Commit | Line | Data |
---|---|---|
a0d0e21e LW |
1 | =head1 NAME |
2 | ||
3 | perltrap - Perl traps for the unwary | |
4 | ||
5 | =head1 DESCRIPTION | |
6 | ||
cb1a09d0 AD |
7 | The biggest trap of all is forgetting to use the B<-w> switch; see |
8 | L<perlrun>. The second biggest trap is not making your entire program | |
daff0e37 CS |
9 | runnable under C<use strict>. The third biggest trap is not reading |
10 | the list of changes in this version of Perl; see L<perldelta>. | |
a0d0e21e LW |
11 | |
12 | =head2 Awk Traps | |
13 | ||
14 | Accustomed B<awk> users should take special note of the following: | |
15 | ||
16 | =over 4 | |
17 | ||
18 | =item * | |
19 | ||
20 | The English module, loaded via | |
21 | ||
22 | use English; | |
23 | ||
54310121 | 24 | allows you to refer to special variables (like C<$/>) with names (like |
25 | C<$RS>), as though they were in B<awk>; see L<perlvar> for details. | |
a0d0e21e LW |
26 | |
27 | =item * | |
28 | ||
29 | Semicolons are required after all simple statements in Perl (except | |
30 | at the end of a block). Newline is not a statement delimiter. | |
31 | ||
32 | =item * | |
33 | ||
34 | Curly brackets are required on C<if>s and C<while>s. | |
35 | ||
36 | =item * | |
37 | ||
5db417f7 | 38 | Variables begin with "$", "@" or "%" in Perl. |
a0d0e21e LW |
39 | |
40 | =item * | |
41 | ||
42 | Arrays index from 0. Likewise string positions in substr() and | |
43 | index(). | |
44 | ||
45 | =item * | |
46 | ||
47 | You have to decide whether your array has numeric or string indices. | |
48 | ||
49 | =item * | |
50 | ||
aa689395 | 51 | Hash values do not spring into existence upon mere reference. |
a0d0e21e LW |
52 | |
53 | =item * | |
54 | ||
55 | You have to decide whether you want to use string or numeric | |
56 | comparisons. | |
57 | ||
58 | =item * | |
59 | ||
60 | Reading an input line does not split it for you. You get to split it | |
54310121 | 61 | to an array yourself. And the split() operator has different |
62 | arguments than B<awk>'s. | |
a0d0e21e LW |
63 | |
64 | =item * | |
65 | ||
66 | The current input line is normally in $_, not $0. It generally does | |
67 | not have the newline stripped. ($0 is the name of the program | |
68 | executed.) See L<perlvar>. | |
69 | ||
70 | =item * | |
71 | ||
8b0a4b75 | 72 | $E<lt>I<digit>E<gt> does not refer to fields--it refers to substrings matched |
73 | by the last match pattern. | |
a0d0e21e LW |
74 | |
75 | =item * | |
76 | ||
77 | The print() statement does not add field and record separators unless | |
8b0a4b75 | 78 | you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using |
a0d0e21e LW |
79 | the English module. |
80 | ||
81 | =item * | |
82 | ||
83 | You must open your files before you print to them. | |
84 | ||
85 | =item * | |
86 | ||
87 | The range operator is "..", not comma. The comma operator works as in | |
88 | C. | |
89 | ||
90 | =item * | |
91 | ||
92 | The match operator is "=~", not "~". ("~" is the one's complement | |
93 | operator, as in C.) | |
94 | ||
95 | =item * | |
96 | ||
97 | The exponentiation operator is "**", not "^". "^" is the XOR | |
98 | operator, as in C. (You know, one could get the feeling that B<awk> is | |
99 | basically incompatible with C.) | |
100 | ||
101 | =item * | |
102 | ||
103 | The concatenation operator is ".", not the null string. (Using the | |
5f05dabc | 104 | null string would render C</pat/ /pat/> unparsable, because the third slash |
105 | would be interpreted as a division operator--the tokenizer is in fact | |
8b0a4b75 | 106 | slightly context sensitive for operators like "/", "?", and "E<gt>". |
a0d0e21e LW |
107 | And in fact, "." itself can be the beginning of a number.) |
108 | ||
109 | =item * | |
110 | ||
111 | The C<next>, C<exit>, and C<continue> keywords work differently. | |
112 | ||
113 | =item * | |
114 | ||
115 | ||
116 | The following variables work differently: | |
117 | ||
118 | Awk Perl | |
119 | ARGC $#ARGV or scalar @ARGV | |
120 | ARGV[0] $0 | |
121 | FILENAME $ARGV | |
122 | FNR $. - something | |
123 | FS (whatever you like) | |
124 | NF $#Fld, or some such | |
125 | NR $. | |
126 | OFMT $# | |
127 | OFS $, | |
128 | ORS $\ | |
129 | RLENGTH length($&) | |
130 | RS $/ | |
131 | RSTART length($`) | |
132 | SUBSEP $; | |
133 | ||
134 | =item * | |
135 | ||
136 | You cannot set $RS to a pattern, only a string. | |
137 | ||
138 | =item * | |
139 | ||
140 | When in doubt, run the B<awk> construct through B<a2p> and see what it | |
141 | gives you. | |
142 | ||
143 | =back | |
144 | ||
145 | =head2 C Traps | |
146 | ||
147 | Cerebral C programmers should take note of the following: | |
148 | ||
149 | =over 4 | |
150 | ||
151 | =item * | |
152 | ||
153 | Curly brackets are required on C<if>'s and C<while>'s. | |
154 | ||
155 | =item * | |
156 | ||
157 | You must use C<elsif> rather than C<else if>. | |
158 | ||
159 | =item * | |
160 | ||
54310121 | 161 | The C<break> and C<continue> keywords from C become in |
a0d0e21e LW |
162 | Perl C<last> and C<next>, respectively. |
163 | Unlike in C, these do I<NOT> work within a C<do { } while> construct. | |
164 | ||
165 | =item * | |
166 | ||
167 | There's no switch statement. (But it's easy to build one on the fly.) | |
168 | ||
169 | =item * | |
170 | ||
5db417f7 | 171 | Variables begin with "$", "@" or "%" in Perl. |
a0d0e21e LW |
172 | |
173 | =item * | |
174 | ||
6dbacca0 | 175 | C<printf()> does not implement the "*" format for interpolating |
a0d0e21e LW |
176 | field widths, but it's trivial to use interpolation of double-quoted |
177 | strings to achieve the same effect. | |
178 | ||
179 | =item * | |
180 | ||
181 | Comments begin with "#", not "/*". | |
182 | ||
183 | =item * | |
184 | ||
185 | You can't take the address of anything, although a similar operator | |
5f05dabc | 186 | in Perl is the backslash, which creates a reference. |
a0d0e21e LW |
187 | |
188 | =item * | |
189 | ||
4633a7c4 LW |
190 | C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]> |
191 | ends up in C<$0>. | |
a0d0e21e LW |
192 | |
193 | =item * | |
194 | ||
195 | System calls such as link(), unlink(), rename(), etc. return nonzero for | |
196 | success, not 0. | |
197 | ||
198 | =item * | |
199 | ||
200 | Signal handlers deal with signal names, not numbers. Use C<kill -l> | |
201 | to find their names on your system. | |
202 | ||
203 | =back | |
204 | ||
205 | =head2 Sed Traps | |
206 | ||
207 | Seasoned B<sed> programmers should take note of the following: | |
208 | ||
209 | =over 4 | |
210 | ||
