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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 C<use warnings> or use the B<-w> |
8 | switch; see L<perllexwarn> and L<perlrun>. The second biggest trap is not |
9 | making your entire program runnable under C<use strict>. The third biggest |
10 | trap is not reading the list of changes in this version of Perl; see |
11 | L<perldelta>. |
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12 | |
13 | =head2 Awk Traps |
14 | |
15 | Accustomed B<awk> users should take special note of the following: |
16 | |
17 | =over 4 |
18 | |
19 | =item * |
20 | |
21 | The English module, loaded via |
22 | |
23 | use English; |
24 | |
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25 | allows you to refer to special variables (like C<$/>) with names (like |
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26 | $RS), as though they were in B<awk>; see L<perlvar> for details. |
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27 | |
28 | =item * |
29 | |
30 | Semicolons are required after all simple statements in Perl (except |
31 | at the end of a block). Newline is not a statement delimiter. |
32 | |
33 | =item * |
34 | |
35 | Curly brackets are required on C<if>s and C<while>s. |
36 | |
37 | =item * |
38 | |
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39 | Variables begin with "$", "@" or "%" in Perl. |
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40 | |
41 | =item * |
42 | |
43 | Arrays index from 0. Likewise string positions in substr() and |
44 | index(). |
45 | |
46 | =item * |
47 | |
48 | You have to decide whether your array has numeric or string indices. |
49 | |
50 | =item * |
51 | |
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52 | Hash values do not spring into existence upon mere reference. |
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53 | |
54 | =item * |
55 | |
56 | You have to decide whether you want to use string or numeric |
57 | comparisons. |
58 | |
59 | =item * |
60 | |
61 | Reading an input line does not split it for you. You get to split it |
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62 | to an array yourself. And the split() operator has different |
63 | arguments than B<awk>'s. |
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64 | |
65 | =item * |
66 | |
67 | The current input line is normally in $_, not $0. It generally does |
68 | not have the newline stripped. ($0 is the name of the program |
69 | executed.) See L<perlvar>. |
70 | |
71 | =item * |
72 | |
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73 | $<I<digit>> does not refer to fields--it refers to substrings matched |
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74 | by the last match pattern. |
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75 | |
76 | =item * |
77 | |
78 | The print() statement does not add field and record separators unless |
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79 | you set C<$,> and C<$\>. You can set $OFS and $ORS if you're using |
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80 | the English module. |
81 | |
82 | =item * |
83 | |
84 | You must open your files before you print to them. |
85 | |
86 | =item * |
87 | |
88 | The range operator is "..", not comma. The comma operator works as in |
89 | C. |
90 | |
91 | =item * |
92 | |
93 | The match operator is "=~", not "~". ("~" is the one's complement |
94 | operator, as in C.) |
95 | |
96 | =item * |
97 | |
98 | The exponentiation operator is "**", not "^". "^" is the XOR |
99 | operator, as in C. (You know, one could get the feeling that B<awk> is |
100 | basically incompatible with C.) |
101 | |
102 | =item * |
103 | |
104 | The concatenation operator is ".", not the null string. (Using the |
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105 | null string would render C</pat/ /pat/> unparsable, because the third slash |
106 | would be interpreted as a division operator--the tokenizer is in fact |
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107 | slightly context sensitive for operators like "/", "?", and ">". |
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108 | And in fact, "." itself can be the beginning of a number.) |
109 | |
110 | =item * |
111 | |
112 | The C<next>, C<exit>, and C<continue> keywords work differently. |
113 | |
114 | =item * |
115 | |
116 | |
117 | The following variables work differently: |
118 | |
119 | Awk Perl |
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120 | ARGC scalar @ARGV (compare with $#ARGV) |
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121 | ARGV[0] $0 |
122 | FILENAME $ARGV |
123 | FNR $. - something |
124 | FS (whatever you like) |
125 | NF $#Fld, or some such |
126 | NR $. |
127 | OFMT $# |
128 | OFS $, |
129 | ORS $\ |
130 | RLENGTH length($&) |
131 | RS $/ |
132 | RSTART length($`) |
133 | SUBSEP $; |
134 | |
135 | =item * |
136 | |
137 | You cannot set $RS to a pattern, only a string. |
138 | |
139 | =item * |
140 | |
141 | When in doubt, run the B<awk> construct through B<a2p> and see what it |
142 | gives you. |
143 | |
144 | =back |
145 | |
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146 | =head2 C/C++ Traps |
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147 | |
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148 | Cerebral C and C++ programmers should take note of the following: |
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149 | |
150 | =over 4 |
151 | |
152 | =item * |
153 | |
154 | Curly brackets are required on C<if>'s and C<while>'s. |
155 | |
156 | =item * |
157 | |
158 | You must use C<elsif> rather than C<else if>. |
159 | |
160 | =item * |
161 | |
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162 | The C<break> and C<continue> keywords from C become in Perl C<last> |
163 | and C<next>, respectively. Unlike in C, these do I<not> work within a |
164 | C<do { } while> construct. See L<perlsyn/"Loop Control">. |
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165 | |
166 | =item * |
167 | |
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168 | There's no switch statement. (But it's easy to build one on the fly, |
169 | see L<perlsyn/"Basic BLOCKs and Switch Statements">) |
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170 | |
171 | =item * |
172 | |
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173 | Variables begin with "$", "@" or "%" in Perl. |
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174 | |
175 | =item * |
176 | |
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177 | Comments begin with "#", not "/*". |
178 | |
179 | =item * |
180 | |
181 | You can't take the address of anything, although a similar operator |
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182 | in Perl is the backslash, which creates a reference. |
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183 | |
184 | =item * |
185 | |
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186 | C<ARGV> must be capitalized. C<$ARGV[0]> is C's C<argv[1]>, and C<argv[0]> |
187 | ends up in C<$0>. |
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188 | |
189 | =item * |
190 | |
191 | System calls such as link(), unlink(), rename(), etc. return nonzero for |
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192 | success, not 0. (system(), however, returns zero for success.) |
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193 | |
194 | =item * |
195 | |
196 | Signal handlers deal with signal names, not numbers. Use C<kill -l> |
197 | to find their names on your system. |
198 | |
199 | =back |
200 | |
201 | =head2 Sed Traps |
202 | |
203 | Seasoned B<sed> programmers should take note of the following: |
