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| 128 | .rm #[ #] #H #V #F C |
| 129 | .\" ======================================================================== |
| 130 | .\" |
| 131 | .IX Title "PERLFAQ7 1" |
| 132 | .TH PERLFAQ7 1 "2002-06-08" "perl v5.8.0" "Perl Programmers Reference Guide" |
| 133 | .SH "NAME" |
| 134 | perlfaq7 \- General Perl Language Issues ($Revision: 1.8 $, $Date: 2002/03/26 15:48:32 $) |
| 135 | .SH "DESCRIPTION" |
| 136 | .IX Header "DESCRIPTION" |
| 137 | This section deals with general Perl language issues that don't |
| 138 | clearly fit into any of the other sections. |
| 139 | .Sh "Can I get a BNF/yacc/RE for the Perl language?" |
| 140 | .IX Subsection "Can I get a BNF/yacc/RE for the Perl language?" |
| 141 | There is no \s-1BNF\s0, but you can paw your way through the yacc grammar in |
| 142 | perly.y in the source distribution if you're particularly brave. The |
| 143 | grammar relies on very smart tokenizing code, so be prepared to |
| 144 | venture into toke.c as well. |
| 145 | .PP |
| 146 | In the words of Chaim Frenkel: \*(L"Perl's grammar can not be reduced to \s-1BNF\s0. |
| 147 | The work of parsing perl is distributed between yacc, the lexer, smoke |
| 148 | and mirrors.\*(R" |
| 149 | .Sh "What are all these $@%&* punctuation signs, and how do I know when to use them?" |
| 150 | .IX Subsection "What are all these $@%&* punctuation signs, and how do I know when to use them?" |
| 151 | They are type specifiers, as detailed in perldata: |
| 152 | .PP |
| 153 | .Vb 6 |
| 154 | \& $ for scalar values (number, string or reference) |
| 155 | \& @ for arrays |
| 156 | \& % for hashes (associative arrays) |
| 157 | \& & for subroutines (aka functions, procedures, methods) |
| 158 | \& * for all types of that symbol name. In version 4 you used them like |
| 159 | \& pointers, but in modern perls you can just use references. |
| 160 | .Ve |
| 161 | .PP |
| 162 | There are couple of other symbols that you're likely to encounter that aren't |
| 163 | really type specifiers: |
| 164 | .PP |
| 165 | .Vb 2 |
| 166 | \& <> are used for inputting a record from a filehandle. |
| 167 | \& \e takes a reference to something. |
| 168 | .Ve |
| 169 | .PP |
| 170 | Note that <\s-1FILE\s0> is \fIneither\fR the type specifier for files |
| 171 | nor the name of the handle. It is the \f(CW\*(C`<>\*(C'\fR operator applied |
| 172 | to the handle \s-1FILE\s0. It reads one line (well, record\*(--see |
| 173 | \&\*(L"$/\*(R" in perlvar) from the handle \s-1FILE\s0 in scalar context, or \fIall\fR lines |
| 174 | in list context. When performing open, close, or any other operation |
| 175 | besides \f(CW\*(C`<>\*(C'\fR on files, or even when talking about the handle, do |
| 176 | \&\fInot\fR use the brackets. These are correct: \f(CW\*(C`eof(FH)\*(C'\fR, \f(CW\*(C`seek(FH, 0, |
| 177 | 2)\*(C'\fR and \*(L"copying from \s-1STDIN\s0 to \s-1FILE\s0\*(R". |
| 178 | .Sh "Do I always/never have to quote my strings or use semicolons and commas?" |
| 179 | .IX Subsection "Do I always/never have to quote my strings or use semicolons and commas?" |
| 180 | Normally, a bareword doesn't need to be quoted, but in most cases |
| 181 | probably should be (and must be under \f(CW\*(C`use strict\*(C'\fR). But a hash key |
| 182 | consisting of a simple word (that isn't the name of a defined |
| 183 | subroutine) and the left-hand operand to the \f(CW\*(C`=>\*(C'\fR operator both |
| 184 | count as though they were quoted: |
| 185 | .PP |
| 186 | .Vb 4 |
| 187 | \& This is like this |
| 188 | \& ------------ --------------- |
| 189 | \& $foo{line} $foo{"line"} |
| 190 | \& bar => stuff "bar" => stuff |
| 191 | .Ve |
| 192 | .PP |
| 193 | The final semicolon in a block is optional, as is the final comma in a |
| 194 | list. Good style (see perlstyle) says to put them in except for |
| 195 | one\-liners: |
| 196 | .PP |
| 197 | .Vb 2 |
| 198 | \& if ($whoops) { exit 1 } |
| 199 | \& @nums = (1, 2, 3); |
| 200 | .Ve |
| 201 | .PP |
| 202 | .Vb 7 |
| 203 | \& if ($whoops) { |
| 204 | \& exit 1; |
| 205 | \& } |
| 206 | \& @lines = ( |
| 207 | \& "There Beren came from mountains cold", |
| 208 | \& "And lost he wandered under leaves", |
| 209 | \& ); |
| 210 | .Ve |
| 211 | .Sh "How do I skip some return values?" |
| 212 | .IX Subsection "How do I skip some return values?" |
| 213 | One way is to treat the return values as a list and index into it: |
| 214 | .PP |
| 215 | .Vb 1 |
| 216 | \& $dir = (getpwnam($user))[7]; |
| 217 | .Ve |
| 218 | .PP |
| 219 | Another way is to use undef as an element on the left\-hand\-side: |
| 220 | .PP |
| 221 | .Vb 1 |
| 222 | \& ($dev, $ino, undef, undef, $uid, $gid) = stat($file); |
| 223 | .Ve |
| 224 | .Sh "How do I temporarily block warnings?" |
| 225 | .IX Subsection "How do I temporarily block warnings?" |
| 226 | If you are running Perl 5.6.0 or better, the \f(CW\*(C`use warnings\*(C'\fR pragma |
| 227 | allows fine control of what warning are produced. |
| 228 | See perllexwarn for more details. |
| 229 | .PP |
| 230 | .Vb 4 |
| 231 | \& { |
| 232 | \& no warnings; # temporarily turn off warnings |
| 233 | \& $a = $b + $c; # I know these might be undef |
| 234 | \& } |
| 235 | .Ve |
| 236 | .PP |
| 237 | If you have an older version of Perl, the \f(CW$^W\fR variable (documented |
| 238 | in perlvar) controls runtime warnings for a block: |
| 239 | .PP |
| 240 | .Vb 4 |
| 241 | \& { |
| 242 | \& local $^W = 0; # temporarily turn off warnings |
| 243 | \& $a = $b + $c; # I know these might be undef |
| 244 | \& } |
| 245 | .Ve |
| 246 | .PP |
| 247 | Note that like all the punctuation variables, you cannot currently |
| 248 | use \fImy()\fR on \f(CW$^W\fR, only \fIlocal()\fR. |
| 249 | .Sh "What's an extension?" |
| 250 | .IX Subsection "What's an extension?" |
| 251 | An extension is a way of calling compiled C code from Perl. Reading |
