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perldelta item on reliable exception handling
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1=encoding utf8
2
3=head1 NAME
4
5[ this is a template for a new perldelta file. Any text flagged as
6XXX needs to be processed before release. ]
7
8perldelta - what is new for perl v5.13.1
9
10=head1 DESCRIPTION
11
12This document describes differences between the 5.13.0 release and
13the 5.13.1 release.
14
15If you are upgrading from an earlier release such as 5.10, first read
16L<perl5120delta>, which describes differences between 5.10 and
175.12.
18
19=head1 Notice
20
21XXX Any important notices here
22
23=head1 Incompatible Changes
24
df5278db 25=head2 "C<\cI<X>>"
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27The backslash-c construct was designed as a way of specifying
28non-printable characters, but there were no restrictions (on ASCII
29platforms) on what the character following the C<c> could be. Now, that
30character must be one of the ASCII characters.
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32=head2 localised tied hashes, arrays and scalars are no longed tied
33
34In the following:
35
36 tie @a, ...;
37 {
38 local @a;
39 # here, @a is a now a new, untied array
40 }
41 # here, @a refers again to the old, tied array
42
43The new local array used to be made tied too, which was fairly pointless,
44and has now been fixed. This fix could however potentially cause a change
45in behaviour of some code.
46
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47=head1 Core Enhancements
48
49XXX New core language features go here. Summarise user-visible core language
50enhancements. Particularly prominent performance optimisations could go
51here, but most should go in the L</Performance Enhancements> section.
52
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53=head2 Exception Handling Reliability
54
55Several changes have been made to the way C<die>, C<warn>, and C<$@>
56behave, in order to make them more reliable and consistent.
57
58When an exception is thrown inside an C<eval>, the exception is no
59longer at risk of being clobbered by code running during unwinding
60(e.g., destructors). Previously, the exception was written into C<$@>
61early in the throwing process, and would be overwritten if C<eval> was
62used internally in the destructor for an object that had to be freed
63while exiting from the outer C<eval>. Now the exception is written
64into C<$@> last thing before exiting the outer C<eval>, so the code
65running immediately thereafter can rely on the value in C<$@> correctly
66corresponding to that C<eval>.
67
68Likewise, a C<local $@> inside an C<eval> will no longer clobber any
69exception thrown in its scope. Previously, the restoration of C<$@> upon
70unwinding would overwrite any exception being thrown. Now the exception
71gets to the C<eval> anyway. So C<local $@> is safe inside an C<eval>,
72albeit of rather limited use.
73
74Exceptions thrown from object destructors no longer modify the C<$@>
75of the surrounding context. (If the surrounding context was exception
76unwinding, this used to be another way to clobber the exception being
77thrown. Due to the above change it no longer has that significance,
78but there are other situations where C<$@> is significant.) Previously
79such an exception was sometimes emitted as a warning, and then either
80string-appended to the surrounding C<$@> or completely replaced the
81surrounding C<$@>, depending on whether that exception and the surrounding
82C<$@> were strings or objects. Now, an exception in this situation is
83always emitted as a warning, leaving the surrounding C<$@> untouched.
84In addition to object destructors, this also affects any function call
85performed by XS code using the C<G_KEEPERR> flag.
86
87C<$@> is also no longer used as an internal temporary variable when
88preparing to C<die>. Previously it was internally necessary to put
89any exception object (any non-string exception) into C<$@> first,
90before it could be used as an exception. (The C API still offers the
91old option, so an XS module might still clobber C<$@> in the old way.)
92This change together with the foregoing means that, in various places,
93C<$@> may be observed to contain its previously-assigned value, rather
94than having been overwritten by recent exception-related activity.
95
96Warnings for C<warn> can now be objects, in the same way as exceptions
97for C<die>. If an object-based warning gets the default handling,
98of writing to standard error, it will of course still be stringified
99along the way. But a C<$SIG{__WARN__}> handler will now receive an
100object-based warning as an object, where previously it was passed the
101result of stringifying the object.
102
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103=head1 New Platforms
104
105XXX List any platforms that this version of perl compiles on, that previous
106versions did not. These will either be enabled by new files in the F<hints/>
107directories, or new subdirectories and F<README> files at the top level of the
108source tree.
109
110=head1 Modules and Pragmata
111
112XXX All changes to installed files in F<cpan/>, F<dist/>, F<ext/> and F<lib/>
113go here. If Module::CoreList is updated, generate an initial draft of the
114following sections using F<Porting/corelist-perldelta.pl>, which prints stub
115entries to STDOUT. Results can be pasted in place of the '=head2' entries
116below. A paragraph summary for important changes should then be added by hand.
117In an ideal world, dual-life modules would have a F<Changes> file that could be
118cribbed.