204 | |
205 | =over 4 |
206 | |
207 | =item * |
208 | |
209 | Backreferences in substitutions use "$" rather than "\". |
210 | |
211 | =item * |
212 | |
213 | The pattern matching metacharacters "(", ")", and "|" do not have backslashes |
214 | in front. |
215 | |
216 | =item * |
217 | |
218 | The range operator is C<...>, rather than comma. |
219 | |
220 | =back |
221 | |
222 | =head2 Shell Traps |
223 | |
224 | Sharp shell programmers should take note of the following: |
225 | |
226 | =over 4 |
227 | |
228 | =item * |
229 | |
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230 | The backtick operator does variable interpolation without regard to |
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231 | the presence of single quotes in the command. |
232 | |
233 | =item * |
234 | |
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235 | The backtick operator does no translation of the return value, unlike B<csh>. |
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236 | |
237 | =item * |
238 | |
239 | Shells (especially B<csh>) do several levels of substitution on each |
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240 | command line. Perl does substitution in only certain constructs |
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241 | such as double quotes, backticks, angle brackets, and search patterns. |
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242 | |
243 | =item * |
244 | |
245 | Shells interpret scripts a little bit at a time. Perl compiles the |
246 | entire program before executing it (except for C<BEGIN> blocks, which |
247 | execute at compile time). |
248 | |
249 | =item * |
250 | |
251 | The arguments are available via @ARGV, not $1, $2, etc. |
252 | |
253 | =item * |
254 | |
255 | The environment is not automatically made available as separate scalar |
256 | variables. |
257 | |
258 | =back |
259 | |
260 | =head2 Perl Traps |
261 | |
262 | Practicing Perl Programmers should take note of the following: |
263 | |
264 | =over 4 |
265 | |
266 | =item * |
267 | |
268 | Remember that many operations behave differently in a list |
269 | context than they do in a scalar one. See L<perldata> for details. |
270 | |
271 | =item * |
272 | |
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273 | Avoid barewords if you can, especially all lowercase ones. |
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274 | You can't tell by just looking at it whether a bareword is |
275 | a function or a string. By using quotes on strings and |
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276 | parentheses on function calls, you won't ever get them confused. |
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277 | |
278 | =item * |
279 | |
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280 | You cannot discern from mere inspection which builtins |
281 | are unary operators (like chop() and chdir()) |
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282 | and which are list operators (like print() and unlink()). |
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283 | (Unless prototyped, user-defined subroutines can B<only> be list |
284 | operators, never unary ones.) See L<perlop> and L<perlsub>. |
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285 | |
286 | =item * |
287 | |
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288 | People have a hard time remembering that some functions |
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289 | default to $_, or @ARGV, or whatever, but that others which |
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290 | you might expect to do not. |
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291 | |
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292 | =item * |
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293 | |
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294 | The <FH> construct is not the name of the filehandle, it is a readline |
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295 | operation on that handle. The data read is assigned to $_ only if the |
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296 | file read is the sole condition in a while loop: |
297 | |
298 | while (<FH>) { } |
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299 | while (defined($_ = <FH>)) { }.. |
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300 | <FH>; # data discarded! |
301 | |
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302 | =item * |
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303 | |
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304 | Remember not to use C<=> when you need C<=~>; |
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305 | these two constructs are quite different: |
306 | |
307 | $x = /foo/; |
308 | $x =~ /foo/; |
309 | |
310 | =item * |
311 | |
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312 | The C<do {}> construct isn't a real loop that you can use |
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313 | loop control on. |
314 | |
315 | =item * |
316 | |
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317 | Use C<my()> for local variables whenever you can get away with |
318 | it (but see L<perlform> for where you can't). |
319 | Using C<local()> actually gives a local value to a global |
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320 | variable, which leaves you open to unforeseen side-effects |
321 | of dynamic scoping. |
322 | |
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323 | =item * |
324 | |
325 | If you localize an exported variable in a module, its exported value will |
326 | not change. The local name becomes an alias to a new value but the |
327 | external name is still an alias for the original. |
328 | |
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329 | =back |
330 | |
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331 | =head2 Perl4 to Perl5 Traps |
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332 | |
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333 | Practicing Perl4 Programmers should take note of the following |
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334 | Perl4-to-Perl5 specific traps. |
335 | |
336 | They're crudely ordered according to the following list: |
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337 | |
338 | =over 4 |
339 | |
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340 | =item Discontinuance, Deprecation, and BugFix traps |
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341 | |
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342 | Anything that's been fixed as a perl4 bug, removed as a perl4 feature |
343 | or deprecated as a perl4 feature with the intent to encourage usage of |
344 | some other perl5 feature. |
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345 | |
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346 | =item Parsing Traps |
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347 | |
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348 | Traps that appear to stem from the new parser. |
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349 | |
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350 | =item Numerical Traps |
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351 | |
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352 | Traps having to do with numerical or mathematical operators. |
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353 | |
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354 | =item General data type traps |
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355 | |
