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1 | =head1 NAME |
2 | |
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3 | perlfaq7 - Perl Language Issues ($Revision: 1.24 $, $Date: 1999/01/08 05:32:11 $) |
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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 | |
21 | =head2 What are all these $@%* punctuation signs, and how do I know when to use them? |
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) |
28 | * for all types of that symbol name. In version 4 you used them like |
29 | pointers, but in modern perls you can just use references. |
30 | |
31 | While there are a few places where you don't actually need these type |
32 | specifiers, you should always use them. |
33 | |
34 | A couple of others that you're likely to encounter that aren't |
35 | really type specifiers are: |
36 | |
37 | <> are used for inputting a record from a filehandle. |
38 | \ takes a reference to something. |
39 | |
40 | Note that E<lt>FILEE<gt> is I<neither> the type specifier for files |
41 | nor the name of the handle. It is the C<E<lt>E<gt>> operator applied |
42 | to the handle FILE. It reads one line (well, record - see |
43 | L<perlvar/$/>) from the handle FILE in scalar context, or I<all> lines |
44 | in list context. When performing open, close, or any other operation |
45 | besides C<E<lt>E<gt>> on files, or even talking about the handle, do |
46 | I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0, |
47 | 2)> and "copying from STDIN to FILE". |
48 | |
49 | =head2 Do I always/never have to quote my strings or use semicolons and commas? |
50 | |
51 | Normally, a bareword doesn't need to be quoted, but in most cases |
52 | probably should be (and must be under C<use strict>). But a hash key |
53 | consisting of a simple word (that isn't the name of a defined |
54 | subroutine) and the left-hand operand to the C<=E<gt>> operator both |
55 | count as though they were quoted: |
56 | |
57 | This is like this |
58 | ------------ --------------- |
59 | $foo{line} $foo{"line"} |
60 | bar => stuff "bar" => stuff |
61 | |
62 | The final semicolon in a block is optional, as is the final comma in a |
63 | list. Good style (see L<perlstyle>) says to put them in except for |
64 | one-liners: |
65 | |
66 | if ($whoops) { exit 1 } |
67 | @nums = (1, 2, 3); |
68 | |
69 | if ($whoops) { |
70 | exit 1; |
71 | } |
72 | @lines = ( |
73 | "There Beren came from mountains cold", |
74 | "And lost he wandered under leaves", |
75 | ); |
76 | |
77 | =head2 How do I skip some return values? |
78 | |
79 | One way is to treat the return values as a list and index into it: |
80 | |
81 | $dir = (getpwnam($user))[7]; |
82 | |
83 | Another way is to use undef as an element on the left-hand-side: |
84 | |
85 | ($dev, $ino, undef, undef, $uid, $gid) = stat($file); |
86 | |
87 | =head2 How do I temporarily block warnings? |
88 | |
89 | The C<$^W> variable (documented in L<perlvar>) controls |
90 | runtime warnings for a block: |
91 | |
92 | { |
93 | local $^W = 0; # temporarily turn off warnings |
94 | $a = $b + $c; # I know these might be undef |
95 | } |
96 | |
97 | Note that like all the punctuation variables, you cannot currently |
98 | use my() on C<$^W>, only local(). |
99 | |
100 | A new C<use warnings> pragma is in the works to provide finer control |
101 | over all this. The curious should check the perl5-porters mailing list |
102 | archives for details. |
103 | |
104 | =head2 What's an extension? |
105 | |
106 | A way of calling compiled C code from Perl. Reading L<perlxstut> |
107 | is a good place to learn more about extensions. |
108 | |
109 | =head2 Why do Perl operators have different precedence than C operators? |
110 | |
111 | Actually, they don't. All C operators that Perl copies have the same |
112 | precedence in Perl as they do in C. The problem is with operators that C |
113 | doesn't have, especially functions that give a list context to everything |
114 | on their right, eg print, chmod, exec, and so on. Such functions are |
115 | called "list operators" and appear as such in the precedence table in |
116 | L<perlop>. |
117 | |
118 | A common mistake is to write: |
119 | |
120 | unlink $file || die "snafu"; |
121 | |
122 | This gets interpreted as: |
123 | |
124 | unlink ($file || die "snafu"); |
