Commit | Line | Data |
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1 | =head1 NAME |
2 | |
3 | perltrap - Perl traps for the unwary |
4 | |
5 | =head1 DESCRIPTION |
6 | |
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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 |
9 | runnable under C<use strict>. |
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10 | |
11 | =head2 Awk Traps |
12 | |
13 | Accustomed B<awk> users should take special note of the following: |
14 | |
15 | =over 4 |
16 | |
17 | =item * |
18 | |
19 | The English module, loaded via |
20 | |
21 | use English; |
22 | |
23 | allows you to refer to special variables (like $RS) as |
24 | though they were in B<awk>; see L<perlvar> for details. |
25 | |
26 | =item * |
27 | |
28 | Semicolons are required after all simple statements in Perl (except |
29 | at the end of a block). Newline is not a statement delimiter. |
30 | |
31 | =item * |
32 | |
33 | Curly brackets are required on C<if>s and C<while>s. |
34 | |
35 | =item * |
36 | |
37 | Variables begin with "$" or "@" in Perl. |
38 | |
39 | =item * |
40 | |
41 | Arrays index from 0. Likewise string positions in substr() and |
42 | index(). |
43 | |
44 | =item * |
45 | |
46 | You have to decide whether your array has numeric or string indices. |
47 | |
48 | =item * |
49 | |
50 | Associative array values do not spring into existence upon mere |
51 | reference. |
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 |
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61 | yourself to an array. And the split() operator has different |
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62 | arguments. |
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 | |
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72 | $E<lt>I<digit>E<gt> does not refer to fields--it refers to substrings matched |
73 | by the last match pattern. |
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74 | |
75 | =item * |
76 | |
77 | The print() statement does not add field and record separators unless |
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78 | you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using |
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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 |
104 | null string would render C</pat/ /pat/> unparsable, since the third slash |
105 | would be interpreted as a division operator--the tokener is in fact |
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106 | slightly context sensitive for operators like "/", "?", and "E<gt>". |
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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 $RS to a pattern, only a string. |
137 | |
138 | =item * |
139 | |
140 | When in doubt, run the B<awk> construct through B<a2p> and see what it |
141 | gives you. |
142 | |
143 | =back |
144 | |
145 | =head2 C Traps |
146 | |
147 | Cerebral C programmers should take note of the following: |
148 | |
149 | =over 4 |
150 | |
151 | =item * |
152 | |
153 | Curly brackets are required on C<if>'s and C<while>'s. |
154 | |
155 | =item * |
156 | |
157 | You must use C<elsif> rather than C<else if>. |
158 | |
159 | =item * |
160 | |
161 | The C<break> and C<continue> keywords from C become in |
162 | Perl C<last> and C<next>, respectively. |
163 | Unlike in C, these do I<NOT> work within a C<do { } while> construct. |
164 | |
165 | =item * |
166 | |
167 | There's no switch statement. (But it's easy to build one on the fly.) |
168 | |
169 | =item * |
170 | |
171 | Variables begin with "$" or "@" in Perl. |
172 | |
173 | =item * |
174 | |
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175 | C<printf()> does not implement the "*" format for interpolating |
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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 |
186 | in Perl 5 is the backslash, which creates a reference. |
187 | |
188 | =item * |
189 | |
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190 | C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]> |
191 | ends up in C<$0>. |
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192 | |
193 | =item * |
194 | |
195 | System calls such as link(), unlink(), rename(), etc. return nonzero for |
196 | success, not 0. |
197 | |
198 | =item * |
199 | |
200 | Signal handlers deal with signal names, not numbers. Use C<kill -l> |
201 | to find their names on your system. |
202 | |
203 | =back |
204 | |
205 | =head2 Sed Traps |
206 | |
207 | Seasoned B<sed> programmers should take note of the following: |
208 | |
209 | =over 4 |
210 | |
211 | =item * |
212 | |
213 | Backreferences in substitutions use "$" 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 | |
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234 | The backtick operator does variable interpolation without regard to |
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235 | the presence of single quotes in the command. |
236 | |
237 | =item * |
238 | |
239 | The backtick operator does no translation of the return value, unlike B<csh>. |
240 | |
241 | =item * |
242 | |
243 | Shells (especially B<csh>) do several levels of substitution on each |