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356 | Traps involving perl standard data types. |
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357 | |
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358 | =item Context Traps - scalar, list contexts |
359 | |
360 | Traps related to context within lists, scalar statements/declarations. |
361 | |
362 | =item Precedence Traps |
363 | |
364 | Traps related to the precedence of parsing, evaluation, and execution of |
365 | code. |
366 | |
367 | =item General Regular Expression Traps using s///, etc. |
368 | |
369 | Traps related to the use of pattern matching. |
370 | |
371 | =item Subroutine, Signal, Sorting Traps |
372 | |
373 | Traps related to the use of signals and signal handlers, general subroutines, |
374 | and sorting, along with sorting subroutines. |
375 | |
376 | =item OS Traps |
377 | |
378 | OS-specific traps. |
379 | |
380 | =item DBM Traps |
381 | |
382 | Traps specific to the use of C<dbmopen()>, and specific dbm implementations. |
383 | |
384 | =item Unclassified Traps |
385 | |
386 | Everything else. |
387 | |
388 | =back |
389 | |
390 | If you find an example of a conversion trap that is not listed here, |
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391 | please submit it to <F<perlbug@perl.org>> for inclusion. |
9f1b1f2d |
392 | Also note that at least some of these can be caught with the |
393 | C<use warnings> pragma or the B<-w> switch. |
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394 | |
395 | =head2 Discontinuance, Deprecation, and BugFix traps |
396 | |
397 | Anything that has been discontinued, deprecated, or fixed as |
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398 | a bug from perl4. |
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399 | |
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400 | =over 4 |
401 | |
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402 | =item * Discontinuance |
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403 | |
404 | Symbols starting with "_" are no longer forced into package main, except |
405 | for C<$_> itself (and C<@_>, etc.). |
406 | |
407 | package test; |
408 | $_legacy = 1; |
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409 | |
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410 | package main; |
411 | print "\$_legacy is ",$_legacy,"\n"; |
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412 | |
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413 | # perl4 prints: $_legacy is 1 |
414 | # perl5 prints: $_legacy is |
415 | |
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416 | =item * Deprecation |
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417 | |
418 | Double-colon is now a valid package separator in a variable name. Thus these |
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419 | behave differently in perl4 vs. perl5, because the packages don't exist. |
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420 | |
421 | $a=1;$b=2;$c=3;$var=4; |
422 | print "$a::$b::$c "; |
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423 | print "$var::abc::xyz\n"; |
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424 | |
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425 | # perl4 prints: 1::2::3 4::abc::xyz |
426 | # perl5 prints: 3 |
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427 | |
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428 | Given that C<::> is now the preferred package delimiter, it is debatable |
429 | whether this should be classed as a bug or not. |
430 | (The older package delimiter, ' ,is used here) |
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431 | |
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432 | $x = 10 ; |
433 | print "x=${'x}\n" ; |
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434 | |
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435 | # perl4 prints: x=10 |
436 | # perl5 prints: Can't find string terminator "'" anywhere before EOF |
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437 | |
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438 | You can avoid this problem, and remain compatible with perl4, if you |
439 | always explicitly include the package name: |
440 | |
441 | $x = 10 ; |
442 | print "x=${main'x}\n" ; |
443 | |
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444 | Also see precedence traps, for parsing C<$:>. |
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445 | |
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446 | =item * BugFix |
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447 | |
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448 | The second and third arguments of C<splice()> are now evaluated in scalar |
449 | context (as the Camel says) rather than list context. |
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450 | |
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451 | sub sub1{return(0,2) } # return a 2-element list |
452 | sub sub2{ return(1,2,3)} # return a 3-element list |
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453 | @a1 = ("a","b","c","d","e"); |
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454 | @a2 = splice(@a1,&sub1,&sub2); |
455 | print join(' ',@a2),"\n"; |
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456 | |
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457 | # perl4 prints: a b |
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458 | # perl5 prints: c d e |
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459 | |
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460 | =item * Discontinuance |
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461 | |
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462 | You can't do a C<goto> into a block that is optimized away. Darn. |
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463 | |
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464 | goto marker1; |
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465 | |
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466 | for(1){ |
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467 | marker1: |
468 | print "Here I is!\n"; |
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469 | } |
470 | |
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471 | # perl4 prints: Here I is! |
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472 | # perl5 errors: Can't "goto" into the middle of a foreach loop |
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473 | |
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474 | =item * Discontinuance |
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475 | |
476 | It is no longer syntactically legal to use whitespace as the name |
477 | of a variable, or as a delimiter for any kind of quote construct. |
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478 | Double darn. |
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479 | |
480 | $a = ("foo bar"); |
481 | $b = q baz ; |
482 | print "a is $a, b is $b\n"; |
54310121 |
483 | |
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484 | # perl4 prints: a is foo bar, b is baz |
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485 | # perl5 errors: Bareword found where operator expected |
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486 | |
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487 | =item * Discontinuance |
488 | |
489 | The archaic while/if BLOCK BLOCK syntax is no longer supported. |
490 | |
491 | if { 1 } { |
492 | print "True!"; |
493 | } |
494 | else { |
495 | print "False!"; |
496 | } |
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497 | |
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498 | # perl4 prints: True! |
