Commit | Line | Data |
68dc0745 |
1 | =head1 NAME |
2 | |
fc36a67e |
3 | perlfaq7 - Perl Language Issues ($Revision: 1.18 $, $Date: 1997/04/24 22:44:14 $) |
68dc0745 |
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 | |
12 | No, in the words of Chaim Frenkel: "Perl's grammar can not be reduced |
13 | to BNF. The work of parsing perl is distributed between yacc, the |
14 | lexer, smoke and mirrors." |
15 | |
16 | =head2 What are all these $@%* punctuation signs, and how do I know when to use them? |
17 | |
18 | They are type specifiers, as detailed in L<perldata>: |
19 | |
20 | $ for scalar values (number, string or reference) |
21 | @ for arrays |
22 | % for hashes (associative arrays) |
23 | * for all types of that symbol name. In version 4 you used them like |
24 | pointers, but in modern perls you can just use references. |
25 | |
26 | While there are a few places where you don't actually need these type |
27 | specifiers, you should always use them. |
28 | |
29 | A couple of others that you're likely to encounter that aren't |
30 | really type specifiers are: |
31 | |
32 | <> are used for inputting a record from a filehandle. |
33 | \ takes a reference to something. |
34 | |
35 | Note that E<lt>FILEE<gt> is I<neither> the type specifier for files |
36 | nor the name of the handle. It is the C<E<lt>E<gt>> operator applied |
37 | to the handle FILE. It reads one line (well, record - see |
38 | L<perlvar/$/>) from the handle FILE in scalar context, or I<all> lines |
39 | in list context. When performing open, close, or any other operation |
40 | besides C<E<lt>E<gt>> on files, or even talking about the handle, do |
41 | I<not> use the brackets. These are correct: C<eof(FH)>, C<seek(FH, 0, |
42 | 2)> and "copying from STDIN to FILE". |
43 | |
44 | =head2 Do I always/never have to quote my strings or use semicolons and commas? |
45 | |
46 | Normally, a bareword doesn't need to be quoted, but in most cases |
47 | probably should be (and must be under C<use strict>). But a hash key |
48 | consisting of a simple word (that isn't the name of a defined |
49 | subroutine) and the left-hand operand to the C<=E<gt>> operator both |
50 | count as though they were quoted: |
51 | |
52 | This is like this |
53 | ------------ --------------- |
54 | $foo{line} $foo{"line"} |
55 | bar => stuff "bar" => stuff |
56 | |
57 | The final semicolon in a block is optional, as is the final comma in a |
58 | list. Good style (see L<perlstyle>) says to put them in except for |
59 | one-liners: |
60 | |
61 | if ($whoops) { exit 1 } |
62 | @nums = (1, 2, 3); |
63 | |
64 | if ($whoops) { |
65 | exit 1; |
66 | } |
67 | @lines = ( |
68 | "There Beren came from mountains cold", |
69 | "And lost he wandered under leaves", |
70 | ); |
71 | |
72 | =head2 How do I skip some return values? |
73 | |
74 | One way is to treat the return values as a list and index into it: |
75 | |
76 | $dir = (getpwnam($user))[7]; |
77 | |
78 | Another way is to use undef as an element on the left-hand-side: |
79 | |
80 | ($dev, $ino, undef, undef, $uid, $gid) = stat($file); |
81 | |
82 | =head2 How do I temporarily block warnings? |
83 | |
84 | The C<$^W> variable (documented in L<perlvar>) controls |
85 | runtime warnings for a block: |
86 | |
87 | { |
88 | local $^W = 0; # temporarily turn off warnings |
89 | $a = $b + $c; # I know these might be undef |
90 | } |
91 | |
92 | Note that like all the punctuation variables, you cannot currently |
93 | use my() on C<$^W>, only local(). |
94 | |
95 | A new C<use warnings> pragma is in the works to provide finer control |
96 | over all this. The curious should check the perl5-porters mailing list |
97 | archives for details. |
98 | |
99 | =head2 What's an extension? |
100 | |
101 | A way of calling compiled C code from Perl. Reading L<perlxstut> |
102 | is a good place to learn more about extensions. |
103 | |
104 | =head2 Why do Perl operators have different precedence than C operators? |
105 | |
106 | Actually, they don't. All C operators that Perl copies have the same |