| 252 | perlxstut is a good place to learn more about extensions. |
| 253 | .Sh "Why do Perl operators have different precedence than C operators?" |
| 254 | .IX Subsection "Why do Perl operators have different precedence than C operators?" |
| 255 | Actually, they don't. All C operators that Perl copies have the same |
| 256 | precedence in Perl as they do in C. The problem is with operators that C |
| 257 | doesn't have, especially functions that give a list context to everything |
| 258 | on their right, eg. print, chmod, exec, and so on. Such functions are |
| 259 | called \*(L"list operators\*(R" and appear as such in the precedence table in |
| 260 | perlop. |
| 261 | .PP |
| 262 | A common mistake is to write: |
| 263 | .PP |
| 264 | .Vb 1 |
| 265 | \& unlink $file || die "snafu"; |
| 266 | .Ve |
| 267 | .PP |
| 268 | This gets interpreted as: |
| 269 | .PP |
| 270 | .Vb 1 |
| 271 | \& unlink ($file || die "snafu"); |
| 272 | .Ve |
| 273 | .PP |
| 274 | To avoid this problem, either put in extra parentheses or use the |
| 275 | super low precedence \f(CW\*(C`or\*(C'\fR operator: |
| 276 | .PP |
| 277 | .Vb 2 |
| 278 | \& (unlink $file) || die "snafu"; |
| 279 | \& unlink $file or die "snafu"; |
| 280 | .Ve |
| 281 | .PP |
| 282 | The \*(L"English\*(R" operators (\f(CW\*(C`and\*(C'\fR, \f(CW\*(C`or\*(C'\fR, \f(CW\*(C`xor\*(C'\fR, and \f(CW\*(C`not\*(C'\fR) |
| 283 | deliberately have precedence lower than that of list operators for |
| 284 | just such situations as the one above. |
| 285 | .PP |
| 286 | Another operator with surprising precedence is exponentiation. It |
| 287 | binds more tightly even than unary minus, making \f(CW\*(C`\-2**2\*(C'\fR product a |
| 288 | negative not a positive four. It is also right\-associating, meaning |
| 289 | that \f(CW\*(C`2**3**2\*(C'\fR is two raised to the ninth power, not eight squared. |
| 290 | .PP |
| 291 | Although it has the same precedence as in C, Perl's \f(CW\*(C`?:\*(C'\fR operator |
| 292 | produces an lvalue. This assigns \f(CW$x\fR to either \f(CW$a\fR or \f(CW$b\fR, depending |
| 293 | on the trueness of \f(CW$maybe:\fR |
| 294 | .PP |
| 295 | .Vb 1 |
| 296 | \& ($maybe ? $a : $b) = $x; |
| 297 | .Ve |
| 298 | .Sh "How do I declare/create a structure?" |
| 299 | .IX Subsection "How do I declare/create a structure?" |
| 300 | In general, you don't \*(L"declare\*(R" a structure. Just use a (probably |
| 301 | anonymous) hash reference. See perlref and perldsc for details. |
| 302 | Here's an example: |
| 303 | .PP |
| 304 | .Vb 3 |
| 305 | \& $person = {}; # new anonymous hash |
| 306 | \& $person->{AGE} = 24; # set field AGE to 24 |
| 307 | \& $person->{NAME} = "Nat"; # set field NAME to "Nat" |
| 308 | .Ve |
| 309 | .PP |
| 310 | If you're looking for something a bit more rigorous, try perltoot. |
| 311 | .Sh "How do I create a module?" |
| 312 | .IX Subsection "How do I create a module?" |
| 313 | A module is a package that lives in a file of the same name. For |
| 314 | example, the Hello::There module would live in Hello/There.pm. For |
| 315 | details, read perlmod. You'll also find Exporter helpful. If |
| 316 | you're writing a C or mixed-language module with both C and Perl, then |
| 317 | you should study perlxstut. |
| 318 | .PP |
| 319 | The \f(CW\*(C`h2xs\*(C'\fR program will create stubs for all the important stuff for you: |
| 320 | .PP |
| 321 | .Vb 1 |
| 322 | \& % h2xs -XA -n My::Module |
| 323 | .Ve |
| 324 | .PP |
| 325 | The \f(CW\*(C`\-X\*(C'\fR switch tells \f(CW\*(C`h2xs\*(C'\fR that you are not using \f(CW\*(C`XS\*(C'\fR extension |
| 326 | code. The \f(CW\*(C`\-A\*(C'\fR switch tells \f(CW\*(C`h2xs\*(C'\fR that you are not using the |
| 327 | AutoLoader, and the \f(CW\*(C`\-n\*(C'\fR switch specifies the name of the module. |
| 328 | See h2xs for more details. |
| 329 | .Sh "How do I create a class?" |
| 330 | .IX Subsection "How do I create a class?" |
| 331 | See perltoot for an introduction to classes and objects, as well as |
| 332 | perlobj and perlbot. |
| 333 | .Sh "How can I tell if a variable is tainted?" |
| 334 | .IX Subsection "How can I tell if a variable is tainted?" |
| 335 | You can use the \fItainted()\fR function of the Scalar::Util module, available |
| 336 | from \s-1CPAN\s0 (or included with Perl since release 5.8.0). |
| 337 | See also \*(L"Laundering and Detecting Tainted Data\*(R" in perlsec. |
| 338 | .Sh "What's a closure?" |
| 339 | .IX Subsection "What's a closure?" |
| 340 | Closures are documented in perlref. |
| 341 | .PP |
| 342 | \&\fIClosure\fR is a computer science term with a precise but |
| 343 | hard-to-explain meaning. Closures are implemented in Perl as anonymous |
| 344 | subroutines with lasting references to lexical variables outside their |
| 345 | own scopes. These lexicals magically refer to the variables that were |
| 346 | around when the subroutine was defined (deep binding). |
| 347 | .PP |
| 348 | Closures make sense in any programming language where you can have the |
| 349 | return value of a function be itself a function, as you can in Perl. |
| 350 | Note that some languages provide anonymous functions but are not |
| 351 | capable of providing proper closures: the Python language, for |
| 352 | example. For more information on closures, check out any textbook on |
| 353 | functional programming. Scheme is a language that not only supports |
| 354 | but encourages closures. |
| 355 | .PP |
| 356 | Here's a classic function-generating function: |
| 357 | .PP |
| 358 | .Vb 3 |
| 359 | \& sub add_function_generator { |
| 360 | \& return sub { shift + shift }; |
| 361 | \& } |
| 362 | .Ve |
| 363 | .PP |
| 364 | .Vb 2 |
| 365 | \& $add_sub = add_function_generator(); |
| 366 | \& $sum = $add_sub->(4,5); # $sum is 9 now. |
| 367 | .Ve |
| 368 | .PP |
| 369 | The closure works as a \fIfunction template\fR with some customization |
| 370 | slots left out to be filled later. The anonymous subroutine returned |