499 | # perl5 errors: syntax error at test.pl line 1, near "if {" |
500 | |
501 | =item * BugFix |
502 | |
503 | The C<**> operator now binds more tightly than unary minus. |
504 | It was documented to work this way before, but didn't. |
505 | |
506 | print -4**2,"\n"; |
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507 | |
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508 | # perl4 prints: 16 |
509 | # perl5 prints: -16 |
510 | |
54310121 |
511 | =item * Discontinuance |
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512 | |
513 | The meaning of C<foreach{}> has changed slightly when it is iterating over a |
514 | list which is not an array. This used to assign the list to a |
515 | temporary array, but no longer does so (for efficiency). This means |
516 | that you'll now be iterating over the actual values, not over copies of |
517 | the values. Modifications to the loop variable can change the original |
518 | values. |
519 | |
520 | @list = ('ab','abc','bcd','def'); |
521 | foreach $var (grep(/ab/,@list)){ |
522 | $var = 1; |
523 | } |
524 | print (join(':',@list)); |
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525 | |
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526 | # perl4 prints: ab:abc:bcd:def |
527 | # perl5 prints: 1:1:bcd:def |
528 | |
529 | To retain Perl4 semantics you need to assign your list |
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530 | explicitly to a temporary array and then iterate over that. For |
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531 | example, you might need to change |
532 | |
533 | foreach $var (grep(/ab/,@list)){ |
534 | |
535 | to |
536 | |
537 | foreach $var (@tmp = grep(/ab/,@list)){ |
538 | |
539 | Otherwise changing $var will clobber the values of @list. (This most often |
540 | happens when you use C<$_> for the loop variable, and call subroutines in |
541 | the loop that don't properly localize C<$_>.) |
542 | |
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543 | =item * Discontinuance |
544 | |
545 | C<split> with no arguments now behaves like C<split ' '> (which doesn't |
546 | return an initial null field if $_ starts with whitespace), it used to |
547 | behave like C<split /\s+/> (which does). |
548 | |
549 | $_ = ' hi mom'; |
550 | print join(':', split); |
551 | |
552 | # perl4 prints: :hi:mom |
553 | # perl5 prints: hi:mom |
554 | |
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555 | =item * BugFix |
556 | |
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557 | Perl 4 would ignore any text which was attached to an B<-e> switch, |
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558 | always taking the code snippet from the following arg. Additionally, it |
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559 | would silently accept an B<-e> switch without a following arg. Both of |
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560 | these behaviors have been fixed. |
561 | |
562 | perl -e'print "attached to -e"' 'print "separate arg"' |
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563 | |
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564 | # perl4 prints: separate arg |
565 | # perl5 prints: attached to -e |
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566 | |
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567 | perl -e |
568 | |
569 | # perl4 prints: |
570 | # perl5 dies: No code specified for -e. |
571 | |
572 | =item * Discontinuance |
573 | |
574 | In Perl 4 the return value of C<push> was undocumented, but it was |
575 | actually the last value being pushed onto the target list. In Perl 5 |
576 | the return value of C<push> is documented, but has changed, it is the |
577 | number of elements in the resulting list. |
578 | |
579 | @x = ('existing'); |
580 | print push(@x, 'first new', 'second new'); |
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581 | |
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582 | # perl4 prints: second new |
583 | # perl5 prints: 3 |
584 | |
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585 | =item * Deprecation |
586 | |
587 | Some error messages will be different. |
588 | |
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589 | =item * Discontinuance |
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590 | |
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591 | In Perl 4, if in list context the delimiters to the first argument of |
592 | C<split()> were C<??>, the result would be placed in C<@_> as well as |
593 | being returned. Perl 5 has more respect for your subroutine arguments. |
594 | |
595 | =item * Discontinuance |
596 | |
6dbacca0 |
597 | Some bugs may have been inadvertently removed. :-) |
598 | |
599 | =back |
600 | |
601 | =head2 Parsing Traps |
602 | |
603 | Perl4-to-Perl5 traps from having to do with parsing. |
604 | |
605 | =over 4 |
606 | |
607 | =item * Parsing |
608 | |
609 | Note the space between . and = |
610 | |
611 | $string . = "more string"; |
612 | print $string; |
54310121 |
613 | |
6dbacca0 |
614 | # perl4 prints: more string |
615 | # perl5 prints: syntax error at - line 1, near ". =" |
616 | |
617 | =item * Parsing |
618 | |
619 | Better parsing in perl 5 |
620 | |
621 | sub foo {} |
622 | &foo |
623 | print("hello, world\n"); |
54310121 |
624 | |
6dbacca0 |
625 | # perl4 prints: hello, world |
626 | # perl5 prints: syntax error |
627 | |
628 | =item * Parsing |
629 | |
630 | "if it looks like a function, it is a function" rule. |
631 | |
632 | print |
633 | ($foo == 1) ? "is one\n" : "is zero\n"; |
54310121 |
634 | |
6dbacca0 |
635 | # perl4 prints: is zero |
636 | # perl5 warns: "Useless use of a constant in void context" if using -w |
637 | |
c12982c8 |
638 | =item * Parsing |
639 | |
640 | String interpolation of the C<$#array> construct differs when braces |
641 | are to used around the name. |
642 | |
9fda99eb |
643 | @a = (1..3); |
c12982c8 |
644 | print "${#a}"; |
645 | |
646 | # perl4 prints: 2 |
647 | # perl5 fails with syntax error |
648 | |
649 | @ = (1..3); |
650 | print "$#{a}"; |
651 | |
652 | # perl4 prints: {a} |
653 | # perl5 prints: 2 |
654 | |
bf1f8817 |
655 | =item * Parsing |
656 | |
657 | When perl sees C<map {> (or C<grep {>), it has to guess whether the C<{> |
658 | starts a BLOCK or a hash reference. If it guesses wrong, it will report |
659 | a syntax error near the C<}> and the missing (or unexpected) comma. |
660 | |
661 | Use unary C<+> before C<{> on a hash reference, and unary C<+> applied |
662 | to the first thing in a BLOCK (after C<{>), for perl to guess right all |
663 | the time. (See L<perlfunc/map>.) |
664 | |
6dbacca0 |
665 | =back |
666 | |
667 | =head2 Numerical Traps |
668 | |
669 | Perl4-to-Perl5 traps having to do with numerical operators, |
670 | operands, or output from same. |
671 | |
672 | =over 5 |
673 | |
674 | =item * Numerical |
675 | |
a9709c40 |
676 | Formatted output and significant digits. In general, Perl 5 |
677 | tries to be more precise. For example, on a Solaris Sparc: |
6dbacca0 |
678 | |
679 | print 7.373504 - 0, "\n"; |
54310121 |
680 | printf "%20.18f\n", 7.373504 - 0; |
681 | |
6dbacca0 |
682 | # Perl4 prints: |
a9709c40 |
683 | 7.3750399999999996141 |
684 | 7.375039999999999614 |
54310121 |
685 | |
6dbacca0 |
686 | # Perl5 prints: |
687 | 7.373504 |
a9709c40 |
688 | 7.375039999999999614 |
689 | |
690 | Notice how the first result looks better in Perl 5. |
691 | |
692 | Your results may vary, since your floating point formatting routines |
693 | and even floating point format may be slightly different. |
6dbacca0 |
694 | |
695 | =item * Numerical |
696 | |
5f05dabc |
