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1 | package Try::Tiny; |
2 | |
3 | use strict; |
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4 | #use warnings; |
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5 | |
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6 | use vars qw(@EXPORT @EXPORT_OK $VERSION @ISA); |
7 | |
8 | BEGIN { |
9 | require Exporter; |
10 | @ISA = qw(Exporter); |
11 | } |
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12 | |
13 | $VERSION = "0.01"; |
14 | |
15 | $VERSION = eval $VERSION; |
16 | |
17 | @EXPORT = @EXPORT_OK = qw(try catch); |
18 | |
19 | sub try (&;$) { |
20 | my ( $try, $catch ) = @_; |
21 | |
22 | # we need to save this here, the eval block will be in scalar context due |
23 | # to $failed |
24 | my $wantarray = wantarray; |
25 | |
26 | my ( @ret, $error, $failed ); |
27 | |
28 | # FIXME consider using local $SIG{__DIE__} to accumilate all errors. It's |
29 | # not perfect, but we could provide a list of additional errors for |
30 | # $catch->(); |
31 | |
32 | { |
33 | # localize $@ to prevent clobbering of previous value by a successful |
34 | # eval. |
35 | local $@; |
36 | |
37 | # failed will be true if the eval dies, because 1 will not be returned |
38 | # from the eval body |
39 | $failed = not eval { |
40 | |
41 | # evaluate the try block in the correct context |
42 | if ( $wantarray ) { |
43 | @ret = $try->(); |
44 | } elsif ( defined $wantarray ) { |
45 | $ret[0] = $try->(); |
46 | } else { |
47 | $try->(); |
48 | }; |
49 | |
50 | return 1; # properly set $fail to false |
51 | }; |
52 | |
53 | # copy $@ to $error, when we leave this scope local $@ will revert $@ |
54 | # back to its previous value |
55 | $error = $@; |
56 | } |
57 | |
58 | # at this point $failed contains a true value if the eval died even if some |
59 | # destructor overwrite $@ as the eval was unwinding. |
60 | if ( $failed ) { |
61 | # if we got an error, invoke the catch block. |
62 | if ( $catch ) { |
63 | # This works like given($error), but is backwards compatible and |
64 | # sets $_ in the dynamic scope for the body of C<$catch> |
65 | for ($error) { |
66 | return $catch->($error); |
67 | } |
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68 | |
69 | # in case when() was used without an explicit return, the C<for> |
70 | # loop will be aborted and there's no useful return value |
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71 | } |
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72 | |
73 | return; |
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74 | } else { |
75 | # no failure, $@ is back to what it was, everything is fine |
76 | return $wantarray ? @ret : $ret[0]; |
77 | } |
78 | } |
79 | |
80 | sub catch (&) { |
81 | return $_[0]; |
82 | } |
83 | |
84 | |
85 | __PACKAGE__ |
86 | |
87 | __END__ |
88 | |
89 | =pod |
90 | |
91 | =head1 NAME |
92 | |
93 | Try::Tiny - minimal try/catch with proper localization of $@ |
94 | |
95 | =head1 SYNOPSIS |
96 | |
97 | # handle errors with a catch handler |
98 | try { |
99 | die "foo"; |
100 | } catch { |
101 | warn "caught error: $_"; |
102 | }; |
103 | |
104 | # just silence errors |
105 | try { |
106 | die "foo"; |
107 | }; |
108 | |
109 | =head1 DESCRIPTION |
110 | |
111 | This module provides bare bones C<try>/C<catch> statements that are designed to |
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112 | minimize common mistakes with eval blocks, and NOTHING else. |
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113 | |
114 | This is unlike L<TryCatch> which provides a nice syntax and avoids adding |
115 | another call stack layer, and supports calling C<return> from the try block to |
116 | return from the parent subroutine. These extra features come at a cost of a few |
117 | dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are |
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118 | occasionally problematic, and the additional catch filtering uses L<Moose> |
119 | type constraints which may not be desirable either. |
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120 | |
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121 | The main focus of this module is to provide simple and reliable error handling |
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122 | for those having a hard time installing L<TryCatch>, but who still want to |
123 | write correct C<eval> blocks without 5 lines of boilerplate each time. |
124 | |
125 | It's designed to work as correctly as possible in light of the various |
126 | pathological edge cases (see L<BACKGROUND>) and to be compatible with any style |
127 | of error values (simple strings, references, objects, overloaded objects, etc). |
128 | |
129 | =head1 EXPORTS |
130 | |
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131 | All functions are exported by default using L<Exporter>. |
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132 | |
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133 | In the future L<Sub::Exporter> may be used to allow the keywords to be renamed, |
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134 | but this technically does not satisfy Adam Kennedy's definition of "Tiny". |
135 | |
136 | =over 4 |
137 | |
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138 | =item try (&;$) |
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139 | |
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140 | Takes one mandatory try subroutine and one optional catch subroutine. |
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141 | |
142 | The mandatory subroutine is evaluated in the context of an C<eval> block. |
143 | |
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144 | If no error occurred the value from the first block is returned, preserving |
145 | list/scalar context. |
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146 | |
147 | If there was an error and the second subroutine was given it will be invoked |
148 | with the error in C<$_> (localized) and as that block's first and only |
149 | argument. |
150 | |
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151 | Note that the error may be false, but if that happens the C<catch> block will |
152 | still be invoked.. |
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153 | |
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154 | =item catch (&) |
155 | |
156 | Intended to be used in the second argument position of C<try>. |
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157 | |
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158 | Just returns the subroutine it was given. |
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159 | |
160 | catch { ... } |
161 | |
162 | is the same as |
163 | |
164 | sub { ... } |
165 | |
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166 | =back |
167 | |
168 | =head1 BACKGROUND |
169 | |
