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68dc0745 | 1 | =head1 NAME |
2 | ||
f05bbc40 | 3 | perlfaq7 - General Perl Language Issues ($Revision: 1.8 $, $Date: 2002/03/26 15:48:32 $) |
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 | ||
c8db1d39 TC |
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." | |
68dc0745 | 20 | |
d92eb7b0 | 21 | =head2 What are all these $@%&* punctuation signs, and how do I know when to use them? |
68dc0745 | 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) | |
d92eb7b0 | 28 | & for subroutines (aka functions, procedures, methods) |
68dc0745 | 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 | ||
a6dd486b JB |
32 | There are couple of other symbols that you're likely to encounter that aren't |
33 | really type specifiers: | |
68dc0745 | 34 | |
35 | <> are used for inputting a record from a filehandle. | |
36 | \ takes a reference to something. | |
37 | ||
c47ff5f1 GS |
38 | Note that <FILE> is I<neither> the type specifier for files |
39 | nor the name of the handle. It is the C<< <> >> operator applied | |
a6dd486b | 40 | to the handle FILE. It reads one line (well, record--see |
68dc0745 | 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 | |
a6dd486b | 43 | besides C<< <> >> on files, or even when talking about the handle, do |
68dc0745 | 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 | |
c47ff5f1 | 52 | subroutine) and the left-hand operand to the C<< => >> operator both |
68dc0745 | 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 | ||
9f1b1f2d GS |
87 | If you are running Perl 5.6.0 or better, the C<use warnings> pragma |
88 | allows fine control of what warning are produced. | |
89 | See L<perllexwarn> for more details. | |
90 | ||
91 | { | |
92 | no warnings; # temporarily turn off warnings | |
93 | $a = $b + $c; # I know these might be undef | |
94 | } | |
95 | ||
96 | If you have an older version of Perl, the C<$^W> variable (documented | |
97 | in L<perlvar>) controls runtime warnings for a block: | |
68dc0745 | 98 | |
99 | { | |
100 | local $^W = 0; # temporarily turn off warnings | |
101 | $a = $b + $c; # I know these might be undef | |
102 | } | |
103 | ||
104 | Note that like all the punctuation variables, you cannot currently | |
105 | use my() on C<$^W>, only local(). | |
106 | ||
68dc0745 | 107 | =head2 What's an extension? |
108 | ||
a6dd486b JB |
109 | An extension is a way of calling compiled C code from Perl. Reading |
110 | L<perlxstut> is a good place to learn more about extensions. | |
68dc0745 | 111 | |
112 | =head2 Why do Perl operators have different precedence than C operators? | |
113 | ||
114 | Actually, they don't. All C operators that Perl copies have the same | |
115 | precedence in Perl as they do in C. The problem is with operators that C | |
116 | doesn't have, especially functions that give a list context to everything | |
a6dd486b | 117 | on their right, eg. print, chmod, exec, and so on. Such functions are |
68dc0745 | 118 | called "list operators" and appear as such in the precedence table in |
119 | L<perlop>. | |
120 | ||
121 | A common mistake is to write: | |
122 | ||
123 | unlink $file || die "snafu"; | |
124 | ||
125 | This gets interpreted as: | |
126 | ||
127 | unlink ($file || die "snafu"); | |
128 | ||
129 | To avoid this problem, either put in extra parentheses or use the | |
130 | super low precedence C<or> operator: | |
131 | ||
132 | (unlink $file) || die "snafu"; | |
133 | unlink $file or die "snafu"; | |
134 | ||
135 | The "English" operators (C<and>, C<or>, C<xor>, and C<not>) | |
136 | deliberately have precedence lower than that of list operators for | |
137 | just such situations as the one above. | |
138 | ||
139 | Another operator with surprising precedence is exponentiation. It | |
140 | binds more tightly even than unary minus, making C<-2**2> product a | |
141 | negative not a positive four. It is also right-associating, meaning | |
142 | that C<2**3**2> is two raised to the ninth power, not eight squared. | |
143 | ||
c8db1d39 TC |
144 | Although it has the same precedence as in C, Perl's C<?:> operator |
145 | produces an lvalue. This assigns $x to either $a or $b, depending | |
146 | on the trueness of $maybe: | |
147 | ||
148 | ($maybe ? $a : $b) = $x; | |
149 | ||
68dc0745 | 150 | =head2 How do I declare/create a structure? |
151 | ||
152 | In general, you don't "declare" a structure. Just use a (probably | |
153 | anonymous) hash reference. See L<perlref> and L<perldsc> for details. | |
154 | Here's an example: | |
155 | ||
156 | $person = {}; # new anonymous hash | |
157 | $person->{AGE} = 24; # set field AGE to 24 | |
158 | $person->{NAME} = "Nat"; # set field NAME to "Nat" | |
159 | ||
160 | If you're looking for something a bit more rigorous, try L<perltoot>. | |