244 | command line. Perl does substitution only in certain constructs |
245 | such as double quotes, backticks, angle brackets, and search patterns. |
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 | |
277 | Avoid barewords if you can, especially all lower-case ones. |
278 | You can't tell just by looking at it whether a bareword is |
279 | a function or a string. By using quotes on strings and |
280 | parens on function calls, you won't ever get them confused. |
281 | |
282 | =item * |
283 | |
284 | You cannot discern from mere inspection which built-ins |
285 | are unary operators (like chop() and chdir()) |
286 | and which are list operators (like print() and unlink()). |
287 | (User-defined subroutines can B<only> be list operators, never |
288 | unary ones.) See L<perlop>. |
289 | |
290 | =item * |
291 | |
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292 | People have a hard time remembering that some functions |
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293 | default to $_, or @ARGV, or whatever, but that others which |
294 | you might expect to do not. |
295 | |
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296 | =item * |
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297 | |
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298 | The E<lt>FHE<gt> construct is not the name of the filehandle, it is a readline |
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299 | operation on that handle. The data read is only assigned to $_ if the |
300 | file read is the sole condition in a while loop: |
301 | |
302 | while (<FH>) { } |
303 | while ($_ = <FH>) { }.. |
304 | <FH>; # data discarded! |
305 | |
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306 | =item * |
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307 | |
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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 | |
316 | The C<do {}> construct isn't a real loop that you can use |
317 | loop control on. |
318 | |
319 | =item * |
320 | |
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321 | Use C<my()> for local variables whenever you can get away with |
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322 | it (but see L<perlform> for where you can't). |
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323 | Using C<local()> actually gives a local value to a global |
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324 | variable, which leaves you open to unforeseen side-effects |
325 | of dynamic scoping. |
326 | |
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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 | |
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333 | =back |
334 | |
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335 | =head2 Perl4 to Perl5 Traps |
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336 | |
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337 | Practicing Perl4 Programmers should take note of the following |
338 | Perl4-to-Perl5 specific traps. |
339 | |
340 | They're crudely ordered according to the following list: |
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341 | |
342 | =over 4 |
343 | |
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344 | =item Discontinuance, Deprecation, and BugFix traps |
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345 | |
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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. |
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349 | |
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350 | =item Parsing Traps |
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351 | |
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352 | Traps that appear to stem from the new parser. |
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353 | |
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354 | =item Numerical Traps |
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355 | |
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356 | Traps having to do with numerical or mathematical operators. |
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357 | |
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358 | =item General data type traps |
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359 | |
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360 | Traps involving perl standard data types. |
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361 | |
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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, |
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 C<-w>. |
397 | |
398 | =head2 Discontinuance, Deprecation, and BugFix traps |
399 | |
400 | Anything that has been discontinued, deprecated, or fixed as |
401 | a bug from perl4. |
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402 | |
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403 | =over 4 |
404 | |
405 | =item * Discontinuance |
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 | $_legacy = 1; |
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412 | |
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413 | package main; |