| 371 | by \fIadd_function_generator()\fR isn't technically a closure because it |
| 372 | refers to no lexicals outside its own scope. |
| 373 | .PP |
| 374 | Contrast this with the following \fImake_adder()\fR function, in which the |
| 375 | returned anonymous function contains a reference to a lexical variable |
| 376 | outside the scope of that function itself. Such a reference requires |
| 377 | that Perl return a proper closure, thus locking in for all time the |
| 378 | value that the lexical had when the function was created. |
| 379 | .PP |
| 380 | .Vb 4 |
| 381 | \& sub make_adder { |
| 382 | \& my $addpiece = shift; |
| 383 | \& return sub { shift + $addpiece }; |
| 384 | \& } |
| 385 | .Ve |
| 386 | .PP |
| 387 | .Vb 2 |
| 388 | \& $f1 = make_adder(20); |
| 389 | \& $f2 = make_adder(555); |
| 390 | .Ve |
| 391 | .PP |
| 392 | Now \f(CW\*(C`&$f1($n)\*(C'\fR is always 20 plus whatever \f(CW$n\fR you pass in, whereas |
| 393 | \&\f(CW\*(C`&$f2($n)\*(C'\fR is always 555 plus whatever \f(CW$n\fR you pass in. The \f(CW$addpiece\fR |
| 394 | in the closure sticks around. |
| 395 | .PP |
| 396 | Closures are often used for less esoteric purposes. For example, when |
| 397 | you want to pass in a bit of code into a function: |
| 398 | .PP |
| 399 | .Vb 2 |
| 400 | \& my $line; |
| 401 | \& timeout( 30, sub { $line = <STDIN> } ); |
| 402 | .Ve |
| 403 | .PP |
| 404 | If the code to execute had been passed in as a string, |
| 405 | \&\f(CW'$line = <STDIN>'\fR, there would have been no way for the |
| 406 | hypothetical \fItimeout()\fR function to access the lexical variable |
| 407 | \&\f(CW$line\fR back in its caller's scope. |
| 408 | .Sh "What is variable suicide and how can I prevent it?" |
| 409 | .IX Subsection "What is variable suicide and how can I prevent it?" |
| 410 | Variable suicide is when you (temporarily or permanently) lose the |
| 411 | value of a variable. It is caused by scoping through \fImy()\fR and \fIlocal()\fR |
| 412 | interacting with either closures or aliased \fIforeach()\fR iterator |
| 413 | variables and subroutine arguments. It used to be easy to |
| 414 | inadvertently lose a variable's value this way, but now it's much |
| 415 | harder. Take this code: |
| 416 | .PP |
| 417 | .Vb 6 |
| 418 | \& my $f = "foo"; |
| 419 | \& sub T { |
| 420 | \& while ($i++ < 3) { my $f = $f; $f .= "bar"; print $f, "\en" } |
| 421 | \& } |
| 422 | \& T; |
| 423 | \& print "Finally $f\en"; |
| 424 | .Ve |
| 425 | .PP |
| 426 | The \f(CW$f\fR that has \*(L"bar\*(R" added to it three times should be a new \f(CW$f\fR |
| 427 | (\f(CW\*(C`my $f\*(C'\fR should create a new local variable each time through the loop). |
| 428 | It isn't, however. This was a bug, now fixed in the latest releases |
| 429 | (tested against 5.004_05, 5.005_03, and 5.005_56). |
| 430 | .Sh "How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?" |
| 431 | .IX Subsection "How can I pass/return a {Function, FileHandle, Array, Hash, Method, Regex}?" |
| 432 | With the exception of regexes, you need to pass references to these |
| 433 | objects. See \*(L"Pass by Reference\*(R" in perlsub for this particular |
| 434 | question, and perlref for information on references. |
| 435 | .PP |
| 436 | See ``Passing Regexes'', below, for information on passing regular |
| 437 | expressions. |
| 438 | .IP "Passing Variables and Functions" 4 |
| 439 | .IX Item "Passing Variables and Functions" |
| 440 | Regular variables and functions are quite easy to pass: just pass in a |
| 441 | reference to an existing or anonymous variable or function: |
| 442 | .Sp |
| 443 | .Vb 1 |
| 444 | \& func( \e$some_scalar ); |
| 445 | .Ve |
| 446 | .Sp |
| 447 | .Vb 2 |
| 448 | \& func( \e@some_array ); |
| 449 | \& func( [ 1 .. 10 ] ); |
| 450 | .Ve |
| 451 | .Sp |
| 452 | .Vb 2 |
| 453 | \& func( \e%some_hash ); |
| 454 | \& func( { this => 10, that => 20 } ); |
| 455 | .Ve |
| 456 | .Sp |
| 457 | .Vb 2 |
| 458 | \& func( \e&some_func ); |
| 459 | \& func( sub { $_[0] ** $_[1] } ); |
| 460 | .Ve |
| 461 | .IP "Passing Filehandles" 4 |
| 462 | .IX Item "Passing Filehandles" |
| 463 | To pass filehandles to subroutines, use the \f(CW*FH\fR or \f(CW\*(C`\e*FH\*(C'\fR notations. |
| 464 | These are \*(L"typeglobs\*(R"\-\-see \*(L"Typeglobs and Filehandles\*(R" in perldata |
| 465 | and especially \*(L"Pass by Reference\*(R" in perlsub for more information. |
| 466 | .Sp |
| 467 | Here's an excerpt: |
| 468 | .Sp |
| 469 | If you're passing around filehandles, you could usually just use the bare |
| 470 | typeglob, like *STDOUT, but typeglobs references would be better because |
| 471 | they'll still work properly under \f(CW\*(C`use strict 'refs'\*(C'\fR. For example: |
| 472 | .Sp |
| 473 | .Vb 5 |
| 474 | \& splutter(\e*STDOUT); |
| 475 | \& sub splutter { |
| 476 | \& my $fh = shift; |
| 477 | \& print $fh "her um well a hmmm\en"; |
| 478 | \& } |
| 479 | .Ve |
| 480 | .Sp |
| 481 | .Vb 5 |
| 482 | \& $rec = get_rec(\e*STDIN); |
| 483 | \& sub get_rec { |
| 484 | \& my $fh = shift; |
| 485 | \& return scalar <$fh>; |
| 486 | \& } |
| 487 | .Ve |
| 488 | .Sp |
| 489 | If you're planning on generating new filehandles, you could do this: |
| 490 | .Sp |
| 491 | .Vb 7 |
| 492 | \& sub openit { |
| 493 | \& my $path = shift; |
| 494 | \& local *FH; |
| 495 | \& return open (FH, $path) ? *FH : undef; |
| 496 | \& } |
| 497 | \& $fh = openit('< /etc/motd'); |
| 498 | \& print <$fh>; |
| 499 | .Ve |
| 500 | .IP "Passing Regexes" 4 |
| 501 | .IX Item "Passing Regexes" |
| 502 | To pass regexes around, you'll need to be using a release of Perl |
| 503 | sufficiently recent as to support the \f(CW\*(C`qr//\*(C'\fR construct, pass around |
| 504 | strings and use an exception-trapping eval, or else be very, very clever. |
| 505 | .Sp |
| 506 | Here's an example of how to pass in a string to be regex compared |
| 507 | using \f(CW\*(C`qr//\*(C'\fR: |
