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1 | package Try::Tiny; |
2 | |
3 | use strict; |
ae53da51 |
4 | #use warnings; |
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5 | |
ae53da51 |
6 | use vars qw(@EXPORT @EXPORT_OK $VERSION @ISA); |
7 | |
8 | BEGIN { |
9 | require Exporter; |
10 | @ISA = qw(Exporter); |
11 | } |
3176feef |
12 | |
308df09e |
13 | $VERSION = "0.09"; |
3176feef |
14 | |
15 | $VERSION = eval $VERSION; |
16 | |
7195fc08 |
17 | @EXPORT = @EXPORT_OK = qw(try catch finally); |
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18 | |
6f114080 |
19 | $Carp::Internal{+__PACKAGE__}++; |
20 | |
7195fc08 |
21 | # Need to prototype as @ not $$ because of the way Perl evaluates the prototype. |
22 | # Keeping it at $$ means you only ever get 1 sub because we need to eval in a list |
23 | # context & not a scalar one |
24 | |
25 | sub try (&;@) { |
26 | my ( $try, @code_refs ) = @_; |
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27 | |
28 | # we need to save this here, the eval block will be in scalar context due |
29 | # to $failed |
30 | my $wantarray = wantarray; |
31 | |
b611f396 |
32 | my ( $catch, @finally ); |
7195fc08 |
33 | |
34 | # find labeled blocks in the argument list. |
35 | # catch and finally tag the blocks by blessing a scalar reference to them. |
36 | foreach my $code_ref (@code_refs) { |
37 | next unless $code_ref; |
38 | |
39 | my $ref = ref($code_ref); |
40 | |
41 | if ( $ref eq 'Try::Tiny::Catch' ) { |
42 | $catch = ${$code_ref}; |
43 | } elsif ( $ref eq 'Try::Tiny::Finally' ) { |
b611f396 |
44 | push @finally, ${$code_ref}; |
7195fc08 |
45 | } else { |
46 | use Carp; |
47 | confess("Unknown code ref type given '${ref}'. Check your usage & try again"); |
48 | } |
49 | } |
50 | |
1d64c1ad |
51 | # save the value of $@ so we can set $@ back to it in the beginning of the eval |
511c05ca |
52 | my $prev_error = $@; |
53 | |
3176feef |
54 | my ( @ret, $error, $failed ); |
55 | |
1d64c1ad |
56 | # FIXME consider using local $SIG{__DIE__} to accumulate all errors. It's |
3176feef |
57 | # not perfect, but we could provide a list of additional errors for |
58 | # $catch->(); |
59 | |
60 | { |
61 | # localize $@ to prevent clobbering of previous value by a successful |
62 | # eval. |
63 | local $@; |
64 | |
65 | # failed will be true if the eval dies, because 1 will not be returned |
66 | # from the eval body |
67 | $failed = not eval { |
511c05ca |
68 | $@ = $prev_error; |
3176feef |
69 | |
70 | # evaluate the try block in the correct context |
71 | if ( $wantarray ) { |
72 | @ret = $try->(); |
73 | } elsif ( defined $wantarray ) { |
74 | $ret[0] = $try->(); |
75 | } else { |
76 | $try->(); |
77 | }; |
78 | |
79 | return 1; # properly set $fail to false |
80 | }; |
81 | |
1d64c1ad |
82 | # copy $@ to $error; when we leave this scope, local $@ will revert $@ |
3176feef |
83 | # back to its previous value |
84 | $error = $@; |
85 | } |
86 | |
82ef0e61 |
87 | # set up a scope guard to invoke the finally block at the end |
b611f396 |
88 | my @guards = |
89 | map { Try::Tiny::ScopeGuard->_new($_, $failed ? $error : ()) } |
90 | @finally; |
82ef0e61 |
91 | |
1d64c1ad |
92 | # at this point $failed contains a true value if the eval died, even if some |
93 | # destructor overwrote $@ as the eval was unwinding. |
3176feef |
94 | if ( $failed ) { |
95 | # if we got an error, invoke the catch block. |
96 | if ( $catch ) { |
97 | # This works like given($error), but is backwards compatible and |
98 | # sets $_ in the dynamic scope for the body of C<$catch> |
99 | for ($error) { |
82ef0e61 |
100 | return $catch->($error); |
3176feef |
101 | } |
44599111 |
102 | |
103 | # in case when() was used without an explicit return, the C<for> |
104 | # loop will be aborted and there's no useful return value |
3176feef |
105 | } |
44599111 |
106 | |
107 | return; |
3176feef |
108 | } else { |
109 | # no failure, $@ is back to what it was, everything is fine |
110 | return $wantarray ? @ret : $ret[0]; |
111 | } |
112 | } |
113 | |
7195fc08 |
114 | sub catch (&;@) { |
115 | my ( $block, @rest ) = @_; |
116 | |
117 | return ( |
118 | bless(\$block, 'Try::Tiny::Catch'), |
119 | @rest, |