414 | print "\$_legacy is ",$_legacy,"\n"; |
415 | |
416 | # perl4 prints: $_legacy is 1 |
417 | # perl5 prints: $_legacy is |
418 | |
419 | =item * Deprecation |
420 | |
421 | Double-colon is now a valid package separator in a variable name. Thus these |
422 | behave differently in perl4 vs. perl5, since the packages don't exist. |
423 | |
424 | $a=1;$b=2;$c=3;$var=4; |
425 | print "$a::$b::$c "; |
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426 | print "$var::abc::xyz\n"; |
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427 | |
428 | # perl4 prints: 1::2::3 4::abc::xyz |
429 | # perl5 prints: 3 |
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430 | |
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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) |
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434 | |
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435 | $x = 10 ; |
436 | print "x=${'x}\n" ; |
437 | |
438 | # perl4 prints: x=10 |
439 | # perl5 prints: Can't find string terminator "'" anywhere before EOF |
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440 | |
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441 | Also see precedence traps, for parsing C<$:>. |
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442 | |
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443 | =item * BugFix |
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444 | |
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445 | The second and third arguments of C<splice()> are now evaluated in scalar |
446 | context (as the Camel says) rather than list context. |
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447 | |
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448 | sub sub1{return(0,2) } # return a 2-elem array |
449 | sub sub2{ return(1,2,3)} # return a 3-elem array |
450 | @a1 = ("a","b","c","d","e"); |
451 | @a2 = splice(@a1,&sub1,&sub2); |
452 | print join(' ',@a2),"\n"; |
453 | |
454 | # perl4 prints: a b |
455 | # perl5 prints: c d e |
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456 | |
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457 | =item * Discontinuance |
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458 | |
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459 | You can't do a C<goto> into a block that is optimized away. Darn. |
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460 | |
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461 | goto marker1; |
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462 | |
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463 | for(1){ |
464 | marker1: |
465 | print "Here I is!\n"; |
466 | } |
467 | |
468 | # perl4 prints: Here I is! |
469 | # perl5 dumps core (SEGV) |
470 | |
471 | =item * Discontinuance |
472 | |
473 | It is no longer syntactically legal to use whitespace as the name |
474 | of a variable, or as a delimiter for any kind of quote construct. |
475 | Double darn. |
476 | |
477 | $a = ("foo bar"); |
478 | $b = q baz ; |
479 | print "a is $a, b is $b\n"; |
480 | |
481 | # perl4 prints: a is foo bar, b is baz |
482 | # perl5 errors: Bare word found where operator expected |
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483 | |
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484 | =item * Discontinuance |
485 | |
486 | The archaic while/if BLOCK BLOCK syntax is no longer supported. |
487 | |
488 | if { 1 } { |
489 | print "True!"; |
490 | } |
491 | else { |
492 | print "False!"; |
493 | } |
494 | |
495 | # perl4 prints: True! |
496 | # perl5 errors: syntax error at test.pl line 1, near "if {" |
497 | |
498 | =item * BugFix |
499 | |
500 | The C<**> operator now binds more tightly than unary minus. |
501 | It was documented to work this way before, but didn't. |
502 | |
503 | print -4**2,"\n"; |
504 | |
505 | # perl4 prints: 16 |
506 | # perl5 prints: -16 |
507 | |
508 | =item * Discontinuance |
509 | |
510 | The meaning of C<foreach{}> has changed slightly when it is iterating over a |
511 | list which is not an array. This used to assign the list to a |
512 | temporary array, but no longer does so (for efficiency). This means |
513 | that you'll now be iterating over the actual values, not over copies of |
514 | the values. Modifications to the loop variable can change the original |
515 | values. |
516 | |
517 | @list = ('ab','abc','bcd','def'); |
518 | foreach $var (grep(/ab/,@list)){ |
519 | $var = 1; |
520 | } |
521 | print (join(':',@list)); |
522 | |
523 | # perl4 prints: ab:abc:bcd:def |
524 | # perl5 prints: 1:1:bcd:def |
525 | |
526 | To retain Perl4 semantics you need to assign your list |
527 | explicitly to a temporary array and then iterate over that. For |
528 | example, you might need to change |
529 | |
530 | foreach $var (grep(/ab/,@list)){ |
531 | |
532 | to |
533 | |
534 | foreach $var (@tmp = grep(/ab/,@list)){ |
535 | |
536 | Otherwise changing $var will clobber the values of @list. (This most often |
537 | happens when you use C<$_> for the loop variable, and call subroutines in |
538 | the loop that don't properly localize C<$_>.) |
539 | |
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540 | =item * Discontinuance |
541 | |