161 | ||
162 | =head2 How do I create a module? | |
163 | ||
164 | A module is a package that lives in a file of the same name. For | |
165 | example, the Hello::There module would live in Hello/There.pm. For | |
166 | details, read L<perlmod>. You'll also find L<Exporter> helpful. If | |
167 | you're writing a C or mixed-language module with both C and Perl, then | |
168 | you should study L<perlxstut>. | |
169 | ||
6761e064 | 170 | The C<h2xs> program will create stubs for all the important stuff for you: |
65acb1b1 TC |
171 | |
172 | % h2xs -XA -n My::Module | |
7207e29d | 173 | |
6761e064 JH |
174 | The C<-X> switch tells C<h2xs> that you are not using C<XS> extension |
175 | code. The C<-A> switch tells C<h2xs> that you are not using the | |
176 | AutoLoader, and the C<-n> switch specifies the name of the module. | |
177 | See L<h2xs> for more details. | |
65acb1b1 | 178 | |
68dc0745 | 179 | =head2 How do I create a class? |
180 | ||
181 | See L<perltoot> for an introduction to classes and objects, as well as | |
182 | L<perlobj> and L<perlbot>. | |
183 | ||
184 | =head2 How can I tell if a variable is tainted? | |
185 | ||
213329dd JH |
186 | You can use the tainted() function of the Scalar::Util module, available |
187 | from CPAN (or included with Perl since release 5.8.0). | |
188 | See also L<perlsec/"Laundering and Detecting Tainted Data">. | |
68dc0745 | 189 | |
190 | =head2 What's a closure? | |
191 | ||
192 | Closures are documented in L<perlref>. | |
193 | ||
194 | I<Closure> is a computer science term with a precise but | |
195 | hard-to-explain meaning. Closures are implemented in Perl as anonymous | |
196 | subroutines with lasting references to lexical variables outside their | |
197 | own scopes. These lexicals magically refer to the variables that were | |
198 | around when the subroutine was defined (deep binding). | |
199 | ||
200 | Closures make sense in any programming language where you can have the | |
201 | return value of a function be itself a function, as you can in Perl. | |
202 | Note that some languages provide anonymous functions but are not | |
a6dd486b | 203 | capable of providing proper closures: the Python language, for |
68dc0745 | 204 | example. For more information on closures, check out any textbook on |
205 | functional programming. Scheme is a language that not only supports | |
206 | but encourages closures. | |
207 | ||
208 | Here's a classic function-generating function: | |
209 | ||
210 | sub add_function_generator { | |
211 | return sub { shift + shift }; | |
212 | } | |
213 | ||
214 | $add_sub = add_function_generator(); | |
c8db1d39 | 215 | $sum = $add_sub->(4,5); # $sum is 9 now. |
68dc0745 | 216 | |
217 | The closure works as a I<function template> with some customization | |
218 | slots left out to be filled later. The anonymous subroutine returned | |
219 | by add_function_generator() isn't technically a closure because it | |
220 | refers to no lexicals outside its own scope. | |
221 | ||
222 | Contrast this with the following make_adder() function, in which the | |
223 | returned anonymous function contains a reference to a lexical variable | |
224 | outside the scope of that function itself. Such a reference requires | |
225 | that Perl return a proper closure, thus locking in for all time the | |
226 | value that the lexical had when the function was created. | |
227 | ||
228 | sub make_adder { | |
229 | my $addpiece = shift; | |
230 | return sub { shift + $addpiece }; | |
231 | } | |
232 | ||
233 | $f1 = make_adder(20); | |
234 | $f2 = make_adder(555); | |
235 | ||
236 | Now C<&$f1($n)> is always 20 plus whatever $n you pass in, whereas | |
237 | C<&$f2($n)> is always 555 plus whatever $n you pass in. The $addpiece | |
238 | in the closure sticks around. | |
239 | ||
240 | Closures are often used for less esoteric purposes. For example, when | |
241 | you want to pass in a bit of code into a function: | |
242 | ||
243 | my $line; | |
244 | timeout( 30, sub { $line = <STDIN> } ); | |
245 | ||
c47ff5f1 GS |
246 | If the code to execute had been passed in as a string, |
247 | C<< '$line = <STDIN>' >>, there would have been no way for the | |
248 | hypothetical timeout() function to access the lexical variable | |
249 | $line back in its caller's scope. | |
68dc0745 | 250 | |
46fc3d4c | 251 | =head2 What is variable suicide and how can I prevent it? |
252 | ||
253 | Variable suicide is when you (temporarily or permanently) lose the | |
254 | value of a variable. It is caused by scoping through my() and local() | |
368c9434 | 255 | interacting with either closures or aliased foreach() iterator |
46fc3d4c | 256 | variables and subroutine arguments. It used to be easy to |
257 | inadvertently lose a variable's value this way, but now it's much | |