107 | precedence in Perl as they do in C. The problem is with operators that C |
108 | doesn't have, especially functions that give a list context to everything |
109 | on their right, eg print, chmod, exec, and so on. Such functions are |
110 | called "list operators" and appear as such in the precedence table in |
111 | L<perlop>. |
112 | |
113 | A common mistake is to write: |
114 | |
115 | unlink $file || die "snafu"; |
116 | |
117 | This gets interpreted as: |
118 | |
119 | unlink ($file || die "snafu"); |
120 | |
121 | To avoid this problem, either put in extra parentheses or use the |
122 | super low precedence C<or> operator: |
123 | |
124 | (unlink $file) || die "snafu"; |
125 | unlink $file or die "snafu"; |
126 | |
127 | The "English" operators (C<and>, C<or>, C<xor>, and C<not>) |
128 | deliberately have precedence lower than that of list operators for |
129 | just such situations as the one above. |
130 | |
131 | Another operator with surprising precedence is exponentiation. It |
132 | binds more tightly even than unary minus, making C<-2**2> product a |
133 | negative not a positive four. It is also right-associating, meaning |
134 | that C<2**3**2> is two raised to the ninth power, not eight squared. |
135 | |
136 | =head2 How do I declare/create a structure? |
137 | |
138 | In general, you don't "declare" a structure. Just use a (probably |
139 | anonymous) hash reference. See L<perlref> and L<perldsc> for details. |
140 | Here's an example: |
141 | |
142 | $person = {}; # new anonymous hash |
143 | $person->{AGE} = 24; # set field AGE to 24 |
144 | $person->{NAME} = "Nat"; # set field NAME to "Nat" |
145 | |
146 | If you're looking for something a bit more rigorous, try L<perltoot>. |
147 | |
148 | =head2 How do I create a module? |
149 | |
150 | A module is a package that lives in a file of the same name. For |
151 | example, the Hello::There module would live in Hello/There.pm. For |
152 | details, read L<perlmod>. You'll also find L<Exporter> helpful. If |
153 | you're writing a C or mixed-language module with both C and Perl, then |
154 | you should study L<perlxstut>. |
155 | |
156 | Here's a convenient template you might wish you use when starting your |
157 | own module. Make sure to change the names appropriately. |
158 | |
159 | package Some::Module; # assumes Some/Module.pm |
160 | |
161 | use strict; |
162 | |
163 | BEGIN { |
164 | use Exporter (); |
165 | use vars qw($VERSION @ISA @EXPORT @EXPORT_OK %EXPORT_TAGS); |
166 | |
167 | ## set the version for version checking; uncomment to use |
168 | ## $VERSION = 1.00; |
169 | |
170 | # if using RCS/CVS, this next line may be preferred, |
171 | # but beware two-digit versions. |
fc36a67e |
172 | $VERSION = do{my@r=q$Revision: 1.18 $=~/\d+/g;sprintf '%d.'.'%02d'x$#r,@r}; |
68dc0745 |
173 | |
174 | @ISA = qw(Exporter); |
175 | @EXPORT = qw(&func1 &func2 &func3); |
176 | %EXPORT_TAGS = ( ); # eg: TAG => [ qw!name1 name2! ], |
177 | |
178 | # your exported package globals go here, |
179 | # as well as any optionally exported functions |
180 | @EXPORT_OK = qw($Var1 %Hashit); |
181 | } |
182 | use vars @EXPORT_OK; |
183 | |
184 | # non-exported package globals go here |
185 | use vars qw( @more $stuff ); |
186 | |
187 | # initialize package globals, first exported ones |
188 | $Var1 = ''; |
189 | %Hashit = (); |
190 | |
191 | # then the others (which are still accessible as $Some::Module::stuff) |
192 | $stuff = ''; |
193 | @more = (); |
194 | |
195 | # all file-scoped lexicals must be created before |
196 | # the functions below that use them. |
197 | |
198 | # file-private lexicals go here |
199 | my $priv_var = ''; |
200 | my %secret_hash = (); |
201 | |
202 | # here's a file-private function as a closure, |
203 | # callable as &$priv_func; it cannot be prototyped. |
204 | my $priv_func = sub { |
205 | # stuff goes here. |
206 | }; |
207 | |
208 | # make all your functions, whether exported or not; |
209 | # remember to put something interesting in the {} stubs |
210 | sub func1 {} # no prototype |