120 | ); |
3176feef |
121 | } |
122 | |
7195fc08 |
123 | sub finally (&;@) { |
124 | my ( $block, @rest ) = @_; |
125 | |
126 | return ( |
127 | bless(\$block, 'Try::Tiny::Finally'), |
128 | @rest, |
129 | ); |
130 | } |
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131 | |
b611f396 |
132 | { |
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133 | package # hide from PAUSE |
134 | Try::Tiny::ScopeGuard; |
b611f396 |
135 | |
136 | sub _new { |
137 | shift; |
138 | bless [ @_ ]; |
139 | } |
140 | |
141 | sub DESTROY { |
142 | my @guts = @{ shift() }; |
143 | my $code = shift @guts; |
144 | $code->(@guts); |
145 | } |
82ef0e61 |
146 | } |
147 | |
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148 | __PACKAGE__ |
149 | |
150 | __END__ |
151 | |
152 | =pod |
153 | |
154 | =head1 NAME |
155 | |
156 | Try::Tiny - minimal try/catch with proper localization of $@ |
157 | |
158 | =head1 SYNOPSIS |
159 | |
160 | # handle errors with a catch handler |
161 | try { |
162 | die "foo"; |
163 | } catch { |
2dc64249 |
164 | warn "caught error: $_"; # not $@ |
3176feef |
165 | }; |
166 | |
167 | # just silence errors |
168 | try { |
169 | die "foo"; |
170 | }; |
171 | |
172 | =head1 DESCRIPTION |
173 | |
7195fc08 |
174 | This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to |
1f7c5af6 |
175 | minimize common mistakes with eval blocks, and NOTHING else. |
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176 | |
177 | This is unlike L<TryCatch> which provides a nice syntax and avoids adding |
178 | another call stack layer, and supports calling C<return> from the try block to |
179 | return from the parent subroutine. These extra features come at a cost of a few |
180 | dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are |
1f7c5af6 |
181 | occasionally problematic, and the additional catch filtering uses L<Moose> |
182 | type constraints which may not be desirable either. |
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183 | |
1f7c5af6 |
184 | The main focus of this module is to provide simple and reliable error handling |
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185 | for those having a hard time installing L<TryCatch>, but who still want to |
186 | write correct C<eval> blocks without 5 lines of boilerplate each time. |
187 | |
188 | It's designed to work as correctly as possible in light of the various |
189 | pathological edge cases (see L<BACKGROUND>) and to be compatible with any style |
190 | of error values (simple strings, references, objects, overloaded objects, etc). |
191 | |
a5cd5f73 |
192 | If the try block dies, it returns the value of the last statement executed in |
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193 | the catch block, if there is one. Otherwise, it returns C<undef> in scalar |
a5cd5f73 |
194 | context or the empty list in list context. The following two examples both |
195 | assign C<"bar"> to C<$x>. |
196 | |
197 | my $x = try { die "foo" } catch { "bar" }; |
198 | |
199 | my $x = eval { die "foo" } || "bar"; |
200 | |
7195fc08 |
201 | You can add finally blocks making the following true. |
202 | |
203 | my $x; |
204 | try { die 'foo' } finally { $x = 'bar' }; |
205 | try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' }; |
206 | |
207 | Finally blocks are always executed making them suitable for cleanup code |
9610f171 |
208 | which cannot be handled using local. You can add as many finally blocks to a |
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209 | given try block as you like. |
7195fc08 |
210 | |
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211 | =head1 EXPORTS |
212 | |
1f7c5af6 |
213 | All functions are exported by default using L<Exporter>. |
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214 | |
7195fc08 |
215 | If you need to rename the C<try>, C<catch> or C<finally> keyword consider using |
6157bcb8 |
216 | L<Sub::Import> to get L<Sub::Exporter>'s flexibility. |
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217 | |
218 | =over 4 |
219 | |
7195fc08 |
220 | =item try (&;@) |
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221 | |
7195fc08 |
222 | Takes one mandatory try subroutine, an optional catch subroutine & finally |