258 | harder. Take this code: | |
259 | ||
260 | my $f = "foo"; | |
261 | sub T { | |
262 | while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\n" } | |
263 | } | |
264 | T; | |
265 | print "Finally $f\n"; | |
266 | ||
267 | The $f that has "bar" added to it three times should be a new C<$f> | |
d92eb7b0 GS |
268 | (C<my $f> should create a new local variable each time through the loop). |
269 | It isn't, however. This was a bug, now fixed in the latest releases | |
270 | (tested against 5.004_05, 5.005_03, and 5.005_56). | |
46fc3d4c | 271 | |
d92eb7b0 | 272 | =head2 How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}? |
68dc0745 | 273 | |
d92eb7b0 | 274 | With the exception of regexes, you need to pass references to these |
68dc0745 | 275 | objects. See L<perlsub/"Pass by Reference"> for this particular |
276 | question, and L<perlref> for information on references. | |
277 | ||
a6dd486b JB |
278 | See ``Passing Regexes'', below, for information on passing regular |
279 | expressions. | |
280 | ||
68dc0745 | 281 | =over 4 |
282 | ||
283 | =item Passing Variables and Functions | |
284 | ||
a6dd486b | 285 | Regular variables and functions are quite easy to pass: just pass in a |
68dc0745 | 286 | reference to an existing or anonymous variable or function: |
287 | ||
288 | func( \$some_scalar ); | |
289 | ||
65acb1b1 | 290 | func( \@some_array ); |
68dc0745 | 291 | func( [ 1 .. 10 ] ); |
292 | ||
293 | func( \%some_hash ); | |
294 | func( { this => 10, that => 20 } ); | |
295 | ||
296 | func( \&some_func ); | |
297 | func( sub { $_[0] ** $_[1] } ); | |
298 | ||
299 | =item Passing Filehandles | |
300 | ||
c8db1d39 | 301 | To pass filehandles to subroutines, use the C<*FH> or C<\*FH> notations. |
a6dd486b | 302 | These are "typeglobs"--see L<perldata/"Typeglobs and Filehandles"> |
c8db1d39 TC |
303 | and especially L<perlsub/"Pass by Reference"> for more information. |
304 | ||
305 | Here's an excerpt: | |
306 | ||
307 | If you're passing around filehandles, you could usually just use the bare | |
308 | typeglob, like *STDOUT, but typeglobs references would be better because | |
309 | they'll still work properly under C<use strict 'refs'>. For example: | |
68dc0745 | 310 | |
c8db1d39 TC |
311 | splutter(\*STDOUT); |
312 | sub splutter { | |
313 | my $fh = shift; | |
314 | print $fh "her um well a hmmm\n"; | |
315 | } | |
316 | ||
317 | $rec = get_rec(\*STDIN); | |
318 | sub get_rec { | |
319 | my $fh = shift; | |
320 | return scalar <$fh>; | |
321 | } | |
322 | ||
323 | If you're planning on generating new filehandles, you could do this: | |
324 | ||
325 | sub openit { | |
fc40549d | 326 | my $path = shift; |
c8db1d39 TC |
327 | local *FH; |
328 | return open (FH, $path) ? *FH : undef; | |
329 | } | |
330 | $fh = openit('< /etc/motd'); | |
331 | print <$fh>; | |
68dc0745 | 332 | |
d92eb7b0 GS |
333 | =item Passing Regexes |
334 | ||
335 | To pass regexes around, you'll need to be using a release of Perl | |
336 | sufficiently recent as to support the C<qr//> construct, pass around | |
337 | strings and use an exception-trapping eval, or else be very, very clever. | |
68dc0745 | 338 | |
d92eb7b0 GS |
339 | Here's an example of how to pass in a string to be regex compared |
340 | using C<qr//>: | |
68dc0745 | 341 | |
342 | sub compare($$) { | |
d92eb7b0 GS |
343 | my ($val1, $regex) = @_; |
344 | my $retval = $val1 =~ /$regex/; | |
345 | return $retval; | |
346 | } | |
347 | $match = compare("old McDonald", qr/d.*D/i); | |
348 | ||
349 | Notice how C<qr//> allows flags at the end. That pattern was compiled | |
350 | at compile time, although it was executed later. The nifty C<qr//> | |
351 | notation wasn't introduced until the 5.005 release. Before that, you | |
352 | had to approach this problem much less intuitively. For example, here | |
353 | it is again if you don't have C<qr//>: | |
354 | ||
355 | sub compare($$) { | |
356 | my ($val1, $regex) = @_; | |
357 | my $retval = eval { $val1 =~ /$regex/ }; | |
68dc0745 | 358 | die if $@; |
359 | return $retval; | |
360 | } | |
361 | ||
d92eb7b0 | 362 | $match = compare("old McDonald", q/($?i)d.*D/); |
68dc0745 | 363 | |
364 | Make sure you never say something like this: | |
365 | ||
d92eb7b0 | 366 | return eval "\$val =~ /$regex/"; # WRONG |
68dc0745 | 367 | |
d92eb7b0 | 368 | or someone can sneak shell escapes into the regex due to the double |
68dc0745 | 369 | interpolation of the eval and the double-quoted string. For example: |
370 | ||
371 | $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; | |
372 | ||
373 | eval "\$string =~ /$pattern_of_evil/"; | |
374 | ||
375 | Those preferring to be very, very clever might see the O'Reilly book, | |
376 | I<Mastering Regular Expressions>, by Jeffrey Friedl. Page 273's | |