125 | |
126 | To avoid this problem, either put in extra parentheses or use the |
127 | super low precedence C<or> operator: |
128 | |
129 | (unlink $file) || die "snafu"; |
130 | unlink $file or die "snafu"; |
131 | |
132 | The "English" operators (C<and>, C<or>, C<xor>, and C<not>) |
133 | deliberately have precedence lower than that of list operators for |
134 | just such situations as the one above. |
135 | |
136 | Another operator with surprising precedence is exponentiation. It |
137 | binds more tightly even than unary minus, making C<-2**2> product a |
138 | negative not a positive four. It is also right-associating, meaning |
139 | that C<2**3**2> is two raised to the ninth power, not eight squared. |
140 | |
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141 | Although it has the same precedence as in C, Perl's C<?:> operator |
142 | produces an lvalue. This assigns $x to either $a or $b, depending |
143 | on the trueness of $maybe: |
144 | |
145 | ($maybe ? $a : $b) = $x; |
146 | |
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147 | =head2 How do I declare/create a structure? |
148 | |
149 | In general, you don't "declare" a structure. Just use a (probably |
150 | anonymous) hash reference. See L<perlref> and L<perldsc> for details. |
151 | Here's an example: |
152 | |
153 | $person = {}; # new anonymous hash |
154 | $person->{AGE} = 24; # set field AGE to 24 |
155 | $person->{NAME} = "Nat"; # set field NAME to "Nat" |
156 | |
157 | If you're looking for something a bit more rigorous, try L<perltoot>. |
158 | |
159 | =head2 How do I create a module? |
160 | |
161 | A module is a package that lives in a file of the same name. For |
162 | example, the Hello::There module would live in Hello/There.pm. For |
163 | details, read L<perlmod>. You'll also find L<Exporter> helpful. If |
164 | you're writing a C or mixed-language module with both C and Perl, then |
165 | you should study L<perlxstut>. |
166 | |
167 | Here's a convenient template you might wish you use when starting your |
168 | own module. Make sure to change the names appropriately. |
169 | |
170 | package Some::Module; # assumes Some/Module.pm |
171 | |
172 | use strict; |
173 | |
174 | BEGIN { |
175 | use Exporter (); |
176 | use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS); |
177 | |
178 | ## set the version for version checking; uncomment to use |
179 | ## $VERSION = 1.00; |
180 | |
181 | # if using RCS/CVS, this next line may be preferred, |
182 | # but beware two-digit versions. |
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183 | $VERSION = do{my@r=q$Revision: 1.24 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r}; |
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184 | |
185 | @ISA = qw(Exporter); |
186 | @EXPORT = qw(&func1 &func2 &func3); |
187 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], |
188 | |
189 | # your exported package globals go here, |
190 | # as well as any optionally exported functions |
191 | @EXPORT_OK = qw($Var1 %Hashit); |
192 | } |
193 | use vars @EXPORT_OK; |
194 | |
195 | # non-exported package globals go here |
196 | use vars qw( @more $stuff ); |
197 | |
198 | # initialize package globals, first exported ones |
199 | $Var1 = ''; |
200 | %Hashit = (); |
201 | |
202 | # then the others (which are still accessible as $Some::Module::stuff) |
203 | $stuff = ''; |
204 | @more = (); |
205 | |
206 | # all file-scoped lexicals must be created before |
207 | # the functions below that use them. |
208 | |
209 | # file-private lexicals go here |
210 | my $priv_var = ''; |
211 | my %secret_hash = (); |
212 | |
213 | # here's a file-private function as a closure, |
214 | # callable as &$priv_func; it cannot be prototyped. |
215 | my $priv_func = sub { |
216 | # stuff goes here. |
217 | }; |
218 | |
219 | # make all your functions, whether exported or not; |
220 | # remember to put something interesting in the {} stubs |
221 | sub func1 {} # no prototype |
222 | sub func2() {} # proto'd void |
223 | sub func3($$) {} # proto'd to 2 scalars |
224 | |
225 | # this one isn't exported, but could be called! |
226 | sub func4(\%) {} # proto'd to 1 hash ref |
227 | |
228 | END { } # module clean-up code here (global destructor) |
229 | |
230 | 1; # modules must return true |
231 | |