211 | sub func2() {} # proto'd void |
212 | sub func3($$) {} # proto'd to 2 scalars |
213 | |
214 | # this one isn't exported, but could be called! |
215 | sub func4(\%) {} # proto'd to 1 hash ref |
216 | |
217 | END { } # module clean-up code here (global destructor) |
218 | |
219 | 1; # modules must return true |
220 | |
221 | =head2 How do I create a class? |
222 | |
223 | See L<perltoot> for an introduction to classes and objects, as well as |
224 | L<perlobj> and L<perlbot>. |
225 | |
226 | =head2 How can I tell if a variable is tainted? |
227 | |
228 | See L<perlsec/"Laundering and Detecting Tainted Data">. Here's an |
229 | example (which doesn't use any system calls, because the kill() |
230 | is given no processes to signal): |
231 | |
232 | sub is_tainted { |
233 | return ! eval { join('',@_), kill 0; 1; }; |
234 | } |
235 | |
236 | This is not C<-w> clean, however. There is no C<-w> clean way to |
237 | detect taintedness - take this as a hint that you should untaint |
238 | all possibly-tainted data. |
239 | |
240 | =head2 What's a closure? |
241 | |
242 | Closures are documented in L<perlref>. |
243 | |
244 | I<Closure> is a computer science term with a precise but |
245 | hard-to-explain meaning. Closures are implemented in Perl as anonymous |
246 | subroutines with lasting references to lexical variables outside their |
247 | own scopes. These lexicals magically refer to the variables that were |
248 | around when the subroutine was defined (deep binding). |
249 | |
250 | Closures make sense in any programming language where you can have the |
251 | return value of a function be itself a function, as you can in Perl. |
252 | Note that some languages provide anonymous functions but are not |
253 | capable of providing proper closures; the Python language, for |
254 | example. For more information on closures, check out any textbook on |
255 | functional programming. Scheme is a language that not only supports |
256 | but encourages closures. |
257 | |
258 | Here's a classic function-generating function: |
259 | |
260 | sub add_function_generator { |
261 | return sub { shift + shift }; |
262 | } |
263 | |
264 | $add_sub = add_function_generator(); |
265 | $sum = &$add_sub(4,5); # $sum is 9 now. |
266 | |
267 | The closure works as a I<function template> with some customization |
268 | slots left out to be filled later. The anonymous subroutine returned |
269 | by add_function_generator() isn't technically a closure because it |
270 | refers to no lexicals outside its own scope. |
271 | |
272 | Contrast this with the following make_adder() function, in which the |
273 | returned anonymous function contains a reference to a lexical variable |
274 | outside the scope of that function itself. Such a reference requires |
275 | that Perl return a proper closure, thus locking in for all time the |
276 | value that the lexical had when the function was created. |
277 | |
278 | sub make_adder { |
279 | my $addpiece = shift; |
280 | return sub { shift + $addpiece }; |
281 | } |
282 | |
283 | $f1 = make_adder(20); |
284 | $f2 = make_adder(555); |
285 | |
286 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas |
287 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece |
288 | in the closure sticks around. |
289 | |
290 | Closures are often used for less esoteric purposes. For example, when |
291 | you want to pass in a bit of code into a function: |
292 | |
293 | my $line; |
294 | timeout( 30, sub { $line = <STDIN> } ); |
295 | |
296 | If the code to execute had been passed in as a string, C<'$line = |
297 | E<lt>STDINE<gt>'>, there would have been no way for the hypothetical |
298 | timeout() function to access the lexical variable $line back in its |
299 | caller's scope. |
300 | |
46fc3d4c |
301 | =head2 What is variable suicide and how can I prevent it? |
302 | |
303 | Variable suicide is when you (temporarily or permanently) lose the |
304 | value of a variable. It is caused by scoping through my() and local() |
305 | interacting with either closures or aliased foreach() interator |
