a = $b + $c; # I know these might be undef |
93 | } |
94 | |
95 | Note that like all the punctuation variables, you cannot currently |
96 | use my() on C<$^W>, only local(). |
97 | |
98 | A new C<use warnings> pragma is in the works to provide finer control |
99 | over all this. The curious should check the perl5-porters mailing list |
100 | archives for details. |
101 | |
102 | =head2 What's an extension? |
103 | |
104 | A way of calling compiled C code from Perl. Reading L<perlxstut> |
105 | is a good place to learn more about extensions. |
106 | |
107 | =head2 Why do Perl operators have different precedence than C operators? |
108 | |
109 | Actually, they don't. All C operators that Perl copies have the same |
110 | precedence in Perl as they do in C. The problem is with operators that C |
111 | doesn't have, especially functions that give a list context to everything |
112 | on their right, eg print, chmod, exec, and so on. Such functions are |
113 | called "list operators" and appear as such in the precedence table in |
114 | L<perlop>. |
115 | |
116 | A common mistake is to write: |
117 | |
118 | unlink $file || die "snafu"; |
119 | |
120 | This gets interpreted as: |
121 | |
122 | unlink ($file || die "snafu"); |
123 | |
124 | To avoid this problem, either put in extra parentheses or use the |
125 | super low precedence C<or> operator: |
126 | |
127 | (unlink $file) || die "snafu"; |
128 | unlink $file or die "snafu"; |
129 | |
130 | The "English" operators (C<and>, C<or>, C<xor>, and C<not>) |
131 | deliberately have precedence lower than that of list operators for |
132 | just such situations as the one above. |
133 | |
134 | Another operator with surprising precedence is exponentiation. It |
135 | binds more tightly even than unary minus, making C<-2**2> product a |
136 | negative not a positive four. It is also right-associating, meaning |
137 | that C<2**3**2> is two raised to the ninth power, not eight squared. |
138 | |
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139 | Although it has the same precedence as in C, Perl's C<?:> operator |
140 | produces an lvalue. This assigns $x to either $a or $b, depending |
141 | on the trueness of $maybe: |
142 | |
143 | ($maybe ? $a : $b) = $x; |
144 | |
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145 | =head2 How do I declare/create a structure? |
146 | |
147 | In general, you don't "declare" a structure. Just use a (probably |
148 | anonymous) hash reference. See L<perlref> and L<perldsc> for details. |
149 | Here's an example: |
150 | |
151 | $person = {}; # new anonymous hash |
152 | $person->{AGE} = 24; # set field AGE to 24 |
153 | $person->{NAME} = "Nat"; # set field NAME to "Nat" |
154 | |
155 | If you're looking for something a bit more rigorous, try L<perltoot>. |
156 | |
157 | =head2 How do I create a module? |
158 | |
159 | A module is a package that lives in a file of the same name. For |
160 | example, the Hello::There module would live in Hello/There.pm. For |
161 | details, read L<perlmod>. You'll also find L<Exporter> helpful. If |
162 | you're writing a C or mixed-language module with both C and Perl, then |
163 | you should study L<perlxstut>. |
164 | |
165 | Here's a convenient template you might wish you use when starting your |
166 | own module. Make sure to change the names appropriately. |
167 | |
168 | package Some::Module; # assumes Some/Module.pm |
169 | |
170 | use strict; |
171 | |
172 | BEGIN { |
173 | use Exporter (); |
174 | use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS); |
175 | |
176 | ## set the version for version checking; uncomment to use |
177 | ## $VERSION = 1.00; |
178 | |
179 | # if using RCS/CVS, this next line may be preferred, |
180 | # but beware two-digit versions. |
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181 | $VERSION = do{my@r=q$Revision: 1.28 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r}; |
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182 | |
183 | @ISA = qw(Exporter); |
184 | @EXPORT = qw(&func1 &func2 &func3); |
185 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], |
186 | |
187 | # your exported package globals go here, |
188 | # as well as any optionally exported functions |
189 | @EXPORT_OK = qw($Var1 %Hashit); |
190 | } |
191 | use vars @EXPORT_OK; |
192 | |
193 | # non-exported package globals go here |
194 | use vars qw( @more $stuff ); |
195 | |
196 | # initialize package globals, first exported ones |
197 | $Var1 = ''; |
198 | %Hashit = (); |
199 | |
200 | # then the others (which are still accessible as $Some::Module::stuff) |
201 | $stuff = ''; |
202 | @more = (); |
203 | |
204 | # all file-scoped lexicals must be created before |