377 | Build_MatchMany_Function() is particularly interesting. A complete | |
378 | citation of this book is given in L<perlfaq2>. | |
379 | ||
380 | =item Passing Methods | |
381 | ||
382 | To pass an object method into a subroutine, you can do this: | |
383 | ||
384 | call_a_lot(10, $some_obj, "methname") | |
385 | sub call_a_lot { | |
386 | my ($count, $widget, $trick) = @_; | |
387 | for (my $i = 0; $i < $count; $i++) { | |
388 | $widget->$trick(); | |
389 | } | |
390 | } | |
391 | ||
a6dd486b JB |
392 | Or, you can use a closure to bundle up the object, its |
393 | method call, and arguments: | |
68dc0745 | 394 | |
395 | my $whatnot = sub { $some_obj->obfuscate(@args) }; | |
396 | func($whatnot); | |
397 | sub func { | |
398 | my $code = shift; | |
399 | &$code(); | |
400 | } | |
401 | ||
402 | You could also investigate the can() method in the UNIVERSAL class | |
403 | (part of the standard perl distribution). | |
404 | ||
405 | =back | |
406 | ||
407 | =head2 How do I create a static variable? | |
408 | ||
409 | As with most things in Perl, TMTOWTDI. What is a "static variable" in | |
410 | other languages could be either a function-private variable (visible | |
411 | only within a single function, retaining its value between calls to | |
412 | that function), or a file-private variable (visible only to functions | |
413 | within the file it was declared in) in Perl. | |
414 | ||
415 | Here's code to implement a function-private variable: | |
416 | ||
417 | BEGIN { | |
418 | my $counter = 42; | |
419 | sub prev_counter { return --$counter } | |
420 | sub next_counter { return $counter++ } | |
421 | } | |
422 | ||
423 | Now prev_counter() and next_counter() share a private variable $counter | |
424 | that was initialized at compile time. | |
425 | ||
426 | To declare a file-private variable, you'll still use a my(), putting | |
a6dd486b JB |
427 | the declaration at the outer scope level at the top of the file. |
428 | Assume this is in file Pax.pm: | |
68dc0745 | 429 | |
430 | package Pax; | |
431 | my $started = scalar(localtime(time())); | |
432 | ||
433 | sub begun { return $started } | |
434 | ||
435 | When C<use Pax> or C<require Pax> loads this module, the variable will | |
436 | be initialized. It won't get garbage-collected the way most variables | |
437 | going out of scope do, because the begun() function cares about it, | |
438 | but no one else can get it. It is not called $Pax::started because | |
439 | its scope is unrelated to the package. It's scoped to the file. You | |
440 | could conceivably have several packages in that same file all | |
441 | accessing the same private variable, but another file with the same | |
442 | package couldn't get to it. | |
443 | ||
c2611fb3 | 444 | See L<perlsub/"Persistent Private Variables"> for details. |
c8db1d39 | 445 | |
68dc0745 | 446 | =head2 What's the difference between dynamic and lexical (static) scoping? Between local() and my()? |
447 | ||
a6dd486b JB |
448 | C<local($x)> saves away the old value of the global variable C<$x> |
449 | and assigns a new value for the duration of the subroutine I<which is | |
68dc0745 | 450 | visible in other functions called from that subroutine>. This is done |
451 | at run-time, so is called dynamic scoping. local() always affects global | |
452 | variables, also called package variables or dynamic variables. | |
453 | ||
454 | C<my($x)> creates a new variable that is only visible in the current | |
a6dd486b | 455 | subroutine. This is done at compile-time, so it is called lexical or |
68dc0745 | 456 | static scoping. my() always affects private variables, also called |
457 | lexical variables or (improperly) static(ly scoped) variables. | |
458 | ||
459 | For instance: | |
460 | ||
461 | sub visible { | |
462 | print "var has value $var\n"; | |
463 | } | |
464 | ||
465 | sub dynamic { | |
466 | local $var = 'local'; # new temporary value for the still-global | |
467 | visible(); # variable called $var | |
468 | } | |
469 | ||
470 | sub lexical { | |
471 | my $var = 'private'; # new private variable, $var | |
472 | visible(); # (invisible outside of sub scope) | |
473 | } | |
474 | ||
475 | $var = 'global'; | |
476 | ||
477 | visible(); # prints global | |
478 | dynamic(); # prints local | |
479 | lexical(); # prints global | |
480 | ||
481 | Notice how at no point does the value "private" get printed. That's | |
482 | because $var only has that value within the block of the lexical() | |
483 | function, and it is hidden from called subroutine. | |
484 | ||
485 | In summary, local() doesn't make what you think of as private, local | |
486 | variables. It gives a global variable a temporary value. my() is | |
487 | what you're looking for if you want private variables. | |
488 | ||
13a2d996 SP |