223 | subroutine. |
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224 | |
225 | The mandatory subroutine is evaluated in the context of an C<eval> block. |
226 | |
1f7c5af6 |
227 | If no error occurred the value from the first block is returned, preserving |
228 | list/scalar context. |
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229 | |
230 | If there was an error and the second subroutine was given it will be invoked |
231 | with the error in C<$_> (localized) and as that block's first and only |
232 | argument. |
233 | |
2dc64249 |
234 | C<$@> does B<not> contain the error. Inside the C<catch> block it has the same |
235 | value it had before the C<try> block was executed. |
236 | |
1f7c5af6 |
237 | Note that the error may be false, but if that happens the C<catch> block will |
1d64c1ad |
238 | still be invoked. |
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239 | |
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240 | Once all execution is finished then the finally block if given will execute. |
241 | |
242 | =item catch (&;$) |
1f7c5af6 |
243 | |
244 | Intended to be used in the second argument position of C<try>. |
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245 | |
7195fc08 |
246 | Returns a reference to the subroutine it was given but blessed as |
247 | C<Try::Tiny::Catch> which allows try to decode correctly what to do |
248 | with this code reference. |
3176feef |
249 | |
250 | catch { ... } |
251 | |
2dc64249 |
252 | Inside the catch block the caught error is stored in C<$_>, while previous |
253 | value of C<$@> is still available for use. This value may or may not be |
254 | meaningful depending on what happened before the C<try>, but it might be a good |
255 | idea to preserve it in an error stack. |
ac4f5f9f |
256 | |
0a0641f9 |
257 | For code that captures C<$@> when throwing new errors (i.e. |
258 | L<Class::Throwable>), you'll need to do: |
259 | |
260 | local $@ = $_; |
261 | |
7195fc08 |
262 | =item finally (&;$) |
263 | |
264 | try { ... } |
265 | catch { ... } |
266 | finally { ... }; |
267 | |
268 | Or |
269 | |
270 | try { ... } |
271 | finally { ... }; |
272 | |
273 | Or even |
274 | |
275 | try { ... } |
276 | finally { ... } |
277 | catch { ... }; |
278 | |
279 | Intended to be the second or third element of C<try>. Finally blocks are always |
280 | executed in the event of a successful C<try> or if C<catch> is run. This allows |
281 | you to locate cleanup code which cannot be done via C<local()> e.g. closing a file |
282 | handle. |
283 | |
d6e0f0df |
284 | When invoked, the finally block is passed the error that was caught. If no |
285 | error was caught, it is passed nothing. In other words, the following code |
286 | does just what you would expect: |
287 | |
288 | try { |
289 | die_sometimes(); |
290 | } catch { |
291 | # ...code run in case of error |
292 | } finally { |
293 | if (@_) { |
294 | print "The try block died with: @_\n"; |
295 | } else { |
296 | print "The try block ran without error.\n"; |
297 | } |
298 | }; |
299 | |
7195fc08 |
300 | B<You must always do your own error handling in the finally block>. C<Try::Tiny> will |
301 | not do anything about handling possible errors coming from code located in these |
302 | blocks. |
303 | |
304 | In the same way C<catch()> blesses the code reference this subroutine does the same |
305 | except it bless them as C<Try::Tiny::Finally>. |
306 | |
3176feef |
307 | =back |
308 | |
309 | =head1 BACKGROUND |
310 | |
311 | There are a number of issues with C<eval>. |
312 | |
313 | =head2 Clobbering $@ |
314 | |
315 | When you run an eval block and it succeeds, C<$@> will be cleared, potentially |
a717a876 |
316 | clobbering an error that is currently being caught. |
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317 | |
1f7c5af6 |
318 | This causes action at a distance, clearing previous errors your caller may have |
319 | not yet handled. |
320 | |
321 | C<$@> must be properly localized before invoking C<eval> in order to avoid this |
322 | issue. |
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323 | |
8e5b4441 |