205 | # the functions below that use them. |
206 | |
207 | # file-private lexicals go here |
208 | my $priv_var = ''; |
209 | my %secret_hash = (); |
210 | |
211 | # here's a file-private function as a closure, |
212 | # callable as &$priv_func; it cannot be prototyped. |
213 | my $priv_func = sub { |
214 | # stuff goes here. |
215 | }; |
216 | |
217 | # make all your functions, whether exported or not; |
218 | # remember to put something interesting in the {} stubs |
219 | sub func1 {} # no prototype |
220 | sub func2() {} # proto'd void |
221 | sub func3($$) {} # proto'd to 2 scalars |
222 | |
223 | # this one isn't exported, but could be called! |
224 | sub func4(\%) {} # proto'd to 1 hash ref |
225 | |
226 | END { } # module clean-up code here (global destructor) |
227 | |
228 | 1; # modules must return true |
229 | |
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230 | The h2xs program will create stubs for all the important stuff for you: |
231 | |
232 | % h2xs -XA -n My::Module |
233 | |
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234 | =head2 How do I create a class? |
235 | |
236 | See L<perltoot> for an introduction to classes and objects, as well as |
237 | L<perlobj> and L<perlbot>. |
238 | |
239 | =head2 How can I tell if a variable is tainted? |
240 | |
241 | See L<perlsec/"Laundering and Detecting Tainted Data">. Here's an |
242 | example (which doesn't use any system calls, because the kill() |
243 | is given no processes to signal): |
244 | |
245 | sub is_tainted { |
246 | return ! eval { join('',@_), kill 0; 1; }; |
247 | } |
248 | |
249 | This is not C<-w> clean, however. There is no C<-w> clean way to |
250 | detect taintedness - take this as a hint that you should untaint |
251 | all possibly-tainted data. |
252 | |
253 | =head2 What's a closure? |
254 | |
255 | Closures are documented in L<perlref>. |
256 | |
257 | I<Closure> is a computer science term with a precise but |
258 | hard-to-explain meaning. Closures are implemented in Perl as anonymous |
259 | subroutines with lasting references to lexical variables outside their |
260 | own scopes. These lexicals magically refer to the variables that were |
261 | around when the subroutine was defined (deep binding). |
262 | |
263 | Closures make sense in any programming language where you can have the |
264 | return value of a function be itself a function, as you can in Perl. |
265 | Note that some languages provide anonymous functions but are not |
266 | capable of providing proper closures; the Python language, for |
267 | example. For more information on closures, check out any textbook on |
268 | functional programming. Scheme is a language that not only supports |
269 | but encourages closures. |
270 | |
271 | Here's a classic function-generating function: |
272 | |
273 | sub add_function_generator { |
274 | return sub { shift + shift }; |
275 | } |
276 | |
277 | $add_sub = add_function_generator(); |
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278 | $sum = $add_sub->(4,5); # $sum is 9 now. |
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279 | |
280 | The closure works as a I<function template> with some customization |
281 | slots left out to be filled later. The anonymous subroutine returned |
282 | by add_function_generator() isn't technically a closure because it |
283 | refers to no lexicals outside its own scope. |
284 | |
285 | Contrast this with the following make_adder() function, in which the |
286 | returned anonymous function contains a reference to a lexical variable |
287 | outside the scope of that function itself. Such a reference requires |
288 | that Perl return a proper closure, thus locking in for all time the |
289 | value that the lexical had when the function was created. |
290 | |
291 | sub make_adder { |
292 | my $addpiece = shift; |
293 | return sub { shift + $addpiece }; |
294 | } |
295 | |
296 | $f1 = make_adder(20); |
297 | $f2 = make_adder(555); |
298 | |
299 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas |
300 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece |
301 | in the closure sticks around. |
302 | |
303 | Closures are often used for less esoteric purposes. For example, when |
304 | you want to pass in a bit of code into a function: |
305 | |
306 | my $line; |
307 | timeout( 30, sub { $line = <STDIN> } ); |
308 | |
309 | If the code to execute had been passed in as a string, C<'$line = |
310 | E<lt>STDINE<gt>'>, there would have been no way for the hypothetical |
311 | timeout() function to access the lexical variable $line back in its |
312 | caller's scope. |
313 | |
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314 | =head2 What is variable suicide and how can I prevent it? |