489 | See L<perlsub/"Private Variables via my()"> and |
490 | L<perlsub/"Temporary Values via local()"> for excruciating details. | |
68dc0745 | 491 | |
492 | =head2 How can I access a dynamic variable while a similarly named lexical is in scope? | |
493 | ||
494 | You can do this via symbolic references, provided you haven't set | |
495 | C<use strict "refs">. So instead of $var, use C<${'var'}>. | |
496 | ||
497 | local $var = "global"; | |
498 | my $var = "lexical"; | |
499 | ||
500 | print "lexical is $var\n"; | |
501 | ||
502 | no strict 'refs'; | |
503 | print "global is ${'var'}\n"; | |
504 | ||
505 | If you know your package, you can just mention it explicitly, as in | |
506 | $Some_Pack::var. Note that the notation $::var is I<not> the dynamic | |
507 | $var in the current package, but rather the one in the C<main> | |
508 | package, as though you had written $main::var. Specifying the package | |
509 | directly makes you hard-code its name, but it executes faster and | |
510 | avoids running afoul of C<use strict "refs">. | |
511 | ||
512 | =head2 What's the difference between deep and shallow binding? | |
513 | ||
514 | In deep binding, lexical variables mentioned in anonymous subroutines | |
515 | are the same ones that were in scope when the subroutine was created. | |
516 | In shallow binding, they are whichever variables with the same names | |
517 | happen to be in scope when the subroutine is called. Perl always uses | |
518 | deep binding of lexical variables (i.e., those created with my()). | |
519 | However, dynamic variables (aka global, local, or package variables) | |
520 | are effectively shallowly bound. Consider this just one more reason | |
521 | not to use them. See the answer to L<"What's a closure?">. | |
522 | ||
c47ff5f1 | 523 | =head2 Why doesn't "my($foo) = <FILE>;" work right? |
68dc0745 | 524 | |
c8db1d39 | 525 | C<my()> and C<local()> give list context to the right hand side |
c47ff5f1 | 526 | of C<=>. The <FH> read operation, like so many of Perl's |
c8db1d39 TC |
527 | functions and operators, can tell which context it was called in and |
528 | behaves appropriately. In general, the scalar() function can help. | |
529 | This function does nothing to the data itself (contrary to popular myth) | |
530 | but rather tells its argument to behave in whatever its scalar fashion is. | |
531 | If that function doesn't have a defined scalar behavior, this of course | |
532 | doesn't help you (such as with sort()). | |
68dc0745 | 533 | |
534 | To enforce scalar context in this particular case, however, you need | |
535 | merely omit the parentheses: | |
536 | ||
537 | local($foo) = <FILE>; # WRONG | |
538 | local($foo) = scalar(<FILE>); # ok | |
539 | local $foo = <FILE>; # right | |
540 | ||
541 | You should probably be using lexical variables anyway, although the | |
542 | issue is the same here: | |
543 | ||
544 | my($foo) = <FILE>; # WRONG | |
545 | my $foo = <FILE>; # right | |
546 | ||
54310121 | 547 | =head2 How do I redefine a builtin function, operator, or method? |
68dc0745 | 548 | |
549 | Why do you want to do that? :-) | |
550 | ||
551 | If you want to override a predefined function, such as open(), | |
552 | then you'll have to import the new definition from a different | |
4a4eefd0 | 553 | module. See L<perlsub/"Overriding Built-in Functions">. There's |
65acb1b1 | 554 | also an example in L<perltoot/"Class::Template">. |
68dc0745 | 555 | |
556 | If you want to overload a Perl operator, such as C<+> or C<**>, | |
557 | then you'll want to use the C<use overload> pragma, documented | |
558 | in L<overload>. | |
559 | ||
560 | If you're talking about obscuring method calls in parent classes, | |
561 | see L<perltoot/"Overridden Methods">. | |
562 | ||
563 | =head2 What's the difference between calling a function as &foo and foo()? | |
564 | ||
565 | When you call a function as C<&foo>, you allow that function access to | |
a6dd486b JB |
566 | your current @_ values, and you bypass prototypes. |
567 | The function doesn't get an empty @_--it gets yours! While not | |
68dc0745 | 568 | strictly speaking a bug (it's documented that way in L<perlsub>), it |
569 | would be hard to consider this a feature in most cases. | |
570 | ||
c8db1d39 | 571 | When you call your function as C<&foo()>, then you I<do> get a new @_, |
68dc0745 | 572 | but prototyping is still circumvented. |
573 | ||
574 | Normally, you want to call a function using C<foo()>. You may only | |
575 | omit the parentheses if the function is already known to the compiler | |
576 | because it already saw the definition (C<use> but not C<require>), | |
577 | or via a forward reference or C<use subs> declaration. Even in this | |
578 | case, you get a clean @_ without any of the old values leaking through | |