211 | =item * | |
212 | ||
213 | Backreferences in substitutions use "$" rather than "\". | |
214 | ||
215 | =item * | |
216 | ||
217 | The pattern matching metacharacters "(", ")", and "|" do not have backslashes | |
218 | in front. | |
219 | ||
220 | =item * | |
221 | ||
222 | The range operator is C<...>, rather than comma. | |
223 | ||
224 | =back | |
225 | ||
226 | =head2 Shell Traps | |
227 | ||
228 | Sharp shell programmers should take note of the following: | |
229 | ||
230 | =over 4 | |
231 | ||
232 | =item * | |
233 | ||
54310121 | 234 | The backtick operator does variable interpolation without regard to |
a0d0e21e LW |
235 | the presence of single quotes in the command. |
236 | ||
237 | =item * | |
238 | ||
54310121 | 239 | The backtick operator does no translation of the return value, unlike B<csh>. |
a0d0e21e LW |
240 | |
241 | =item * | |
242 | ||
243 | Shells (especially B<csh>) do several levels of substitution on each | |
5f05dabc | 244 | command line. Perl does substitution in only certain constructs |
54310121 | 245 | such as double quotes, backticks, angle brackets, and search patterns. |
a0d0e21e LW |
246 | |
247 | =item * | |
248 | ||
249 | Shells interpret scripts a little bit at a time. Perl compiles the | |
250 | entire program before executing it (except for C<BEGIN> blocks, which | |
251 | execute at compile time). | |
252 | ||
253 | =item * | |
254 | ||
255 | The arguments are available via @ARGV, not $1, $2, etc. | |
256 | ||
257 | =item * | |
258 | ||
259 | The environment is not automatically made available as separate scalar | |
260 | variables. | |
261 | ||
262 | =back | |
263 | ||
264 | =head2 Perl Traps | |
265 | ||
266 | Practicing Perl Programmers should take note of the following: | |
267 | ||
268 | =over 4 | |
269 | ||
270 | =item * | |
271 | ||
272 | Remember that many operations behave differently in a list | |
273 | context than they do in a scalar one. See L<perldata> for details. | |
274 | ||
275 | =item * | |
276 | ||
68dc0745 | 277 | Avoid barewords if you can, especially all lowercase ones. |
54310121 | 278 | You can't tell by just looking at it whether a bareword is |
279 | a function or a string. By using quotes on strings and | |
5f05dabc | 280 | parentheses on function calls, you won't ever get them confused. |
a0d0e21e LW |
281 | |
282 | =item * | |
283 | ||
54310121 | 284 | You cannot discern from mere inspection which builtins |
285 | are unary operators (like chop() and chdir()) | |
a0d0e21e | 286 | and which are list operators (like print() and unlink()). |
5f05dabc | 287 | (User-defined subroutines can be B<only> list operators, never |
a0d0e21e LW |
288 | unary ones.) See L<perlop>. |
289 | ||
290 | =item * | |
291 | ||
748a9306 | 292 | People have a hard time remembering that some functions |
a0d0e21e | 293 | default to $_, or @ARGV, or whatever, but that others which |
54310121 | 294 | you might expect to do not. |
a0d0e21e | 295 | |
6dbacca0 | 296 | =item * |
a0d0e21e | 297 | |
8b0a4b75 | 298 | The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline |
5f05dabc | 299 | operation on that handle. The data read is assigned to $_ only if the |
748a9306 LW |
300 | file read is the sole condition in a while loop: |
301 | ||
302 | while (<FH>) { } | |
54310121 | 303 | while (defined($_ = <FH>)) { }.. |
748a9306 LW |
304 | <FH>; # data discarded! |
305 | ||
6dbacca0 | 306 | =item * |
748a9306 | 307 | |
a0d0e21e LW |
308 | Remember not to use "C<=>" when you need "C<=~>"; |
309 | these two constructs are quite different: | |
310 | ||
311 | $x = /foo/; | |
312 | $x =~ /foo/; | |
313 | ||
314 | =item * | |
315 | ||
54310121 | 316 | The C<do {}> construct isn't a real loop that you can use |
a0d0e21e LW |
317 | loop control on. |
318 | ||
319 | =item * | |
320 | ||
54310121 | 321 | Use C<my()> for local variables whenever you can get away with |
322 | it (but see L<perlform> for where you can't). | |
323 | Using C<local()> actually gives a local value to a global | |
a0d0e21e LW |
324 | variable, which leaves you open to unforeseen side-effects |
325 | of dynamic scoping. | |
326 | ||
c07a80fd | 327 | =item * |
328 | ||
329 | If you localize an exported variable in a module, its exported value will | |
330 | not change. The local name becomes an alias to a new value but the | |
331 | external name is still an alias for the original. | |
332 | ||
a0d0e21e LW |
333 | =back |
334 | ||
5f05dabc | 335 | =head2 Perl4 to Perl5 Traps |
a0d0e21e | 336 | |
54310121 | 337 | Practicing Perl4 Programmers should take note of the following |
6dbacca0 | 338 | Perl4-to-Perl5 specific traps. |
339 | ||
340 | They're crudely ordered according to the following list: | |
a0d0e21e LW |
341 | |
342 | =over 4 | |
343 | ||
6dbacca0 | 344 | =item Discontinuance, Deprecation, and BugFix traps |
a0d0e21e | 345 | |
6dbacca0 | 346 | Anything that's been fixed as a perl4 bug, removed as a perl4 feature |
347 | or deprecated as a perl4 feature with the intent to encourage usage of | |
348 | some other perl5 feature. | |
a0d0e21e | 349 | |
6dbacca0 | 350 | =item Parsing Traps |
748a9306 | 351 | |
6dbacca0 | 352 | Traps that appear to stem from the new parser. |
a0d0e21e | 353 | |
6dbacca0 | 354 | =item Numerical Traps |
a0d0e21e | 355 | |
6dbacca0 | 356 | Traps having to do with numerical or mathematical operators. |
a0d0e21e | 357 | |
6dbacca0 | 358 | =item General data type traps |
a0d0e21e | 359 | |
6dbacca0 | 360 | Traps involving perl standard data types. |
a0d0e21e | 361 | |
6dbacca0 | 362 | =item Context Traps - scalar, list contexts |
363 | ||
364 | Traps related to context within lists, scalar statements/declarations. | |
365 | ||
366 | =item Precedence Traps | |
367 | ||
368 | Traps related to the precedence of parsing, evaluation, and execution of | |
369 | code. | |
370 | ||
371 | =item General Regular Expression Traps using s///, etc. | |
372 | ||
373 | Traps related to the use of pattern matching. | |
374 | ||
375 | =item Subroutine, Signal, Sorting Traps | |
376 | ||
377 | Traps related to the use of signals and signal handlers, general subroutines, | |
378 | and sorting, along with sorting subroutines. | |
379 | ||
380 | =item OS Traps | |
381 | ||
382 | OS-specific traps. | |
383 | ||
384 | =item DBM Traps | |
385 | ||
386 | Traps specific to the use of C<dbmopen()>, and specific dbm implementations. | |
387 | ||
388 | =item Unclassified Traps | |
389 | ||
390 | Everything else. | |
391 | ||
392 | =back | |
393 | ||
394 | If you find an example of a conversion trap that is not listed here, | |