315 | |
316 | Variable suicide is when you (temporarily or permanently) lose the |
317 | value of a variable. It is caused by scoping through my() and local() |
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318 | interacting with either closures or aliased foreach() iterator |
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319 | variables and subroutine arguments. It used to be easy to |
320 | inadvertently lose a variable's value this way, but now it's much |
321 | harder. Take this code: |
322 | |
323 | my $f = "foo"; |
324 | sub T { |
325 | while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } |
326 | } |
327 | T; |
328 | print "Finally $f\n"; |
329 | |
330 | The $f that has "bar" added to it three times should be a new C<$f> |
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331 | (C<my $f> should create a new local variable each time through the loop). |
332 | It isn't, however. This was a bug, now fixed in the latest releases |
333 | (tested against 5.004_05, 5.005_03, and 5.005_56). |
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334 | |
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335 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? |
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336 | |
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337 | With the exception of regexes, you need to pass references to these |
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338 | objects. See L<perlsub/"Pass by Reference"> for this particular |
339 | question, and L<perlref> for information on references. |
340 | |
341 | =over 4 |
342 | |
343 | =item Passing Variables and Functions |
344 | |
345 | Regular variables and functions are quite easy: just pass in a |
346 | reference to an existing or anonymous variable or function: |
347 | |
348 | func( \$some_scalar ); |
349 | |
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350 | func( \@some_array ); |
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351 | func( [ 1 .. 10 ] ); |
352 | |
353 | func( \%some_hash ); |
354 | func( { this => 10, that => 20 } ); |
355 | |
356 | func( \&some_func ); |
357 | func( sub { $_[0] ** $_[1] } ); |
358 | |
359 | =item Passing Filehandles |
360 | |
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361 | To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations. |
362 | These are "typeglobs" - see L<perldata/"Typeglobs and Filehandles"> |
363 | and especially L<perlsub/"Pass by Reference"> for more information. |
364 | |
365 | Here's an excerpt: |
366 | |
367 | If you're passing around filehandles, you could usually just use the bare |
368 | typeglob, like *STDOUT, but typeglobs references would be better because |
369 | they'll still work properly under C<use strict 'refs'>. For example: |
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370 | |
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371 | splutter(\*STDOUT); |
372 | sub splutter { |
373 | my $fh = shift; |
374 | print $fh "her um well a hmmm\n"; |
375 | } |
376 | |
377 | $rec = get_rec(\*STDIN); |
378 | sub get_rec { |
379 | my $fh = shift; |
380 | return scalar <$fh>; |
381 | } |
382 | |
383 | If you're planning on generating new filehandles, you could do this: |
384 | |
385 | sub openit { |
386 | my $name = shift; |
387 | local *FH; |
388 | return open (FH, $path) ? *FH : undef; |
389 | } |
390 | $fh = openit('< /etc/motd'); |
391 | print <$fh>; |
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392 | |
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393 | =item Passing Regexes |
394 | |
395 | To pass regexes around, you'll need to be using a release of Perl |
396 | sufficiently recent as to support the C<qr//> construct, pass around |
397 | strings and use an exception-trapping eval, or else be very, very clever. |
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398 | |
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399 | Here's an example of how to pass in a string to be regex compared |
400 | using C<qr//>: |
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401 | |
402 | sub compare($$) { |
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403 | my ($val1, $regex) = @_; |
404 | my $retval = $val1 =~ /$regex/; |
405 | return $retval; |
406 | } |
407 | $match = compare("old McDonald", qr/d.*D/i); |
408 | |
409 | Notice how C<qr//> allows flags at the end. That pattern was compiled |
410 | at compile time, although it was executed later. The nifty C<qr//> |
411 | notation wasn't introduced until the 5.005 release. Before that, you |
412 | had to approach this problem much less intuitively. For example, here |
413 | it is again if you don't have C<qr//>: |