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232 | The h2xs program will create stubs for all the important stuff for you: |
233 | |
234 | % h2xs -XA -n My::Module |
235 | |
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236 | =head2 How do I create a class? |
237 | |
238 | See L<perltoot> for an introduction to classes and objects, as well as |
239 | L<perlobj> and L<perlbot>. |
240 | |
241 | =head2 How can I tell if a variable is tainted? |
242 | |
243 | See L<perlsec/"Laundering and Detecting Tainted Data">. Here's an |
244 | example (which doesn't use any system calls, because the kill() |
245 | is given no processes to signal): |
246 | |
247 | sub is_tainted { |
248 | return ! eval { join('',@_), kill 0; 1; }; |
249 | } |
250 | |
251 | This is not C<-w> clean, however. There is no C<-w> clean way to |
252 | detect taintedness - take this as a hint that you should untaint |
253 | all possibly-tainted data. |
254 | |
255 | =head2 What's a closure? |
256 | |
257 | Closures are documented in L<perlref>. |
258 | |
259 | I<Closure> is a computer science term with a precise but |
260 | hard-to-explain meaning. Closures are implemented in Perl as anonymous |
261 | subroutines with lasting references to lexical variables outside their |
262 | own scopes. These lexicals magically refer to the variables that were |
263 | around when the subroutine was defined (deep binding). |
264 | |
265 | Closures make sense in any programming language where you can have the |
266 | return value of a function be itself a function, as you can in Perl. |
267 | Note that some languages provide anonymous functions but are not |
268 | capable of providing proper closures; the Python language, for |
269 | example. For more information on closures, check out any textbook on |
270 | functional programming. Scheme is a language that not only supports |
271 | but encourages closures. |
272 | |
273 | Here's a classic function-generating function: |
274 | |
275 | sub add_function_generator { |
276 | return sub { shift + shift }; |
277 | } |
278 | |
279 | $add_sub = add_function_generator(); |
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280 | $sum = $add_sub->(4,5); # $sum is 9 now. |
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281 | |
282 | The closure works as a I<function template> with some customization |
283 | slots left out to be filled later. The anonymous subroutine returned |
284 | by add_function_generator() isn't technically a closure because it |
285 | refers to no lexicals outside its own scope. |
286 | |
287 | Contrast this with the following make_adder() function, in which the |
288 | returned anonymous function contains a reference to a lexical variable |
289 | outside the scope of that function itself. Such a reference requires |
290 | that Perl return a proper closure, thus locking in for all time the |
291 | value that the lexical had when the function was created. |
292 | |
293 | sub make_adder { |
294 | my $addpiece = shift; |
295 | return sub { shift + $addpiece }; |
296 | } |
297 | |
298 | $f1 = make_adder(20); |
299 | $f2 = make_adder(555); |
300 | |
301 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas |
302 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece |
303 | in the closure sticks around. |
304 | |
305 | Closures are often used for less esoteric purposes. For example, when |
306 | you want to pass in a bit of code into a function: |
307 | |
308 | my $line; |
309 | timeout( 30, sub { $line = <STDIN> } ); |
310 | |
311 | If the code to execute had been passed in as a string, C<'$line = |
312 | E<lt>STDINE<gt>'>, there would have been no way for the hypothetical |
313 | timeout() function to access the lexical variable $line back in its |
314 | caller's scope. |
315 | |
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316 | =head2 What is variable suicide and how can I prevent it? |
317 | |
318 | Variable suicide is when you (temporarily or permanently) lose the |
319 | value of a variable. It is caused by scoping through my() and local() |
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320 | interacting with either closures or aliased foreach() iterator |
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321 | variables and subroutine arguments. It used to be easy to |
322 | inadvertently lose a variable's value this way, but now it's much |