306 | variables and subroutine arguments. It used to be easy to |
307 | inadvertently lose a variable's value this way, but now it's much |
308 | harder. Take this code: |
309 | |
310 | my $f = "foo"; |
311 | sub T { |
312 | while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } |
313 | } |
314 | T; |
315 | print "Finally $f\n"; |
316 | |
317 | The $f that has "bar" added to it three times should be a new C<$f> |
318 | (C<my $f> should create a new local variable each time through the |
319 | loop). It isn't, however. This is a bug, and will be fixed. |
320 | |
68dc0745 |
321 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regexp}? |
322 | |
323 | With the exception of regexps, you need to pass references to these |
324 | objects. See L<perlsub/"Pass by Reference"> for this particular |
325 | question, and L<perlref> for information on references. |
326 | |
327 | =over 4 |
328 | |
329 | =item Passing Variables and Functions |
330 | |
331 | Regular variables and functions are quite easy: just pass in a |
332 | reference to an existing or anonymous variable or function: |
333 | |
334 | func( \$some_scalar ); |
335 | |
336 | func( \$some_array ); |
337 | func( [ 1 .. 10 ] ); |
338 | |
339 | func( \%some_hash ); |
340 | func( { this => 10, that => 20 } ); |
341 | |
342 | func( \&some_func ); |
343 | func( sub { $_[0] ** $_[1] } ); |
344 | |
345 | =item Passing Filehandles |
346 | |
347 | To create filehandles you can pass to subroutines, you can use C<*FH> |
348 | or C<\*FH> notation ("typeglobs" - see L<perldata> for more information), |
349 | or create filehandles dynamically using the old FileHandle or the new |
350 | IO::File modules, both part of the standard Perl distribution. |
351 | |
352 | use Fcntl; |
353 | use IO::File; |
354 | my $fh = new IO::File $filename, O_WRONLY|O_APPEND; |
355 | or die "Can't append to $filename: $!"; |
356 | func($fh); |
357 | |
358 | =item Passing Regexps |
359 | |
360 | To pass regexps around, you'll need to either use one of the highly |
361 | experimental regular expression modules from CPAN (Nick Ing-Simmons's |
46fc3d4c |
362 | Regexp or Ilya Zakharevich's Devel::Regexp), pass around strings |
363 | and use an exception-trapping eval, or else be be very, very clever. |
364 | Here's an example of how to pass in a string to be regexp compared: |
68dc0745 |
365 | |
366 | sub compare($$) { |
367 | my ($val1, $regexp) = @_; |
368 | my $retval = eval { $val =~ /$regexp/ }; |
369 | die if $@; |
370 | return $retval; |
371 | } |
372 | |
373 | $match = compare("old McDonald", q/d.*D/); |
374 | |
375 | Make sure you never say something like this: |
376 | |
377 | return eval "\$val =~ /$regexp/"; # WRONG |
378 | |
379 | or someone can sneak shell escapes into the regexp due to the double |
380 | interpolation of the eval and the double-quoted string. For example: |
381 | |
382 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; |
383 | |
384 | eval "\$string =~ /$pattern_of_evil/"; |
385 | |
386 | Those preferring to be very, very clever might see the O'Reilly book, |
387 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's |
388 | Build_MatchMany_Function() is particularly interesting. A complete |
389 | citation of this book is given in L<perlfaq2>. |
390 | |
391 | =item Passing Methods |
392 | |
393 | To pass an object method into a subroutine, you can do this: |
394 | |
395 | call_a_lot(10, $some_obj, "methname") |
396 | sub call_a_lot { |
397 | my ($count, $widget, $trick) = @_; |
398 | for (my $i = 0; $i < $count; $i++) { |
399 | $widget->$trick(); |
400 | } |
401 | } |
402 | |
403 | or you can use a closure to bundle up the object and its method call |
404 | and arguments: |
405 | |
406 | my $whatnot = sub { $some_obj->obfuscate(@args) }; |
407 | func($whatnot); |
408 | sub func { |
409 | my $code = shift; |
410 | &$code(); |
411 | } |
412 | |
413 | You could also investigate the can() method in the UNIVERSAL class |
414 | (part of the standard perl distribution). |
415 | |
416 | =back |
417 | |