| 508 | .Sp |
| 509 | .Vb 6 |
| 510 | \& sub compare($$) { |
| 511 | \& my ($val1, $regex) = @_; |
| 512 | \& my $retval = $val1 =~ /$regex/; |
| 513 | \& return $retval; |
| 514 | \& } |
| 515 | \& $match = compare("old McDonald", qr/d.*D/i); |
| 516 | .Ve |
| 517 | .Sp |
| 518 | Notice how \f(CW\*(C`qr//\*(C'\fR allows flags at the end. That pattern was compiled |
| 519 | at compile time, although it was executed later. The nifty \f(CW\*(C`qr//\*(C'\fR |
| 520 | notation wasn't introduced until the 5.005 release. Before that, you |
| 521 | had to approach this problem much less intuitively. For example, here |
| 522 | it is again if you don't have \f(CW\*(C`qr//\*(C'\fR: |
| 523 | .Sp |
| 524 | .Vb 6 |
| 525 | \& sub compare($$) { |
| 526 | \& my ($val1, $regex) = @_; |
| 527 | \& my $retval = eval { $val1 =~ /$regex/ }; |
| 528 | \& die if $@; |
| 529 | \& return $retval; |
| 530 | \& } |
| 531 | .Ve |
| 532 | .Sp |
| 533 | .Vb 1 |
| 534 | \& $match = compare("old McDonald", q/($?i)d.*D/); |
| 535 | .Ve |
| 536 | .Sp |
| 537 | Make sure you never say something like this: |
| 538 | .Sp |
| 539 | .Vb 1 |
| 540 | \& return eval "\e$val =~ /$regex/"; # WRONG |
| 541 | .Ve |
| 542 | .Sp |
| 543 | or someone can sneak shell escapes into the regex due to the double |
| 544 | interpolation of the eval and the double-quoted string. For example: |
| 545 | .Sp |
| 546 | .Vb 1 |
| 547 | \& $pattern_of_evil = 'danger ${ system("rm -rf * &") } danger'; |
| 548 | .Ve |
| 549 | .Sp |
| 550 | .Vb 1 |
| 551 | \& eval "\e$string =~ /$pattern_of_evil/"; |
| 552 | .Ve |
| 553 | .Sp |
| 554 | Those preferring to be very, very clever might see the O'Reilly book, |
| 555 | \&\fIMastering Regular Expressions\fR, by Jeffrey Friedl. Page 273's |
| 556 | \&\fIBuild_MatchMany_Function()\fR is particularly interesting. A complete |
| 557 | citation of this book is given in perlfaq2. |
| 558 | .IP "Passing Methods" 4 |
| 559 | .IX Item "Passing Methods" |
| 560 | To pass an object method into a subroutine, you can do this: |
| 561 | .Sp |
| 562 | .Vb 7 |
| 563 | \& call_a_lot(10, $some_obj, "methname") |
| 564 | \& sub call_a_lot { |
| 565 | \& my ($count, $widget, $trick) = @_; |
| 566 | \& for (my $i = 0; $i < $count; $i++) { |
| 567 | \& $widget->$trick(); |
| 568 | \& } |
| 569 | \& } |
| 570 | .Ve |
| 571 | .Sp |
| 572 | Or, you can use a closure to bundle up the object, its |
| 573 | method call, and arguments: |
| 574 | .Sp |
| 575 | .Vb 6 |
| 576 | \& my $whatnot = sub { $some_obj->obfuscate(@args) }; |
| 577 | \& func($whatnot); |
| 578 | \& sub func { |
| 579 | \& my $code = shift; |
| 580 | \& &$code(); |
| 581 | \& } |
| 582 | .Ve |
| 583 | .Sp |
| 584 | You could also investigate the \fIcan()\fR method in the \s-1UNIVERSAL\s0 class |
| 585 | (part of the standard perl distribution). |
| 586 | .Sh "How do I create a static variable?" |
| 587 | .IX Subsection "How do I create a static variable?" |
| 588 | As with most things in Perl, \s-1TMTOWTDI\s0. What is a \*(L"static variable\*(R" in |
| 589 | other languages could be either a function-private variable (visible |
| 590 | only within a single function, retaining its value between calls to |
| 591 | that function), or a file-private variable (visible only to functions |
| 592 | within the file it was declared in) in Perl. |
| 593 | .PP |
| 594 | Here's code to implement a function-private variable: |
| 595 | .PP |
| 596 | .Vb 5 |
| 597 | \& BEGIN { |
| 598 | \& my $counter = 42; |
| 599 | \& sub prev_counter { return --$counter } |
| 600 | \& sub next_counter { return $counter++ } |
| 601 | \& } |
| 602 | .Ve |
| 603 | .PP |
| 604 | Now \fIprev_counter()\fR and \fInext_counter()\fR share a private variable \f(CW$counter\fR |
| 605 | that was initialized at compile time. |
| 606 | .PP |
| 607 | To declare a file-private variable, you'll still use a \fImy()\fR, putting |
| 608 | the declaration at the outer scope level at the top of the file. |
| 609 | Assume this is in file Pax.pm: |
| 610 | .PP |
| 611 | .Vb 2 |
| 612 | \& package Pax; |
| 613 | \& my $started = scalar(localtime(time())); |
| 614 | .Ve |
| 615 | .PP |
| 616 | .Vb 1 |
| 617 | \& sub begun { return $started } |
| 618 | .Ve |
| 619 | .PP |
| 620 | When \f(CW\*(C`use Pax\*(C'\fR or \f(CW\*(C`require Pax\*(C'\fR loads this module, the variable will |
| 621 | be initialized. It won't get garbage-collected the way most variables |
| 622 | going out of scope do, because the \fIbegun()\fR function cares about it, |
| 623 | but no one else can get it. It is not called \f(CW$Pax::started\fR because |
| 624 | its scope is unrelated to the package. It's scoped to the file. You |
| 625 | could conceivably have several packages in that same file all |
| 626 | accessing the same private variable, but another file with the same |
| 627 | package couldn't get to it. |
| 628 | .PP |
| 629 | See \*(L"Persistent Private Variables\*(R" in perlsub for details. |
| 630 | .Sh "What's the difference between dynamic and lexical (static) scoping? Between \fIlocal()\fP and \fImy()\fP?" |
| 631 | .IX Subsection "What's the difference between dynamic and lexical (static) scoping? Between local() and my()?" |
| 632 | \&\f(CW\*(C`local($x)\*(C'\fR saves away the old value of the global variable \f(CW$x\fR |
| 633 | and assigns a new value for the duration of the subroutine \fIwhich is |
| 634 | visible in other functions called from that subroutine\fR. This is done |
| 635 | at run\-time, so is called dynamic scoping. \fIlocal()\fR always affects global |
| 636 | variables, also called package variables or dynamic variables. |
| 637 | .PP |
| 638 | \&\f(CW\*(C`my($x)\*(C'\fR creates a new variable that is only visible in the current |
| 639 | subroutine. This is done at compile\-time, so it is called lexical or |
| 640 | static scoping. \fImy()\fR always affects private variables, also called |
| 641 | lexical variables or (improperly) static(ly scoped) variables. |