323 | harder. Take this code: |
324 | |
325 | my $f = "foo"; |
326 | sub T { |
327 | while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } |
328 | } |
329 | T; |
330 | print "Finally $f\n"; |
331 | |
332 | The $f that has "bar" added to it three times should be a new C<$f> |
333 | (C<my $f> should create a new local variable each time through the |
334 | loop). It isn't, however. This is a bug, and will be fixed. |
335 | |
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336 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regexp}? |
337 | |
338 | With the exception of regexps, you need to pass references to these |
339 | objects. See L<perlsub/"Pass by Reference"> for this particular |
340 | question, and L<perlref> for information on references. |
341 | |
342 | =over 4 |
343 | |
344 | =item Passing Variables and Functions |
345 | |
346 | Regular variables and functions are quite easy: just pass in a |
347 | reference to an existing or anonymous variable or function: |
348 | |
349 | func( \$some_scalar ); |
350 | |
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351 | func( \@some_array ); |
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352 | func( [ 1 .. 10 ] ); |
353 | |
354 | func( \%some_hash ); |
355 | func( { this => 10, that => 20 } ); |
356 | |
357 | func( \&some_func ); |
358 | func( sub { $_[0] ** $_[1] } ); |
359 | |
360 | =item Passing Filehandles |
361 | |
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362 | To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations. |
363 | These are "typeglobs" - see L<perldata/"Typeglobs and Filehandles"> |
364 | and especially L<perlsub/"Pass by Reference"> for more information. |
365 | |
366 | Here's an excerpt: |
367 | |
368 | If you're passing around filehandles, you could usually just use the bare |
369 | typeglob, like *STDOUT, but typeglobs references would be better because |
370 | they'll still work properly under C<use strict 'refs'>. For example: |
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371 | |
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372 | splutter(\*STDOUT); |
373 | sub splutter { |
374 | my $fh = shift; |
375 | print $fh "her um well a hmmm\n"; |
376 | } |
377 | |
378 | $rec = get_rec(\*STDIN); |
379 | sub get_rec { |
380 | my $fh = shift; |
381 | return scalar <$fh>; |
382 | } |
383 | |
384 | If you're planning on generating new filehandles, you could do this: |
385 | |
386 | sub openit { |
387 | my $name = shift; |
388 | local *FH; |
389 | return open (FH, $path) ? *FH : undef; |
390 | } |
391 | $fh = openit('< /etc/motd'); |
392 | print <$fh>; |
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393 | |
394 | =item Passing Regexps |
395 | |
396 | To pass regexps around, you'll need to either use one of the highly |
397 | experimental regular expression modules from CPAN (Nick Ing-Simmons's |
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398 | Regexp or Ilya Zakharevich's Devel::Regexp), pass around strings |
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399 | and use an exception-trapping eval, or else be very, very clever. |
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400 | Here's an example of how to pass in a string to be regexp compared: |
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401 | |
402 | sub compare($$) { |
403 | my ($val1, $regexp) = @_; |
404 | my $retval = eval { $val =~ /$regexp/ }; |
405 | die if $@; |
406 | return $retval; |
407 | } |
408 | |
409 | $match = compare("old McDonald", q/d.*D/); |
410 | |
411 | Make sure you never say something like this: |
412 | |
413 | return eval "\$val =~ /$regexp/"; # WRONG |
414 | |
415 | or someone can sneak shell escapes into the regexp due to the double |
416 | interpolation of the eval and the double-quoted string. For example: |
417 | |
418 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; |
419 | |
420 | eval "\$string =~ /$pattern_of_evil/"; |
421 | |
422 | Those preferring to be very, very clever might see the O'Reilly book, |
423 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's |
424 | Build_MatchMany_Function() is particularly interesting. A complete |
425 | citation of this book is given in L<perlfaq2>. |
426 | |
427 | =item Passing Methods |
428 | |
429 | To pass an object method into a subroutine, you can do this: |