324 | More specifically, C<$@> is clobbered at the beginning of the C<eval>, which |
511c05ca |
325 | also makes it impossible to capture the previous error before you die (for |
326 | instance when making exception objects with error stacks). |
327 | |
328 | For this reason C<try> will actually set C<$@> to its previous value (before |
1d64c1ad |
329 | the localization) in the beginning of the C<eval> block. |
511c05ca |
330 | |
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331 | =head2 Localizing $@ silently masks errors |
332 | |
333 | Inside an eval block C<die> behaves sort of like: |
334 | |
335 | sub die { |
91254b51 |
336 | $@ = $_[0]; |
3176feef |
337 | return_undef_from_eval(); |
338 | } |
339 | |
340 | This means that if you were polite and localized C<$@> you can't die in that |
1f7c5af6 |
341 | scope, or your error will be discarded (printing "Something's wrong" instead). |
3176feef |
342 | |
343 | The workaround is very ugly: |
344 | |
345 | my $error = do { |
346 | local $@; |
347 | eval { ... }; |
348 | $@; |
349 | }; |
350 | |
351 | ... |
352 | die $error; |
353 | |
354 | =head2 $@ might not be a true value |
355 | |
356 | This code is wrong: |
357 | |
358 | if ( $@ ) { |
359 | ... |
360 | } |
361 | |
1f7c5af6 |
362 | because due to the previous caveats it may have been unset. |
363 | |
1d64c1ad |
364 | C<$@> could also be an overloaded error object that evaluates to false, but |
365 | that's asking for trouble anyway. |
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366 | |
367 | The classic failure mode is: |
368 | |
369 | sub Object::DESTROY { |
370 | eval { ... } |
371 | } |
372 | |
373 | eval { |
374 | my $obj = Object->new; |
375 | |
376 | die "foo"; |
377 | }; |
378 | |
379 | if ( $@ ) { |
380 | |
381 | } |
382 | |
1f7c5af6 |
383 | In this case since C<Object::DESTROY> is not localizing C<$@> but still uses |
1d64c1ad |
384 | C<eval>, it will set C<$@> to C<"">. |
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385 | |
1f7c5af6 |
386 | The destructor is called when the stack is unwound, after C<die> sets C<$@> to |
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387 | C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has |
1f7c5af6 |
388 | been cleared by C<eval> in the destructor. |
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389 | |
1f7c5af6 |
390 | The workaround for this is even uglier than the previous ones. Even though we |
391 | can't save the value of C<$@> from code that doesn't localize, we can at least |
392 | be sure the eval was aborted due to an error: |
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393 | |
394 | my $failed = not eval { |
395 | ... |
396 | |
397 | return 1; |
398 | }; |
399 | |
1f7c5af6 |
400 | This is because an C<eval> that caught a C<die> will always return a false |
401 | value. |
3176feef |
402 | |
f9b91e2c |
403 | =head1 SHINY SYNTAX |
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404 | |
1f7c5af6 |
405 | Using Perl 5.10 you can use L<perlsyn/"Switch statements">. |
3176feef |
406 | |
1f7c5af6 |
407 | The C<catch> block is invoked in a topicalizer context (like a C<given> block), |
408 | but note that you can't return a useful value from C<catch> using the C<when> |
27293e40 |
409 | blocks without an explicit C<return>. |
3176feef |
410 | |
411 | This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to |
412 | concisely match errors: |
413 | |
414 | try { |
415 | require Foo; |
416 | } catch { |
1f7c5af6 |
417 | when (/^Can't locate .*?\.pm in \@INC/) { } # ignore |
3176feef |
418 | default { die $_ } |
deb85b37 |
419 | }; |
3176feef |
420 | |
421 | =head1 CAVEATS |
422 | |
423 | =over 4 |
424 | |
425 | =item * |
426 | |
013dca8f |
427 | C<@_> is not available within the C<try> block, so you need to copy your |
428 | arglist. In case you want to work with argument values directly via C<@_> |
429 | aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference: |
318cb1eb |
430 | |
431 | sub foo { |
432 | my ( $self, @args ) = @_; |
433 | try { $self->bar(@args) } |
434 | } |
013dca8f |
435 | |
436 | or |
437 | |
438 | sub bar_in_place { |
439 | my $self = shift; |
440 | my $args = \@_; |
441 | try { $_ = $self->bar($_) for @$args } |