579 | where they don't belong. | |
580 | ||
581 | =head2 How do I create a switch or case statement? | |
582 | ||
583 | This is explained in more depth in the L<perlsyn>. Briefly, there's | |
584 | no official case statement, because of the variety of tests possible | |
585 | in Perl (numeric comparison, string comparison, glob comparison, | |
83df6a1d JH |
586 | regex matching, overloaded comparisons, ...). |
587 | Larry couldn't decide how best to do this, so he left it out, even | |
588 | though it's been on the wish list since perl1. | |
68dc0745 | 589 | |
83df6a1d JH |
590 | Starting from Perl 5.8 to get switch and case one can use the |
591 | Switch extension and say: | |
592 | ||
593 | use Switch; | |
594 | ||
595 | after which one has switch and case. It is not as fast as it could be | |
596 | because it's not really part of the language (it's done using source | |
597 | filters) but it is available, and it's very flexible. | |
598 | ||
599 | But if one wants to use pure Perl, the general answer is to write a | |
600 | construct like this: | |
c8db1d39 TC |
601 | |
602 | for ($variable_to_test) { | |
603 | if (/pat1/) { } # do something | |
604 | elsif (/pat2/) { } # do something else | |
605 | elsif (/pat3/) { } # do something else | |
606 | else { } # default | |
607 | } | |
68dc0745 | 608 | |
c8db1d39 TC |
609 | Here's a simple example of a switch based on pattern matching, this |
610 | time lined up in a way to make it look more like a switch statement. | |
8305e449 | 611 | We'll do a multiway conditional based on the type of reference stored |
c8db1d39 TC |
612 | in $whatchamacallit: |
613 | ||
614 | SWITCH: for (ref $whatchamacallit) { | |
68dc0745 | 615 | |
616 | /^$/ && die "not a reference"; | |
617 | ||
618 | /SCALAR/ && do { | |
619 | print_scalar($$ref); | |
620 | last SWITCH; | |
621 | }; | |
622 | ||
623 | /ARRAY/ && do { | |
624 | print_array(@$ref); | |
625 | last SWITCH; | |
626 | }; | |
627 | ||
628 | /HASH/ && do { | |
629 | print_hash(%$ref); | |
630 | last SWITCH; | |
631 | }; | |
632 | ||
633 | /CODE/ && do { | |
634 | warn "can't print function ref"; | |
635 | last SWITCH; | |
636 | }; | |
637 | ||
638 | # DEFAULT | |
639 | ||
640 | warn "User defined type skipped"; | |
641 | ||
642 | } | |
643 | ||
c8db1d39 TC |
644 | See C<perlsyn/"Basic BLOCKs and Switch Statements"> for many other |
645 | examples in this style. | |
646 | ||
647 | Sometimes you should change the positions of the constant and the variable. | |
648 | For example, let's say you wanted to test which of many answers you were | |
649 | given, but in a case-insensitive way that also allows abbreviations. | |
650 | You can use the following technique if the strings all start with | |
a6dd486b | 651 | different characters or if you want to arrange the matches so that |
c8db1d39 TC |
652 | one takes precedence over another, as C<"SEND"> has precedence over |
653 | C<"STOP"> here: | |
654 | ||
655 | chomp($answer = <>); | |
656 | if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" } | |
657 | elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" } | |
658 | elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" } | |
659 | elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" } | |
660 | elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" } | |
661 | ||
662 | A totally different approach is to create a hash of function references. | |
663 | ||
664 | my %commands = ( | |
665 | "happy" => \&joy, | |
666 | "sad", => \&sullen, | |
667 | "done" => sub { die "See ya!" }, | |
668 | "mad" => \&angry, | |
669 | ); | |
670 | ||
671 | print "How are you? "; | |
672 | chomp($string = <STDIN>); | |
673 | if ($commands{$string}) { | |
674 | $commands{$string}->(); | |
675 | } else { | |
676 | print "No such command: $string\n"; | |
677 | } | |
678 | ||
68dc0745 | 679 | =head2 How can I catch accesses to undefined variables/functions/methods? |
680 | ||
681 | The AUTOLOAD method, discussed in L<perlsub/"Autoloading"> and | |
682 | L<perltoot/"AUTOLOAD: Proxy Methods">, lets you capture calls to | |
683 | undefined functions and methods. | |
684 | ||
685 | When it comes to undefined variables that would trigger a warning | |
686 | under C<-w>, you can use a handler to trap the pseudo-signal | |
687 | C<__WARN__> like this: | |
688 | ||
689 | $SIG{__WARN__} = sub { | |
690 | ||
c8db1d39 | 691 | for ( $_[0] ) { # voici un switch statement |
68dc0745 | 692 | |
693 | /Use of uninitialized value/ && do { | |
694 | # promote warning to a fatal | |
695 | die $_; | |
696 | }; | |
697 | ||
698 | # other warning cases to catch could go here; | |
699 | ||
700 | warn $_; | |
701 | } | |
702 | ||
703 | }; | |
704 | ||
705 | =head2 Why can't a method included in this same file be found? | |
706 | ||