697 | This specific item has been deleted. It demonstrated how the auto-increment |
5e378fdf |
698 | operator would not catch when a number went over the signed int limit. Fixed |
a6006777 |
699 | in version 5.003_04. But always be wary when using large integers. |
700 | If in doubt: |
6dbacca0 |
701 | |
5e378fdf |
702 | use Math::BigInt; |
6dbacca0 |
703 | |
54310121 |
704 | =item * Numerical |
6dbacca0 |
705 | |
706 | Assignment of return values from numeric equality tests |
707 | does not work in perl5 when the test evaluates to false (0). |
d1be9408 |
708 | Logical tests now return a null, instead of 0 |
a6006777 |
709 | |
6dbacca0 |
710 | $p = ($test == 1); |
711 | print $p,"\n"; |
a6006777 |
712 | |
6dbacca0 |
713 | # perl4 prints: 0 |
714 | # perl5 prints: |
715 | |
dc848c6f |
716 | Also see L<"General Regular Expression Traps using s///, etc."> |
717 | for another example of this new feature... |
6dbacca0 |
718 | |
651ad3b1 |
719 | =item * Bitwise string ops |
720 | |
721 | When bitwise operators which can operate upon either numbers or |
722 | strings (C<& | ^ ~>) are given only strings as arguments, perl4 would |
723 | treat the operands as bitstrings so long as the program contained a call |
724 | to the C<vec()> function. perl5 treats the string operands as bitstrings. |
725 | (See L<perlop/Bitwise String Operators> for more details.) |
726 | |
727 | $fred = "10"; |
728 | $barney = "12"; |
729 | $betty = $fred & $barney; |
730 | print "$betty\n"; |
731 | # Uncomment the next line to change perl4's behavior |
732 | # ($dummy) = vec("dummy", 0, 0); |
733 | |
734 | # Perl4 prints: |
735 | 8 |
736 | |
737 | # Perl5 prints: |
738 | 10 |
739 | |
740 | # If vec() is used anywhere in the program, both print: |
741 | 10 |
742 | |
6dbacca0 |
743 | =back |
744 | |
745 | =head2 General data type traps |
746 | |
747 | Perl4-to-Perl5 traps involving most data-types, and their usage |
748 | within certain expressions and/or context. |
749 | |
750 | =over 5 |
751 | |
752 | =item * (Arrays) |
753 | |
754 | Negative array subscripts now count from the end of the array. |
755 | |
756 | @a = (1, 2, 3, 4, 5); |
757 | print "The third element of the array is $a[3] also expressed as $a[-2] \n"; |
54310121 |
758 | |
6dbacca0 |
759 | # perl4 prints: The third element of the array is 4 also expressed as |
760 | # perl5 prints: The third element of the array is 4 also expressed as 4 |
761 | |
762 | =item * (Arrays) |
763 | |
764 | Setting C<$#array> lower now discards array elements, and makes them |
765 | impossible to recover. |
766 | |
54310121 |
767 | @a = (a,b,c,d,e); |
6dbacca0 |
768 | print "Before: ",join('',@a); |
54310121 |
769 | $#a =1; |
6dbacca0 |
770 | print ", After: ",join('',@a); |
771 | $#a =3; |
772 | print ", Recovered: ",join('',@a),"\n"; |
54310121 |
773 | |
6dbacca0 |
774 | # perl4 prints: Before: abcde, After: ab, Recovered: abcd |
775 | # perl5 prints: Before: abcde, After: ab, Recovered: ab |
776 | |
777 | =item * (Hashes) |
778 | |
779 | Hashes get defined before use |
780 | |
54310121 |
781 | local($s,@a,%h); |
6dbacca0 |
782 | die "scalar \$s defined" if defined($s); |
783 | die "array \@a defined" if defined(@a); |
784 | die "hash \%h defined" if defined(%h); |
54310121 |
785 | |
6dbacca0 |
786 | # perl4 prints: |
787 | # perl5 dies: hash %h defined |
788 | |
475342a6 |
789 | Perl will now generate a warning when it sees defined(@a) and |
790 | defined(%h). |
791 | |
6dbacca0 |
792 | =item * (Globs) |
793 | |
794 | glob assignment from variable to variable will fail if the assigned |
795 | variable is localized subsequent to the assignment |
796 | |
797 | @a = ("This is Perl 4"); |
798 | *b = *a; |
799 | local(@a); |
800 | print @b,"\n"; |
54310121 |
801 | |
6dbacca0 |
802 | # perl4 prints: This is Perl 4 |
803 | # perl5 prints: |
54310121 |
804 | |
a3cb178b |
805 | =item * (Globs) |
54310121 |
806 | |
a3cb178b |
807 | Assigning C<undef> to a glob has no effect in Perl 5. In Perl 4 |
808 | it undefines the associated scalar (but may have other side effects |
9fda99eb |
809 | including SEGVs). Perl 5 will also warn if C<undef> is assigned to a |
810 | typeglob. (Note that assigning C<undef> to a typeglob is different |
811 | than calling the C<undef> function on a typeglob (C<undef *foo>), which |
812 | has quite a few effects. |
813 | |
814 | $foo = "bar"; |
815 | *foo = undef; |
816 | print $foo; |
817 | |
818 | # perl4 prints: |
819 | # perl4 warns: "Use of uninitialized variable" if using -w |
820 | # perl5 prints: bar |
821 | # perl5 warns: "Undefined value assigned to typeglob" if using -w |
5e378fdf |
822 | |
6dbacca0 |
823 | =item * (Scalar String) |
824 | |
825 | Changes in unary negation (of strings) |
826 | This change effects both the return value and what it |
827 | does to auto(magic)increment. |
828 | |
829 | $x = "aaa"; |
830 | print ++$x," : "; |
831 | print -$x," : "; |
832 | print ++$x,"\n"; |
54310121 |
833 | |
6dbacca0 |
834 | # perl4 prints: aab : -0 : 1 |
835 | # perl5 prints: aab : -aab : aac |
836 | |
837 | =item * (Constants) |
838 | |
839 | perl 4 lets you modify constants: |
840 | |
841 | $foo = "x"; |
842 | &mod($foo); |
843 | for ($x = 0; $x < 3; $x++) { |
844 | &mod("a"); |
845 | } |
846 | sub mod { |
847 | print "before: $_[0]"; |
848 | $_[0] = "m"; |
849 | print " after: $_[0]\n"; |
850 | } |
54310121 |
851 | |
6dbacca0 |
852 | # perl4: |
853 | # before: x after: m |
854 | # before: a after: m |
855 | # before: m after: m |
856 | # before: m after: m |
54310121 |
857 | |
6dbacca0 |
858 | # Perl5: |
859 | # before: x after: m |
860 | # Modification of a read-only value attempted at foo.pl line 12. |
861 | # before: a |
862 | |
863 | =item * (Scalars) |
864 | |
865 | The behavior is slightly different for: |
866 | |
867 | print "$x", defined $x |
54310121 |
868 | |
6dbacca0 |
869 | # perl 4: 1 |
870 | # perl 5: <no output, $x is not called into existence> |
871 | |
872 | =item * (Variable Suicide) |
873 | |
874 | Variable suicide behavior is more consistent under Perl 5. |
aa689395 |
875 | Perl5 exhibits the same behavior for hashes and scalars, |
5f05dabc |
876 | that perl4 exhibits for only scalars. |
6dbacca0 |
877 | |
878 | $aGlobal{ "aKey" } = "global value"; |
879 | print "MAIN:", $aGlobal{"aKey"}, "\n"; |
880 | $GlobalLevel = 0; |
881 | &test( *aGlobal ); |
882 | |
883 | sub test { |
884 | local( *theArgument ) = @_; |
885 | local( %aNewLocal ); # perl 4 != 5.001l,m |
54310121 |
886 | $aNewLocal{"aKey"} = "this should never appear"; |
6dbacca0 |
887 | print "SUB: ", $theArgument{"aKey"}, "\n"; |
888 | $aNewLocal{"aKey"} = "level $GlobalLevel"; # what should print |
889 | $GlobalLevel++; |
890 | if( $GlobalLevel<4 ) { |
891 | &test( *aNewLocal ); |
892 | } |
893 | } |
54310121 |
894 | |
6dbacca0 |
895 | # Perl4: |
896 | # MAIN:global value |
897 | # SUB: global value |
898 | # SUB: level 0 |
899 | # SUB: level 1 |
900 | # SUB: level 2 |
54310121 |
901 | |
6dbacca0 |
902 | # Perl5: |
903 | # MAIN:global value |
904 | # SUB: global value |
905 | # SUB: this should never appear |
906 | # SUB: this should never appear |
907 | # SUB: this should never appear |
908 | |
84dc3c4d |
909 | =back |
6dbacca0 |
910 | |
911 | =head2 Context Traps - scalar, list contexts |
912 | |
913 | =over 5 |
914 | |
915 | =item * (list context) |
916 | |
917 | The elements of argument lists for formats are now evaluated in list |
918 | context. This means you can interpolate list values now. |
919 | |
920 | @fmt = ("foo","bar","baz"); |
921 | format STDOUT= |
922 | @<<<<< @||||| @>>>>> |
923 | @fmt; |
924 | . |
54310121 |
925 | write; |
926 | |
6dbacca0 |
927 | # perl4 errors: Please use commas to separate fields in file |
928 | # perl5 prints: foo bar baz |