| 642 | .PP |
| 643 | For instance: |
| 644 | .PP |
| 645 | .Vb 3 |
| 646 | \& sub visible { |
| 647 | \& print "var has value $var\en"; |
| 648 | \& } |
| 649 | .Ve |
| 650 | .PP |
| 651 | .Vb 4 |
| 652 | \& sub dynamic { |
| 653 | \& local $var = 'local'; # new temporary value for the still-global |
| 654 | \& visible(); # variable called $var |
| 655 | \& } |
| 656 | .Ve |
| 657 | .PP |
| 658 | .Vb 4 |
| 659 | \& sub lexical { |
| 660 | \& my $var = 'private'; # new private variable, $var |
| 661 | \& visible(); # (invisible outside of sub scope) |
| 662 | \& } |
| 663 | .Ve |
| 664 | .PP |
| 665 | .Vb 1 |
| 666 | \& $var = 'global'; |
| 667 | .Ve |
| 668 | .PP |
| 669 | .Vb 3 |
| 670 | \& visible(); # prints global |
| 671 | \& dynamic(); # prints local |
| 672 | \& lexical(); # prints global |
| 673 | .Ve |
| 674 | .PP |
| 675 | Notice how at no point does the value \*(L"private\*(R" get printed. That's |
| 676 | because \f(CW$var\fR only has that value within the block of the \fIlexical()\fR |
| 677 | function, and it is hidden from called subroutine. |
| 678 | .PP |
| 679 | In summary, \fIlocal()\fR doesn't make what you think of as private, local |
| 680 | variables. It gives a global variable a temporary value. \fImy()\fR is |
| 681 | what you're looking for if you want private variables. |
| 682 | .PP |
| 683 | See \*(L"Private Variables via \fImy()\fR\*(R" in perlsub and |
| 684 | \&\*(L"Temporary Values via \fIlocal()\fR\*(R" in perlsub for excruciating details. |
| 685 | .Sh "How can I access a dynamic variable while a similarly named lexical is in scope?" |
| 686 | .IX Subsection "How can I access a dynamic variable while a similarly named lexical is in scope?" |
| 687 | You can do this via symbolic references, provided you haven't set |
| 688 | \&\f(CW\*(C`use strict "refs"\*(C'\fR. So instead of \f(CW$var\fR, use \f(CW\*(C`${'var'}\*(C'\fR. |
| 689 | .PP |
| 690 | .Vb 2 |
| 691 | \& local $var = "global"; |
| 692 | \& my $var = "lexical"; |
| 693 | .Ve |
| 694 | .PP |
| 695 | .Vb 1 |
| 696 | \& print "lexical is $var\en"; |
| 697 | .Ve |
| 698 | .PP |
| 699 | .Vb 2 |
| 700 | \& no strict 'refs'; |
| 701 | \& print "global is ${'var'}\en"; |
| 702 | .Ve |
| 703 | .PP |
| 704 | If you know your package, you can just mention it explicitly, as in |
| 705 | \&\f(CW$Some_Pack::var\fR. Note that the notation \f(CW$::var\fR is \fInot\fR the dynamic |
| 706 | \&\f(CW$var\fR in the current package, but rather the one in the \f(CW\*(C`main\*(C'\fR |
| 707 | package, as though you had written \f(CW$main::var\fR. Specifying the package |
| 708 | directly makes you hard-code its name, but it executes faster and |
| 709 | avoids running afoul of \f(CW\*(C`use strict "refs"\*(C'\fR. |
| 710 | .Sh "What's the difference between deep and shallow binding?" |
| 711 | .IX Subsection "What's the difference between deep and shallow binding?" |
| 712 | In deep binding, lexical variables mentioned in anonymous subroutines |
| 713 | are the same ones that were in scope when the subroutine was created. |
| 714 | In shallow binding, they are whichever variables with the same names |
| 715 | happen to be in scope when the subroutine is called. Perl always uses |
| 716 | deep binding of lexical variables (i.e., those created with \fImy()\fR). |
| 717 | However, dynamic variables (aka global, local, or package variables) |
| 718 | are effectively shallowly bound. Consider this just one more reason |
| 719 | not to use them. See the answer to \*(L"What's a closure?\*(R". |
| 720 | .ie n .Sh "Why doesn't ""my($foo) = <\s-1FILE\s0>;"" work right?" |
| 721 | .el .Sh "Why doesn't ``my($foo) = <\s-1FILE\s0>;'' work right?" |
| 722 | .IX Subsection "Why doesn't my($foo) = <FILE>; work right?" |
| 723 | \&\f(CW\*(C`my()\*(C'\fR and \f(CW\*(C`local()\*(C'\fR give list context to the right hand side |
| 724 | of \f(CW\*(C`=\*(C'\fR. The <\s-1FH\s0> read operation, like so many of Perl's |
| 725 | functions and operators, can tell which context it was called in and |
| 726 | behaves appropriately. In general, the \fIscalar()\fR function can help. |
| 727 | This function does nothing to the data itself (contrary to popular myth) |
| 728 | but rather tells its argument to behave in whatever its scalar fashion is. |
| 729 | If that function doesn't have a defined scalar behavior, this of course |
| 730 | doesn't help you (such as with \fIsort()\fR). |
| 731 | .PP |
| 732 | To enforce scalar context in this particular case, however, you need |
| 733 | merely omit the parentheses: |
| 734 | .PP |
| 735 | .Vb 3 |
| 736 | \& local($foo) = <FILE>; # WRONG |
| 737 | \& local($foo) = scalar(<FILE>); # ok |
| 738 | \& local $foo = <FILE>; # right |
| 739 | .Ve |
| 740 | .PP |
| 741 | You should probably be using lexical variables anyway, although the |
| 742 | issue is the same here: |
| 743 | .PP |
| 744 | .Vb 2 |
| 745 | \& my($foo) = <FILE>; # WRONG |
| 746 | \& my $foo = <FILE>; # right |
| 747 | .Ve |
| 748 | .Sh "How do I redefine a builtin function, operator, or method?" |
| 749 | .IX Subsection "How do I redefine a builtin function, operator, or method?" |
| 750 | Why do you want to do that? :\-) |
| 751 | .PP |
| 752 | If you want to override a predefined function, such as \fIopen()\fR, |
| 753 | then you'll have to import the new definition from a different |
| 754 | module. See \*(L"Overriding Built-in Functions\*(R" in perlsub. There's |
| 755 | also an example in \*(L"Class::Template\*(R" in perltoot. |
| 756 | .PP |
| 757 | If you want to overload a Perl operator, such as \f(CW\*(C`+\*(C'\fR or \f(CW\*(C`**\*(C'\fR, |
| 758 | then you'll want to use the \f(CW\*(C`use overload\*(C'\fR pragma, documented |
| 759 | in overload. |
| 760 | .PP |
| 761 | If you're talking about obscuring method calls in parent classes, |
| 762 | see \*(L"Overridden Methods\*(R" in perltoot. |