414 | |
415 | sub compare($$) { |
416 | my ($val1, $regex) = @_; |
417 | my $retval = eval { $val1 =~ /$regex/ }; |
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418 | die if $@; |
419 | return $retval; |
420 | } |
421 | |
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422 | $match = compare("old McDonald", q/($?i)d.*D/); |
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423 | |
424 | Make sure you never say something like this: |
425 | |
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426 | return eval "\$val =~ /$regex/"; # WRONG |
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427 | |
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428 | or someone can sneak shell escapes into the regex due to the double |
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429 | interpolation of the eval and the double-quoted string. For example: |
430 | |
431 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; |
432 | |
433 | eval "\$string =~ /$pattern_of_evil/"; |
434 | |
435 | Those preferring to be very, very clever might see the O'Reilly book, |
436 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's |
437 | Build_MatchMany_Function() is particularly interesting. A complete |
438 | citation of this book is given in L<perlfaq2>. |
439 | |
440 | =item Passing Methods |
441 | |
442 | To pass an object method into a subroutine, you can do this: |
443 | |
444 | call_a_lot(10, $some_obj, "methname") |
445 | sub call_a_lot { |
446 | my ($count, $widget, $trick) = @_; |
447 | for (my $i = 0; $i < $count; $i++) { |
448 | $widget->$trick(); |
449 | } |
450 | } |
451 | |
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452 | Or you can use a closure to bundle up the object and its method call |
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453 | and arguments: |
454 | |
455 | my $whatnot = sub { $some_obj->obfuscate(@args) }; |
456 | func($whatnot); |
457 | sub func { |
458 | my $code = shift; |
459 | &$code(); |
460 | } |
461 | |
462 | You could also investigate the can() method in the UNIVERSAL class |
463 | (part of the standard perl distribution). |
464 | |
465 | =back |
466 | |
467 | =head2 How do I create a static variable? |
468 | |
469 | As with most things in Perl, TMTOWTDI. What is a "static variable" in |
470 | other languages could be either a function-private variable (visible |
471 | only within a single function, retaining its value between calls to |
472 | that function), or a file-private variable (visible only to functions |
473 | within the file it was declared in) in Perl. |
474 | |
475 | Here's code to implement a function-private variable: |
476 | |
477 | BEGIN { |
478 | my $counter = 42; |
479 | sub prev_counter { return --$counter } |
480 | sub next_counter { return $counter++ } |
481 | } |
482 | |
483 | Now prev_counter() and next_counter() share a private variable $counter |
484 | that was initialized at compile time. |
485 | |
486 | To declare a file-private variable, you'll still use a my(), putting |
487 | it at the outer scope level at the top of the file. Assume this is in |
488 | file Pax.pm: |
489 | |
490 | package Pax; |
491 | my $started = scalar(localtime(time())); |
492 | |
493 | sub begun { return $started } |
494 | |
495 | When C<use Pax> or C<require Pax> loads this module, the variable will |
496 | be initialized. It won't get garbage-collected the way most variables |
497 | going out of scope do, because the begun() function cares about it, |
498 | but no one else can get it. It is not called $Pax::started because |
499 | its scope is unrelated to the package. It's scoped to the file. You |
500 | could conceivably have several packages in that same file all |
501 | accessing the same private variable, but another file with the same |
502 | package couldn't get to it. |
503 | |
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504 | See L<perlsub/"Persistent Private Variables"> for details. |
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505 | |
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506 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
507 | |
508 | C<local($x)> saves away the old value of the global variable C<$x>, |
509 | and assigns a new value for the duration of the subroutine, I<which is |
510 | visible in other functions called from that subroutine>. This is done |
511 | at run-time, so is called dynamic scoping. local() always affects global |
512 | variables, also called package variables or dynamic variables. |
513 | |
514 | C<my($x)> creates a new variable that is only visible in the current |
515 | subroutine. This is done at compile-time, so is called lexical or |
