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Comparing Guard/Guard.pm (file contents):
Revision 1.3 by root, Sat Dec 13 17:50:29 2008 UTC vs.
Revision 1.7 by root, Sat Dec 13 18:44:16 2008 UTC

… …
3Guard - safe cleanup blocks 3Guard - safe cleanup blocks
4 4
5=head1 SYNOPSIS 5=head1 SYNOPSIS
6 6
7 use Guard; 7 use Guard;
8
9 # temporarily chdir to "/etc" directory, but make sure
10 # to go back to "/" no matter how myfun exits:
11 sub myfun {
12 scope_guard { chdir "/" };
13 chdir "/etc";
14
15 call_function_that_might_die_or_other_fun_stuff;
16 }
8 17
9=head1 DESCRIPTION 18=head1 DESCRIPTION
10 19
11This module implements so-called "guards". A guard is something (usually 20This module implements so-called "guards". A guard is something (usually
12an object) that "guards" a resource, ensuring that it is cleaned up when 21an object) that "guards" a resource, ensuring that it is cleaned up when
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60 die if $@; 69 die if $@;
61 70
62Except it is much faster, and the whole thing gets executed even when the 71Except it is much faster, and the whole thing gets executed even when the
63BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means. 72BLOCK calls C<exit>, C<goto>, C<last> or escapes via other means.
64 73
74If multiple BLOCKs are registered to the same scope, they will be executed
75in reverse order. Stuff like C<local> is managed via the same mechanism,
76so variables C<local>ised after calling C<scope_guard> will be restored
77when the guard runs.
78
65See B<EXCEPTIONS>, below, for an explanation of exception handling 79See B<EXCEPTIONS>, below, for an explanation of exception handling
66(C<die>) within guard blocks. 80(C<die>) within guard blocks.
67 81
68Example: Temporarily change the directory to F</etc> and make sure it's 82Example: temporarily change the timezone for the current process,
69set back to F</> when the function returns: 83ensuring it will be reset when the C<if> scope is exited:
70 84
71 sub dosomething { 85 use Guard;
72 scope_guard { chdir "/" }; 86 use POSIX ();
73 chdir "/etc";
74 87
75 ... 88 if ($need_to_switch_tz) {
89 # make sure we call tzset after $ENV{TZ} has been restored
90 scope_guard { POSIX::tzset };
91
92 # localise after the scope_guard, so it gets undone in time
93 local $ENV{TZ} = "Europe/London";
94 POSIX::tzset;
95
96 # do something with the new timezone
76 } 97 }
77 98
78=item my $guard = guard BLOCK 99=item my $guard = guard BLOCK
79 100
80Behaves the same as C<scope_guard>, except that instead of executing 101Behaves the same as C<scope_guard>, except that instead of executing
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128 149
129=back 150=back
130 151
131=head1 EXCEPTIONS 152=head1 EXCEPTIONS
132 153
133Guard blocks should not normally throw exceptions (e.g. C<die>), after 154Guard blocks should not normally throw exceptions (that is, C<die>). After
134all, they are usually used to clean up after such exceptions. However, if 155all, they are usually used to clean up after such exceptions. However, if
135something truly exceptional is happening, a guard block should be allowed 156something truly exceptional is happening, a guard block should be allowed
136to die. Also, programming errors are a large source of exceptions, and the 157to die. Also, programming errors are a large source of exceptions, and the
137programmer certainly wants to know about those. 158programmer certainly wants to know about those.
138 159
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155 Marc Lehmann <schmorp@schmorp.de> 176 Marc Lehmann <schmorp@schmorp.de>
156 http://home.schmorp.de/ 177 http://home.schmorp.de/
157 178
158=head1 THANKS 179=head1 THANKS
159 180
160To Marco Maisenhelder, who reminded me of the C<$Guard::DIED> solution to 181Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED>
161the problem of exceptions. 182solution to the problem of exceptions.
162 183
163=cut 184=cut
164 185

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