| 763 | .Sh "What's the difference between calling a function as &foo and \fIfoo()\fP?" |
| 764 | .IX Subsection "What's the difference between calling a function as &foo and foo()?" |
| 765 | When you call a function as \f(CW&foo\fR, you allow that function access to |
| 766 | your current \f(CW@_\fR values, and you bypass prototypes. |
| 767 | The function doesn't get an empty \f(CW@_\fR\-\-it gets yours! While not |
| 768 | strictly speaking a bug (it's documented that way in perlsub), it |
| 769 | would be hard to consider this a feature in most cases. |
| 770 | .PP |
| 771 | When you call your function as \f(CW\*(C`&foo()\*(C'\fR, then you \fIdo\fR get a new \f(CW@_\fR, |
| 772 | but prototyping is still circumvented. |
| 773 | .PP |
| 774 | Normally, you want to call a function using \f(CW\*(C`foo()\*(C'\fR. You may only |
| 775 | omit the parentheses if the function is already known to the compiler |
| 776 | because it already saw the definition (\f(CW\*(C`use\*(C'\fR but not \f(CW\*(C`require\*(C'\fR), |
| 777 | or via a forward reference or \f(CW\*(C`use subs\*(C'\fR declaration. Even in this |
| 778 | case, you get a clean \f(CW@_\fR without any of the old values leaking through |
| 779 | where they don't belong. |
| 780 | .Sh "How do I create a switch or case statement?" |
| 781 | .IX Subsection "How do I create a switch or case statement?" |
| 782 | This is explained in more depth in the perlsyn. Briefly, there's |
| 783 | no official case statement, because of the variety of tests possible |
| 784 | in Perl (numeric comparison, string comparison, glob comparison, |
| 785 | regex matching, overloaded comparisons, ...). |
| 786 | Larry couldn't decide how best to do this, so he left it out, even |
| 787 | though it's been on the wish list since perl1. |
| 788 | .PP |
| 789 | Starting from Perl 5.8 to get switch and case one can use the |
| 790 | Switch extension and say: |
| 791 | .PP |
| 792 | .Vb 1 |
| 793 | \& use Switch; |
| 794 | .Ve |
| 795 | .PP |
| 796 | after which one has switch and case. It is not as fast as it could be |
| 797 | because it's not really part of the language (it's done using source |
| 798 | filters) but it is available, and it's very flexible. |
| 799 | .PP |
| 800 | But if one wants to use pure Perl, the general answer is to write a |
| 801 | construct like this: |
| 802 | .PP |
| 803 | .Vb 6 |
| 804 | \& for ($variable_to_test) { |
| 805 | \& if (/pat1/) { } # do something |
| 806 | \& elsif (/pat2/) { } # do something else |
| 807 | \& elsif (/pat3/) { } # do something else |
| 808 | \& else { } # default |
| 809 | \& } |
| 810 | .Ve |
| 811 | .PP |
| 812 | Here's a simple example of a switch based on pattern matching, this |
| 813 | time lined up in a way to make it look more like a switch statement. |
| 814 | We'll do a multiway conditional based on the type of reference stored |
| 815 | in \f(CW$whatchamacallit:\fR |
| 816 | .PP |
| 817 | .Vb 1 |
| 818 | \& SWITCH: for (ref $whatchamacallit) { |
| 819 | .Ve |
| 820 | .PP |
| 821 | .Vb 1 |
| 822 | \& /^$/ && die "not a reference"; |
| 823 | .Ve |
| 824 | .PP |
| 825 | .Vb 4 |
| 826 | \& /SCALAR/ && do { |
| 827 | \& print_scalar($$ref); |
| 828 | \& last SWITCH; |
| 829 | \& }; |
| 830 | .Ve |
| 831 | .PP |
| 832 | .Vb 4 |
| 833 | \& /ARRAY/ && do { |
| 834 | \& print_array(@$ref); |
| 835 | \& last SWITCH; |
| 836 | \& }; |
| 837 | .Ve |
| 838 | .PP |
| 839 | .Vb 4 |
| 840 | \& /HASH/ && do { |
| 841 | \& print_hash(%$ref); |
| 842 | \& last SWITCH; |
| 843 | \& }; |
| 844 | .Ve |
| 845 | .PP |
| 846 | .Vb 4 |
| 847 | \& /CODE/ && do { |
| 848 | \& warn "can't print function ref"; |
| 849 | \& last SWITCH; |
| 850 | \& }; |
| 851 | .Ve |
| 852 | .PP |
| 853 | .Vb 1 |
| 854 | \& # DEFAULT |
| 855 | .Ve |
| 856 | .PP |
| 857 | .Vb 1 |
| 858 | \& warn "User defined type skipped"; |
| 859 | .Ve |
| 860 | .PP |
| 861 | .Vb 1 |
| 862 | \& } |
| 863 | .Ve |
| 864 | .PP |
| 865 | See \f(CW\*(C`perlsyn/"Basic BLOCKs and Switch Statements"\*(C'\fR for many other |
| 866 | examples in this style. |
| 867 | .PP |
| 868 | Sometimes you should change the positions of the constant and the variable. |
| 869 | For example, let's say you wanted to test which of many answers you were |
| 870 | given, but in a case-insensitive way that also allows abbreviations. |
| 871 | You can use the following technique if the strings all start with |
| 872 | different characters or if you want to arrange the matches so that |
| 873 | one takes precedence over another, as \f(CW"SEND"\fR has precedence over |
| 874 | \&\f(CW"STOP"\fR here: |
| 875 | .PP |
| 876 | .Vb 6 |
| 877 | \& chomp($answer = <>); |
| 878 | \& if ("SEND" =~ /^\eQ$answer/i) { print "Action is send\en" } |
| 879 | \& elsif ("STOP" =~ /^\eQ$answer/i) { print "Action is stop\en" } |
| 880 | \& elsif ("ABORT" =~ /^\eQ$answer/i) { print "Action is abort\en" } |
| 881 | \& elsif ("LIST" =~ /^\eQ$answer/i) { print "Action is list\en" } |
| 882 | \& elsif ("EDIT" =~ /^\eQ$answer/i) { print "Action is edit\en" } |
| 883 | .Ve |
| 884 | .PP |
| 885 | A totally different approach is to create a hash of function references. |
| 886 | .PP |
| 887 | .Vb 6 |
| 888 | \& my %commands = ( |
| 889 | \& "happy" => \e&joy, |
| 890 | \& "sad", => \e&sullen, |
| 891 | \& "done" => sub { die "See ya!" }, |
| 892 | \& "mad" => \e&angry, |
| 893 | \& ); |
| 894 | .Ve |
| 895 | .PP |
| 896 | .Vb 7 |
| 897 | \& print "How are you? "; |
| 898 | \& chomp($string = <STDIN>); |
| 899 | \& if ($commands{$string}) { |
| 900 | \& $commands{$string}->(); |
| 901 | \& } else { |
| 902 | \& print "No such command: $string\en"; |
| 903 | \& } |
| 904 | .Ve |
| 905 | .Sh "How can I catch accesses to undefined variables/functions/methods?" |
| 906 | .IX Subsection "How can I catch accesses to undefined variables/functions/methods?" |
| 907 | The \s-1AUTOLOAD\s0 method, discussed in \*(L"Autoloading\*(R" in perlsub and |