418 | =head2 How do I create a static variable? |
419 | |
420 | As with most things in Perl, TMTOWTDI. What is a "static variable" in |
421 | other languages could be either a function-private variable (visible |
422 | only within a single function, retaining its value between calls to |
423 | that function), or a file-private variable (visible only to functions |
424 | within the file it was declared in) in Perl. |
425 | |
426 | Here's code to implement a function-private variable: |
427 | |
428 | BEGIN { |
429 | my $counter = 42; |
430 | sub prev_counter { return --$counter } |
431 | sub next_counter { return $counter++ } |
432 | } |
433 | |
434 | Now prev_counter() and next_counter() share a private variable $counter |
435 | that was initialized at compile time. |
436 | |
437 | To declare a file-private variable, you'll still use a my(), putting |
438 | it at the outer scope level at the top of the file. Assume this is in |
439 | file Pax.pm: |
440 | |
441 | package Pax; |
442 | my $started = scalar(localtime(time())); |
443 | |
444 | sub begun { return $started } |
445 | |
446 | When C<use Pax> or C<require Pax> loads this module, the variable will |
447 | be initialized. It won't get garbage-collected the way most variables |
448 | going out of scope do, because the begun() function cares about it, |
449 | but no one else can get it. It is not called $Pax::started because |
450 | its scope is unrelated to the package. It's scoped to the file. You |
451 | could conceivably have several packages in that same file all |
452 | accessing the same private variable, but another file with the same |
453 | package couldn't get to it. |
454 | |
455 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
456 | |
457 | C<local($x)> saves away the old value of the global variable C<$x>, |
458 | and assigns a new value for the duration of the subroutine, I<which is |
459 | visible in other functions called from that subroutine>. This is done |
460 | at run-time, so is called dynamic scoping. local() always affects global |
461 | variables, also called package variables or dynamic variables. |
462 | |
463 | C<my($x)> creates a new variable that is only visible in the current |
464 | subroutine. This is done at compile-time, so is called lexical or |
465 | static scoping. my() always affects private variables, also called |
466 | lexical variables or (improperly) static(ly scoped) variables. |
467 | |
468 | For instance: |
469 | |
470 | sub visible { |
471 | print "var has value $var\n"; |
472 | } |
473 | |
474 | sub dynamic { |
475 | local $var = 'local'; # new temporary value for the still-global |
476 | visible(); # variable called $var |
477 | } |
478 | |
479 | sub lexical { |
480 | my $var = 'private'; # new private variable, $var |
481 | visible(); # (invisible outside of sub scope) |
482 | } |
483 | |
484 | $var = 'global'; |
485 | |
486 | visible(); # prints global |
487 | dynamic(); # prints local |
488 | lexical(); # prints global |
489 | |
490 | Notice how at no point does the value "private" get printed. That's |
491 | because $var only has that value within the block of the lexical() |
492 | function, and it is hidden from called subroutine. |
493 | |
494 | In summary, local() doesn't make what you think of as private, local |
495 | variables. It gives a global variable a temporary value. my() is |
496 | what you're looking for if you want private variables. |
497 | |
498 | See also L<perlsub>, which explains this all in more detail. |
499 | |
500 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? |
501 | |
502 | You can do this via symbolic references, provided you haven't set |
503 | C<use strict "refs">. So instead of $var, use C<${'var'}>. |
504 | |
505 | local $var = "global"; |
506 | my $var = "lexical"; |
507 | |
508 | print "lexical is $var\n"; |
509 | |
510 | no strict 'refs'; |
511 | print "global is ${'var'}\n"; |
512 | |
513 | If you know your package, you can just mention it explicitly, as in |
514 | $Some_Pack::var. Note that the notation $::var is I<not> the dynamic |
