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10 | # to go back to "/" no matter how myfun exits: |
10 | # to go back to "/" no matter how myfun exits: |
11 | sub myfun { |
11 | sub myfun { |
12 | scope_guard { chdir "/" }; |
12 | scope_guard { chdir "/" }; |
13 | chdir "/etc"; |
13 | chdir "/etc"; |
14 | |
14 | |
15 | call_function_that_might_die_or_other_fun_stuff; |
15 | code_that_might_die_or_does_other_fun_stuff; |
16 | } |
16 | } |
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17 | |
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18 | # create an object that, when the last reference to it is gone, |
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19 | # invokes the given codeblock: |
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20 | my $guard = guard { print "destroyed!\n" }; |
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21 | undef $guard; # probably destroyed here |
17 | |
22 | |
18 | =head1 DESCRIPTION |
23 | =head1 DESCRIPTION |
19 | |
24 | |
20 | This module implements so-called "guards". A guard is something (usually |
25 | This module implements so-called "guards". A guard is something (usually |
21 | an object) that "guards" a resource, ensuring that it is cleaned up when |
26 | an object) that "guards" a resource, ensuring that it is cleaned up when |
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34 | |
39 | |
35 | =cut |
40 | =cut |
36 | |
41 | |
37 | package Guard; |
42 | package Guard; |
38 | |
43 | |
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44 | no warnings; |
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45 | |
39 | BEGIN { |
46 | BEGIN { |
40 | $VERSION = '0.1'; |
47 | $VERSION = '1.02'; |
41 | @ISA = qw(Exporter); |
48 | @ISA = qw(Exporter); |
42 | @EXPORT = qw(guard scope_guard); |
49 | @EXPORT = qw(guard scope_guard); |
43 | |
50 | |
44 | require Exporter; |
51 | require Exporter; |
45 | |
52 | |
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133 | |
140 | |
134 | The advantage of doing this with a guard instead of simply calling C<< |
141 | The advantage of doing this with a guard instead of simply calling C<< |
135 | $sem->down >> in the callback is that you can opt not to create the timer, |
142 | $sem->down >> in the callback is that you can opt not to create the timer, |
136 | or your code can throw an exception before it can create the timer, or you |
143 | or your code can throw an exception before it can create the timer, or you |
137 | can create multiple timers or other event watchers and only when the last |
144 | can create multiple timers or other event watchers and only when the last |
138 | one gets executed will the lock be unlocked. |
145 | one gets executed will the lock be unlocked. Using the C<guard>, you do |
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146 | not have to worry about catching all the places where you have to unlock |
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147 | the semaphore. |
139 | |
148 | |
140 | =item Guard::cancel $guard |
149 | =item $guard->cancel |
141 | |
150 | |
142 | Calling this function will "disable" the guard object returned by the |
151 | Calling this function will "disable" the guard object returned by the |
143 | C<guard> function, i.e. it will free the BLOCK originally passed to |
152 | C<guard> function, i.e. it will free the BLOCK originally passed to |
144 | C<guard >and will arrange for the BLOCK not to be executed. |
153 | C<guard >and will arrange for the BLOCK not to be executed. |
145 | |
154 | |
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158 | all, they are usually used to clean up after such exceptions. However, if |
167 | all, they are usually used to clean up after such exceptions. However, if |
159 | something truly exceptional is happening, a guard block should be allowed |
168 | something truly exceptional is happening, a guard block should be allowed |
160 | to die. Also, programming errors are a large source of exceptions, and the |
169 | to die. Also, programming errors are a large source of exceptions, and the |
161 | programmer certainly wants to know about those. |
170 | programmer certainly wants to know about those. |
162 | |
171 | |
163 | Since in most cases, the block executing when the guard gets executes does |
172 | Since in most cases, the block executing when the guard gets executed does |
164 | not know or does not care about the guard blocks, it makes little sense to |
173 | not know or does not care about the guard blocks, it makes little sense to |
165 | let containing code handle the exception. |
174 | let containing code handle the exception. |
166 | |
175 | |
167 | Therefore, whenever a guard block throws an exception, it will be caught, |
176 | Therefore, whenever a guard block throws an exception, it will be caught, |
168 | and this module will call the code reference stored in C<$Guard::DIED> |
177 | followed by calling the code reference stored in C<$Guard::DIED> (with |
169 | (with C<$@> set to the actual exception), which is similar to how most |
178 | C<$@> set to the actual exception), which is similar to how most event |
170 | event loops handle this case. |
179 | loops handle this case. |
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180 | |
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181 | The default for C<$Guard::DIED> is to call C<warn "$@">. |
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182 | |
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183 | The C<$@> variable will be restored to its value before the guard call in |
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184 | all cases, so guards will not disturb C<$@> in any way. |
171 | |
185 | |
172 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
186 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
173 | not guaranteed, but right now, the exception will simply be ignored). |
187 | not guaranteed, but right now, the exception will simply be ignored). |
174 | |
188 | |
175 | The default for C<$Guard::DIED> is to call C<warn "$@">. |
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176 | |
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177 | =head1 AUTHOR |
189 | =head1 AUTHOR |
178 | |
190 | |
179 | Marc Lehmann <schmorp@schmorp.de> |
191 | Marc Lehmann <schmorp@schmorp.de> |
180 | http://home.schmorp.de/ |
192 | http://home.schmorp.de/ |
181 | |
193 | |
182 | =head1 THANKS |
194 | =head1 THANKS |
183 | |
195 | |
184 | Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> |
196 | Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> |
185 | solution to the problem of exceptions. |
197 | solution to the problem of exceptions. |
186 | |
198 | |
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199 | =head1 SEE ALSO |
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200 | |
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201 | L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement |
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202 | dynamic, not scoped guards, and have a lot higher CPU, memory and typing |
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203 | overhead. |
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204 | |
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205 | L<Hook::Scope>, which has apparently never been finished and corrupts |
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206 | memory when used. |
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207 | |
187 | =cut |
208 | =cut |
188 | |
209 | |