430 | |
431 | call_a_lot(10, $some_obj, "methname") |
432 | sub call_a_lot { |
433 | my ($count, $widget, $trick) = @_; |
434 | for (my $i = 0; $i < $count; $i++) { |
435 | $widget->$trick(); |
436 | } |
437 | } |
438 | |
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439 | Or you can use a closure to bundle up the object and its method call |
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440 | and arguments: |
441 | |
442 | my $whatnot = sub { $some_obj->obfuscate(@args) }; |
443 | func($whatnot); |
444 | sub func { |
445 | my $code = shift; |
446 | &$code(); |
447 | } |
448 | |
449 | You could also investigate the can() method in the UNIVERSAL class |
450 | (part of the standard perl distribution). |
451 | |
452 | =back |
453 | |
454 | =head2 How do I create a static variable? |
455 | |
456 | As with most things in Perl, TMTOWTDI. What is a "static variable" in |
457 | other languages could be either a function-private variable (visible |
458 | only within a single function, retaining its value between calls to |
459 | that function), or a file-private variable (visible only to functions |
460 | within the file it was declared in) in Perl. |
461 | |
462 | Here's code to implement a function-private variable: |
463 | |
464 | BEGIN { |
465 | my $counter = 42; |
466 | sub prev_counter { return --$counter } |
467 | sub next_counter { return $counter++ } |
468 | } |
469 | |
470 | Now prev_counter() and next_counter() share a private variable $counter |
471 | that was initialized at compile time. |
472 | |
473 | To declare a file-private variable, you'll still use a my(), putting |
474 | it at the outer scope level at the top of the file. Assume this is in |
475 | file Pax.pm: |
476 | |
477 | package Pax; |
478 | my $started = scalar(localtime(time())); |
479 | |
480 | sub begun { return $started } |
481 | |
482 | When C<use Pax> or C<require Pax> loads this module, the variable will |
483 | be initialized. It won't get garbage-collected the way most variables |
484 | going out of scope do, because the begun() function cares about it, |
485 | but no one else can get it. It is not called $Pax::started because |
486 | its scope is unrelated to the package. It's scoped to the file. You |
487 | could conceivably have several packages in that same file all |
488 | accessing the same private variable, but another file with the same |
489 | package couldn't get to it. |
490 | |
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491 | See L<perlsub/"Peristent Private Variables"> for details. |
492 | |
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493 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
494 | |
495 | C<local($x)> saves away the old value of the global variable C<$x>, |
496 | and assigns a new value for the duration of the subroutine, I<which is |
497 | visible in other functions called from that subroutine>. This is done |
498 | at run-time, so is called dynamic scoping. local() always affects global |
499 | variables, also called package variables or dynamic variables. |
500 | |
501 | C<my($x)> creates a new variable that is only visible in the current |
502 | subroutine. This is done at compile-time, so is called lexical or |
503 | static scoping. my() always affects private variables, also called |
504 | lexical variables or (improperly) static(ly scoped) variables. |
505 | |
506 | For instance: |
507 | |
508 | sub visible { |
509 | print "var has value $var\n"; |
510 | } |
511 | |
512 | sub dynamic { |
513 | local $var = 'local'; # new temporary value for the still-global |
514 | visible(); # variable called $var |
515 | } |
516 | |
517 | sub lexical { |
518 | my $var = 'private'; # new private variable, $var |
519 | visible(); # (invisible outside of sub scope) |
520 | } |
521 | |
522 | $var = 'global'; |
523 | |
524 | visible(); # prints global |
525 | dynamic(); # prints local |
526 | lexical(); # prints global |
527 | |
528 | Notice how at no point does the value "private" get printed. That's |
529 | because $var only has that value within the block of the lexical() |
530 | function, and it is hidden from called subroutine. |
531 | |
532 | In summary, local() doesn't make what you think of as private, local |
533 | variables. It gives a global variable a temporary value. my() is |