516 | static scoping. my() always affects private variables, also called |
517 | lexical variables or (improperly) static(ly scoped) variables. |
518 | |
519 | For instance: |
520 | |
521 | sub visible { |
522 | print "var has value $var\n"; |
523 | } |
524 | |
525 | sub dynamic { |
526 | local $var = 'local'; # new temporary value for the still-global |
527 | visible(); # variable called $var |
528 | } |
529 | |
530 | sub lexical { |
531 | my $var = 'private'; # new private variable, $var |
532 | visible(); # (invisible outside of sub scope) |
533 | } |
534 | |
535 | $var = 'global'; |
536 | |
537 | visible(); # prints global |
538 | dynamic(); # prints local |
539 | lexical(); # prints global |
540 | |
541 | Notice how at no point does the value "private" get printed. That's |
542 | because $var only has that value within the block of the lexical() |
543 | function, and it is hidden from called subroutine. |
544 | |
545 | In summary, local() doesn't make what you think of as private, local |
546 | variables. It gives a global variable a temporary value. my() is |
547 | what you're looking for if you want private variables. |
548 | |
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549 | See L<perlsub/"Private Variables via my()"> and L<perlsub/"Temporary |
550 | Values via local()"> for excruciating details. |
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551 | |
552 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? |
553 | |
554 | You can do this via symbolic references, provided you haven't set |
555 | C<use strict "refs">. So instead of $var, use C<${'var'}>. |
556 | |
557 | local $var = "global"; |
558 | my $var = "lexical"; |
559 | |
560 | print "lexical is $var\n"; |
561 | |
562 | no strict 'refs'; |
563 | print "global is ${'var'}\n"; |
564 | |
565 | If you know your package, you can just mention it explicitly, as in |
566 | $Some_Pack::var. Note that the notation $::var is I<not> the dynamic |
567 | $var in the current package, but rather the one in the C<main> |
568 | package, as though you had written $main::var. Specifying the package |
569 | directly makes you hard-code its name, but it executes faster and |
570 | avoids running afoul of C<use strict "refs">. |
571 | |
572 | =head2 What's the difference between deep and shallow binding? |
573 | |
574 | In deep binding, lexical variables mentioned in anonymous subroutines |
575 | are the same ones that were in scope when the subroutine was created. |
576 | In shallow binding, they are whichever variables with the same names |
577 | happen to be in scope when the subroutine is called. Perl always uses |
578 | deep binding of lexical variables (i.e., those created with my()). |
579 | However, dynamic variables (aka global, local, or package variables) |
580 | are effectively shallowly bound. Consider this just one more reason |
581 | not to use them. See the answer to L<"What's a closure?">. |
582 | |
65acb1b1 |
583 | =head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right? |
68dc0745 |
584 | |
c8db1d39 |
585 | C<my()> and C<local()> give list context to the right hand side |
586 | of C<=>. The E<lt>FHE<gt> read operation, like so many of Perl's |
587 | functions and operators, can tell which context it was called in and |
588 | behaves appropriately. In general, the scalar() function can help. |
589 | This function does nothing to the data itself (contrary to popular myth) |
590 | but rather tells its argument to behave in whatever its scalar fashion is. |
591 | If that function doesn't have a defined scalar behavior, this of course |
592 | doesn't help you (such as with sort()). |
68dc0745 |
593 | |
594 | To enforce scalar context in this particular case, however, you need |
595 | merely omit the parentheses: |
596 | |
597 | local($foo) = <FILE>; # WRONG |
598 | local($foo) = scalar(<FILE>); # ok |
599 | local $foo = <FILE>; # right |
600 | |
601 | You should probably be using lexical variables anyway, although the |
602 | issue is the same here: |
603 | |
604 | my($foo) = <FILE>; # WRONG |
605 | my $foo = <FILE>; # right |
606 | |
54310121 |
607 | =head2 How do I redefine a builtin function, operator, or method? |
68dc0745 |
608 | |
609 | Why do you want to do that? :-) |
610 | |
611 | If you want to override a predefined function, such as open(), |
612 | then you'll have to import the new definition from a different |
613 | module. See L<perlsub/"Overriding Builtin Functions">. There's |
65acb1b1 |
614 | also an example in L<perltoot/"Class::Template">. |
68dc0745 |
615 | |
616 | If you want to overload a Perl operator, such as C<+> or C<**>, |
617 | then you'll want to use the C<use overload> pragma, documented |
618 | in L<overload>. |
619 | |