707 | Some possible reasons: your inheritance is getting confused, you've | |
708 | misspelled the method name, or the object is of the wrong type. Check | |
a6dd486b JB |
709 | out L<perltoot> for details about any of the above cases. You may |
710 | also use C<print ref($object)> to find out the class C<$object> was | |
711 | blessed into. | |
68dc0745 | 712 | |
713 | Another possible reason for problems is because you've used the | |
714 | indirect object syntax (eg, C<find Guru "Samy">) on a class name | |
715 | before Perl has seen that such a package exists. It's wisest to make | |
716 | sure your packages are all defined before you start using them, which | |
717 | will be taken care of if you use the C<use> statement instead of | |
a6dd486b | 718 | C<require>. If not, make sure to use arrow notation (eg., |
c47ff5f1 | 719 | C<< Guru->find("Samy") >>) instead. Object notation is explained in |
68dc0745 | 720 | L<perlobj>. |
721 | ||
c8db1d39 | 722 | Make sure to read about creating modules in L<perlmod> and |
ae93639c | 723 | the perils of indirect objects in L<perlobj/"Method Invocation">. |
c8db1d39 | 724 | |
68dc0745 | 725 | =head2 How can I find out my current package? |
726 | ||
727 | If you're just a random program, you can do this to find | |
728 | out what the currently compiled package is: | |
729 | ||
c8db1d39 | 730 | my $packname = __PACKAGE__; |
68dc0745 | 731 | |
a6dd486b | 732 | But, if you're a method and you want to print an error message |
68dc0745 | 733 | that includes the kind of object you were called on (which is |
734 | not necessarily the same as the one in which you were compiled): | |
735 | ||
736 | sub amethod { | |
92c2ed05 | 737 | my $self = shift; |
68dc0745 | 738 | my $class = ref($self) || $self; |
739 | warn "called me from a $class object"; | |
740 | } | |
741 | ||
46fc3d4c | 742 | =head2 How can I comment out a large block of perl code? |
743 | ||
f05bbc40 JH |
744 | You can use embedded POD to discard it. The =for directive |
745 | lasts until the next paragraph (two consecutive newlines). | |
46fc3d4c | 746 | |
747 | # program is here | |
748 | ||
749 | =for nobody | |
750 | This paragraph is commented out | |
751 | ||
752 | # program continues | |
753 | ||
f05bbc40 JH |
754 | The =begin and =end directives can contain multiple |
755 | paragraphs. | |
756 | ||
46fc3d4c | 757 | =begin comment text |
758 | ||
759 | all of this stuff | |
760 | ||
761 | here will be ignored | |
762 | by everyone | |
763 | ||
764 | =end comment text | |
765 | ||
f05bbc40 JH |
766 | The pod directives cannot go just anywhere. You must put a |
767 | pod directive where the parser is expecting a new statement, | |
768 | not just in the middle of an expression or some other | |
769 | arbitrary s grammar production. | |
fc36a67e | 770 | |
f05bbc40 | 771 | See L<perlpod> for more details. |
c8db1d39 | 772 | |
65acb1b1 TC |
773 | =head2 How do I clear a package? |
774 | ||
775 | Use this code, provided by Mark-Jason Dominus: | |
776 | ||
777 | sub scrub_package { | |
778 | no strict 'refs'; | |
779 | my $pack = shift; | |
780 | die "Shouldn't delete main package" | |
781 | if $pack eq "" || $pack eq "main"; | |
782 | my $stash = *{$pack . '::'}{HASH}; | |
783 | my $name; | |
784 | foreach $name (keys %$stash) { | |
785 | my $fullname = $pack . '::' . $name; | |
786 | # Get rid of everything with that name. | |
787 | undef $$fullname; | |
788 | undef @$fullname; | |
789 | undef %$fullname; | |
790 | undef &$fullname; | |
791 | undef *$fullname; | |
792 | } | |
793 | } | |
794 | ||
795 | Or, if you're using a recent release of Perl, you can | |
796 | just use the Symbol::delete_package() function instead. | |
797 | ||
d92eb7b0 GS |
798 | =head2 How can I use a variable as a variable name? |
799 | ||
800 | Beginners often think they want to have a variable contain the name | |
801 | of a variable. | |
802 | ||
803 | $fred = 23; | |
804 | $varname = "fred"; | |
805 | ++$$varname; # $fred now 24 | |
806 | ||
807 | This works I<sometimes>, but it is a very bad idea for two reasons. | |
808 | ||
a6dd486b JB |
809 | The first reason is that this technique I<only works on global |
810 | variables>. That means that if $fred is a lexical variable created | |
811 | with my() in the above example, the code wouldn't work at all: you'd | |
812 | accidentally access the global and skip right over the private lexical | |
813 | altogether. Global variables are bad because they can easily collide | |
814 | accidentally and in general make for non-scalable and confusing code. | |
d92eb7b0 GS |
815 | |
816 | Symbolic references are forbidden under the C<use strict> pragma. | |
817 | They are not true references and consequently are not reference counted | |
818 | or garbage collected. | |
819 | ||
820 | The other reason why using a variable to hold the name of another | |