| 908 | \&\*(L"\s-1AUTOLOAD:\s0 Proxy Methods\*(R" in perltoot, lets you capture calls to |
| 909 | undefined functions and methods. |
| 910 | .PP |
| 911 | When it comes to undefined variables that would trigger a warning |
| 912 | under \f(CW\*(C`\-w\*(C'\fR, you can use a handler to trap the pseudo-signal |
| 913 | \&\f(CW\*(C`_\|_WARN_\|_\*(C'\fR like this: |
| 914 | .PP |
| 915 | .Vb 1 |
| 916 | \& $SIG{__WARN__} = sub { |
| 917 | .Ve |
| 918 | .PP |
| 919 | .Vb 1 |
| 920 | \& for ( $_[0] ) { # voici un switch statement |
| 921 | .Ve |
| 922 | .PP |
| 923 | .Vb 4 |
| 924 | \& /Use of uninitialized value/ && do { |
| 925 | \& # promote warning to a fatal |
| 926 | \& die $_; |
| 927 | \& }; |
| 928 | .Ve |
| 929 | .PP |
| 930 | .Vb 1 |
| 931 | \& # other warning cases to catch could go here; |
| 932 | .Ve |
| 933 | .PP |
| 934 | .Vb 2 |
| 935 | \& warn $_; |
| 936 | \& } |
| 937 | .Ve |
| 938 | .PP |
| 939 | .Vb 1 |
| 940 | \& }; |
| 941 | .Ve |
| 942 | .Sh "Why can't a method included in this same file be found?" |
| 943 | .IX Subsection "Why can't a method included in this same file be found?" |
| 944 | Some possible reasons: your inheritance is getting confused, you've |
| 945 | misspelled the method name, or the object is of the wrong type. Check |
| 946 | out perltoot for details about any of the above cases. You may |
| 947 | also use \f(CW\*(C`print ref($object)\*(C'\fR to find out the class \f(CW$object\fR was |
| 948 | blessed into. |
| 949 | .PP |
| 950 | Another possible reason for problems is because you've used the |
| 951 | indirect object syntax (eg, \f(CW\*(C`find Guru "Samy"\*(C'\fR) on a class name |
| 952 | before Perl has seen that such a package exists. It's wisest to make |
| 953 | sure your packages are all defined before you start using them, which |
| 954 | will be taken care of if you use the \f(CW\*(C`use\*(C'\fR statement instead of |
| 955 | \&\f(CW\*(C`require\*(C'\fR. If not, make sure to use arrow notation (eg., |
| 956 | \&\f(CW\*(C`Guru\->find("Samy")\*(C'\fR) instead. Object notation is explained in |
| 957 | perlobj. |
| 958 | .PP |
| 959 | Make sure to read about creating modules in perlmod and |
| 960 | the perils of indirect objects in \*(L"Method Invocation\*(R" in perlobj. |
| 961 | .Sh "How can I find out my current package?" |
| 962 | .IX Subsection "How can I find out my current package?" |
| 963 | If you're just a random program, you can do this to find |
| 964 | out what the currently compiled package is: |
| 965 | .PP |
| 966 | .Vb 1 |
| 967 | \& my $packname = __PACKAGE__; |
| 968 | .Ve |
| 969 | .PP |
| 970 | But, if you're a method and you want to print an error message |
| 971 | that includes the kind of object you were called on (which is |
| 972 | not necessarily the same as the one in which you were compiled): |
| 973 | .PP |
| 974 | .Vb 5 |
| 975 | \& sub amethod { |
| 976 | \& my $self = shift; |
| 977 | \& my $class = ref($self) || $self; |
| 978 | \& warn "called me from a $class object"; |
| 979 | \& } |
| 980 | .Ve |
| 981 | .Sh "How can I comment out a large block of perl code?" |
| 982 | .IX Subsection "How can I comment out a large block of perl code?" |
| 983 | You can use embedded \s-1POD\s0 to discard it. The =for directive |
| 984 | lasts until the next paragraph (two consecutive newlines). |
| 985 | .PP |
| 986 | .Vb 1 |
| 987 | \& # program is here |
| 988 | .Ve |
| 989 | .PP |
| 990 | .Vb 2 |
| 991 | \& =for nobody |
| 992 | \& This paragraph is commented out |
| 993 | .Ve |
| 994 | .PP |
| 995 | .Vb 1 |
| 996 | \& # program continues |
| 997 | .Ve |
| 998 | .PP |
| 999 | The =begin and =end directives can contain multiple |
| 1000 | paragraphs. |
| 1001 | .PP |
| 1002 | .Vb 1 |
| 1003 | \& =begin comment text |
| 1004 | .Ve |
| 1005 | .PP |
| 1006 | .Vb 1 |
| 1007 | \& all of this stuff |
| 1008 | .Ve |
| 1009 | .PP |
| 1010 | .Vb 2 |
| 1011 | \& here will be ignored |
| 1012 | \& by everyone |
| 1013 | .Ve |
| 1014 | .PP |
| 1015 | .Vb 1 |
| 1016 | \& =end comment text |
| 1017 | .Ve |
| 1018 | .PP |
| 1019 | The pod directives cannot go just anywhere. You must put a |
| 1020 | pod directive where the parser is expecting a new statement, |
| 1021 | not just in the middle of an expression or some other |
| 1022 | arbitrary s grammar production. |
| 1023 | .PP |
| 1024 | See perlpod for more details. |
| 1025 | .Sh "How do I clear a package?" |
| 1026 | .IX Subsection "How do I clear a package?" |
| 1027 | Use this code, provided by Mark-Jason Dominus: |
| 1028 | .PP |
| 1029 | .Vb 17 |
| 1030 | \& sub scrub_package { |
| 1031 | \& no strict 'refs'; |
| 1032 | \& my $pack = shift; |
| 1033 | \& die "Shouldn't delete main package" |
| 1034 | \& if $pack eq "" || $pack eq "main"; |
| 1035 | \& my $stash = *{$pack . '::'}{HASH}; |
| 1036 | \& my $name; |
| 1037 | \& foreach $name (keys %$stash) { |
| 1038 | \& my $fullname = $pack . '::' . $name; |
| 1039 | \& # Get rid of everything with that name. |
| 1040 | \& undef $$fullname; |
| 1041 | \& undef @$fullname; |
| 1042 | \& undef %$fullname; |
| 1043 | \& undef &$fullname; |
| 1044 | \& undef *$fullname; |
| 1045 | \& } |
| 1046 | \& } |
| 1047 | .Ve |
| 1048 | .PP |
| 1049 | Or, if you're using a recent release of Perl, you can |
| 1050 | just use the \fISymbol::delete_package()\fR function instead. |
| 1051 | .Sh "How can I use a variable as a variable name?" |
| 1052 | .IX Subsection "How can I use a variable as a variable name?" |
| 1053 | Beginners often think they want to have a variable contain the name |
| 1054 | of a variable. |
| 1055 | .PP |
| 1056 | .Vb 3 |
| 1057 | \& $fred = 23; |
| 1058 | \& $varname = "fred"; |
| 1059 | \& ++$$varname; # $fred now 24 |
| 1060 | .Ve |
| 1061 | .PP |
| 1062 | This works \fIsometimes\fR, but it is a very bad idea for two reasons. |
| 1063 | .PP |
| 1064 | The first reason is that this technique \fIonly works on global |