515 | $var in the current package, but rather the one in the C<main> |
516 | package, as though you had written $main::var. Specifying the package |
517 | directly makes you hard-code its name, but it executes faster and |
518 | avoids running afoul of C<use strict "refs">. |
519 | |
520 | =head2 What's the difference between deep and shallow binding? |
521 | |
522 | In deep binding, lexical variables mentioned in anonymous subroutines |
523 | are the same ones that were in scope when the subroutine was created. |
524 | In shallow binding, they are whichever variables with the same names |
525 | happen to be in scope when the subroutine is called. Perl always uses |
526 | deep binding of lexical variables (i.e., those created with my()). |
527 | However, dynamic variables (aka global, local, or package variables) |
528 | are effectively shallowly bound. Consider this just one more reason |
529 | not to use them. See the answer to L<"What's a closure?">. |
530 | |
531 | =head2 Why doesn't "local($foo) = <FILE>;" work right? |
532 | |
533 | C<local()> gives list context to the right hand side of C<=>. The |
534 | E<lt>FHE<gt> read operation, like so many of Perl's functions and |
535 | operators, can tell which context it was called in and behaves |
536 | appropriately. In general, the scalar() function can help. This |
537 | function does nothing to the data itself (contrary to popular myth) |
538 | but rather tells its argument to behave in whatever its scalar fashion |
539 | is. If that function doesn't have a defined scalar behavior, this of |
540 | course doesn't help you (such as with sort()). |
541 | |
542 | To enforce scalar context in this particular case, however, you need |
543 | merely omit the parentheses: |
544 | |
545 | local($foo) = <FILE>; # WRONG |
546 | local($foo) = scalar(<FILE>); # ok |
547 | local $foo = <FILE>; # right |
548 | |
549 | You should probably be using lexical variables anyway, although the |
550 | issue is the same here: |
551 | |
552 | my($foo) = <FILE>; # WRONG |
553 | my $foo = <FILE>; # right |
554 | |
54310121 |
555 | =head2 How do I redefine a builtin function, operator, or method? |
68dc0745 |
556 | |
557 | Why do you want to do that? :-) |
558 | |
559 | If you want to override a predefined function, such as open(), |
560 | then you'll have to import the new definition from a different |
561 | module. See L<perlsub/"Overriding Builtin Functions">. There's |
46fc3d4c |
562 | also an example in L<perltoot/"Class::Template">. |
68dc0745 |
563 | |
564 | If you want to overload a Perl operator, such as C<+> or C<**>, |
565 | then you'll want to use the C<use overload> pragma, documented |
566 | in L<overload>. |
567 | |
568 | If you're talking about obscuring method calls in parent classes, |
569 | see L<perltoot/"Overridden Methods">. |
570 | |
571 | =head2 What's the difference between calling a function as &foo and foo()? |
572 | |
573 | When you call a function as C<&foo>, you allow that function access to |
574 | your current @_ values, and you by-pass prototypes. That means that |
575 | the function doesn't get an empty @_, it gets yours! While not |
576 | strictly speaking a bug (it's documented that way in L<perlsub>), it |
577 | would be hard to consider this a feature in most cases. |
578 | |
579 | When you call your function as C<&foo()>, then you do get a new @_, |
580 | but prototyping is still circumvented. |
581 | |
582 | Normally, you want to call a function using C<foo()>. You may only |
583 | omit the parentheses if the function is already known to the compiler |
584 | because it already saw the definition (C<use> but not C<require>), |
585 | or via a forward reference or C<use subs> declaration. Even in this |
586 | case, you get a clean @_ without any of the old values leaking through |
587 | where they don't belong. |
588 | |
589 | =head2 How do I create a switch or case statement? |
590 | |
591 | This is explained in more depth in the L<perlsyn>. Briefly, there's |
592 | no official case statement, because of the variety of tests possible |