620 | If you're talking about obscuring method calls in parent classes, |
621 | see L<perltoot/"Overridden Methods">. |
622 | |
623 | =head2 What's the difference between calling a function as &foo and foo()? |
624 | |
625 | When you call a function as C<&foo>, you allow that function access to |
626 | your current @_ values, and you by-pass prototypes. That means that |
627 | the function doesn't get an empty @_, it gets yours! While not |
628 | strictly speaking a bug (it's documented that way in L<perlsub>), it |
629 | would be hard to consider this a feature in most cases. |
630 | |
c8db1d39 |
631 | When you call your function as C<&foo()>, then you I<do> get a new @_, |
68dc0745 |
632 | but prototyping is still circumvented. |
633 | |
634 | Normally, you want to call a function using C<foo()>. You may only |
635 | omit the parentheses if the function is already known to the compiler |
636 | because it already saw the definition (C<use> but not C<require>), |
637 | or via a forward reference or C<use subs> declaration. Even in this |
638 | case, you get a clean @_ without any of the old values leaking through |
639 | where they don't belong. |
640 | |
641 | =head2 How do I create a switch or case statement? |
642 | |
643 | This is explained in more depth in the L<perlsyn>. Briefly, there's |
644 | no official case statement, because of the variety of tests possible |
645 | in Perl (numeric comparison, string comparison, glob comparison, |
d92eb7b0 |
646 | regex matching, overloaded comparisons, ...). Larry couldn't decide |
68dc0745 |
647 | how best to do this, so he left it out, even though it's been on the |
648 | wish list since perl1. |
649 | |
c8db1d39 |
650 | The general answer is to write a construct like this: |
651 | |
652 | for ($variable_to_test) { |
653 | if (/pat1/) { } # do something |
654 | elsif (/pat2/) { } # do something else |
655 | elsif (/pat3/) { } # do something else |
656 | else { } # default |
657 | } |
68dc0745 |
658 | |
c8db1d39 |
659 | Here's a simple example of a switch based on pattern matching, this |
660 | time lined up in a way to make it look more like a switch statement. |
661 | We'll do a multi-way conditional based on the type of reference stored |
662 | in $whatchamacallit: |
663 | |
664 | SWITCH: for (ref $whatchamacallit) { |
68dc0745 |
665 | |
666 | /^$/ && die "not a reference"; |
667 | |
668 | /SCALAR/ && do { |
669 | print_scalar($$ref); |
670 | last SWITCH; |
671 | }; |
672 | |
673 | /ARRAY/ && do { |
674 | print_array(@$ref); |
675 | last SWITCH; |
676 | }; |
677 | |
678 | /HASH/ && do { |
679 | print_hash(%$ref); |
680 | last SWITCH; |
681 | }; |
682 | |
683 | /CODE/ && do { |
684 | warn "can't print function ref"; |
685 | last SWITCH; |
686 | }; |
687 | |
688 | # DEFAULT |
689 | |
690 | warn "User defined type skipped"; |
691 | |
692 | } |
693 | |
c8db1d39 |
694 | See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other |
695 | examples in this style. |
696 | |
697 | Sometimes you should change the positions of the constant and the variable. |
698 | For example, let's say you wanted to test which of many answers you were |
699 | given, but in a case-insensitive way that also allows abbreviations. |
700 | You can use the following technique if the strings all start with |
701 | different characters, or if you want to arrange the matches so that |
702 | one takes precedence over another, as C<"SEND"> has precedence over |
703 | C<"STOP"> here: |
704 | |
705 | chomp($answer = <>); |
706 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } |
707 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } |
708 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } |
709 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } |
710 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } |
711 | |
712 | A totally different approach is to create a hash of function references. |
713 | |
714 | my %commands = ( |
715 | "happy" => \&joy, |
716 | "sad", => \&sullen, |
717 | "done" => sub { die "See ya!" }, |
718 | "mad" => \&angry, |
719 | ); |
720 | |
721 | print "How are you? "; |
722 | chomp($string = <STDIN>); |
723 | if ($commands{$string}) { |
724 | $commands{$string}->(); |
725 | } else { |
726 | print "No such command: $string\n"; |
727 | } |
728 | |
68dc0745 |
729 | =head2 How can I catch accesses to undefined variables/functions/methods? |
730 | |
731 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and |
732 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to |
733 | undefined functions and methods. |
734 | |
735 | When it comes to undefined variables that would trigger a warning |
736 | under C<-w>, you can use a handler to trap the pseudo-signal |
737 | C<__WARN__> like this: |