442 | } |
318cb1eb |
443 | |
444 | =item * |
445 | |
446 | C<return> returns from the C<try> block, not from the parent sub (note that |
447 | this is also how C<eval> works, but not how L<TryCatch> works): |
448 | |
449 | sub bar { |
450 | try { return "foo" }; |
451 | return "baz"; |
452 | } |
453 | |
454 | say bar(); # "baz" |
455 | |
456 | =item * |
457 | |
1f7c5af6 |
458 | C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp> |
c12e626f |
459 | will not report this when using full stack traces, though, because |
460 | C<%Carp::Internal> is used. This lack of magic is considered a feature. |
3176feef |
461 | |
462 | =item * |
463 | |
57c50f41 |
464 | The value of C<$_> in the C<catch> block is not guaranteed to be the value of |
465 | the exception thrown (C<$@>) in the C<try> block. There is no safe way to |
466 | ensure this, since C<eval> may be used unhygenically in destructors. The only |
467 | guarantee is that the C<catch> will be called if an exception is thrown. |
3176feef |
468 | |
a5cd5f73 |
469 | =item * |
470 | |
471 | The return value of the C<catch> block is not ignored, so if testing the result |
472 | of the expression for truth on success, be sure to return a false value from |
473 | the C<catch> block: |
474 | |
475 | my $obj = try { |
476 | MightFail->new; |
477 | } catch { |
478 | ... |
479 | |
480 | return; # avoid returning a true value; |
481 | }; |
482 | |
483 | return unless $obj; |
484 | |
eaca95b7 |
485 | =item * |
486 | |
487 | C<$SIG{__DIE__}> is still in effect. |
488 | |
489 | Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of |
490 | C<eval> blocks, since it isn't people have grown to rely on it. Therefore in |
491 | the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for |
492 | the scope of the error throwing code. |
493 | |
cbfb5327 |
494 | =item * |
495 | |
496 | Lexical C<$_> may override the one set by C<catch>. |
497 | |
498 | For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some |
499 | confusing behavior: |
500 | |
501 | given ($foo) { |
502 | when (...) { |
503 | try { |
504 | ... |
505 | } catch { |
506 | warn $_; # will print $foo, not the error |
507 | warn $_[0]; # instead, get the error like this |
508 | } |
509 | } |
510 | } |
511 | |
3176feef |
512 | =back |
513 | |
514 | =head1 SEE ALSO |
515 | |
516 | =over 4 |
517 | |
518 | =item L<TryCatch> |
519 | |
520 | Much more feature complete, more convenient semantics, but at the cost of |
521 | implementation complexity. |
522 | |
9bc603cb |
523 | =item L<autodie> |
524 | |
525 | Automatic error throwing for builtin functions and more. Also designed to |
526 | work well with C<given>/C<when>. |
527 | |
f8227e43 |
528 | =item L<Throwable> |
529 | |
530 | A lightweight role for rolling your own exception classes. |
531 | |
3176feef |
532 | =item L<Error> |
533 | |
534 | Exception object implementation with a C<try> statement. Does not localize |
535 | C<$@>. |
536 | |
537 | =item L<Exception::Class::TryCatch> |
538 | |
539 | Provides a C<catch> statement, but properly calling C<eval> is your |
540 | responsibility. |
541 | |
542 | The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the |
1d64c1ad |
543 | issues with C<$@>, but you still need to localize to prevent clobbering. |
3176feef |
544 | |
545 | =back |
546 | |
faecd5a0 |
547 | =head1 LIGHTNING TALK |
548 | |
549 | I gave a lightning talk about this module, you can see the slides (Firefox |
550 | only): |
551 | |
e9140680 |
552 | L<http://nothingmuch.woobling.org/talks/takahashi.xul?data=yapc_asia_2009/try_tiny.txt> |
faecd5a0 |
553 | |
554 | Or read the source: |
555 | |
556 | L<http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml> |
557 | |
3176feef |
558 | =head1 VERSION CONTROL |
559 | |
560 | L<http://github.com/nothingmuch/try-tiny/> |
561 | |
562 | =head1 AUTHOR |
563 | |
564 | Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt> |
565 | |
566 | =head1 COPYRIGHT |
567 | |
c4e1eb12 |
568 | Copyright (c) 2009 Yuval Kogman. All rights reserved. |
3176feef |
569 | This program is free software; you can redistribute |
c4e1eb12 |
570 | it and/or modify it under the terms of the MIT license. |
3176feef |
571 | |
572 | =cut |
573 | |