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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 | } |
17 | |
17 | |
18 | =head1 DESCRIPTION |
18 | =head1 DESCRIPTION |
19 | |
19 | |
20 | This module implements so-called "guards". A guard is something (usually |
20 | This module implements so-called "guards". A guard is something (usually |
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34 | |
34 | |
35 | =cut |
35 | =cut |
36 | |
36 | |
37 | package Guard; |
37 | package Guard; |
38 | |
38 | |
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39 | no warnings; |
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40 | |
39 | BEGIN { |
41 | BEGIN { |
40 | $VERSION = '0.1'; |
42 | $VERSION = '1.0'; |
41 | @ISA = qw(Exporter); |
43 | @ISA = qw(Exporter); |
42 | @EXPORT = qw(guard scope_guard); |
44 | @EXPORT = qw(guard scope_guard); |
43 | |
45 | |
44 | require Exporter; |
46 | require Exporter; |
45 | |
47 | |
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107 | |
109 | |
108 | See the EXCEPTIONS section for an explanation of how exceptions |
110 | See the EXCEPTIONS section for an explanation of how exceptions |
109 | (i.e. C<die>) are handled inside guard blocks. |
111 | (i.e. C<die>) are handled inside guard blocks. |
110 | |
112 | |
111 | Example: acquire a Coro::Semaphore for a second by registering a |
113 | Example: acquire a Coro::Semaphore for a second by registering a |
112 | timer. The timer callback references the guard used to unlock it again. |
114 | timer. The timer callback references the guard used to unlock it |
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115 | again. (Please ignore the fact that C<Coro::Semaphore> has a C<guard> |
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116 | method that does this already): |
113 | |
117 | |
114 | use Guard; |
118 | use Guard; |
115 | use AnyEvent; |
119 | use AnyEvent; |
116 | use Coro::Semaphore; |
120 | use Coro::Semaphore; |
117 | |
121 | |
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131 | |
135 | |
132 | The advantage of doing this with a guard instead of simply calling C<< |
136 | The advantage of doing this with a guard instead of simply calling C<< |
133 | $sem->down >> in the callback is that you can opt not to create the timer, |
137 | $sem->down >> in the callback is that you can opt not to create the timer, |
134 | or your code can throw an exception before it can create the timer, or you |
138 | or your code can throw an exception before it can create the timer, or you |
135 | can create multiple timers or other event watchers and only when the last |
139 | can create multiple timers or other event watchers and only when the last |
136 | one gets executed will the lock be unlocked. |
140 | one gets executed will the lock be unlocked. Using the C<guard>, you do |
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141 | not have to worry about catching all the places where you have to unlock |
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142 | the semaphore. |
137 | |
143 | |
138 | =item Guard::cancel $guard |
144 | =item $guard->cancel |
139 | |
145 | |
140 | Calling this function will "disable" the guard object returned by the |
146 | Calling this function will "disable" the guard object returned by the |
141 | C<guard> function, i.e. it will free the BLOCK originally passed to |
147 | C<guard> function, i.e. it will free the BLOCK originally passed to |
142 | C<guard >and will arrange for the BLOCK not to be executed. |
148 | C<guard >and will arrange for the BLOCK not to be executed. |
143 | |
149 | |
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156 | all, they are usually used to clean up after such exceptions. However, if |
162 | all, they are usually used to clean up after such exceptions. However, if |
157 | something truly exceptional is happening, a guard block should be allowed |
163 | something truly exceptional is happening, a guard block should be allowed |
158 | to die. Also, programming errors are a large source of exceptions, and the |
164 | to die. Also, programming errors are a large source of exceptions, and the |
159 | programmer certainly wants to know about those. |
165 | programmer certainly wants to know about those. |
160 | |
166 | |
161 | Since in most cases, the block executing when the guard gets executes does |
167 | Since in most cases, the block executing when the guard gets executed does |
162 | not know or does not care about the guard blocks, it makes little sense to |
168 | not know or does not care about the guard blocks, it makes little sense to |
163 | let containing code handle the exception. |
169 | let containing code handle the exception. |
164 | |
170 | |
165 | Therefore, whenever a guard block throws an exception, it will be caught, |
171 | Therefore, whenever a guard block throws an exception, it will be caught, |
166 | and this module will call the code reference stored in C<$Guard::DIED> |
172 | followed by calling the code reference stored in C<$Guard::DIED> (with |
167 | (with C<$@> set to the actual exception), which is similar to how most |
173 | C<$@> set to the actual exception), which is similar to how most event |
168 | event loops handle this case. |
174 | loops handle this case. |
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175 | |
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176 | The default for C<$Guard::DIED> is to call C<warn "$@">. |
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177 | |
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178 | The C<$@> variable will be restored to its value before the guard call in |
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179 | all cases, so guards will not disturb C<$@> in any way. |
169 | |
180 | |
170 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
181 | The code reference stored in C<$Guard::DIED> should not die (behaviour is |
171 | not guaranteed, but right now, the exception will simply be ignored). |
182 | not guaranteed, but right now, the exception will simply be ignored). |
172 | |
183 | |
173 | The default for C<$Guard::DIED> is to call C<warn "$@">. |
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174 | |
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175 | =head1 AUTHOR |
184 | =head1 AUTHOR |
176 | |
185 | |
177 | Marc Lehmann <schmorp@schmorp.de> |
186 | Marc Lehmann <schmorp@schmorp.de> |
178 | http://home.schmorp.de/ |
187 | http://home.schmorp.de/ |
179 | |
188 | |
180 | =head1 THANKS |
189 | =head1 THANKS |
181 | |
190 | |
182 | Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> |
191 | Thanks to Marco Maisenhelder, who reminded me of the C<$Guard::DIED> |
183 | solution to the problem of exceptions. |
192 | solution to the problem of exceptions. |
184 | |
193 | |
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194 | =head1 SEE ALSO |
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195 | |
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196 | L<Scope::Guard> and L<Sub::ScopeFinalizer>, which actually implement |
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197 | dynamic, not scoped guards, and have a lot higher CPU, memory and typing |
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198 | overhead. |
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199 | |
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200 | L<Hook::Scope>, which has apparently never been finished and corrupts |
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201 | memory when used. |
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202 | |
185 | =cut |
203 | =cut |
186 | |
204 | |