534 | what you're looking for if you want private variables. |
535 | |
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536 | See L<perlsub/"Private Variables via my()"> and L<perlsub/"Temporary |
537 | Values via local()"> for excruciating details. |
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538 | |
539 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? |
540 | |
541 | You can do this via symbolic references, provided you haven't set |
542 | C<use strict "refs">. So instead of $var, use C<${'var'}>. |
543 | |
544 | local $var = "global"; |
545 | my $var = "lexical"; |
546 | |
547 | print "lexical is $var\n"; |
548 | |
549 | no strict 'refs'; |
550 | print "global is ${'var'}\n"; |
551 | |
552 | If you know your package, you can just mention it explicitly, as in |
553 | $Some_Pack::var. Note that the notation $::var is I<not> the dynamic |
554 | $var in the current package, but rather the one in the C<main> |
555 | package, as though you had written $main::var. Specifying the package |
556 | directly makes you hard-code its name, but it executes faster and |
557 | avoids running afoul of C<use strict "refs">. |
558 | |
559 | =head2 What's the difference between deep and shallow binding? |
560 | |
561 | In deep binding, lexical variables mentioned in anonymous subroutines |
562 | are the same ones that were in scope when the subroutine was created. |
563 | In shallow binding, they are whichever variables with the same names |
564 | happen to be in scope when the subroutine is called. Perl always uses |
565 | deep binding of lexical variables (i.e., those created with my()). |
566 | However, dynamic variables (aka global, local, or package variables) |
567 | are effectively shallowly bound. Consider this just one more reason |
568 | not to use them. See the answer to L<"What's a closure?">. |
569 | |
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570 | =head2 Why doesn't "my($foo) = E<lt>FILEE<gt>;" work right? |
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571 | |
c8db1d39 |
572 | C<my()> and C<local()> give list context to the right hand side |
573 | of C<=>. The E<lt>FHE<gt> read operation, like so many of Perl's |
574 | functions and operators, can tell which context it was called in and |
575 | behaves appropriately. In general, the scalar() function can help. |
576 | This function does nothing to the data itself (contrary to popular myth) |
577 | but rather tells its argument to behave in whatever its scalar fashion is. |
578 | If that function doesn't have a defined scalar behavior, this of course |
579 | doesn't help you (such as with sort()). |
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580 | |
581 | To enforce scalar context in this particular case, however, you need |
582 | merely omit the parentheses: |
583 | |
584 | local($foo) = <FILE>; # WRONG |
585 | local($foo) = scalar(<FILE>); # ok |
586 | local $foo = <FILE>; # right |
587 | |
588 | You should probably be using lexical variables anyway, although the |
589 | issue is the same here: |
590 | |
591 | my($foo) = <FILE>; # WRONG |
592 | my $foo = <FILE>; # right |
593 | |
54310121 |
594 | =head2 How do I redefine a builtin function, operator, or method? |
68dc0745 |
595 | |
596 | Why do you want to do that? :-) |
597 | |
598 | If you want to override a predefined function, such as open(), |
599 | then you'll have to import the new definition from a different |
600 | module. See L<perlsub/"Overriding Builtin Functions">. There's |
65acb1b1 |
601 | also an example in L<perltoot/"Class::Template">. |
68dc0745 |
602 | |
603 | If you want to overload a Perl operator, such as C<+> or C<**>, |
604 | then you'll want to use the C<use overload> pragma, documented |
605 | in L<overload>. |
606 | |
607 | If you're talking about obscuring method calls in parent classes, |
608 | see L<perltoot/"Overridden Methods">. |
609 | |
610 | =head2 What's the difference between calling a function as &foo and foo()? |
611 | |
612 | When you call a function as C<&foo>, you allow that function access to |
613 | your current @_ values, and you by-pass prototypes. That means that |
614 | the function doesn't get an empty @_, it gets yours! While not |
615 | strictly speaking a bug (it's documented that way in L<perlsub>), it |
616 | would be hard to consider this a feature in most cases. |