119
120=head2 New Modules and Pragmata
121
122=head2 Pragmata Changes
123
124=head2 Updated Modules
125
126=head2 Removed Modules and Pragmata
127
128=head1 Utility Changes
129
130XXX Changes to installed programs such as F<perlbug> and F<xsubpp> go
131here. Most of these are built within the directories F<utils> and F<x2p>.
132
133=over 4
134
135=item F<XXX>
136
137XXX
138
139=back
140
141=head1 New Documentation
142
143XXX Changes which create B<new> files in F<pod/> go here.
144
145=over 4
146
147=item L<XXX>
148
149XXX
150
151=back
152
153=head1 Changes to Existing Documentation
154
155XXX Changes which significantly change existing files in F<pod/> go here.
156Any changes to F<pod/perldiag.pod> should go in L</New or Changed Diagnostics>.
157
158
159=head1 Performance Enhancements
160
161XXX Changes which enhance performance without changing behaviour go here. There
162may well be none in a stable release.
163
164=over 4
165
166=item *
167
168XXX
169
170=back
171
172=head1 Installation and Configuration Improvements
173
174XXX Changes to F<Configure>, F<installperl>, F<installman>, and analogous tools
175go here.
176
177=head2 Configuration improvements
178
179XXX
180
181=head2 Compilation improvements
182
183XXX
184
185=head2 Platform Specific Changes
186
187=over 4
188
189=item XXX-some-platform
190
191XXX
192
193=back
194
195=head1 Selected Bug Fixes
196
197XXX Important bug fixes in the core language are summarised here.
198Bug fixes in files in F<ext/> and F<lib/> are best summarised in
199L</Modules and Pragmata>.
200
201=over 4
202
203=item *
204
205XXX
206
207=back
208
209=head1 New or Changed Diagnostics
210
211XXX New or changed warnings emitted by the core's C<C> code go here.
212
213=over 4
214
215=item C<XXX>
216
217XXX
218
219=back
220
221=head1 Changed Internals
222
223XXX Changes which affect the interface available to C<XS> code go here.
224
225=over 4
226
227=item *
228
229XXX
230
231=back
232
233=head1 New Tests
234
235XXX Changes which create B<new> files in F<t/> go here. Changes to
236existing files in F<t/> aren't worth summarising, although the bugs that
237they represent may be.
238
239=over 4
240
241=item F<XXX>
242
243XXX
244
245=back
246
247=head1 Known Problems
248
249XXX Descriptions of platform agnostic bugs we know we can't fix go here. Any
250tests that had to be C<TODO>ed for the release would be noted here, unless
251they were specific to a particular platform (see below).
252
253This is a list of some significant unfixed bugs, which are regressions
254from either 5.XXX.XXX or 5.XXX.XXX.
255
256=over 4
257
258=item *
259
260XXX
261
262=back
263
264=head1 Deprecations
265
266XXX Add any new known deprecations here.
267
268The following items are now deprecated.
269
270=over 4
271
272=item *
273
274XXX
275
276=back
277
278=head1 Platform Specific Notes
279
280XXX Any changes specific to a particular platform. VMS and Win32 are the usual
281stars here. It's probably best to group changes under the same section layout
282as the main perldelta
283
284=head1 Obituary
285
286XXX If any significant core contributor has died, we've added a short obituary
287here.
288
289=head1 Acknowledgements
290
291XXX The list of people to thank goes here.
292
293
294=head1 Reporting Bugs
295
296If you find what you think is a bug, you might check the articles
297recently posted to the comp.lang.perl.misc newsgroup and the perl
298bug database at http://rt.perl.org/perlbug/ . There may also be
299information at http://www.perl.org/ , the Perl Home Page.
300
301If you believe you have an unreported bug, please run the B<perlbug>
302program included with your release. Be sure to trim your bug down
303to a tiny but sufficient test case. Your bug report, along with the
304output of C<perl -V>, will be sent off to perlbug@perl.org to be
305analysed by the Perl porting team.
306
307If the bug you are reporting has security implications, which make it
308inappropriate to send to a publicly archived mailing list, then please send
309it to perl5-security-report@perl.org. This points to a closed subscription
310unarchived mailing list, which includes all the core committers, who be able
311to help assess the impact of issues, figure out a resolution, and help
312co-ordinate the release of patches to mitigate or fix the problem across all
313platforms on which Perl is supported. Please only use this address for
314security issues in the Perl core, not for modules independently
315distributed on CPAN.
316
317=head1 SEE ALSO
318
319The F<Changes> file for an explanation of how to view exhaustive details
320on what changed.
321
322The F<INSTALL> file for how to build Perl.
323
324The F<README> file for general stuff.
325
326The F<Artistic> and F<Copying> files for copyright information.
327
328=cut