a6dd486b | 821 | variable is a bad idea is that the question often stems from a lack of |
d92eb7b0 GS |
822 | understanding of Perl data structures, particularly hashes. By using |
823 | symbolic references, you are just using the package's symbol-table hash | |
824 | (like C<%main::>) instead of a user-defined hash. The solution is to | |
825 | use your own hash or a real reference instead. | |
826 | ||
827 | $fred = 23; | |
828 | $varname = "fred"; | |
829 | $USER_VARS{$varname}++; # not $$varname++ | |
830 | ||
831 | There we're using the %USER_VARS hash instead of symbolic references. | |
832 | Sometimes this comes up in reading strings from the user with variable | |
833 | references and wanting to expand them to the values of your perl | |
834 | program's variables. This is also a bad idea because it conflates the | |
835 | program-addressable namespace and the user-addressable one. Instead of | |
836 | reading a string and expanding it to the actual contents of your program's | |
837 | own variables: | |
838 | ||
839 | $str = 'this has a $fred and $barney in it'; | |
840 | $str =~ s/(\$\w+)/$1/eeg; # need double eval | |
841 | ||
a6dd486b | 842 | it would be better to keep a hash around like %USER_VARS and have |
d92eb7b0 GS |
843 | variable references actually refer to entries in that hash: |
844 | ||
845 | $str =~ s/\$(\w+)/$USER_VARS{$1}/g; # no /e here at all | |
846 | ||
847 | That's faster, cleaner, and safer than the previous approach. Of course, | |
848 | you don't need to use a dollar sign. You could use your own scheme to | |
849 | make it less confusing, like bracketed percent symbols, etc. | |
850 | ||
851 | $str = 'this has a %fred% and %barney% in it'; | |
852 | $str =~ s/%(\w+)%/$USER_VARS{$1}/g; # no /e here at all | |
853 | ||
a6dd486b JB |
854 | Another reason that folks sometimes think they want a variable to |
855 | contain the name of a variable is because they don't know how to build | |
856 | proper data structures using hashes. For example, let's say they | |
857 | wanted two hashes in their program: %fred and %barney, and that they | |
858 | wanted to use another scalar variable to refer to those by name. | |
d92eb7b0 GS |
859 | |
860 | $name = "fred"; | |
861 | $$name{WIFE} = "wilma"; # set %fred | |
862 | ||
863 | $name = "barney"; | |
864 | $$name{WIFE} = "betty"; # set %barney | |
865 | ||
866 | This is still a symbolic reference, and is still saddled with the | |
867 | problems enumerated above. It would be far better to write: | |
868 | ||
869 | $folks{"fred"}{WIFE} = "wilma"; | |
870 | $folks{"barney"}{WIFE} = "betty"; | |
871 | ||
872 | And just use a multilevel hash to start with. | |
873 | ||
874 | The only times that you absolutely I<must> use symbolic references are | |
875 | when you really must refer to the symbol table. This may be because it's | |
876 | something that can't take a real reference to, such as a format name. | |
877 | Doing so may also be important for method calls, since these always go | |
878 | through the symbol table for resolution. | |
879 | ||
880 | In those cases, you would turn off C<strict 'refs'> temporarily so you | |
881 | can play around with the symbol table. For example: | |
882 | ||
883 | @colors = qw(red blue green yellow orange purple violet); | |
884 | for my $name (@colors) { | |
885 | no strict 'refs'; # renege for the block | |
886 | *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; | |
887 | } | |
888 | ||
889 | All those functions (red(), blue(), green(), etc.) appear to be separate, | |
890 | but the real code in the closure actually was compiled only once. | |
891 | ||
892 | So, sometimes you might want to use symbolic references to directly | |
893 | manipulate the symbol table. This doesn't matter for formats, handles, and | |
a6dd486b JB |
894 | subroutines, because they are always global--you can't use my() on them. |
895 | For scalars, arrays, and hashes, though--and usually for subroutines-- | |
896 | you probably only want to use hard references. | |
d92eb7b0 | 897 | |
68dc0745 | 898 | =head1 AUTHOR AND COPYRIGHT |
899 | ||
0bc0ad85 | 900 | Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington. |
5a964f20 TC |
901 | All rights reserved. |
902 | ||
5a7beb56 JH |
903 | This documentation is free; you can redistribute it and/or modify it |
904 | under the same terms as Perl itself. | |
5a964f20 TC |
905 | |
906 | Irrespective of its distribution, all code examples in this file | |
907 | are hereby placed into the public domain. You are permitted and | |
908 | encouraged to use this code in your own programs for fun | |
909 | or for profit as you see fit. A simple comment in the code giving | |
910 | credit would be courteous but is not required. | |
a6dd486b | 911 |