617 | |
c8db1d39 |
618 | When you call your function as C<&foo()>, then you I<do> get a new @_, |
68dc0745 |
619 | but prototyping is still circumvented. |
620 | |
621 | Normally, you want to call a function using C<foo()>. You may only |
622 | omit the parentheses if the function is already known to the compiler |
623 | because it already saw the definition (C<use> but not C<require>), |
624 | or via a forward reference or C<use subs> declaration. Even in this |
625 | case, you get a clean @_ without any of the old values leaking through |
626 | where they don't belong. |
627 | |
628 | =head2 How do I create a switch or case statement? |
629 | |
630 | This is explained in more depth in the L<perlsyn>. Briefly, there's |
631 | no official case statement, because of the variety of tests possible |
632 | in Perl (numeric comparison, string comparison, glob comparison, |
633 | regexp matching, overloaded comparisons, ...). Larry couldn't decide |
634 | how best to do this, so he left it out, even though it's been on the |
635 | wish list since perl1. |
636 | |
c8db1d39 |
637 | The general answer is to write a construct like this: |
638 | |
639 | for ($variable_to_test) { |
640 | if (/pat1/) { } # do something |
641 | elsif (/pat2/) { } # do something else |
642 | elsif (/pat3/) { } # do something else |
643 | else { } # default |
644 | } |
68dc0745 |
645 | |
c8db1d39 |
646 | Here's a simple example of a switch based on pattern matching, this |
647 | time lined up in a way to make it look more like a switch statement. |
648 | We'll do a multi-way conditional based on the type of reference stored |
649 | in $whatchamacallit: |
650 | |
651 | SWITCH: for (ref $whatchamacallit) { |
68dc0745 |
652 | |
653 | /^$/ && die "not a reference"; |
654 | |
655 | /SCALAR/ && do { |
656 | print_scalar($$ref); |
657 | last SWITCH; |
658 | }; |
659 | |
660 | /ARRAY/ && do { |
661 | print_array(@$ref); |
662 | last SWITCH; |
663 | }; |
664 | |
665 | /HASH/ && do { |
666 | print_hash(%$ref); |
667 | last SWITCH; |
668 | }; |
669 | |
670 | /CODE/ && do { |
671 | warn "can't print function ref"; |
672 | last SWITCH; |
673 | }; |
674 | |
675 | # DEFAULT |
676 | |
677 | warn "User defined type skipped"; |
678 | |
679 | } |
680 | |
c8db1d39 |
681 | See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other |
682 | examples in this style. |
683 | |
684 | Sometimes you should change the positions of the constant and the variable. |
685 | For example, let's say you wanted to test which of many answers you were |
686 | given, but in a case-insensitive way that also allows abbreviations. |
687 | You can use the following technique if the strings all start with |
688 | different characters, or if you want to arrange the matches so that |
689 | one takes precedence over another, as C<"SEND"> has precedence over |
690 | C<"STOP"> here: |
691 | |
692 | chomp($answer = <>); |
693 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } |
694 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } |
695 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } |
696 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } |
697 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } |
698 | |
699 | A totally different approach is to create a hash of function references. |
700 | |
701 | my %commands = ( |
702 | "happy" => \&joy, |
703 | "sad", => \&sullen, |
704 | "done" => sub { die "See ya!" }, |
705 | "mad" => \&angry, |
706 | ); |
707 | |
708 | print "How are you? "; |
709 | chomp($string = <STDIN>); |
710 | if ($commands{$string}) { |
711 | $commands{$string}->(); |
712 | } else { |
713 | print "No such command: $string\n"; |
714 | } |
715 | |
68dc0745 |
716 | =head2 How can I catch accesses to undefined variables/functions/methods? |
717 | |
718 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and |
719 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to |
720 | undefined functions and methods. |
721 | |
722 | When it comes to undefined variables that would trigger a warning |
723 | under C<-w>, you can use a handler to trap the pseudo-signal |
724 | C<__WARN__> like this: |
725 | |
726 | $SIG{__WARN__} = sub { |
727 | |
c8db1d39 |
728 | for ( $_[0] ) { # voici un switch statement |
68dc0745 |
729 | |
730 | /Use of uninitialized value/ && do { |
731 | # promote warning to a fatal |