738 | |
739 | $SIG{__WARN__} = sub { |
740 | |
c8db1d39 |
741 | for ( $_[0] ) { # voici un switch statement |
68dc0745 |
742 | |
743 | /Use of uninitialized value/ && do { |
744 | # promote warning to a fatal |
745 | die $_; |
746 | }; |
747 | |
748 | # other warning cases to catch could go here; |
749 | |
750 | warn $_; |
751 | } |
752 | |
753 | }; |
754 | |
755 | =head2 Why can't a method included in this same file be found? |
756 | |
757 | Some possible reasons: your inheritance is getting confused, you've |
758 | misspelled the method name, or the object is of the wrong type. Check |
759 | out L<perltoot> for details on these. You may also use C<print |
760 | ref($object)> to find out the class C<$object> was blessed into. |
761 | |
762 | Another possible reason for problems is because you've used the |
763 | indirect object syntax (eg, C<find Guru "Samy">) on a class name |
764 | before Perl has seen that such a package exists. It's wisest to make |
765 | sure your packages are all defined before you start using them, which |
766 | will be taken care of if you use the C<use> statement instead of |
767 | C<require>. If not, make sure to use arrow notation (eg, |
7b8d334a |
768 | C<Guru-E<gt>find("Samy")>) instead. Object notation is explained in |
68dc0745 |
769 | L<perlobj>. |
770 | |
c8db1d39 |
771 | Make sure to read about creating modules in L<perlmod> and |
772 | the perils of indirect objects in L<perlobj/"WARNING">. |
773 | |
68dc0745 |
774 | =head2 How can I find out my current package? |
775 | |
776 | If you're just a random program, you can do this to find |
777 | out what the currently compiled package is: |
778 | |
c8db1d39 |
779 | my $packname = __PACKAGE__; |
68dc0745 |
780 | |
781 | But if you're a method and you want to print an error message |
782 | that includes the kind of object you were called on (which is |
783 | not necessarily the same as the one in which you were compiled): |
784 | |
785 | sub amethod { |
92c2ed05 |
786 | my $self = shift; |
68dc0745 |
787 | my $class = ref($self) || $self; |
788 | warn "called me from a $class object"; |
789 | } |
790 | |
46fc3d4c |
791 | =head2 How can I comment out a large block of perl code? |
792 | |
793 | Use embedded POD to discard it: |
794 | |
795 | # program is here |
796 | |
797 | =for nobody |
798 | This paragraph is commented out |
799 | |
800 | # program continues |
801 | |
802 | =begin comment text |
803 | |
804 | all of this stuff |
805 | |
806 | here will be ignored |
807 | by everyone |
808 | |
809 | =end comment text |
810 | |
fc36a67e |
811 | =cut |
812 | |
c8db1d39 |
813 | This can't go just anywhere. You have to put a pod directive where |
814 | the parser is expecting a new statement, not just in the middle |
815 | of an expression or some other arbitrary yacc grammar production. |
816 | |
65acb1b1 |
817 | =head2 How do I clear a package? |
818 | |
819 | Use this code, provided by Mark-Jason Dominus: |
820 | |
821 | sub scrub_package { |
822 | no strict 'refs'; |
823 | my $pack = shift; |
824 | die "Shouldn't delete main package" |
825 | if $pack eq "" || $pack eq "main"; |
826 | my $stash = *{$pack . '::'}{HASH}; |
827 | my $name; |
828 | foreach $name (keys %$stash) { |
829 | my $fullname = $pack . '::' . $name; |
830 | # Get rid of everything with that name. |
831 | undef $$fullname; |
832 | undef @$fullname; |
833 | undef %$fullname; |
834 | undef &$fullname; |
835 | undef *$fullname; |
836 | } |
837 | } |
838 | |
839 | Or, if you're using a recent release of Perl, you can |
840 | just use the Symbol::delete_package() function instead. |
841 | |
d92eb7b0 |
842 | =head2 How can I use a variable as a variable name? |
843 | |
844 | Beginners often think they want to have a variable contain the name |
845 | of a variable. |
846 | |
847 | $fred = 23; |
848 | $varname = "fred"; |
849 | ++$$varname; # $fred now 24 |
850 | |
851 | This works I<sometimes>, but it is a very bad idea for two reasons. |
852 | |
853 | The first reason is that they I<only work on global variables>. |
854 | That means above that if $fred is a lexical variable created with my(), |
855 | that the code won't work at all: you'll accidentally access the global |
856 | and skip right over the private lexical altogether. Global variables |
857 | are bad because they can easily collide accidentally and in general make |
858 | for non-scalable and confusing code. |
859 | |
860 | Symbolic references are forbidden under the C<use strict> pragma. |
861 | They are not true references and consequently are not reference counted |
862 | or garbage collected. |
863 | |
864 | The other reason why using a variable to hold the name of another |