542 | C<split> with no arguments now behaves like C<split ' '> (which doesn't |
543 | return an initial null field if $_ starts with whitespace), it used to |
544 | behave like C<split /\s+/> (which does). |
545 | |
546 | $_ = ' hi mom'; |
547 | print join(':', split); |
548 | |
549 | # perl4 prints: :hi:mom |
550 | # perl5 prints: hi:mom |
551 | |
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552 | =item * Deprecation |
553 | |
554 | Some error messages will be different. |
555 | |
556 | =item * Discontinuance |
557 | |
558 | Some bugs may have been inadvertently removed. :-) |
559 | |
560 | =back |
561 | |
562 | =head2 Parsing Traps |
563 | |
564 | Perl4-to-Perl5 traps from having to do with parsing. |
565 | |
566 | =over 4 |
567 | |
568 | =item * Parsing |
569 | |
570 | Note the space between . and = |
571 | |
572 | $string . = "more string"; |
573 | print $string; |
574 | |
575 | # perl4 prints: more string |
576 | # perl5 prints: syntax error at - line 1, near ". =" |
577 | |
578 | =item * Parsing |
579 | |
580 | Better parsing in perl 5 |
581 | |
582 | sub foo {} |
583 | &foo |
584 | print("hello, world\n"); |
585 | |
586 | # perl4 prints: hello, world |
587 | # perl5 prints: syntax error |
588 | |
589 | =item * Parsing |
590 | |
591 | "if it looks like a function, it is a function" rule. |
592 | |
593 | print |
594 | ($foo == 1) ? "is one\n" : "is zero\n"; |
595 | |
596 | # perl4 prints: is zero |
597 | # perl5 warns: "Useless use of a constant in void context" if using -w |
598 | |
599 | =back |
600 | |
601 | =head2 Numerical Traps |
602 | |
603 | Perl4-to-Perl5 traps having to do with numerical operators, |
604 | operands, or output from same. |
605 | |
606 | =over 5 |
607 | |
608 | =item * Numerical |
609 | |
610 | Formatted output and significant digits |
611 | |
612 | print 7.373504 - 0, "\n"; |
613 | printf "%20.18f\n", 7.373504 - 0; |
614 | |
615 | # Perl4 prints: |
616 | 7.375039999999996141 |
617 | 7.37503999999999614 |
618 | |
619 | # Perl5 prints: |
620 | 7.373504 |
621 | 7.37503999999999614 |
622 | |
623 | =item * Numerical |
624 | |
5e378fdf |
625 | This specific item has been deleted. It demonstrated how the autoincrement |
626 | operator would not catch when a number went over the signed int limit. Fixed |
627 | in 5.003_04. But always be wary when using large ints. If in doubt: |
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628 | |
5e378fdf |
629 | use Math::BigInt; |
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630 | |
631 | =item * Numerical |
632 | |
633 | Assignment of return values from numeric equality tests |
634 | does not work in perl5 when the test evaluates to false (0). |
635 | Logical tests now return an null, instead of 0 |
636 | |
637 | $p = ($test == 1); |
638 | print $p,"\n"; |
639 | |
640 | # perl4 prints: 0 |
641 | # perl5 prints: |
642 | |
643 | Also see the L<General Regular Expression Traps> tests for another example |
644 | of this new feature... |
645 | |
646 | =back |
647 | |
648 | =head2 General data type traps |
649 | |
650 | Perl4-to-Perl5 traps involving most data-types, and their usage |
651 | within certain expressions and/or context. |
652 | |
653 | =over 5 |
654 | |
655 | =item * (Arrays) |
656 | |
657 | Negative array subscripts now count from the end of the array. |
658 | |
659 | @a = (1, 2, 3, 4, 5); |
660 | print "The third element of the array is $a[3] also expressed as $a[-2] \n"; |
661 | |
662 | # perl4 prints: The third element of the array is 4 also expressed as |
663 | # perl5 prints: The third element of the array is 4 also expressed as 4 |
664 | |
665 | =item * (Arrays) |
666 | |
667 | Setting C<$#array> lower now discards array elements, and makes them |
668 | impossible to recover. |
669 | |
670 | @a = (a,b,c,d,e); |
671 | print "Before: ",join('',@a); |
672 | $#a =1; |
673 | print ", After: ",join('',@a); |
674 | $#a =3; |
675 | print ", Recovered: ",join('',@a),"\n"; |
676 | |
677 | # perl4 prints: Before: abcde, After: ab, Recovered: abcd |
678 | # perl5 prints: Before: abcde, After: ab, Recovered: ab |
679 | |
680 | =item * (Hashes) |
681 | |
682 | Hashes get defined before use |
683 | |
684 | local($s,@a,%h); |
685 | die "scalar \$s defined" if defined($s); |
686 | die "array \@a defined" if defined(@a); |
687 | die "hash \%h defined" if defined(%h); |
688 | |
689 | # perl4 prints: |
690 | # perl5 dies: hash %h defined |
691 | |
692 | =item * (Globs) |
693 | |
694 | glob assignment from variable to variable will fail if the assigned |
695 | variable is localized subsequent to the assignment |
696 | |
697 | @a = ("This is Perl 4"); |
698 | *b = *a; |
699 | local(@a); |
700 | print @b,"\n"; |
701 | |
702 | # perl4 prints: This is Perl 4 |
703 | # perl5 prints: |
704 | |
705 | # Another example |
706 | |
707 | *fred = *barney; # fred is aliased to barney |
708 | @barney = (1, 2, 4); |
709 | # @fred; |
710 | print "@fred"; # should print "1, 2, 4" |
711 | |
712 | # perl4 prints: 1 2 4 |
713 | # perl5 prints: Literal @fred now requires backslash |
5e378fdf |
714 | |
6dbacca0 |
715 | =item * (Scalar String) |
716 | |
717 | Changes in unary negation (of strings) |
718 | This change effects both the return value and what it |
719 | does to auto(magic)increment. |