732 | die $_; |
733 | }; |
734 | |
735 | # other warning cases to catch could go here; |
736 | |
737 | warn $_; |
738 | } |
739 | |
740 | }; |
741 | |
742 | =head2 Why can't a method included in this same file be found? |
743 | |
744 | Some possible reasons: your inheritance is getting confused, you've |
745 | misspelled the method name, or the object is of the wrong type. Check |
746 | out L<perltoot> for details on these. You may also use C<print |
747 | ref($object)> to find out the class C<$object> was blessed into. |
748 | |
749 | Another possible reason for problems is because you've used the |
750 | indirect object syntax (eg, C<find Guru "Samy">) on a class name |
751 | before Perl has seen that such a package exists. It's wisest to make |
752 | sure your packages are all defined before you start using them, which |
753 | will be taken care of if you use the C<use> statement instead of |
754 | C<require>. If not, make sure to use arrow notation (eg, |
7b8d334a |
755 | C<Guru-E<gt>find("Samy")>) instead. Object notation is explained in |
68dc0745 |
756 | L<perlobj>. |
757 | |
c8db1d39 |
758 | Make sure to read about creating modules in L<perlmod> and |
759 | the perils of indirect objects in L<perlobj/"WARNING">. |
760 | |
68dc0745 |
761 | =head2 How can I find out my current package? |
762 | |
763 | If you're just a random program, you can do this to find |
764 | out what the currently compiled package is: |
765 | |
c8db1d39 |
766 | my $packname = __PACKAGE__; |
68dc0745 |
767 | |
768 | But if you're a method and you want to print an error message |
769 | that includes the kind of object you were called on (which is |
770 | not necessarily the same as the one in which you were compiled): |
771 | |
772 | sub amethod { |
92c2ed05 |
773 | my $self = shift; |
68dc0745 |
774 | my $class = ref($self) || $self; |
775 | warn "called me from a $class object"; |
776 | } |
777 | |
46fc3d4c |
778 | =head2 How can I comment out a large block of perl code? |
779 | |
780 | Use embedded POD to discard it: |
781 | |
782 | # program is here |
783 | |
784 | =for nobody |
785 | This paragraph is commented out |
786 | |
787 | # program continues |
788 | |
789 | =begin comment text |
790 | |
791 | all of this stuff |
792 | |
793 | here will be ignored |
794 | by everyone |
795 | |
796 | =end comment text |
797 | |
fc36a67e |
798 | =cut |
799 | |
c8db1d39 |
800 | This can't go just anywhere. You have to put a pod directive where |
801 | the parser is expecting a new statement, not just in the middle |
802 | of an expression or some other arbitrary yacc grammar production. |
803 | |
65acb1b1 |
804 | =head2 How do I clear a package? |
805 | |
806 | Use this code, provided by Mark-Jason Dominus: |
807 | |
808 | sub scrub_package { |
809 | no strict 'refs'; |
810 | my $pack = shift; |
811 | die "Shouldn't delete main package" |
812 | if $pack eq "" || $pack eq "main"; |
813 | my $stash = *{$pack . '::'}{HASH}; |
814 | my $name; |
815 | foreach $name (keys %$stash) { |
816 | my $fullname = $pack . '::' . $name; |
817 | # Get rid of everything with that name. |
818 | undef $$fullname; |
819 | undef @$fullname; |
820 | undef %$fullname; |
821 | undef &$fullname; |
822 | undef *$fullname; |
823 | } |
824 | } |
825 | |
826 | Or, if you're using a recent release of Perl, you can |
827 | just use the Symbol::delete_package() function instead. |
828 | |
68dc0745 |
829 | =head1 AUTHOR AND COPYRIGHT |
830 | |
65acb1b1 |
831 | Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. |
5a964f20 |
832 | All rights reserved. |
833 | |
834 | When included as part of the Standard Version of Perl, or as part of |
835 | its complete documentation whether printed or otherwise, this work |
836 | may be distributed only under the terms of Perl's Artistic License. |
837 | Any distribution of this file or derivatives thereof I<outside> |
838 | of that package require that special arrangements be made with |
839 | copyright holder. |
840 | |
841 | Irrespective of its distribution, all code examples in this file |
842 | are hereby placed into the public domain. You are permitted and |
843 | encouraged to use this code in your own programs for fun |
844 | or for profit as you see fit. A simple comment in the code giving |
845 | credit would be courteous but is not required. |
65acb1b1 |
846 | |