9607fc9c | 395 | please submit it to Bill Middleton <F<wjm@best.com>> for inclusion. |
396 | Also note that at least some of these can be caught with B<-w>. | |
6dbacca0 | 397 | |
398 | =head2 Discontinuance, Deprecation, and BugFix traps | |
399 | ||
400 | Anything that has been discontinued, deprecated, or fixed as | |
54310121 | 401 | a bug from perl4. |
a0d0e21e | 402 | |
6dbacca0 | 403 | =over 4 |
404 | ||
54310121 | 405 | =item * Discontinuance |
6dbacca0 | 406 | |
407 | Symbols starting with "_" are no longer forced into package main, except | |
408 | for C<$_> itself (and C<@_>, etc.). | |
409 | ||
410 | package test; | |
411 | $_legacy = 1; | |
cb1a09d0 | 412 | |
6dbacca0 | 413 | package main; |
414 | print "\$_legacy is ",$_legacy,"\n"; | |
54310121 | 415 | |
6dbacca0 | 416 | # perl4 prints: $_legacy is 1 |
417 | # perl5 prints: $_legacy is | |
418 | ||
54310121 | 419 | =item * Deprecation |
6dbacca0 | 420 | |
421 | Double-colon is now a valid package separator in a variable name. Thus these | |
5f05dabc | 422 | behave differently in perl4 vs. perl5, because the packages don't exist. |
6dbacca0 | 423 | |
424 | $a=1;$b=2;$c=3;$var=4; | |
425 | print "$a::$b::$c "; | |
cb1a09d0 | 426 | print "$var::abc::xyz\n"; |
6dbacca0 | 427 | |
428 | # perl4 prints: 1::2::3 4::abc::xyz | |
429 | # perl5 prints: 3 | |
cb1a09d0 | 430 | |
6dbacca0 | 431 | Given that C<::> is now the preferred package delimiter, it is debatable |
432 | whether this should be classed as a bug or not. | |
433 | (The older package delimiter, ' ,is used here) | |
cb1a09d0 | 434 | |
6dbacca0 | 435 | $x = 10 ; |
436 | print "x=${'x}\n" ; | |
54310121 | 437 | |
6dbacca0 | 438 | # perl4 prints: x=10 |
439 | # perl5 prints: Can't find string terminator "'" anywhere before EOF | |
a0d0e21e | 440 | |
5e77893f MG |
441 | You can avoid this problem, and remain compatible with perl4, if you |
442 | always explicitly include the package name: | |
443 | ||
444 | $x = 10 ; | |
445 | print "x=${main'x}\n" ; | |
446 | ||
54310121 | 447 | Also see precedence traps, for parsing C<$:>. |
a0d0e21e | 448 | |
6dbacca0 | 449 | =item * BugFix |
a0d0e21e | 450 | |
6dbacca0 | 451 | The second and third arguments of C<splice()> are now evaluated in scalar |
452 | context (as the Camel says) rather than list context. | |
a0d0e21e | 453 | |
1d2dff63 GS |
454 | sub sub1{return(0,2) } # return a 2-element list |
455 | sub sub2{ return(1,2,3)} # return a 3-element list | |
54310121 | 456 | @a1 = ("a","b","c","d","e"); |
6dbacca0 | 457 | @a2 = splice(@a1,&sub1,&sub2); |
458 | print join(' ',@a2),"\n"; | |
54310121 | 459 | |
6dbacca0 | 460 | # perl4 prints: a b |
54310121 | 461 | # perl5 prints: c d e |
a0d0e21e | 462 | |
54310121 | 463 | =item * Discontinuance |
a0d0e21e | 464 | |
6dbacca0 | 465 | You can't do a C<goto> into a block that is optimized away. Darn. |
a0d0e21e | 466 | |
6dbacca0 | 467 | goto marker1; |
a0d0e21e | 468 | |
54310121 | 469 | for(1){ |
6dbacca0 | 470 | marker1: |
471 | print "Here I is!\n"; | |
54310121 | 472 | } |
473 | ||
6dbacca0 | 474 | # perl4 prints: Here I is! |
475 | # perl5 dumps core (SEGV) | |
476 | ||
54310121 | 477 | =item * Discontinuance |
6dbacca0 | 478 | |
479 | It is no longer syntactically legal to use whitespace as the name | |
480 | of a variable, or as a delimiter for any kind of quote construct. | |
54310121 | 481 | Double darn. |
6dbacca0 | 482 | |
483 | $a = ("foo bar"); | |
484 | $b = q baz ; | |
485 | print "a is $a, b is $b\n"; | |
54310121 | 486 | |
6dbacca0 | 487 | # perl4 prints: a is foo bar, b is baz |
54310121 | 488 | # perl5 errors: Bareword found where operator expected |
5e378fdf | 489 | |
6dbacca0 | 490 | =item * Discontinuance |
491 | ||
492 | The archaic while/if BLOCK BLOCK syntax is no longer supported. | |
493 | ||
494 | if { 1 } { | |
495 | print "True!"; | |
496 | } | |
497 | else { | |
498 | print "False!"; | |
499 | } | |
54310121 | 500 | |
6dbacca0 | 501 | # perl4 prints: True! |
502 | # perl5 errors: syntax error at test.pl line 1, near "if {" | |
503 | ||
504 | =item * BugFix | |
505 | ||
506 | The C<**> operator now binds more tightly than unary minus. | |
507 | It was documented to work this way before, but didn't. | |
508 | ||
509 | print -4**2,"\n"; | |
54310121 | 510 | |
6dbacca0 | 511 | # perl4 prints: 16 |
512 | # perl5 prints: -16 | |
513 | ||
54310121 | 514 | =item * Discontinuance |
6dbacca0 | 515 | |
516 | The meaning of C<foreach{}> has changed slightly when it is iterating over a | |
517 | list which is not an array. This used to assign the list to a | |
518 | temporary array, but no longer does so (for efficiency). This means | |
519 | that you'll now be iterating over the actual values, not over copies of | |
520 | the values. Modifications to the loop variable can change the original | |
521 | values. | |
522 | ||
523 | @list = ('ab','abc','bcd','def'); | |
524 | foreach $var (grep(/ab/,@list)){ | |
525 | $var = 1; | |
526 | } | |
527 | print (join(':',@list)); | |
54310121 | 528 | |
6dbacca0 | 529 | # perl4 prints: ab:abc:bcd:def |
530 | # perl5 prints: 1:1:bcd:def | |
531 | ||
532 | To retain Perl4 semantics you need to assign your list | |
54310121 | 533 | explicitly to a temporary array and then iterate over that. For |
6dbacca0 | 534 | example, you might need to change |
535 | ||
536 | foreach $var (grep(/ab/,@list)){ | |
537 | ||
538 | to | |
539 | ||
540 | foreach $var (@tmp = grep(/ab/,@list)){ | |
541 | ||
542 | Otherwise changing $var will clobber the values of @list. (This most often | |
543 | happens when you use C<$_> for the loop variable, and call subroutines in | |
544 | the loop that don't properly localize C<$_>.) | |
545 | ||
5e378fdf | 546 | =item * Discontinuance |
547 | ||
548 | C<split> with no arguments now behaves like C<split ' '> (which doesn't | |
549 | return an initial null field if $_ starts with whitespace), it used to | |
550 | behave like C<split /\s+/> (which does). | |
551 | ||
552 | $_ = ' hi mom'; | |
553 | print join(':', split); | |
554 | ||
555 | # perl4 prints: :hi:mom | |
556 | # perl5 prints: hi:mom | |
557 | ||
55497cff | 558 | =item * BugFix |
559 | ||
9607fc9c | 560 | Perl 4 would ignore any text which was attached to an B<-e> switch, |
55497cff | 561 | always taking the code snippet from the following arg. Additionally, it |
9607fc9c | 562 | would silently accept an B<-e> switch without a following arg. Both of |
55497cff | 563 | these behaviors have been fixed. |
564 | ||