929 | |
930 | =item * (scalar context) |
931 | |
54310121 |
932 | The C<caller()> function now returns a false value in a scalar context |
933 | if there is no caller. This lets library files determine if they're |
6dbacca0 |
934 | being required. |
935 | |
936 | caller() ? (print "You rang?\n") : (print "Got a 0\n"); |
54310121 |
937 | |
6dbacca0 |
938 | # perl4 errors: There is no caller |
939 | # perl5 prints: Got a 0 |
5e378fdf |
940 | |
6dbacca0 |
941 | =item * (scalar context) |
942 | |
943 | The comma operator in a scalar context is now guaranteed to give a |
944 | scalar context to its arguments. |
945 | |
946 | @y= ('a','b','c'); |
947 | $x = (1, 2, @y); |
948 | print "x = $x\n"; |
54310121 |
949 | |
6dbacca0 |
950 | # Perl4 prints: x = c # Thinks list context interpolates list |
951 | # Perl5 prints: x = 3 # Knows scalar uses length of list |
952 | |
953 | =item * (list, builtin) |
954 | |
9fda99eb |
955 | C<sprintf()> is prototyped as ($;@), so its first argument is given scalar |
956 | context. Thus, if passed an array, it will probably not do what you want, |
957 | unlike Perl 4: |
6dbacca0 |
958 | |
959 | @z = ('%s%s', 'foo', 'bar'); |
960 | $x = sprintf(@z); |
9fda99eb |
961 | print $x; |
54310121 |
962 | |
9fda99eb |
963 | # perl4 prints: foobar |
964 | # perl5 prints: 3 |
6dbacca0 |
965 | |
9fda99eb |
966 | C<printf()> works the same as it did in Perl 4, though: |
6dbacca0 |
967 | |
9fda99eb |
968 | @z = ('%s%s', 'foo', 'bar'); |
6dbacca0 |
969 | printf STDOUT (@z); |
54310121 |
970 | |
6dbacca0 |
971 | # perl4 prints: foobar |
972 | # perl5 prints: foobar |
973 | |
6dbacca0 |
974 | =back |
975 | |
976 | =head2 Precedence Traps |
977 | |
978 | Perl4-to-Perl5 traps involving precedence order. |
979 | |
f4b17341 |
980 | Perl 4 has almost the same precedence rules as Perl 5 for the operators |
981 | that they both have. Perl 4 however, seems to have had some |
982 | inconsistencies that made the behavior differ from what was documented. |
983 | |
84dc3c4d |
984 | =over 5 |
985 | |
5e378fdf |
986 | =item * Precedence |
987 | |
8dbef698 |
988 | LHS vs. RHS of any assignment operator. LHS is evaluated first |
989 | in perl4, second in perl5; this can affect the relationship |
990 | between side-effects in sub-expressions. |
5e378fdf |
991 | |
992 | @arr = ( 'left', 'right' ); |
993 | $a{shift @arr} = shift @arr; |
994 | print join( ' ', keys %a ); |
995 | |
996 | # perl4 prints: left |
997 | # perl5 prints: right |
998 | |
999 | =item * Precedence |
6dbacca0 |
1000 | |
1001 | These are now semantic errors because of precedence: |
1002 | |
1003 | @list = (1,2,3,4,5); |
1004 | %map = ("a",1,"b",2,"c",3,"d",4); |
1005 | $n = shift @list + 2; # first item in list plus 2 |
1006 | print "n is $n, "; |
1007 | $m = keys %map + 2; # number of items in hash plus 2 |
1008 | print "m is $m\n"; |
54310121 |
1009 | |
6dbacca0 |
1010 | # perl4 prints: n is 3, m is 6 |
1011 | # perl5 errors and fails to compile |
1012 | |
1013 | =item * Precedence |
a0d0e21e |
1014 | |
4633a7c4 |
1015 | The precedence of assignment operators is now the same as the precedence |
1016 | of assignment. Perl 4 mistakenly gave them the precedence of the associated |
1017 | operator. So you now must parenthesize them in expressions like |
1018 | |
1019 | /foo/ ? ($a += 2) : ($a -= 2); |
a6006777 |
1020 | |
4633a7c4 |
1021 | Otherwise |
1022 | |
6dbacca0 |
1023 | /foo/ ? $a += 2 : $a -= 2 |
4633a7c4 |
1024 | |
1025 | would be erroneously parsed as |
1026 | |
1027 | (/foo/ ? $a += 2 : $a) -= 2; |
1028 | |
1029 | On the other hand, |
1030 | |
54310121 |
1031 | $a += /foo/ ? 1 : 2; |
4633a7c4 |
1032 | |
1033 | now works as a C programmer would expect. |
1034 | |
6dbacca0 |
1035 | =item * Precedence |
4633a7c4 |
1036 | |
6dbacca0 |
1037 | open FOO || die; |
a0d0e21e |
1038 | |
5f05dabc |
1039 | is now incorrect. You need parentheses around the filehandle. |
1040 | Otherwise, perl5 leaves the statement as its default precedence: |
a0d0e21e |
1041 | |
6dbacca0 |
1042 | open(FOO || die); |
54310121 |
1043 | |
6dbacca0 |
1044 | # perl4 opens or dies |
9fda99eb |
1045 | # perl5 opens FOO, dying only if 'FOO' is false, i.e. never |
a0d0e21e |
1046 | |
6dbacca0 |
1047 | =item * Precedence |
a0d0e21e |
1048 | |
6dbacca0 |
1049 | perl4 gives the special variable, C<$:> precedence, where perl5 |
1050 | treats C<$::> as main C<package> |
a0d0e21e |
1051 | |
6dbacca0 |
1052 | $a = "x"; print "$::a"; |
54310121 |
1053 | |
6dbacca0 |
1054 | # perl 4 prints: -:a |
1055 | # perl 5 prints: x |
5e378fdf |
1056 | |
6dbacca0 |
1057 | =item * Precedence |
a0d0e21e |
1058 | |
f4b17341 |
1059 | perl4 had buggy precedence for the file test operators vis-a-vis |
1060 | the assignment operators. Thus, although the precedence table |
1061 | for perl4 leads one to believe C<-e $foo .= "q"> should parse as |
1062 | C<((-e $foo) .= "q")>, it actually parses as C<(-e ($foo .= "q"))>. |
1063 | In perl5, the precedence is as documented. |
54310121 |
1064 | |
1065 | -e $foo .= "q" |
a0d0e21e |
1066 | |
6dbacca0 |
1067 | # perl4 prints: no output |
1068 | # perl5 prints: Can't modify -e in concatenation |
a0d0e21e |
1069 | |
f4b17341 |
1070 | =item * Precedence |
1071 | |
1072 | In perl4, keys(), each() and values() were special high-precedence operators |
1073 | that operated on a single hash, but in perl5, they are regular named unary |
1074 | operators. As documented, named unary operators have lower precedence |
1075 | than the arithmetic and concatenation operators C<+ - .>, but the perl4 |
1076 | variants of these operators actually bind tighter than C<+ - .>. |
1077 | Thus, for: |
1078 | |
1079 | %foo = 1..10; |
1080 | print keys %foo - 1 |
1081 | |
1082 | # perl4 prints: 4 |
1083 | # perl5 prints: Type of arg 1 to keys must be hash (not subtraction) |
1084 | |
1085 | The perl4 behavior was probably more useful, if less consistent. |
1086 | |
6dbacca0 |
1087 | =back |
1088 | |
1089 | =head2 General Regular Expression Traps using s///, etc. |
1090 | |
1091 | All types of RE traps. |
1092 | |
1093 | =over 5 |
1094 | |
1095 | =item * Regular Expression |
1096 | |
1097 | C<s'$lhs'$rhs'> now does no interpolation on either side. It used to |
19799a22 |
1098 | interpolate $lhs but not $rhs. (And still does not match a literal |
6dbacca0 |
1099 | '$' in string) |
1100 | |
1101 | $a=1;$b=2; |
1102 | $string = '1 2 $a $b'; |
1103 | $string =~ s'$a'$b'; |
1104 | print $string,"\n"; |
54310121 |
1105 | |
6dbacca0 |
1106 | # perl4 prints: $b 2 $a $b |
1107 | # perl5 prints: 1 2 $a $b |
1108 | |
1109 | =item * Regular Expression |
a0d0e21e |
1110 | |
1111 | C<m//g> now attaches its state to the searched string rather than the |
6dbacca0 |
1112 | regular expression. (Once the scope of a block is left for the sub, the |
1113 | state of the searched string is lost) |
1114 | |
1115 | $_ = "ababab"; |
1116 | while(m/ab/g){ |
1117 | &doit("blah"); |
1118 | } |
1119 | sub doit{local($_) = shift; print "Got $_ "} |
54310121 |
1120 | |
9fda99eb |
1121 | # perl4 prints: Got blah Got blah Got blah Got blah |
6dbacca0 |
1122 | # perl5 prints: infinite loop blah... |
1123 | |
1124 | =item * Regular Expression |
1125 | |
68dc0745 |
1126 | Currently, if you use the C<m//o> qualifier on a regular expression |
1127 | within an anonymous sub, I<all> closures generated from that anonymous |
1128 | sub will use the regular expression as it was compiled when it was used |
1129 | the very first time in any such closure. For instance, if you say |
1130 | |
1131 | sub build_match { |
1132 | my($left,$right) = @_; |
1133 | return sub { $_[0] =~ /$left stuff $right/o; }; |
1134 | } |
9fda99eb |
1135 | $good = build_match('foo','bar'); |