720 | |
721 | $x = "aaa"; |
722 | print ++$x," : "; |
723 | print -$x," : "; |
724 | print ++$x,"\n"; |
725 | |
726 | # perl4 prints: aab : -0 : 1 |
727 | # perl5 prints: aab : -aab : aac |
728 | |
729 | =item * (Constants) |
730 | |
731 | perl 4 lets you modify constants: |
732 | |
733 | $foo = "x"; |
734 | &mod($foo); |
735 | for ($x = 0; $x < 3; $x++) { |
736 | &mod("a"); |
737 | } |
738 | sub mod { |
739 | print "before: $_[0]"; |
740 | $_[0] = "m"; |
741 | print " after: $_[0]\n"; |
742 | } |
743 | |
744 | # perl4: |
745 | # before: x after: m |
746 | # before: a after: m |
747 | # before: m after: m |
748 | # before: m after: m |
749 | |
750 | # Perl5: |
751 | # before: x after: m |
752 | # Modification of a read-only value attempted at foo.pl line 12. |
753 | # before: a |
754 | |
755 | =item * (Scalars) |
756 | |
757 | The behavior is slightly different for: |
758 | |
759 | print "$x", defined $x |
760 | |
761 | # perl 4: 1 |
762 | # perl 5: <no output, $x is not called into existence> |
763 | |
764 | =item * (Variable Suicide) |
765 | |
766 | Variable suicide behavior is more consistent under Perl 5. |
767 | Perl5 exhibits the same behavior for associative arrays and scalars, |
768 | that perl4 exhibits only for scalars. |
769 | |
770 | $aGlobal{ "aKey" } = "global value"; |
771 | print "MAIN:", $aGlobal{"aKey"}, "\n"; |
772 | $GlobalLevel = 0; |
773 | &test( *aGlobal ); |
774 | |
775 | sub test { |
776 | local( *theArgument ) = @_; |
777 | local( %aNewLocal ); # perl 4 != 5.001l,m |
778 | $aNewLocal{"aKey"} = "this should never appear"; |
779 | print "SUB: ", $theArgument{"aKey"}, "\n"; |
780 | $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print |
781 | $GlobalLevel++; |
782 | if( $GlobalLevel<4 ) { |
783 | &test( *aNewLocal ); |
784 | } |
785 | } |
786 | |
787 | # Perl4: |
788 | # MAIN:global value |
789 | # SUB: global value |
790 | # SUB: level 0 |
791 | # SUB: level 1 |
792 | # SUB: level 2 |
793 | |
794 | # Perl5: |
795 | # MAIN:global value |
796 | # SUB: global value |
797 | # SUB: this should never appear |
798 | # SUB: this should never appear |
799 | # SUB: this should never appear |
800 | |
84dc3c4d |
801 | =back |
6dbacca0 |
802 | |
803 | =head2 Context Traps - scalar, list contexts |
804 | |
805 | =over 5 |
806 | |
807 | =item * (list context) |
808 | |
809 | The elements of argument lists for formats are now evaluated in list |
810 | context. This means you can interpolate list values now. |
811 | |
812 | @fmt = ("foo","bar","baz"); |
813 | format STDOUT= |
814 | @<<<<< @||||| @>>>>> |
815 | @fmt; |
816 | . |
817 | write; |
818 | |
819 | # perl4 errors: Please use commas to separate fields in file |
820 | # perl5 prints: foo bar baz |
821 | |
822 | =item * (scalar context) |
823 | |
824 | The C<caller()> function now returns a false value in a scalar context |
825 | if there is no caller. This lets library files determine if they're |
826 | being required. |
827 | |
828 | caller() ? (print "You rang?\n") : (print "Got a 0\n"); |
829 | |
830 | # perl4 errors: There is no caller |
831 | # perl5 prints: Got a 0 |
5e378fdf |
832 | |
6dbacca0 |
833 | =item * (scalar context) |
834 | |
835 | The comma operator in a scalar context is now guaranteed to give a |
836 | scalar context to its arguments. |
837 | |
838 | @y= ('a','b','c'); |
839 | $x = (1, 2, @y); |
840 | print "x = $x\n"; |
841 | |
842 | # Perl4 prints: x = c # Thinks list context interpolates list |
843 | # Perl5 prints: x = 3 # Knows scalar uses length of list |
844 | |
845 | =item * (list, builtin) |
846 | |
847 | C<sprintf()> funkiness (array argument converted to scalar array count) |
848 | This test could be added to t/op/sprintf.t |
849 | |
850 | @z = ('%s%s', 'foo', 'bar'); |
851 | $x = sprintf(@z); |
852 | if ($x eq 'foobar') {print "ok 2\n";} else {print "not ok 2 '$x'\n";} |
853 | |
854 | # perl4 prints: ok 2 |
855 | # perl5 prints: not ok 2 |
856 | |
857 | C<printf()> works fine, though: |
858 | |
859 | printf STDOUT (@z); |
860 | print "\n"; |
861 | |
862 | # perl4 prints: foobar |
863 | # perl5 prints: foobar |
864 | |
865 | Probably a bug. |
866 | |
867 | =back |
868 | |
869 | =head2 Precedence Traps |
870 | |
871 | Perl4-to-Perl5 traps involving precedence order. |
872 | |
84dc3c4d |
873 | =over 5 |
874 | |
5e378fdf |
875 | =item * Precedence |
876 | |
877 | LHS vs. RHS when both sides are getting an op. |
878 | |
879 | @arr = ( 'left', 'right' ); |
880 | $a{shift @arr} = shift @arr; |
881 | print join( ' ', keys %a ); |
882 | |
883 | # perl4 prints: left |
884 | # perl5 prints: right |
885 | |
886 | =item * Precedence |
6dbacca0 |
887 | |
888 | These are now semantic errors because of precedence: |
889 | |
890 | @list = (1,2,3,4,5); |
891 | %map = ("a",1,"b",2,"c",3,"d",4); |
892 | $n = shift @list + 2; # first item in list plus 2 |
893 | print "n is $n, "; |
894 | $m = keys %map + 2; # number of items in hash plus 2 |
895 | print "m is $m\n"; |
896 | |
897 | # perl4 prints: n is 3, m is 6 |
898 | # perl5 errors and fails to compile |
899 | |
900 | =item * Precedence |
a0d0e21e |
901 | |
4633a7c4 |
902 | The precedence of assignment operators is now the same as the precedence |