565 | perl -e'print "attached to -e"' 'print "separate arg"' | |
54310121 | 566 | |
55497cff | 567 | # perl4 prints: separate arg |
568 | # perl5 prints: attached to -e | |
54310121 | 569 | |
55497cff | 570 | perl -e |
571 | ||
572 | # perl4 prints: | |
573 | # perl5 dies: No code specified for -e. | |
574 | ||
575 | =item * Discontinuance | |
576 | ||
577 | In Perl 4 the return value of C<push> was undocumented, but it was | |
578 | actually the last value being pushed onto the target list. In Perl 5 | |
579 | the return value of C<push> is documented, but has changed, it is the | |
580 | number of elements in the resulting list. | |
581 | ||
582 | @x = ('existing'); | |
583 | print push(@x, 'first new', 'second new'); | |
54310121 | 584 | |
55497cff | 585 | # perl4 prints: second new |
586 | # perl5 prints: 3 | |
587 | ||
68dc0745 | 588 | =item * Discontinuance |
589 | ||
590 | In Perl 4 (and versions of Perl 5 before 5.004), C<'\r'> characters in | |
591 | Perl code were silently allowed, although they could cause (mysterious!) | |
592 | failures in certain constructs, particularly here documents. Now, | |
593 | C<'\r'> characters cause an immediate fatal error. (Note: In this | |
594 | example, the notation B<\015> represents the incorrect line | |
595 | ending. Depending upon your text viewer, it will look different.) | |
596 | ||
597 | print "foo";\015 | |
598 | print "bar"; | |
599 | ||
600 | # perl4 prints: foobar | |
601 | # perl5.003 prints: foobar | |
602 | # perl5.004 dies: Illegal character \015 (carriage return) | |
603 | ||
604 | See L<perldiag> for full details. | |
605 | ||
6dbacca0 | 606 | =item * Deprecation |
607 | ||
608 | Some error messages will be different. | |
609 | ||
54310121 | 610 | =item * Discontinuance |
6dbacca0 | 611 | |
612 | Some bugs may have been inadvertently removed. :-) | |
613 | ||
614 | =back | |
615 | ||
616 | =head2 Parsing Traps | |
617 | ||
618 | Perl4-to-Perl5 traps from having to do with parsing. | |
619 | ||
620 | =over 4 | |
621 | ||
622 | =item * Parsing | |
623 | ||
624 | Note the space between . and = | |
625 | ||
626 | $string . = "more string"; | |
627 | print $string; | |
54310121 | 628 | |
6dbacca0 | 629 | # perl4 prints: more string |
630 | # perl5 prints: syntax error at - line 1, near ". =" | |
631 | ||
632 | =item * Parsing | |
633 | ||
634 | Better parsing in perl 5 | |
635 | ||
636 | sub foo {} | |
637 | &foo | |
638 | print("hello, world\n"); | |
54310121 | 639 | |
6dbacca0 | 640 | # perl4 prints: hello, world |
641 | # perl5 prints: syntax error | |
642 | ||
643 | =item * Parsing | |
644 | ||
645 | "if it looks like a function, it is a function" rule. | |
646 | ||
647 | ||
648 | ($foo == 1) ? "is one\n" : "is zero\n"; | |
54310121 | 649 | |
6dbacca0 | 650 | # perl4 prints: is zero |
651 | # perl5 warns: "Useless use of a constant in void context" if using -w | |
652 | ||
c12982c8 GS |
653 | =item * Parsing |
654 | ||
655 | String interpolation of the C<$#array> construct differs when braces | |
656 | are to used around the name. | |
657 | ||
658 | @ = (1..3); | |
659 | print "${#a}"; | |
660 | ||
661 | # perl4 prints: 2 | |
662 | # perl5 fails with syntax error | |
663 | ||
664 | @ = (1..3); | |
665 | print "$#{a}"; | |
666 | ||
667 | # perl4 prints: {a} | |
668 | # perl5 prints: 2 | |
669 | ||
6dbacca0 | 670 | =back |
671 | ||
672 | =head2 Numerical Traps | |
673 | ||
674 | Perl4-to-Perl5 traps having to do with numerical operators, | |
675 | operands, or output from same. | |
676 | ||
677 | =over 5 | |
678 | ||
679 | =item * Numerical | |
680 | ||
681 | Formatted output and significant digits | |
682 | ||
683 | print 7.373504 - 0, "\n"; | |
54310121 | 684 | printf "%20.18f\n", 7.373504 - 0; |
685 | ||
6dbacca0 | 686 | # Perl4 prints: |
687 | 7.375039999999996141 | |
688 | 7.37503999999999614 | |
54310121 | 689 | |
6dbacca0 | 690 | # Perl5 prints: |
691 | 7.373504 | |
692 | 7.37503999999999614 | |
693 | ||
694 | =item * Numerical | |
695 | ||
5f05dabc | 696 | This specific item has been deleted. It demonstrated how the auto-increment |
5e378fdf | 697 | operator would not catch when a number went over the signed int limit. Fixed |
a6006777 | 698 | in version 5.003_04. But always be wary when using large integers. |
699 | If in doubt: | |
6dbacca0 | 700 | |
5e378fdf | 701 | use Math::BigInt; |
6dbacca0 | 702 | |
54310121 | 703 | =item * Numerical |
6dbacca0 | 704 | |
705 | Assignment of return values from numeric equality tests | |
706 | does not work in perl5 when the test evaluates to false (0). | |
707 | Logical tests now return an null, instead of 0 | |
a6006777 | 708 | |
6dbacca0 | 709 | $p = ($test == 1); |
710 | print $p,"\n"; | |
a6006777 | 711 | |
6dbacca0 | 712 | # perl4 prints: 0 |
713 | # perl5 prints: | |
714 | ||
dc848c6f | 715 | Also see L<"General Regular Expression Traps using s///, etc."> |
716 | for another example of this new feature... | |
6dbacca0 | 717 | |
718 | =back | |
719 | ||
720 | =head2 General data type traps | |
721 | ||
722 | Perl4-to-Perl5 traps involving most data-types, and their usage | |
723 | within certain expressions and/or context. | |
724 | ||
725 | =over 5 | |
726 | ||
727 | =item * (Arrays) | |
728 | ||
729 | Negative array subscripts now count from the end of the array. | |
730 | ||
731 | @a = (1, 2, 3, 4, 5); | |
732 | print "The third element of the array is $a[3] also expressed as $a[-2] \n"; | |
54310121 | 733 | |
6dbacca0 | 734 | # perl4 prints: The third element of the array is 4 also expressed as |
735 | # perl5 prints: The third element of the array is 4 also expressed as 4 | |
736 | ||
737 | =item * (Arrays) | |
738 | ||
739 | Setting C<$#array> lower now discards array elements, and makes them | |
740 | impossible to recover. | |
741 | ||
54310121 | 742 | @a = (a,b,c,d,e); |
6dbacca0 | 743 | print "Before: ",join('',@a); |
54310121 | 744 | $#a =1; |
6dbacca0 | 745 | print ", After: ",join('',@a); |
746 | $#a =3; | |
747 | print ", Recovered: ",join('',@a),"\n"; | |
54310121 | 748 | |
6dbacca0 | 749 | # perl4 prints: Before: abcde, After: ab, Recovered: abcd |
750 | # perl5 prints: Before: abcde, After: ab, Recovered: ab | |
751 | ||
752 | =item * (Hashes) | |
753 | ||
754 | Hashes get defined before use | |
755 | ||
54310121 | 756 | local($s,@a,%h); |
6dbacca0 | 757 | die "scalar \$s defined" if defined($s); |
758 | die "array \@a defined" if defined(@a); | |
759 | die "hash \%h defined" if defined(%h); | |
54310121 | 760 | |
6dbacca0 | 761 | # perl4 prints: |
762 | # perl5 dies: hash %h defined | |
763 | ||