170 | There are a number of issues with C<eval>. |
171 | |
172 | =head2 Clobbering $@ |
173 | |
174 | When you run an eval block and it succeeds, C<$@> will be cleared, potentially |
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175 | clobbering an error that is currently being caught. |
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176 | |
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177 | This causes action at a distance, clearing previous errors your caller may have |
178 | not yet handled. |
179 | |
180 | C<$@> must be properly localized before invoking C<eval> in order to avoid this |
181 | issue. |
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182 | |
183 | =head2 Localizing $@ silently masks errors |
184 | |
185 | Inside an eval block C<die> behaves sort of like: |
186 | |
187 | sub die { |
188 | $@_ = $_[0]; |
189 | return_undef_from_eval(); |
190 | } |
191 | |
192 | This means that if you were polite and localized C<$@> you can't die in that |
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193 | scope, or your error will be discarded (printing "Something's wrong" instead). |
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194 | |
195 | The workaround is very ugly: |
196 | |
197 | my $error = do { |
198 | local $@; |
199 | eval { ... }; |
200 | $@; |
201 | }; |
202 | |
203 | ... |
204 | die $error; |
205 | |
206 | =head2 $@ might not be a true value |
207 | |
208 | This code is wrong: |
209 | |
210 | if ( $@ ) { |
211 | ... |
212 | } |
213 | |
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214 | because due to the previous caveats it may have been unset. |
215 | |
216 | C<$@> could also an overloaded error object that evaluates to false, but that's |
217 | asking for trouble anyway. |
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218 | |
219 | The classic failure mode is: |
220 | |
221 | sub Object::DESTROY { |
222 | eval { ... } |
223 | } |
224 | |
225 | eval { |
226 | my $obj = Object->new; |
227 | |
228 | die "foo"; |
229 | }; |
230 | |
231 | if ( $@ ) { |
232 | |
233 | } |
234 | |
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235 | In this case since C<Object::DESTROY> is not localizing C<$@> but still uses |
236 | C<eval> it will set C<$@> to C<"">. |
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237 | |
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238 | The destructor is called when the stack is unwound, after C<die> sets C<$@> to |
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239 | C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has |
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240 | been cleared by C<eval> in the destructor. |
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241 | |
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242 | The workaround for this is even uglier than the previous ones. Even though we |
243 | can't save the value of C<$@> from code that doesn't localize, we can at least |
244 | be sure the eval was aborted due to an error: |
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245 | |
246 | my $failed = not eval { |
247 | ... |
248 | |
249 | return 1; |
250 | }; |
251 | |
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252 | This is because an C<eval> that caught a C<die> will always return a false |
253 | value. |
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254 | |
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255 | =head1 SHINY SYNTAX |
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256 | |
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257 | Using Perl 5.10 you can use L<perlsyn/"Switch statements">. |
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258 | |
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259 | The C<catch> block is invoked in a topicalizer context (like a C<given> block), |
260 | but note that you can't return a useful value from C<catch> using the C<when> |
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261 | blocks without an explicit C<return>. |
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262 | |
263 | This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to |
264 | concisely match errors: |
265 | |
266 | try { |
267 | require Foo; |
268 | } catch { |
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269 | when (/^Can't locate .*?\.pm in \@INC/) { } # ignore |
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270 | default { die $_ } |
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271 | }; |
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272 | |
273 | =head1 CAVEATS |
274 | |
275 | =over 4 |
276 | |
277 | =item * |
278 | |
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279 | C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp> |
280 | will report this when using full stack traces. This lack of magic is considered |
281 | a feature. |
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282 | |
283 | =item * |
284 | |
285 | The value of C<$_> in the C<catch> block is not guaranteed to be preserved, |
286 | there is no safe way to ensure this if C<eval> is used unhygenically in |
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287 | destructors. It's only guaranteeed that the C<catch> will be called. |
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288 | |
289 | =back |
290 | |
291 | =head1 SEE ALSO |
292 | |
293 | =over 4 |
294 | |
295 | =item L<TryCatch> |
296 | |
297 | Much more feature complete, more convenient semantics, but at the cost of |
298 | implementation complexity. |
299 | |
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300 | =item L<Throwable> |
301 | |
302 | A lightweight role for rolling your own exception classes. |
303 | |
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304 | =item L<Error> |
305 | |
306 | Exception object implementation with a C<try> statement. Does not localize |
307 | C<$@>. |
308 | |
309 | =item L<Exception::Class::TryCatch> |
310 | |
311 | Provides a C<catch> statement, but properly calling C<eval> is your |
312 | responsibility. |
313 | |
314 | The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the |
315 | issues with C<$@> but you still need to localize to prevent clobbering. |
316 | |
317 | =back |
318 | |
319 | =head1 VERSION CONTROL |
320 | |
321 | L<http://github.com/nothingmuch/try-tiny/> |
322 | |
323 | =head1 AUTHOR |
324 | |
325 | Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt> |
326 | |
327 | =head1 COPYRIGHT |
328 | |
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329 | Copyright (c) 2009 Yuval Kogman. All rights reserved. |
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330 | This program is free software; you can redistribute |
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331 | it and/or modify it under the terms of the MIT license. |
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332 | |
333 | =cut |
334 | |