903 | of assignment. Perl 4 mistakenly gave them the precedence of the associated |
904 | operator. So you now must parenthesize them in expressions like |
905 | |
906 | /foo/ ? ($a += 2) : ($a -= 2); |
6dbacca0 |
907 | |
4633a7c4 |
908 | Otherwise |
909 | |
6dbacca0 |
910 | /foo/ ? $a += 2 : $a -= 2 |
4633a7c4 |
911 | |
912 | would be erroneously parsed as |
913 | |
914 | (/foo/ ? $a += 2 : $a) -= 2; |
915 | |
916 | On the other hand, |
917 | |
6dbacca0 |
918 | $a += /foo/ ? 1 : 2; |
4633a7c4 |
919 | |
920 | now works as a C programmer would expect. |
921 | |
6dbacca0 |
922 | =item * Precedence |
4633a7c4 |
923 | |
6dbacca0 |
924 | open FOO || die; |
a0d0e21e |
925 | |
6dbacca0 |
926 | is now incorrect. You need parens around the filehandle. |
927 | Otherwise, perl5 leaves the statement as it's default precedence: |
a0d0e21e |
928 | |
6dbacca0 |
929 | open(FOO || die); |
930 | |
931 | # perl4 opens or dies |
932 | # perl5 errors: Precedence problem: open FOO should be open(FOO) |
a0d0e21e |
933 | |
6dbacca0 |
934 | =item * Precedence |
a0d0e21e |
935 | |
6dbacca0 |
936 | perl4 gives the special variable, C<$:> precedence, where perl5 |
937 | treats C<$::> as main C<package> |
a0d0e21e |
938 | |
6dbacca0 |
939 | $a = "x"; print "$::a"; |
940 | |
941 | # perl 4 prints: -:a |
942 | # perl 5 prints: x |
5e378fdf |
943 | |
6dbacca0 |
944 | =item * Precedence |
a0d0e21e |
945 | |
6dbacca0 |
946 | concatenation precedence over filetest operator? |
a0d0e21e |
947 | |
6dbacca0 |
948 | -e $foo .= "q" |
949 | |
950 | # perl4 prints: no output |
951 | # perl5 prints: Can't modify -e in concatenation |
a0d0e21e |
952 | |
6dbacca0 |
953 | =item * Precedence |
a0d0e21e |
954 | |
6dbacca0 |
955 | Assignment to value takes precedence over assignment to key in |
956 | perl5 when using the shift operator on both sides. |
957 | |
958 | @arr = ( 'left', 'right' ); |
959 | $a{shift @arr} = shift @arr; |
960 | print join( ' ', keys %a ); |
961 | |
962 | # perl4 prints: left |
963 | # perl5 prints: right |
964 | |
965 | =back |
966 | |
967 | =head2 General Regular Expression Traps using s///, etc. |
968 | |
969 | All types of RE traps. |
970 | |
971 | =over 5 |
972 | |
973 | =item * Regular Expression |
974 | |
975 | C<s'$lhs'$rhs'> now does no interpolation on either side. It used to |
976 | interpolate C<$lhs> but not C<$rhs>. (And still does not match a literal |
977 | '$' in string) |
978 | |
979 | $a=1;$b=2; |
980 | $string = '1 2 $a $b'; |
981 | $string =~ s'$a'$b'; |
982 | print $string,"\n"; |
983 | |
984 | # perl4 prints: $b 2 $a $b |
985 | # perl5 prints: 1 2 $a $b |
986 | |
987 | =item * Regular Expression |
a0d0e21e |
988 | |
989 | C<m//g> now attaches its state to the searched string rather than the |
6dbacca0 |
990 | regular expression. (Once the scope of a block is left for the sub, the |
991 | state of the searched string is lost) |
992 | |
993 | $_ = "ababab"; |
994 | while(m/ab/g){ |
995 | &doit("blah"); |
996 | } |
997 | sub doit{local($_) = shift; print "Got $_ "} |
998 | |
999 | # perl4 prints: blah blah blah |
1000 | # perl5 prints: infinite loop blah... |
1001 | |
1002 | =item * Regular Expression |
1003 | |
1004 | If no parentheses are used in a match, Perl4 sets C<$+> to |
1005 | the whole match, just like C<$&>. Perl5 does not. |
1006 | |
1007 | "abcdef" =~ /b.*e/; |
1008 | print "\$+ = $+\n"; |
1009 | |
1010 | # perl4 prints: bcde |
1011 | # perl5 prints: |
1012 | |
1013 | =item * Regular Expression |
1014 | |
1015 | substitution now returns the null string if it fails |
1016 | |
1017 | $string = "test"; |
1018 | $value = ($string =~ s/foo//); |
1019 | print $value, "\n"; |
1020 | |
1021 | # perl4 prints: 0 |
1022 | # perl5 prints: |
1023 | |
1024 | Also see L<Numerical Traps> for another example of this new feature. |
1025 | |
1026 | =item * Regular Expression |
1027 | |
1028 | C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no |
1029 | backtick expansion |
1030 | |
1031 | $string = ""; |
1032 | $string =~ s`^`hostname`; |
1033 | print $string, "\n"; |
1034 | |
1035 | # perl4 prints: <the local hostname> |
1036 | # perl5 prints: hostname |
1037 | |
1038 | =item * Regular Expression |
1039 | |
1040 | Stricter parsing of variables used in regular expressions |
1041 | |
1042 | s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o; |
1043 | |
1044 | # perl4: compiles w/o error |
1045 | # perl5: with Scalar found where operator expected ..., near "$opt$plus" |
1046 | |
1047 | an added component of this example, apparently from the same script, is |
1048 | the actual value of the s'd string after the substitution. |
1049 | C<[$opt]> is a character class in perl4 and an array subscript in perl5 |
1050 | |
1051 | $grpc = 'a'; |
1052 | $opt = 'r'; |
1053 | $_ = 'bar'; |
1054 | s/^([^$grpc]*$grpc[$opt]?)/foo/; |
1055 | print ; |
1056 | |
1057 | # perl4 prints: foo |
1058 | # perl5 prints: foobar |
1059 | |
1060 | =item * Regular Expression |
1061 | |
1062 | Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched |
1063 | repeatedly, like C</x/> or C<m!x!>. |
1064 | |
1065 | $test = "once"; |
1066 | sub match { $test =~ m?once?; } |
1067 | &match(); |
1068 | if( &match() ) { |
1069 | # m?x? matches more then once |
1070 | print "perl4\n"; |