764 | =item * (Globs) | |
765 | ||
766 | glob assignment from variable to variable will fail if the assigned | |
767 | variable is localized subsequent to the assignment | |
768 | ||
769 | @a = ("This is Perl 4"); | |
770 | *b = *a; | |
771 | local(@a); | |
772 | print @b,"\n"; | |
54310121 | 773 | |
6dbacca0 | 774 | # perl4 prints: This is Perl 4 |
775 | # perl5 prints: | |
54310121 | 776 | |
a3cb178b | 777 | =item * (Globs) |
54310121 | 778 | |
a3cb178b GS |
779 | Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4 |
780 | it undefines the associated scalar (but may have other side effects | |
781 | including SEGVs). | |
5e378fdf | 782 | |
6dbacca0 | 783 | =item * (Scalar String) |
784 | ||
785 | Changes in unary negation (of strings) | |
786 | This change effects both the return value and what it | |
787 | does to auto(magic)increment. | |
788 | ||
789 | $x = "aaa"; | |
790 | print ++$x," : "; | |
791 | print -$x," : "; | |
792 | print ++$x,"\n"; | |
54310121 | 793 | |
6dbacca0 | 794 | # perl4 prints: aab : -0 : 1 |
795 | # perl5 prints: aab : -aab : aac | |
796 | ||
797 | =item * (Constants) | |
798 | ||
799 | perl 4 lets you modify constants: | |
800 | ||
801 | $foo = "x"; | |
802 | &mod($foo); | |
803 | for ($x = 0; $x < 3; $x++) { | |
804 | &mod("a"); | |
805 | } | |
806 | sub mod { | |
807 | print "before: $_[0]"; | |
808 | $_[0] = "m"; | |
809 | print " after: $_[0]\n"; | |
810 | } | |
54310121 | 811 | |
6dbacca0 | 812 | # perl4: |
813 | # before: x after: m | |
814 | # before: a after: m | |
815 | # before: m after: m | |
816 | # before: m after: m | |
54310121 | 817 | |
6dbacca0 | 818 | # Perl5: |
819 | # before: x after: m | |
820 | # Modification of a read-only value attempted at foo.pl line 12. | |
821 | # before: a | |
822 | ||
823 | =item * (Scalars) | |
824 | ||
825 | The behavior is slightly different for: | |
826 | ||
827 | print "$x", defined $x | |
54310121 | 828 | |
6dbacca0 | 829 | # perl 4: 1 |
830 | # perl 5: <no output, $x is not called into existence> | |
831 | ||
832 | =item * (Variable Suicide) | |
833 | ||
834 | Variable suicide behavior is more consistent under Perl 5. | |
aa689395 | 835 | Perl5 exhibits the same behavior for hashes and scalars, |
5f05dabc | 836 | that perl4 exhibits for only scalars. |
6dbacca0 | 837 | |
838 | $aGlobal{ "aKey" } = "global value"; | |
839 | print "MAIN:", $aGlobal{"aKey"}, "\n"; | |
840 | $GlobalLevel = 0; | |
841 | &test( *aGlobal ); | |
842 | ||
843 | sub test { | |
844 | local( *theArgument ) = @_; | |
845 | local( %aNewLocal ); # perl 4 != 5.001l,m | |
54310121 | 846 | $aNewLocal{"aKey"} = "this should never appear"; |
6dbacca0 | 847 | print "SUB: ", $theArgument{"aKey"}, "\n"; |
848 | $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print | |
849 | $GlobalLevel++; | |
850 | if( $GlobalLevel<4 ) { | |
851 | &test( *aNewLocal ); | |
852 | } | |
853 | } | |
54310121 | 854 | |
6dbacca0 | 855 | # Perl4: |
856 | # MAIN:global value | |
857 | # SUB: global value | |
858 | # SUB: level 0 | |
859 | # SUB: level 1 | |
860 | # SUB: level 2 | |
54310121 | 861 | |
6dbacca0 | 862 | # Perl5: |
863 | # MAIN:global value | |
864 | # SUB: global value | |
865 | # SUB: this should never appear | |
866 | # SUB: this should never appear | |
867 | # SUB: this should never appear | |
868 | ||
84dc3c4d | 869 | =back |
6dbacca0 | 870 | |
871 | =head2 Context Traps - scalar, list contexts | |
872 | ||
873 | =over 5 | |
874 | ||
875 | =item * (list context) | |
876 | ||
877 | The elements of argument lists for formats are now evaluated in list | |
878 | context. This means you can interpolate list values now. | |
879 | ||
880 | @fmt = ("foo","bar","baz"); | |
881 | format STDOUT= | |
882 | @<<<<< @||||| @>>>>> | |
883 | @fmt; | |
884 | . | |
54310121 | 885 | write; |
886 | ||
6dbacca0 | 887 | # perl4 errors: Please use commas to separate fields in file |
888 | # perl5 prints: foo bar baz | |
889 | ||
890 | =item * (scalar context) | |
891 | ||
54310121 | 892 | The C<caller()> function now returns a false value in a scalar context |
893 | if there is no caller. This lets library files determine if they're | |
6dbacca0 | 894 | being required. |
895 | ||
896 | caller() ? (print "You rang?\n") : (print "Got a 0\n"); | |
54310121 | 897 | |
6dbacca0 | 898 | # perl4 errors: There is no caller |
899 | # perl5 prints: Got a 0 | |
5e378fdf | 900 | |
6dbacca0 | 901 | =item * (scalar context) |
902 | ||
903 | The comma operator in a scalar context is now guaranteed to give a | |
904 | scalar context to its arguments. | |
905 | ||
906 | @y= ('a','b','c'); | |
907 | $x = (1, 2, @y); | |
908 | print "x = $x\n"; | |
54310121 | 909 | |
6dbacca0 | 910 | # Perl4 prints: x = c # Thinks list context interpolates list |
911 | # Perl5 prints: x = 3 # Knows scalar uses length of list | |
912 | ||
913 | =item * (list, builtin) | |
914 | ||
915 | C<sprintf()> funkiness (array argument converted to scalar array count) | |
916 | This test could be added to t/op/sprintf.t | |
917 | ||
918 | @z = ('%s%s', 'foo', 'bar'); | |
919 | $x = sprintf(@z); | |
920 | if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";} | |
54310121 | 921 | |
6dbacca0 | 922 | # perl4 prints: ok 2 |
923 | # perl5 prints: not ok 2 | |
924 | ||
925 | C<printf()> works fine, though: | |
926 | ||
927 | printf STDOUT (@z); | |
54310121 | 928 | print "\n"; |
929 | ||
6dbacca0 | 930 | # perl4 prints: foobar |
931 | # perl5 prints: foobar | |
932 | ||
933 | Probably a bug. | |
934 | ||
935 | =back | |
936 | ||
937 | =head2 Precedence Traps | |
938 | ||
939 | Perl4-to-Perl5 traps involving precedence order. | |
940 | ||
f4b17341 GS |
941 | Perl 4 has almost the same precedence rules as Perl 5 for the operators |
942 | that they both have. Perl 4 however, seems to have had some | |
943 | inconsistencies that made the behavior differ from what was documented. | |
944 | ||
84dc3c4d | 945 | =over 5 |
946 | ||
5e378fdf | 947 | =item * Precedence |
948 | ||
8dbef698 JM |
949 | LHS vs. RHS of any assignment operator. LHS is evaluated first |
950 | in perl4, second in perl5; this can affect the relationship | |
951 | between side-effects in sub-expressions. | |
5e378fdf | 952 | |
953 | @arr = ( 'left', 'right' ); | |
954 | $a{shift @arr} = shift @arr; | |
955 | print join( ' ', keys %a ); | |
956 | ||
957 | # perl4 prints: left | |
958 | # perl5 prints: right | |
959 | ||
960 | =item * Precedence | |
6dbacca0 | 961 | |
962 | These are now semantic errors because of precedence: | |
963 | ||
964 | @list = (1,2,3,4,5); | |