| 1065 | variables\fR. That means that if \f(CW$fred\fR is a lexical variable created |
| 1066 | with \fImy()\fR in the above example, the code wouldn't work at all: you'd |
| 1067 | accidentally access the global and skip right over the private lexical |
| 1068 | altogether. Global variables are bad because they can easily collide |
| 1069 | accidentally and in general make for non-scalable and confusing code. |
| 1070 | .PP |
| 1071 | Symbolic references are forbidden under the \f(CW\*(C`use strict\*(C'\fR pragma. |
| 1072 | They are not true references and consequently are not reference counted |
| 1073 | or garbage collected. |
| 1074 | .PP |
| 1075 | The other reason why using a variable to hold the name of another |
| 1076 | variable is a bad idea is that the question often stems from a lack of |
| 1077 | understanding of Perl data structures, particularly hashes. By using |
| 1078 | symbolic references, you are just using the package's symbol-table hash |
| 1079 | (like \f(CW%main::\fR) instead of a user-defined hash. The solution is to |
| 1080 | use your own hash or a real reference instead. |
| 1081 | .PP |
| 1082 | .Vb 3 |
| 1083 | \& $fred = 23; |
| 1084 | \& $varname = "fred"; |
| 1085 | \& $USER_VARS{$varname}++; # not $$varname++ |
| 1086 | .Ve |
| 1087 | .PP |
| 1088 | There we're using the \f(CW%USER_VARS\fR hash instead of symbolic references. |
| 1089 | Sometimes this comes up in reading strings from the user with variable |
| 1090 | references and wanting to expand them to the values of your perl |
| 1091 | program's variables. This is also a bad idea because it conflates the |
| 1092 | program-addressable namespace and the user-addressable one. Instead of |
| 1093 | reading a string and expanding it to the actual contents of your program's |
| 1094 | own variables: |
| 1095 | .PP |
| 1096 | .Vb 2 |
| 1097 | \& $str = 'this has a $fred and $barney in it'; |
| 1098 | \& $str =~ s/(\e$\ew+)/$1/eeg; # need double eval |
| 1099 | .Ve |
| 1100 | .PP |
| 1101 | it would be better to keep a hash around like \f(CW%USER_VARS\fR and have |
| 1102 | variable references actually refer to entries in that hash: |
| 1103 | .PP |
| 1104 | .Vb 1 |
| 1105 | \& $str =~ s/\e$(\ew+)/$USER_VARS{$1}/g; # no /e here at all |
| 1106 | .Ve |
| 1107 | .PP |
| 1108 | That's faster, cleaner, and safer than the previous approach. Of course, |
| 1109 | you don't need to use a dollar sign. You could use your own scheme to |
| 1110 | make it less confusing, like bracketed percent symbols, etc. |
| 1111 | .PP |
| 1112 | .Vb 2 |
| 1113 | \& $str = 'this has a %fred% and %barney% in it'; |
| 1114 | \& $str =~ s/%(\ew+)%/$USER_VARS{$1}/g; # no /e here at all |
| 1115 | .Ve |
| 1116 | .PP |
| 1117 | Another reason that folks sometimes think they want a variable to |
| 1118 | contain the name of a variable is because they don't know how to build |
| 1119 | proper data structures using hashes. For example, let's say they |
| 1120 | wanted two hashes in their program: \f(CW%fred\fR and \f(CW%barney\fR, and that they |
| 1121 | wanted to use another scalar variable to refer to those by name. |
| 1122 | .PP |
| 1123 | .Vb 2 |
| 1124 | \& $name = "fred"; |
| 1125 | \& $$name{WIFE} = "wilma"; # set %fred |
| 1126 | .Ve |
| 1127 | .PP |
| 1128 | .Vb 2 |
| 1129 | \& $name = "barney"; |
| 1130 | \& $$name{WIFE} = "betty"; # set %barney |
| 1131 | .Ve |
| 1132 | .PP |
| 1133 | This is still a symbolic reference, and is still saddled with the |
| 1134 | problems enumerated above. It would be far better to write: |
| 1135 | .PP |
| 1136 | .Vb 2 |
| 1137 | \& $folks{"fred"}{WIFE} = "wilma"; |
| 1138 | \& $folks{"barney"}{WIFE} = "betty"; |
| 1139 | .Ve |
| 1140 | .PP |
| 1141 | And just use a multilevel hash to start with. |
| 1142 | .PP |
| 1143 | The only times that you absolutely \fImust\fR use symbolic references are |
| 1144 | when you really must refer to the symbol table. This may be because it's |
| 1145 | something that can't take a real reference to, such as a format name. |
| 1146 | Doing so may also be important for method calls, since these always go |
| 1147 | through the symbol table for resolution. |
| 1148 | .PP |
| 1149 | In those cases, you would turn off \f(CW\*(C`strict 'refs'\*(C'\fR temporarily so you |
| 1150 | can play around with the symbol table. For example: |
| 1151 | .PP |
| 1152 | .Vb 5 |
| 1153 | \& @colors = qw(red blue green yellow orange purple violet); |
| 1154 | \& for my $name (@colors) { |
| 1155 | \& no strict 'refs'; # renege for the block |
| 1156 | \& *$name = sub { "<FONT COLOR='$name'>@_</FONT>" }; |
| 1157 | \& } |
| 1158 | .Ve |
| 1159 | .PP |
| 1160 | All those functions (\fIred()\fR, \fIblue()\fR, \fIgreen()\fR, etc.) appear to be separate, |
| 1161 | but the real code in the closure actually was compiled only once. |
| 1162 | .PP |
| 1163 | So, sometimes you might want to use symbolic references to directly |
| 1164 | manipulate the symbol table. This doesn't matter for formats, handles, and |
| 1165 | subroutines, because they are always global\*(--you can't use \fImy()\fR on them. |
| 1166 | For scalars, arrays, and hashes, though\*(--and usually for subroutines\*(-- |
| 1167 | you probably only want to use hard references. |
| 1168 | .SH "AUTHOR AND COPYRIGHT" |
| 1169 | .IX Header "AUTHOR AND COPYRIGHT" |
| 1170 | Copyright (c) 1997\-2002 Tom Christiansen and Nathan Torkington. |
| 1171 | All rights reserved. |
| 1172 | .PP |
| 1173 | This documentation is free; you can redistribute it and/or modify it |
| 1174 | under the same terms as Perl itself. |
| 1175 | .PP |
| 1176 | Irrespective of its distribution, all code examples in this file |
| 1177 | are hereby placed into the public domain. You are permitted and |
| 1178 | encouraged to use this code in your own programs for fun |
| 1179 | or for profit as you see fit. A simple comment in the code giving |
| 1180 | credit would be courteous but is not required. |