1071 | } else { |
1072 | # m?x? matches only once |
1073 | print "perl5\n"; |
1074 | } |
1075 | |
1076 | # perl4 prints: perl4 |
1077 | # perl5 prints: perl5 |
a0d0e21e |
1078 | |
a0d0e21e |
1079 | |
6dbacca0 |
1080 | =back |
1081 | |
1082 | =head2 Subroutine, Signal, Sorting Traps |
a0d0e21e |
1083 | |
6dbacca0 |
1084 | The general group of Perl4-to-Perl5 traps having to do with |
1085 | Signals, Sorting, and their related subroutines, as well as |
1086 | general subroutine traps. Includes some OS-Specific traps. |
a0d0e21e |
1087 | |
6dbacca0 |
1088 | =over 5 |
a0d0e21e |
1089 | |
6dbacca0 |
1090 | =item * (Signals) |
a0d0e21e |
1091 | |
6dbacca0 |
1092 | Barewords that used to look like strings to Perl will now look like subroutine |
1093 | calls if a subroutine by that name is defined before the compiler sees them. |
a0d0e21e |
1094 | |
6dbacca0 |
1095 | sub SeeYa { warn"Hasta la vista, baby!" } |
1096 | $SIG{'TERM'} = SeeYa; |
1097 | print "SIGTERM is now $SIG{'TERM'}\n"; |
1098 | |
1099 | # perl4 prints: SIGTERM is main'SeeYa |
1100 | # perl5 prints: SIGTERM is now main::1 |
a0d0e21e |
1101 | |
6dbacca0 |
1102 | Use B<-w> to catch this one |
a0d0e21e |
1103 | |
6dbacca0 |
1104 | =item * (Sort Subroutine) |
a0d0e21e |
1105 | |
6dbacca0 |
1106 | reverse is no longer allowed as the name of a sort subroutine. |
a0d0e21e |
1107 | |
6dbacca0 |
1108 | sub reverse{ print "yup "; $a <=> $b } |
1109 | print sort reverse a,b,c; |
1110 | |
1111 | # perl4 prints: yup yup yup yup abc |
1112 | # perl5 prints: abc |
a0d0e21e |
1113 | |
5e378fdf |
1114 | =item * warn() specifically implies STDERR |
1115 | |
1116 | warn STDERR "Foo!"; |
1117 | |
1118 | # perl4 prints: Foo! |
1119 | # perl5 prints: String found where operator expected |
1120 | |
6dbacca0 |
1121 | =back |
a0d0e21e |
1122 | |
6dbacca0 |
1123 | =head2 OS Traps |
1124 | |
1125 | =over 5 |
1126 | |
1127 | =item * (SysV) |
1128 | |
1129 | Under HPUX, and some other SysV OS's, one had to reset any signal handler, |
1130 | within the signal handler function, each time a signal was handled with |
1131 | perl4. With perl5, the reset is now done correctly. Any code relying |
1132 | on the handler _not_ being reset will have to be reworked. |
1133 | |
1134 | 5.002 and beyond uses sigaction() under SysV |
1135 | |
1136 | sub gotit { |
1137 | print "Got @_... "; |
1138 | } |
1139 | $SIG{'INT'} = 'gotit'; |
1140 | |
1141 | $| = 1; |
1142 | $pid = fork; |
1143 | if ($pid) { |
1144 | kill('INT', $pid); |
1145 | sleep(1); |
1146 | kill('INT', $pid); |
1147 | } else { |
1148 | while (1) {sleep(10);} |
1149 | } |
1150 | |
1151 | # perl4 (HPUX) prints: Got INT... |
1152 | # perl5 (HPUX) prints: Got INT... Got INT... |
1153 | |
1154 | =item * (SysV) |
1155 | |
1156 | Under SysV OS's, C<seek()> on a file opened to append C<E<gt>E<gt>> now does |
1157 | the right thing w.r.t. the fopen() man page. e.g. - When a file is opened |
1158 | for append, it is impossible to overwrite information already in |
1159 | the file. |
1160 | |
1161 | open(TEST,">>seek.test"); |
1162 | $start = tell TEST ; |
1163 | foreach(1 .. 9){ |
1164 | print TEST "$_ "; |
1165 | } |
1166 | $end = tell TEST ; |
1167 | seek(TEST,$start,0); |
1168 | print TEST "18 characters here"; |
1169 | |
1170 | # perl4 (solaris) seek.test has: 18 characters here |
1171 | # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here |
a0d0e21e |
1172 | |
a0d0e21e |
1173 | |
a0d0e21e |
1174 | |
6dbacca0 |
1175 | =back |
a0d0e21e |
1176 | |
6dbacca0 |
1177 | =head2 Interpolation Traps |
a0d0e21e |
1178 | |
8b0a4b75 |
1179 | Perl4-to-Perl5 traps having to do with how things get interpolated |
1180 | within certain expressions, statements, contexts, or whatever. |
1181 | |
6dbacca0 |
1182 | =over 5 |
a0d0e21e |
1183 | |
6dbacca0 |
1184 | =item * Interpolation |
a0d0e21e |
1185 | |
6dbacca0 |
1186 | @ now always interpolates an array in double-quotish strings. |
1187 | |
1188 | print "To: someone@somewhere.com\n"; |
1189 | |
1190 | # perl4 prints: To:someone@somewhere.com |
1191 | # perl5 errors : Literal @somewhere now requires backslash |
1192 | |
1193 | =item * Interpolation |
1194 | |
6dbacca0 |
1195 | Double-quoted strings may no longer end with an unescaped $ or @. |
1196 | |
1197 | $foo = "foo$"; |
1198 | $bar = "bar@"; |
1199 | print "foo is $foo, bar is $bar\n"; |
1200 | |
1201 | # perl4 prints: foo is foo$, bar is bar@ |
1202 | # perl5 errors: Final $ should be \$ or $name |
1203 | |
1204 | Note: perl5 DOES NOT error on the terminating @ in $bar |
1205 | |
1206 | =item * Interpolation |
a0d0e21e |
1207 | |
8b0a4b75 |
1208 | Perl now sometimes evaluates arbitrary expressions inside braces that occur |
1209 | within double quotes (usually when the opening brace is preceded by C<$> |
1210 | or C<@>). |
1211 | |
1212 | @www = "buz"; |
1213 | $foo = "foo"; |
1214 | $bar = "bar"; |
1215 | sub foo { return "bar" }; |
1216 | print "|@{w.w.w}|${main'foo}|"; |
1217 | |
1218 | # perl4 prints: |@{w.w.w}|foo| |
1219 | # perl5 prints: |buz|bar| |
1220 | |
1221 | Note that you can C<use strict;> to ward off such trappiness under perl5. |
1222 | |
1223 | =item * Interpolation |
1224 | |
748a9306 |
1225 | The construct "this is $$x" used to interpolate the pid at that |