965 | %map = ("a",1,"b",2,"c",3,"d",4); | |
966 | $n = shift @list + 2; # first item in list plus 2 | |
967 | print "n is $n, "; | |
968 | $m = keys %map + 2; # number of items in hash plus 2 | |
969 | print "m is $m\n"; | |
54310121 | 970 | |
6dbacca0 | 971 | # perl4 prints: n is 3, m is 6 |
972 | # perl5 errors and fails to compile | |
973 | ||
974 | =item * Precedence | |
a0d0e21e | 975 | |
4633a7c4 LW |
976 | The precedence of assignment operators is now the same as the precedence |
977 | of assignment. Perl 4 mistakenly gave them the precedence of the associated | |
978 | operator. So you now must parenthesize them in expressions like | |
979 | ||
980 | /foo/ ? ($a += 2) : ($a -= 2); | |
a6006777 | 981 | |
4633a7c4 LW |
982 | Otherwise |
983 | ||
6dbacca0 | 984 | /foo/ ? $a += 2 : $a -= 2 |
4633a7c4 LW |
985 | |
986 | would be erroneously parsed as | |
987 | ||
988 | (/foo/ ? $a += 2 : $a) -= 2; | |
989 | ||
990 | On the other hand, | |
991 | ||
54310121 | 992 | $a += /foo/ ? 1 : 2; |
4633a7c4 LW |
993 | |
994 | now works as a C programmer would expect. | |
995 | ||
6dbacca0 | 996 | =item * Precedence |
4633a7c4 | 997 | |
6dbacca0 | 998 | open FOO || die; |
a0d0e21e | 999 | |
5f05dabc | 1000 | is now incorrect. You need parentheses around the filehandle. |
1001 | Otherwise, perl5 leaves the statement as its default precedence: | |
a0d0e21e | 1002 | |
6dbacca0 | 1003 | open(FOO || die); |
54310121 | 1004 | |
6dbacca0 | 1005 | # perl4 opens or dies |
1006 | # perl5 errors: Precedence problem: open FOO should be open(FOO) | |
a0d0e21e | 1007 | |
6dbacca0 | 1008 | =item * Precedence |
a0d0e21e | 1009 | |
6dbacca0 | 1010 | perl4 gives the special variable, C<$:> precedence, where perl5 |
1011 | treats C<$::> as main C<package> | |
a0d0e21e | 1012 | |
6dbacca0 | 1013 | $a = "x"; print "$::a"; |
54310121 | 1014 | |
6dbacca0 | 1015 | # perl 4 prints: -:a |
1016 | # perl 5 prints: x | |
5e378fdf | 1017 | |
6dbacca0 | 1018 | =item * Precedence |
a0d0e21e | 1019 | |
f4b17341 GS |
1020 | perl4 had buggy precedence for the file test operators vis-a-vis |
1021 | the assignment operators. Thus, although the precedence table | |
1022 | for perl4 leads one to believe C<-e $foo .= "q"> should parse as | |
1023 | C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>. | |
1024 | In perl5, the precedence is as documented. | |
54310121 | 1025 | |
1026 | -e $foo .= "q" | |
a0d0e21e | 1027 | |
6dbacca0 | 1028 | # perl4 prints: no output |
1029 | # perl5 prints: Can't modify -e in concatenation | |
a0d0e21e | 1030 | |
f4b17341 GS |
1031 | =item * Precedence |
1032 | ||
1033 | In perl4, keys(), each() and values() were special high-precedence operators | |
1034 | that operated on a single hash, but in perl5, they are regular named unary | |
1035 | operators. As documented, named unary operators have lower precedence | |
1036 | than the arithmetic and concatenation operators C<+ - .>, but the perl4 | |
1037 | variants of these operators actually bind tighter than C<+ - .>. | |
1038 | Thus, for: | |
1039 | ||
1040 | %foo = 1..10; | |
1041 | print keys %foo - 1 | |
1042 | ||
1043 | # perl4 prints: 4 | |
1044 | # perl5 prints: Type of arg 1 to keys must be hash (not subtraction) | |
1045 | ||
1046 | The perl4 behavior was probably more useful, if less consistent. | |
1047 | ||
6dbacca0 | 1048 | =back |
1049 | ||
1050 | =head2 General Regular Expression Traps using s///, etc. | |
1051 | ||
1052 | All types of RE traps. | |
1053 | ||
1054 | =over 5 | |
1055 | ||
1056 | =item * Regular Expression | |
1057 | ||
1058 | C<s'$lhs'$rhs'> now does no interpolation on either side. It used to | |
54310121 | 1059 | interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal |
6dbacca0 | 1060 | '$' in string) |
1061 | ||
1062 | $a=1;$b=2; | |
1063 | $string = '1 2 $a $b'; | |
1064 | $string =~ s'$a'$b'; | |
1065 | print $string,"\n"; | |
54310121 | 1066 | |
6dbacca0 | 1067 | # perl4 prints: $b 2 $a $b |
1068 | # perl5 prints: 1 2 $a $b | |
1069 | ||
1070 | =item * Regular Expression | |
a0d0e21e LW |
1071 | |
1072 | C<m//g> now attaches its state to the searched string rather than the | |
6dbacca0 | 1073 | regular expression. (Once the scope of a block is left for the sub, the |
1074 | state of the searched string is lost) | |
1075 | ||
1076 | $_ = "ababab"; | |
1077 | while(m/ab/g){ | |
1078 | &doit("blah"); | |
1079 | } | |
1080 | sub doit{local($_) = shift; print "Got $_ "} | |
54310121 | 1081 | |
6dbacca0 | 1082 | # perl4 prints: blah blah blah |
1083 | # perl5 prints: infinite loop blah... | |
1084 | ||
1085 | =item * Regular Expression | |
1086 | ||
68dc0745 | 1087 | Currently, if you use the C<m//o> qualifier on a regular expression |
1088 | within an anonymous sub, I<all> closures generated from that anonymous | |
1089 | sub will use the regular expression as it was compiled when it was used | |
1090 | the very first time in any such closure. For instance, if you say | |
1091 | ||
1092 | sub build_match { | |
1093 | my($left,$right) = @_; | |
1094 | return sub { $_[0] =~ /$left stuff $right/o; }; | |
1095 | } | |
1096 | ||
1097 | build_match() will always return a sub which matches the contents of | |
1098 | C<$left> and C<$right> as they were the I<first> time that build_match() | |
1099 | was called, not as they are in the current call. | |
1100 | ||
1101 | This is probably a bug, and may change in future versions of Perl. | |
1102 | ||
1103 | =item * Regular Expression | |
1104 | ||
6dbacca0 | 1105 | If no parentheses are used in a match, Perl4 sets C<$+> to |
1106 | the whole match, just like C<$&>. Perl5 does not. | |
1107 | ||
1108 | "abcdef" =~ /b.*e/; | |
1109 | print "\$+ = $+\n"; | |
54310121 | 1110 | |
6dbacca0 | 1111 | # perl4 prints: bcde |
1112 | # perl5 prints: | |
1113 | ||
1114 | =item * Regular Expression | |
1115 | ||
1116 | substitution now returns the null string if it fails | |
1117 | ||
1118 | $string = "test"; | |
1119 | $value = ($string =~ s/foo//); | |
1120 | print $value, "\n"; | |
54310121 | 1121 | |
6dbacca0 | 1122 | # perl4 prints: 0 |
1123 | # perl5 prints: | |
1124 | ||
1125 | Also see L<Numerical Traps> for another example of this new feature. | |
1126 | ||
1127 | =item * Regular Expression | |
1128 | ||
54310121 | 1129 | C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no |
1130 | backtick expansion | |
6dbacca0 | 1131 | |
1132 | $string = ""; | |
1133 | $string =~ s`^`hostname`; | |
1134 | print $string, "\n"; | |
54310121 | 1135 | |
6dbacca0 | 1136 | # perl4 prints: <the local hostname> |