1136 | $bad = build_match('baz','blarch'); |
1137 | print $good->('foo stuff bar') ? "ok\n" : "not ok\n"; |
1138 | print $bad->('baz stuff blarch') ? "ok\n" : "not ok\n"; |
1139 | print $bad->('foo stuff bar') ? "not ok\n" : "ok\n"; |
1140 | |
1141 | For most builds of Perl5, this will print: |
1142 | ok |
1143 | not ok |
1144 | not ok |
68dc0745 |
1145 | |
1146 | build_match() will always return a sub which matches the contents of |
19799a22 |
1147 | $left and $right as they were the I<first> time that build_match() |
68dc0745 |
1148 | was called, not as they are in the current call. |
1149 | |
68dc0745 |
1150 | =item * Regular Expression |
1151 | |
6dbacca0 |
1152 | If no parentheses are used in a match, Perl4 sets C<$+> to |
1153 | the whole match, just like C<$&>. Perl5 does not. |
1154 | |
1155 | "abcdef" =~ /b.*e/; |
1156 | print "\$+ = $+\n"; |
54310121 |
1157 | |
6dbacca0 |
1158 | # perl4 prints: bcde |
1159 | # perl5 prints: |
1160 | |
1161 | =item * Regular Expression |
1162 | |
1163 | substitution now returns the null string if it fails |
1164 | |
1165 | $string = "test"; |
1166 | $value = ($string =~ s/foo//); |
1167 | print $value, "\n"; |
54310121 |
1168 | |
6dbacca0 |
1169 | # perl4 prints: 0 |
1170 | # perl5 prints: |
1171 | |
1172 | Also see L<Numerical Traps> for another example of this new feature. |
1173 | |
1174 | =item * Regular Expression |
1175 | |
54310121 |
1176 | C<s`lhs`rhs`> (using backticks) is now a normal substitution, with no |
1177 | backtick expansion |
6dbacca0 |
1178 | |
1179 | $string = ""; |
1180 | $string =~ s`^`hostname`; |
1181 | print $string, "\n"; |
54310121 |
1182 | |
6dbacca0 |
1183 | # perl4 prints: <the local hostname> |
1184 | # perl5 prints: hostname |
1185 | |
1186 | =item * Regular Expression |
1187 | |
1188 | Stricter parsing of variables used in regular expressions |
1189 | |
1190 | s/^([^$grpc]*$grpc[$opt$plus$rep]?)//o; |
54310121 |
1191 | |
6dbacca0 |
1192 | # perl4: compiles w/o error |
1193 | # perl5: with Scalar found where operator expected ..., near "$opt$plus" |
1194 | |
1195 | an added component of this example, apparently from the same script, is |
1196 | the actual value of the s'd string after the substitution. |
1197 | C<[$opt]> is a character class in perl4 and an array subscript in perl5 |
1198 | |
54310121 |
1199 | $grpc = 'a'; |
6dbacca0 |
1200 | $opt = 'r'; |
1201 | $_ = 'bar'; |
1202 | s/^([^$grpc]*$grpc[$opt]?)/foo/; |
1203 | print ; |
54310121 |
1204 | |
6dbacca0 |
1205 | # perl4 prints: foo |
1206 | # perl5 prints: foobar |
1207 | |
1208 | =item * Regular Expression |
1209 | |
1210 | Under perl5, C<m?x?> matches only once, like C<?x?>. Under perl4, it matched |
1211 | repeatedly, like C</x/> or C<m!x!>. |
1212 | |
1213 | $test = "once"; |
1214 | sub match { $test =~ m?once?; } |
1215 | &match(); |
1216 | if( &match() ) { |
1217 | # m?x? matches more then once |
1218 | print "perl4\n"; |
54310121 |
1219 | } else { |
6dbacca0 |
1220 | # m?x? matches only once |
54310121 |
1221 | print "perl5\n"; |
6dbacca0 |
1222 | } |
54310121 |
1223 | |
6dbacca0 |
1224 | # perl4 prints: perl4 |
1225 | # perl5 prints: perl5 |
a0d0e21e |
1226 | |
665e98b9 |
1227 | =item * Regular Expression |
1228 | |
1229 | Unlike in Ruby, failed matches in Perl do not reset the match variables |
1230 | ($1, $2, ..., C<$`>, ...). |
a0d0e21e |
1231 | |
6dbacca0 |
1232 | =back |
1233 | |
1234 | =head2 Subroutine, Signal, Sorting Traps |
a0d0e21e |
1235 | |
6dbacca0 |
1236 | The general group of Perl4-to-Perl5 traps having to do with |
1237 | Signals, Sorting, and their related subroutines, as well as |
1238 | general subroutine traps. Includes some OS-Specific traps. |
a0d0e21e |
1239 | |
6dbacca0 |
1240 | =over 5 |
a0d0e21e |
1241 | |
6dbacca0 |
1242 | =item * (Signals) |
a0d0e21e |
1243 | |
6dbacca0 |
1244 | Barewords that used to look like strings to Perl will now look like subroutine |
1245 | calls if a subroutine by that name is defined before the compiler sees them. |
a0d0e21e |
1246 | |
6dbacca0 |
1247 | sub SeeYa { warn"Hasta la vista, baby!" } |
1248 | $SIG{'TERM'} = SeeYa; |
1249 | print "SIGTERM is now $SIG{'TERM'}\n"; |
54310121 |
1250 | |
9fda99eb |
1251 | # perl4 prints: SIGTERM is now main'SeeYa |
1252 | # perl5 prints: SIGTERM is now main::1 (and warns "Hasta la vista, baby!") |
a0d0e21e |
1253 | |
6dbacca0 |
1254 | Use B<-w> to catch this one |
a0d0e21e |
1255 | |
6dbacca0 |
1256 | =item * (Sort Subroutine) |
a0d0e21e |
1257 | |
6dbacca0 |
1258 | reverse is no longer allowed as the name of a sort subroutine. |
a0d0e21e |
1259 | |
6dbacca0 |
1260 | sub reverse{ print "yup "; $a <=> $b } |
9fda99eb |
1261 | print sort reverse (2,1,3); |
54310121 |
1262 | |
9fda99eb |
1263 | # perl4 prints: yup yup 123 |
1264 | # perl5 prints: 123 |
1265 | # perl5 warns (if using -w): Ambiguous call resolved as CORE::reverse() |
a0d0e21e |
1266 | |
b996531f |
1267 | =item * warn() won't let you specify a filehandle. |
1268 | |
1269 | Although it _always_ printed to STDERR, warn() would let you specify a |
1270 | filehandle in perl4. With perl5 it does not. |
5e378fdf |
1271 | |
1272 | warn STDERR "Foo!"; |
1273 | |
1274 | # perl4 prints: Foo! |
54310121 |
1275 | # perl5 prints: String found where operator expected |
5e378fdf |
1276 | |
6dbacca0 |
1277 | =back |
a0d0e21e |
1278 | |
6dbacca0 |
1279 | =head2 OS Traps |
1280 | |
1281 | =over 5 |
1282 | |
1283 | =item * (SysV) |
1284 | |
54310121 |
1285 | Under HPUX, and some other SysV OSes, one had to reset any signal handler, |
1286 | within the signal handler function, each time a signal was handled with |
1287 | perl4. With perl5, the reset is now done correctly. Any code relying |
6dbacca0 |
1288 | on the handler _not_ being reset will have to be reworked. |
1289 | |
a6006777 |
1290 | Since version 5.002, Perl uses sigaction() under SysV. |
6dbacca0 |
1291 | |
1292 | sub gotit { |
54310121 |
1293 | print "Got @_... "; |
1294 | } |
6dbacca0 |
1295 | $SIG{'INT'} = 'gotit'; |
54310121 |
1296 | |
6dbacca0 |
1297 | $| = 1; |
1298 | $pid = fork; |
1299 | if ($pid) { |
1300 | kill('INT', $pid); |
1301 | sleep(1); |
1302 | kill('INT', $pid); |
54310121 |
1303 | } else { |
6dbacca0 |
1304 | while (1) {sleep(10);} |
54310121 |
1305 | } |
1306 | |
6dbacca0 |
1307 | # perl4 (HPUX) prints: Got INT... |
1308 | # perl5 (HPUX) prints: Got INT... Got INT... |
1309 | |
1310 | =item * (SysV) |
1311 | |
c47ff5f1 |
1312 | Under SysV OSes, C<seek()> on a file opened to append C<<< >> >>> now does |
54310121 |
1313 | the right thing w.r.t. the fopen() manpage. e.g., - When a file is opened |
6dbacca0 |
1314 | for append, it is impossible to overwrite information already in |
1315 | the file. |
1316 | |
1317 | open(TEST,">>seek.test"); |
54310121 |
1318 | $start = tell TEST ; |
6dbacca0 |
1319 | foreach(1 .. 9){ |
1320 | print TEST "$_ "; |
1321 | } |
1322 | $end = tell TEST ; |
1323 | seek(TEST,$start,0); |
1324 | print TEST "18 characters here"; |
54310121 |
1325 | |
6dbacca0 |
1326 | # perl4 (solaris) seek.test has: 18 characters here |
1327 | # perl5 (solaris) seek.test has: 1 2 3 4 5 6 7 8 9 18 characters here |
a0d0e21e |
1328 | |
a0d0e21e |
1329 | |
a0d0e21e |
1330 | |
6dbacca0 |
1331 | =back |
a0d0e21e |
1332 | |
6dbacca0 |
1333 | =head2 Interpolation Traps |
a0d0e21e |
1334 | |
8b0a4b75 |
1335 | Perl4-to-Perl5 traps having to do with how things get interpolated |
1336 | within certain expressions, statements, contexts, or whatever. |
1337 | |
6dbacca0 |
1338 | =over 5 |
a0d0e21e |
1339 | |
6dbacca0 |
1340 | =item * Interpolation |
a0d0e21e |
1341 | |
6dbacca0 |
1342 | @ now always interpolates an array in double-quotish strings. |
1343 | |
54310121 |
1344 | print "To: someone@somewhere.com\n"; |