6dbacca0 |
1226 | point, but now apparently tries to dereference C<$x>. C<$$> by itself still |
748a9306 |
1227 | works fine, however. |
1228 | |
6dbacca0 |
1229 | print "this is $$x\n"; |
748a9306 |
1230 | |
6dbacca0 |
1231 | # perl4 prints: this is XXXx (XXX is the current pid) |
1232 | # perl5 prints: this is |
1233 | |
1234 | =item * Interpolation |
1235 | |
1236 | Creation of hashes on the fly with C<eval "EXPR"> now requires either both |
1237 | C<$>'s to be protected in the specification of the hash name, or both curlies |
1238 | to be protected. If both curlies are protected, the result will be compatible |
1239 | with perl4 and perl5. This is a very common practice, and should be changed |
1240 | to use the block form of C<eval{}> if possible. |
c07a80fd |
1241 | |
6dbacca0 |
1242 | $hashname = "foobar"; |
1243 | $key = "baz"; |
1244 | $value = 1234; |
1245 | eval "\$$hashname{'$key'} = q|$value|"; |
1246 | (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope"); |
1247 | |
1248 | # perl4 prints: Yup |
1249 | # perl5 prints: Nope |
1250 | |
1251 | Changing |
1252 | |
1253 | eval "\$$hashname{'$key'} = q|$value|"; |
c07a80fd |
1254 | |
1255 | to |
1256 | |
6dbacca0 |
1257 | eval "\$\$hashname{'$key'} = q|$value|"; |
c07a80fd |
1258 | |
6dbacca0 |
1259 | causes the following result: |
c07a80fd |
1260 | |
6dbacca0 |
1261 | # perl4 prints: Nope |
1262 | # perl5 prints: Yup |
c07a80fd |
1263 | |
6dbacca0 |
1264 | or, changing to |
a0d0e21e |
1265 | |
6dbacca0 |
1266 | eval "\$$hashname\{'$key'\} = q|$value|"; |
1267 | |
1268 | causes the following result: |
1269 | |
1270 | # perl4 prints: Yup |
1271 | # perl5 prints: Yup |
1272 | # and is compatible for both versions |
1273 | |
1274 | |
1275 | =item * Interpolation |
1276 | |
1277 | perl4 programs which unconsciously rely on the bugs in earlier perl versions. |
1278 | |
1279 | perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"' |
1280 | |
1281 | # perl4 prints: This is not perl5 |
1282 | # perl5 prints: This is perl5 |
1283 | |
1284 | =item * Interpolation |
1285 | |
1286 | You also have to be careful about array references. |
1287 | |
1288 | print "$foo{" |
1289 | |
1290 | perl 4 prints: { |
1291 | perl 5 prints: syntax error |
1292 | |
1293 | =item * Interpolation |
1294 | |
1295 | Similarly, watch out for: |
1296 | |
1297 | $foo = "array"; |
1298 | print "\$$foo{bar}\n"; |
1299 | |
1300 | # perl4 prints: $array{bar} |
1301 | # perl5 prints: $ |
1302 | |
1303 | Perl 5 is looking for C<$array{bar}> which doesn't exist, but perl 4 is |
1304 | happy just to expand $foo to "array" by itself. Watch out for this |
1305 | especially in C<eval>'s. |
1306 | |
1307 | =item * Interpolation |
1308 | |
1309 | C<qq()> string passed to C<eval> |
1310 | |
1311 | eval qq( |
1312 | foreach \$y (keys %\$x\) { |
1313 | \$count++; |
1314 | } |
1315 | ); |
1316 | |
1317 | # perl4 runs this ok |
1318 | # perl5 prints: Can't find string terminator ")" |
a0d0e21e |
1319 | |
6dbacca0 |
1320 | =back |
1321 | |
1322 | =head2 DBM Traps |
1323 | |
1324 | General DBM traps. |
1325 | |
1326 | =over 5 |
1327 | |
1328 | =item * DBM |
1329 | |
1330 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) |
1331 | may cause the same script, run under perl5, to fail. The build of perl5 |
1332 | must have been linked with the same dbm/ndbm as the default for C<dbmopen()> |
1333 | to function properly without C<tie>'ing to an extension dbm implementation. |
1334 | |
1335 | dbmopen (%dbm, "file", undef); |
1336 | print "ok\n"; |
1337 | |
1338 | # perl4 prints: ok |
1339 | # perl5 prints: ok (IFF linked with -ldbm or -lndbm) |
1340 | |
1341 | |
1342 | =item * DBM |
1343 | |
1344 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) |
1345 | may cause the same script, run under perl5, to fail. The error generated |
1346 | when exceeding the limit on the key/value size will cause perl5 to exit |
1347 | immediately. |
1348 | |
1349 | dbmopen(DB, "testdb",0600) || die "couldn't open db! $!"; |
1350 | $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm |
1351 | print "YUP\n"; |
1352 | |
1353 | # perl4 prints: |
1354 | dbm store returned -1, errno 28, key "trap" at - line 3. |
1355 | YUP |
1356 | |
1357 | # perl5 prints: |
1358 | dbm store returned -1, errno 28, key "trap" at - line 3. |
a0d0e21e |
1359 | |
1360 | =back |
6dbacca0 |
1361 | |
1362 | =head2 Unclassified Traps |
1363 | |
1364 | Everything else. |
1365 | |
84dc3c4d |
1366 | =over 5 |
1367 | |
6dbacca0 |
1368 | =item * Unclassified |
1369 | |
1370 | C<require>/C<do> trap using returned value |
1371 | |
1372 | If the file doit.pl has: |
1373 | |
1374 | sub foo { |
1375 | $rc = do "./do.pl"; |
1376 | return 8; |
1377 | } |
1378 | print &foo, "\n"; |
1379 | |
1380 | And the do.pl file has the following single line: |
1381 | |
1382 | return 3; |
1383 | |
1384 | Running doit.pl gives the following: |
1385 | |
1386 | # perl 4 prints: 3 (aborts the subroutine early) |
1387 | # perl 5 prints: 8 |
1388 | |
1389 | Same behavior if you replace C<do> with C<require>. |
1390 | |
1391 | =back |
1392 | |
1393 | As always, if any of these are ever officially declared as bugs, |
1394 | they'll be fixed and removed. |
1395 | |