1137 | # perl5 prints: hostname | |
1138 | ||
1139 | =item * Regular Expression | |
1140 | ||
1141 | Stricter parsing of variables used in regular expressions | |
1142 | ||
1143 | s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o; | |
54310121 | 1144 | |
6dbacca0 | 1145 | # perl4: compiles w/o error |
1146 | # perl5: with Scalar found where operator expected ..., near "$opt$plus" | |
1147 | ||
1148 | an added component of this example, apparently from the same script, is | |
1149 | the actual value of the s'd string after the substitution. | |
1150 | C<[$opt]> is a character class in perl4 and an array subscript in perl5 | |
1151 | ||
54310121 | 1152 | $grpc = 'a'; |
6dbacca0 | 1153 | $opt = 'r'; |
1154 | $_ = 'bar'; | |
1155 | s/^([^$grpc]*$grpc[$opt]?)/foo/; | |
1156 | print ; | |
54310121 | 1157 | |
6dbacca0 | 1158 | # perl4 prints: foo |
1159 | # perl5 prints: foobar | |
1160 | ||
1161 | =item * Regular Expression | |
1162 | ||
1163 | Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched | |
1164 | repeatedly, like C</x/> or C<m!x!>. | |
1165 | ||
1166 | $test = "once"; | |
1167 | sub match { $test =~ m?once?; } | |
1168 | &match(); | |
1169 | if( &match() ) { | |
1170 | # m?x? matches more then once | |
1171 | print "perl4\n"; | |
54310121 | 1172 | } else { |
6dbacca0 | 1173 | # m?x? matches only once |
54310121 | 1174 | print "perl5\n"; |
6dbacca0 | 1175 | } |
54310121 | 1176 | |
6dbacca0 | 1177 | # perl4 prints: perl4 |
1178 | # perl5 prints: perl5 | |
a0d0e21e | 1179 | |
a0d0e21e | 1180 | |
6dbacca0 | 1181 | =back |
1182 | ||
1183 | =head2 Subroutine, Signal, Sorting Traps | |
a0d0e21e | 1184 | |
6dbacca0 | 1185 | The general group of Perl4-to-Perl5 traps having to do with |
1186 | Signals, Sorting, and their related subroutines, as well as | |
1187 | general subroutine traps. Includes some OS-Specific traps. | |
a0d0e21e | 1188 | |
6dbacca0 | 1189 | =over 5 |
a0d0e21e | 1190 | |
6dbacca0 | 1191 | =item * (Signals) |
a0d0e21e | 1192 | |
6dbacca0 | 1193 | Barewords that used to look like strings to Perl will now look like subroutine |
1194 | calls if a subroutine by that name is defined before the compiler sees them. | |
a0d0e21e | 1195 | |
6dbacca0 | 1196 | sub SeeYa { warn"Hasta la vista, baby!" } |
1197 | $SIG{'TERM'} = SeeYa; | |
1198 | print "SIGTERM is now $SIG{'TERM'}\n"; | |
54310121 | 1199 | |
6dbacca0 | 1200 | # perl4 prints: SIGTERM is main'SeeYa |
1201 | # perl5 prints: SIGTERM is now main::1 | |
a0d0e21e | 1202 | |
6dbacca0 | 1203 | Use B<-w> to catch this one |
a0d0e21e | 1204 | |
6dbacca0 | 1205 | =item * (Sort Subroutine) |
a0d0e21e | 1206 | |
6dbacca0 | 1207 | reverse is no longer allowed as the name of a sort subroutine. |
a0d0e21e | 1208 | |
6dbacca0 | 1209 | sub reverse{ print "yup "; $a <=> $b } |
54310121 | 1210 | print sort reverse a,b,c; |
1211 | ||
6dbacca0 | 1212 | # perl4 prints: yup yup yup yup abc |
54310121 | 1213 | # perl5 prints: abc |
a0d0e21e | 1214 | |
b996531f | 1215 | =item * warn() won't let you specify a filehandle. |
1216 | ||
1217 | Although it _always_ printed to STDERR, warn() would let you specify a | |
1218 | filehandle in perl4. With perl5 it does not. | |
5e378fdf | 1219 | |
1220 | warn STDERR "Foo!"; | |
1221 | ||
1222 | # perl4 prints: Foo! | |
54310121 | 1223 | # perl5 prints: String found where operator expected |
5e378fdf | 1224 | |
6dbacca0 | 1225 | =back |
a0d0e21e | 1226 | |
6dbacca0 | 1227 | =head2 OS Traps |
1228 | ||
1229 | =over 5 | |
1230 | ||
1231 | =item * (SysV) | |
1232 | ||
54310121 | 1233 | Under HPUX, and some other SysV OSes, one had to reset any signal handler, |
1234 | within the signal handler function, each time a signal was handled with | |
1235 | perl4. With perl5, the reset is now done correctly. Any code relying | |
6dbacca0 | 1236 | on the handler _not_ being reset will have to be reworked. |
1237 | ||
a6006777 | 1238 | Since version 5.002, Perl uses sigaction() under SysV. |
6dbacca0 | 1239 | |
1240 | sub gotit { | |
54310121 | 1241 | print "Got @_... "; |
1242 | } | |
6dbacca0 | 1243 | $SIG{'INT'} = 'gotit'; |
54310121 | 1244 | |
6dbacca0 | 1245 | $| = 1; |
1246 | $pid = fork; | |
1247 | if ($pid) { | |
1248 | kill('INT', $pid); | |
1249 | sleep(1); | |
1250 | kill('INT', $pid); | |
54310121 | 1251 | } else { |
6dbacca0 | 1252 | while (1) {sleep(10);} |
54310121 | 1253 | } |
1254 | ||
6dbacca0 | 1255 | # perl4 (HPUX) prints: Got INT... |
1256 | # perl5 (HPUX) prints: Got INT... Got INT... | |
1257 | ||
1258 | =item * (SysV) | |
1259 | ||
54310121 | 1260 | Under SysV OSes, C<seek()> on a file opened to append C<E<gt>E<gt>> now does |
1261 | the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened | |
6dbacca0 | 1262 | for append, it is impossible to overwrite information already in |
1263 | the file. | |
1264 | ||
1265 | open(TEST,">>seek.test"); | |
54310121 | 1266 | $start = tell TEST ; |
6dbacca0 | 1267 | foreach(1 .. 9){ |
1268 | print TEST "$_ "; | |
1269 | } | |
1270 | $end = tell TEST ; | |
1271 | seek(TEST,$start,0); | |
1272 | print TEST "18 characters here"; | |
54310121 | 1273 | |
6dbacca0 | 1274 | # perl4 (solaris) seek.test has: 18 characters here |
1275 | # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here | |
a0d0e21e | 1276 | |
a0d0e21e | 1277 | |
a0d0e21e | 1278 | |
6dbacca0 | 1279 | =back |
a0d0e21e | 1280 | |
6dbacca0 | 1281 | =head2 Interpolation Traps |
a0d0e21e | 1282 | |
8b0a4b75 | 1283 | Perl4-to-Perl5 traps having to do with how things get interpolated |
1284 | within certain expressions, statements, contexts, or whatever. | |
1285 | ||
6dbacca0 | 1286 | =over 5 |
a0d0e21e | 1287 | |
6dbacca0 | 1288 | =item * Interpolation |
a0d0e21e | 1289 | |
6dbacca0 | 1290 | @ now always interpolates an array in double-quotish strings. |
1291 | ||
54310121 | 1292 | print "To: someone@somewhere.com\n"; |
1293 | ||
6dbacca0 | 1294 | # perl4 prints: To:someone@somewhere.com |
9607fc9c | 1295 | # perl5 errors : In string, @somewhere now must be written as \@somewhere |
6dbacca0 | 1296 | |
1297 | =item * Interpolation | |
1298 | ||
6dbacca0 | 1299 | Double-quoted strings may no longer end with an unescaped $ or @. |
1300 | ||
1301 | $foo = "foo$"; | |
1302 | $bar = "bar@"; | |
1303 | print "foo is $foo, bar is $bar\n"; | |
54310121 | 1304 | |
6dbacca0 | 1305 | # perl4 prints: foo is foo$, bar is bar@ |
1306 | # perl5 errors: Final $ should be \$ or $name | |
1307 | ||
1308 | Note: perl5 DOES NOT error on the terminating @ in $bar | |
1309 | ||
1310 | =item * Interpolation | |
a0d0e21e | 1311 | |