865 | variable a bad idea is that the question often stems from a lack of |
866 | understanding of Perl data structures, particularly hashes. By using |
867 | symbolic references, you are just using the package's symbol-table hash |
868 | (like C<%main::>) instead of a user-defined hash. The solution is to |
869 | use your own hash or a real reference instead. |
870 | |
871 | $fred = 23; |
872 | $varname = "fred"; |
873 | $USER_VARS{$varname}++; # not $$varname++ |
874 | |
875 | There we're using the %USER_VARS hash instead of symbolic references. |
876 | Sometimes this comes up in reading strings from the user with variable |
877 | references and wanting to expand them to the values of your perl |
878 | program's variables. This is also a bad idea because it conflates the |
879 | program-addressable namespace and the user-addressable one. Instead of |
880 | reading a string and expanding it to the actual contents of your program's |
881 | own variables: |
882 | |
883 | $str = 'this has a $fred and $barney in it'; |
884 | $str =~ s/(\$\w+)/$1/eeg; # need double eval |
885 | |
886 | Instead, it would be better to keep a hash around like %USER_VARS and have |
887 | variable references actually refer to entries in that hash: |
888 | |
889 | $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all |
890 | |
891 | That's faster, cleaner, and safer than the previous approach. Of course, |
892 | you don't need to use a dollar sign. You could use your own scheme to |
893 | make it less confusing, like bracketed percent symbols, etc. |
894 | |
895 | $str = 'this has a %fred% and %barney% in it'; |
896 | $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all |
897 | |
898 | Another reason that folks sometimes think they want a variable to contain |
899 | the name of a variable is because they don't know how to build proper |
900 | data structures using hashes. For example, let's say they wanted two |
901 | hashes in their program: %fred and %barney, and to use another scalar |
902 | variable to refer to those by name. |
903 | |
904 | $name = "fred"; |
905 | $$name{WIFE} = "wilma"; # set %fred |
906 | |
907 | $name = "barney"; |
908 | $$name{WIFE} = "betty"; # set %barney |
909 | |
910 | This is still a symbolic reference, and is still saddled with the |
911 | problems enumerated above. It would be far better to write: |
912 | |
913 | $folks{"fred"}{WIFE} = "wilma"; |
914 | $folks{"barney"}{WIFE} = "betty"; |
915 | |
916 | And just use a multilevel hash to start with. |
917 | |
918 | The only times that you absolutely I<must> use symbolic references are |
919 | when you really must refer to the symbol table. This may be because it's |
920 | something that can't take a real reference to, such as a format name. |
921 | Doing so may also be important for method calls, since these always go |
922 | through the symbol table for resolution. |
923 | |
924 | In those cases, you would turn off C<strict 'refs'> temporarily so you |
925 | can play around with the symbol table. For example: |
926 | |
927 | @colors = qw(red blue green yellow orange purple violet); |
928 | for my $name (@colors) { |
929 | no strict 'refs'; # renege for the block |
930 | *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; |
931 | } |
932 | |
933 | All those functions (red(), blue(), green(), etc.) appear to be separate, |
934 | but the real code in the closure actually was compiled only once. |
935 | |
936 | So, sometimes you might want to use symbolic references to directly |
937 | manipulate the symbol table. This doesn't matter for formats, handles, and |
938 | subroutines, because they are always global -- you can't use my() on them. |
939 | But for scalars, arrays, and hashes -- and usually for subroutines -- |
940 | you probably want to use hard references only. |
941 | |
68dc0745 |
942 | =head1 AUTHOR AND COPYRIGHT |
943 | |
65acb1b1 |
944 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 |
945 | All rights reserved. |
946 | |
947 | When included as part of the Standard Version of Perl, or as part of |
948 | its complete documentation whether printed or otherwise, this work |
d92eb7b0 |
949 | may be distributed only under the terms of Perl's Artistic License. |
5a964f20 |
950 | Any distribution of this file or derivatives thereof I<outside> |
951 | of that package require that special arrangements be made with |
952 | copyright holder. |
953 | |
954 | Irrespective of its distribution, all code examples in this file |
955 | are hereby placed into the public domain. You are permitted and |
956 | encouraged to use this code in your own programs for fun |
957 | or for profit as you see fit. A simple comment in the code giving |
958 | credit would be courteous but is not required. |