593 | in Perl (numeric comparison, string comparison, glob comparison, |
594 | regexp matching, overloaded comparisons, ...). Larry couldn't decide |
595 | how best to do this, so he left it out, even though it's been on the |
596 | wish list since perl1. |
597 | |
598 | Here's a simple example of a switch based on pattern matching. We'll |
46fc3d4c |
599 | do a multi-way conditional based on the type of reference stored in |
68dc0745 |
600 | $whatchamacallit: |
601 | |
602 | SWITCH: |
603 | for (ref $whatchamacallit) { |
604 | |
605 | /^$/ && die "not a reference"; |
606 | |
607 | /SCALAR/ && do { |
608 | print_scalar($$ref); |
609 | last SWITCH; |
610 | }; |
611 | |
612 | /ARRAY/ && do { |
613 | print_array(@$ref); |
614 | last SWITCH; |
615 | }; |
616 | |
617 | /HASH/ && do { |
618 | print_hash(%$ref); |
619 | last SWITCH; |
620 | }; |
621 | |
622 | /CODE/ && do { |
623 | warn "can't print function ref"; |
624 | last SWITCH; |
625 | }; |
626 | |
627 | # DEFAULT |
628 | |
629 | warn "User defined type skipped"; |
630 | |
631 | } |
632 | |
633 | =head2 How can I catch accesses to undefined variables/functions/methods? |
634 | |
635 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and |
636 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to |
637 | undefined functions and methods. |
638 | |
639 | When it comes to undefined variables that would trigger a warning |
640 | under C<-w>, you can use a handler to trap the pseudo-signal |
641 | C<__WARN__> like this: |
642 | |
643 | $SIG{__WARN__} = sub { |
644 | |
645 | for ( $_[0] ) { |
646 | |
647 | /Use of uninitialized value/ && do { |
648 | # promote warning to a fatal |
649 | die $_; |
650 | }; |
651 | |
652 | # other warning cases to catch could go here; |
653 | |
654 | warn $_; |
655 | } |
656 | |
657 | }; |
658 | |
659 | =head2 Why can't a method included in this same file be found? |
660 | |
661 | Some possible reasons: your inheritance is getting confused, you've |
662 | misspelled the method name, or the object is of the wrong type. Check |
663 | out L<perltoot> for details on these. You may also use C<print |
664 | ref($object)> to find out the class C<$object> was blessed into. |
665 | |
666 | Another possible reason for problems is because you've used the |
667 | indirect object syntax (eg, C<find Guru "Samy">) on a class name |
668 | before Perl has seen that such a package exists. It's wisest to make |
669 | sure your packages are all defined before you start using them, which |
670 | will be taken care of if you use the C<use> statement instead of |
671 | C<require>. If not, make sure to use arrow notation (eg, |
7b8d334a |
672 | C<Guru-E<gt>find("Samy")>) instead. Object notation is explained in |
68dc0745 |
673 | L<perlobj>. |
674 | |
675 | =head2 How can I find out my current package? |
676 | |
677 | If you're just a random program, you can do this to find |
678 | out what the currently compiled package is: |
679 | |
680 | my $packname = ref bless []; |
681 | |
682 | But if you're a method and you want to print an error message |
683 | that includes the kind of object you were called on (which is |
684 | not necessarily the same as the one in which you were compiled): |
685 | |
686 | sub amethod { |
687 | my $self = shift; |
688 | my $class = ref($self) || $self; |
689 | warn "called me from a $class object"; |
690 | } |
691 | |
46fc3d4c |
692 | =head2 How can I comment out a large block of perl code? |
693 | |
694 | Use embedded POD to discard it: |
695 | |
696 | # program is here |
697 | |
698 | =for nobody |
699 | This paragraph is commented out |
700 | |
701 | # program continues |
702 | |
703 | =begin comment text |
704 | |
705 | all of this stuff |
706 | |
707 | here will be ignored |
708 | by everyone |
709 | |
710 | =end comment text |
711 | |
fc36a67e |
712 | =cut |
713 | |
68dc0745 |
714 | =head1 AUTHOR AND COPYRIGHT |
715 | |
716 | Copyright (c) 1997 Tom Christiansen and Nathan Torkington. |
717 | All rights reserved. See L<perlfaq> for distribution information. |