8b0a4b75 | 1312 | Perl now sometimes evaluates arbitrary expressions inside braces that occur |
1313 | within double quotes (usually when the opening brace is preceded by C<$> | |
1314 | or C<@>). | |
1315 | ||
1316 | @www = "buz"; | |
1317 | $foo = "foo"; | |
1318 | $bar = "bar"; | |
1319 | sub foo { return "bar" }; | |
1320 | print "|@{w.w.w}|${main'foo}|"; | |
1321 | ||
1322 | # perl4 prints: |@{w.w.w}|foo| | |
1323 | # perl5 prints: |buz|bar| | |
1324 | ||
1325 | Note that you can C<use strict;> to ward off such trappiness under perl5. | |
1326 | ||
1327 | =item * Interpolation | |
1328 | ||
748a9306 | 1329 | The construct "this is $$x" used to interpolate the pid at that |
6dbacca0 | 1330 | point, but now apparently tries to dereference C<$x>. C<$$> by itself still |
748a9306 LW |
1331 | works fine, however. |
1332 | ||
6dbacca0 | 1333 | print "this is $$x\n"; |
748a9306 | 1334 | |
6dbacca0 | 1335 | # perl4 prints: this is XXXx (XXX is the current pid) |
1336 | # perl5 prints: this is | |
1337 | ||
1338 | =item * Interpolation | |
1339 | ||
54310121 | 1340 | Creation of hashes on the fly with C<eval "EXPR"> now requires either both |
1341 | C<$>'s to be protected in the specification of the hash name, or both curlies | |
6dbacca0 | 1342 | to be protected. If both curlies are protected, the result will be compatible |
1343 | with perl4 and perl5. This is a very common practice, and should be changed | |
1344 | to use the block form of C<eval{}> if possible. | |
c07a80fd | 1345 | |
6dbacca0 | 1346 | $hashname = "foobar"; |
1347 | $key = "baz"; | |
1348 | $value = 1234; | |
1349 | eval "\$$hashname{'$key'} = q|$value|"; | |
1350 | (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope"); | |
1351 | ||
1352 | # perl4 prints: Yup | |
1353 | # perl5 prints: Nope | |
1354 | ||
1355 | Changing | |
1356 | ||
1357 | eval "\$$hashname{'$key'} = q|$value|"; | |
c07a80fd | 1358 | |
1359 | to | |
1360 | ||
6dbacca0 | 1361 | eval "\$\$hashname{'$key'} = q|$value|"; |
c07a80fd | 1362 | |
6dbacca0 | 1363 | causes the following result: |
c07a80fd | 1364 | |
6dbacca0 | 1365 | # perl4 prints: Nope |
1366 | # perl5 prints: Yup | |
c07a80fd | 1367 | |
6dbacca0 | 1368 | or, changing to |
a0d0e21e | 1369 | |
6dbacca0 | 1370 | eval "\$$hashname\{'$key'\} = q|$value|"; |
1371 | ||
1372 | causes the following result: | |
1373 | ||
1374 | # perl4 prints: Yup | |
1375 | # perl5 prints: Yup | |
1376 | # and is compatible for both versions | |
1377 | ||
1378 | ||
1379 | =item * Interpolation | |
1380 | ||
1381 | perl4 programs which unconsciously rely on the bugs in earlier perl versions. | |
1382 | ||
1383 | perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"' | |
54310121 | 1384 | |
6dbacca0 | 1385 | # perl4 prints: This is not perl5 |
1386 | # perl5 prints: This is perl5 | |
1387 | ||
1388 | =item * Interpolation | |
1389 | ||
54310121 | 1390 | You also have to be careful about array references. |
6dbacca0 | 1391 | |
1392 | print "$foo{" | |
1393 | ||
1394 | perl 4 prints: { | |
1395 | perl 5 prints: syntax error | |
1396 | ||
1397 | =item * Interpolation | |
1398 | ||
1399 | Similarly, watch out for: | |
1400 | ||
1401 | $foo = "array"; | |
1402 | print "\$$foo{bar}\n"; | |
54310121 | 1403 | |
6dbacca0 | 1404 | # perl4 prints: $array{bar} |
1405 | # perl5 prints: $ | |
1406 | ||
1407 | Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is | |
1408 | happy just to expand $foo to "array" by itself. Watch out for this | |
1409 | especially in C<eval>'s. | |
1410 | ||
1411 | =item * Interpolation | |
1412 | ||
1413 | C<qq()> string passed to C<eval> | |
1414 | ||
1415 | eval qq( | |
1416 | foreach \$y (keys %\$x\) { | |
1417 | \$count++; | |
1418 | } | |
1419 | ); | |
54310121 | 1420 | |
6dbacca0 | 1421 | # perl4 runs this ok |
54310121 | 1422 | # perl5 prints: Can't find string terminator ")" |
a0d0e21e | 1423 | |
6dbacca0 | 1424 | =back |
1425 | ||
1426 | =head2 DBM Traps | |
1427 | ||
1428 | General DBM traps. | |
1429 | ||
1430 | =over 5 | |
1431 | ||
1432 | =item * DBM | |
1433 | ||
1434 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) | |
1435 | may cause the same script, run under perl5, to fail. The build of perl5 | |
1436 | must have been linked with the same dbm/ndbm as the default for C<dbmopen()> | |
1437 | to function properly without C<tie>'ing to an extension dbm implementation. | |
1438 | ||
1439 | dbmopen (%dbm, "file", undef); | |
1440 | print "ok\n"; | |
1441 | ||
1442 | # perl4 prints: ok | |
1443 | # perl5 prints: ok (IFF linked with -ldbm or -lndbm) | |
1444 | ||
1445 | ||
1446 | =item * DBM | |
1447 | ||
1448 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) | |
1449 | may cause the same script, run under perl5, to fail. The error generated | |
1450 | when exceeding the limit on the key/value size will cause perl5 to exit | |
1451 | immediately. | |
1452 | ||
1453 | dbmopen(DB, "testdb",0600) || die "couldn't open db! $!"; | |
1454 | $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm | |
1455 | print "YUP\n"; | |
1456 | ||
1457 | # perl4 prints: | |
1458 | dbm store returned -1, errno 28, key "trap" at - line 3. | |
1459 | YUP | |
1460 | ||
1461 | # perl5 prints: | |
1462 | dbm store returned -1, errno 28, key "trap" at - line 3. | |
a0d0e21e LW |
1463 | |
1464 | =back | |
6dbacca0 | 1465 | |
1466 | =head2 Unclassified Traps | |
1467 | ||
1468 | Everything else. | |
1469 | ||
84dc3c4d | 1470 | =over 5 |
1471 | ||
5db417f7 | 1472 | =item * C<require>/C<do> trap using returned value |
6dbacca0 | 1473 | |
1474 | If the file doit.pl has: | |
1475 | ||
1476 | sub foo { | |
1477 | $rc = do "./do.pl"; | |
1478 | return 8; | |
54310121 | 1479 | } |
6dbacca0 | 1480 | print &foo, "\n"; |
1481 | ||
1482 | And the do.pl file has the following single line: | |
1483 | ||
1484 | return 3; | |
1485 | ||
1486 | Running doit.pl gives the following: | |
1487 | ||
1488 | # perl 4 prints: 3 (aborts the subroutine early) | |
54310121 | 1489 | # perl 5 prints: 8 |
6dbacca0 | 1490 | |
1491 | Same behavior if you replace C<do> with C<require>. | |
1492 | ||
5db417f7 TB |
1493 | =item * C<split> on empty string with LIMIT specified |
1494 | ||
1495 | $string = ''; | |
1496 | @list = split(/foo/, $string, 2) | |
1497 | ||
1498 | Perl4 returns a one element list containing the empty string but Perl5 | |
1499 | returns an empty list. | |
1500 | ||
6dbacca0 | 1501 | =back |
1502 | ||
54310121 | 1503 | As always, if any of these are ever officially declared as bugs, |
6dbacca0 | 1504 | they'll be fixed and removed. |
1505 |