1345 | |
6dbacca0 |
1346 | # perl4 prints: To:someone@somewhere.com |
8593bda5 |
1347 | # perl < 5.6.1, error : In string, @somewhere now must be written as \@somewhere |
1348 | # perl >= 5.6.1, warning : Possible unintended interpolation of @somewhere in string |
6dbacca0 |
1349 | |
1350 | =item * Interpolation |
1351 | |
1fa58bec |
1352 | Double-quoted strings may no longer end with an unescaped $. |
6dbacca0 |
1353 | |
1354 | $foo = "foo$"; |
1fa58bec |
1355 | print "foo is $foo\n"; |
54310121 |
1356 | |
1fa58bec |
1357 | # perl4 prints: foo is foo$ |
6dbacca0 |
1358 | # perl5 errors: Final $ should be \$ or $name |
1359 | |
1360 | Note: perl5 DOES NOT error on the terminating @ in $bar |
1361 | |
1362 | =item * Interpolation |
a0d0e21e |
1363 | |
8b0a4b75 |
1364 | Perl now sometimes evaluates arbitrary expressions inside braces that occur |
1365 | within double quotes (usually when the opening brace is preceded by C<$> |
1366 | or C<@>). |
1367 | |
1368 | @www = "buz"; |
1369 | $foo = "foo"; |
1370 | $bar = "bar"; |
1371 | sub foo { return "bar" }; |
1372 | print "|@{w.w.w}|${main'foo}|"; |
1373 | |
1374 | # perl4 prints: |@{w.w.w}|foo| |
1375 | # perl5 prints: |buz|bar| |
1376 | |
1377 | Note that you can C<use strict;> to ward off such trappiness under perl5. |
1378 | |
1379 | =item * Interpolation |
1380 | |
9fda99eb |
1381 | The construct "this is $$x" used to interpolate the pid at that point, but |
1382 | now tries to dereference $x. C<$$> by itself still works fine, however. |
748a9306 |
1383 | |
9fda99eb |
1384 | $s = "a reference"; |
1385 | $x = *s; |
6dbacca0 |
1386 | print "this is $$x\n"; |
748a9306 |
1387 | |
6dbacca0 |
1388 | # perl4 prints: this is XXXx (XXX is the current pid) |
9fda99eb |
1389 | # perl5 prints: this is a reference |
6dbacca0 |
1390 | |
1391 | =item * Interpolation |
1392 | |
54310121 |
1393 | Creation of hashes on the fly with C<eval "EXPR"> now requires either both |
1394 | C<$>'s to be protected in the specification of the hash name, or both curlies |
6dbacca0 |
1395 | to be protected. If both curlies are protected, the result will be compatible |
1396 | with perl4 and perl5. This is a very common practice, and should be changed |
1397 | to use the block form of C<eval{}> if possible. |
c07a80fd |
1398 | |
6dbacca0 |
1399 | $hashname = "foobar"; |
1400 | $key = "baz"; |
1401 | $value = 1234; |
1402 | eval "\$$hashname{'$key'} = q|$value|"; |
1403 | (defined($foobar{'baz'})) ? (print "Yup") : (print "Nope"); |
1404 | |
1405 | # perl4 prints: Yup |
1406 | # perl5 prints: Nope |
1407 | |
1408 | Changing |
1409 | |
1410 | eval "\$$hashname{'$key'} = q|$value|"; |
c07a80fd |
1411 | |
1412 | to |
1413 | |
6dbacca0 |
1414 | eval "\$\$hashname{'$key'} = q|$value|"; |
c07a80fd |
1415 | |
6dbacca0 |
1416 | causes the following result: |
c07a80fd |
1417 | |
6dbacca0 |
1418 | # perl4 prints: Nope |
1419 | # perl5 prints: Yup |
c07a80fd |
1420 | |
6dbacca0 |
1421 | or, changing to |
a0d0e21e |
1422 | |
6dbacca0 |
1423 | eval "\$$hashname\{'$key'\} = q|$value|"; |
1424 | |
1425 | causes the following result: |
1426 | |
1427 | # perl4 prints: Yup |
1428 | # perl5 prints: Yup |
1429 | # and is compatible for both versions |
1430 | |
1431 | |
1432 | =item * Interpolation |
1433 | |
1434 | perl4 programs which unconsciously rely on the bugs in earlier perl versions. |
1435 | |
1436 | perl -e '$bar=q/not/; print "This is $foo{$bar} perl5"' |
54310121 |
1437 | |
6dbacca0 |
1438 | # perl4 prints: This is not perl5 |
1439 | # perl5 prints: This is perl5 |
1440 | |
1441 | =item * Interpolation |
1442 | |
418272e4 |
1443 | You also have to be careful about array and hash brackets during |
1444 | interpolation. |
1445 | |
1446 | print "$foo[" |
1447 | |
1448 | perl 4 prints: [ |
1449 | perl 5 prints: syntax error |
6dbacca0 |
1450 | |
1451 | print "$foo{" |
1452 | |
1453 | perl 4 prints: { |
1454 | perl 5 prints: syntax error |
1455 | |
418272e4 |
1456 | Perl 5 is expecting to find an index or key name following the respective |
1457 | brackets, as well as an ending bracket of the appropriate type. In order |
1458 | to mimic the behavior of Perl 4, you must escape the bracket like so. |
1459 | |
1460 | print "$foo\["; |
1461 | print "$foo\{"; |
1462 | |
6dbacca0 |
1463 | =item * Interpolation |
1464 | |
1465 | Similarly, watch out for: |
1466 | |
9fda99eb |
1467 | $foo = "baz"; |
6dbacca0 |
1468 | print "\$$foo{bar}\n"; |
54310121 |
1469 | |
9fda99eb |
1470 | # perl4 prints: $baz{bar} |
6dbacca0 |
1471 | # perl5 prints: $ |
1472 | |
9fda99eb |
1473 | Perl 5 is looking for C<$foo{bar}> which doesn't exist, but perl 4 is |
1474 | happy just to expand $foo to "baz" by itself. Watch out for this |
6dbacca0 |
1475 | especially in C<eval>'s. |
1476 | |
1477 | =item * Interpolation |
1478 | |
1479 | C<qq()> string passed to C<eval> |
1480 | |
1481 | eval qq( |
1482 | foreach \$y (keys %\$x\) { |
1483 | \$count++; |
1484 | } |
1485 | ); |
54310121 |
1486 | |
6dbacca0 |
1487 | # perl4 runs this ok |
54310121 |
1488 | # perl5 prints: Can't find string terminator ")" |
a0d0e21e |
1489 | |
6dbacca0 |
1490 | =back |
1491 | |
1492 | =head2 DBM Traps |
1493 | |
1494 | General DBM traps. |
1495 | |
1496 | =over 5 |
1497 | |
1498 | =item * DBM |
1499 | |
1500 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) |
1501 | may cause the same script, run under perl5, to fail. The build of perl5 |
1502 | must have been linked with the same dbm/ndbm as the default for C<dbmopen()> |
1503 | to function properly without C<tie>'ing to an extension dbm implementation. |
1504 | |
1505 | dbmopen (%dbm, "file", undef); |
1506 | print "ok\n"; |
1507 | |
1508 | # perl4 prints: ok |
1509 | # perl5 prints: ok (IFF linked with -ldbm or -lndbm) |
1510 | |
1511 | |
1512 | =item * DBM |
1513 | |
1514 | Existing dbm databases created under perl4 (or any other dbm/ndbm tool) |
1515 | may cause the same script, run under perl5, to fail. The error generated |
1516 | when exceeding the limit on the key/value size will cause perl5 to exit |
1517 | immediately. |
1518 | |
1519 | dbmopen(DB, "testdb",0600) || die "couldn't open db! $!"; |
1520 | $DB{'trap'} = "x" x 1024; # value too large for most dbm/ndbm |
1521 | print "YUP\n"; |
1522 | |
1523 | # perl4 prints: |
1524 | dbm store returned -1, errno 28, key "trap" at - line 3. |
1525 | YUP |
1526 | |
1527 | # perl5 prints: |
1528 | dbm store returned -1, errno 28, key "trap" at - line 3. |
a0d0e21e |
1529 | |
1530 | =back |
6dbacca0 |
1531 | |
1532 | =head2 Unclassified Traps |
1533 | |
1534 | Everything else. |
1535 | |
84dc3c4d |
1536 | =over 5 |
1537 | |
5db417f7 |
1538 | =item * C<require>/C<do> trap using returned value |
6dbacca0 |
1539 | |
1540 | If the file doit.pl has: |
1541 | |
1542 | sub foo { |
1543 | $rc = do "./do.pl"; |
1544 | return 8; |
54310121 |
1545 | } |
6dbacca0 |
1546 | print &foo, "\n"; |
1547 | |
1548 | And the do.pl file has the following single line: |
1549 | |
1550 | return 3; |
1551 | |
1552 | Running doit.pl gives the following: |
1553 | |
1554 | # perl 4 prints: 3 (aborts the subroutine early) |
54310121 |
1555 | # perl 5 prints: 8 |
6dbacca0 |
1556 | |
1557 | Same behavior if you replace C<do> with C<require>. |
1558 | |
5db417f7 |
1559 | =item * C<split> on empty string with LIMIT specified |
1560 | |
9fda99eb |
1561 | $string = ''; |
5db417f7 |
1562 | @list = split(/foo/, $string, 2) |
1563 | |
1564 | Perl4 returns a one element list containing the empty string but Perl5 |
1565 | returns an empty list. |
1566 | |
6dbacca0 |
1567 | =back |
1568 | |
54310121 |
1569 | As always, if any of these are ever officially declared as bugs, |
6dbacca0 |
1570 | they'll be fixed and removed. |
1571 | |