Commit | Line | Data |
3176feef |
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 | |
5fb1c15f |
13 | $VERSION = "0.06"; |
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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 | { |
133 | package Try::Tiny::ScopeGuard; |
134 | |
135 | sub _new { |
136 | shift; |
137 | bless [ @_ ]; |
138 | } |
139 | |
140 | sub DESTROY { |
141 | my @guts = @{ shift() }; |
142 | my $code = shift @guts; |
143 | $code->(@guts); |
144 | } |
82ef0e61 |
145 | } |
146 | |
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147 | __PACKAGE__ |
148 | |
149 | __END__ |
150 | |
151 | =pod |
152 | |
153 | =head1 NAME |
154 | |
155 | Try::Tiny - minimal try/catch with proper localization of $@ |
156 | |
157 | =head1 SYNOPSIS |
158 | |
159 | # handle errors with a catch handler |
160 | try { |
161 | die "foo"; |
162 | } catch { |
2dc64249 |
163 | warn "caught error: $_"; # not $@ |
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164 | }; |
165 | |
166 | # just silence errors |
167 | try { |
168 | die "foo"; |
169 | }; |
170 | |
171 | =head1 DESCRIPTION |
172 | |
7195fc08 |
173 | This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to |
1f7c5af6 |
174 | minimize common mistakes with eval blocks, and NOTHING else. |
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175 | |
176 | This is unlike L<TryCatch> which provides a nice syntax and avoids adding |
177 | another call stack layer, and supports calling C<return> from the try block to |
178 | return from the parent subroutine. These extra features come at a cost of a few |
179 | dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are |
1f7c5af6 |
180 | occasionally problematic, and the additional catch filtering uses L<Moose> |
181 | type constraints which may not be desirable either. |
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182 | |
1f7c5af6 |
183 | The main focus of this module is to provide simple and reliable error handling |
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184 | for those having a hard time installing L<TryCatch>, but who still want to |
185 | write correct C<eval> blocks without 5 lines of boilerplate each time. |
186 | |
187 | It's designed to work as correctly as possible in light of the various |
188 | pathological edge cases (see L<BACKGROUND>) and to be compatible with any style |
189 | of error values (simple strings, references, objects, overloaded objects, etc). |
190 | |
a5cd5f73 |
191 | If the try block dies, it returns the value of the last statement executed in |
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192 | the catch block, if there is one. Otherwise, it returns C<undef> in scalar |
a5cd5f73 |
193 | context or the empty list in list context. The following two examples both |
194 | assign C<"bar"> to C<$x>. |
195 | |
196 | my $x = try { die "foo" } catch { "bar" }; |
197 | |
198 | my $x = eval { die "foo" } || "bar"; |
199 | |
7195fc08 |
200 | You can add finally blocks making the following true. |
201 | |
202 | my $x; |
203 | try { die 'foo' } finally { $x = 'bar' }; |
204 | try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' }; |
205 | |
206 | Finally blocks are always executed making them suitable for cleanup code |
9610f171 |
207 | which cannot be handled using local. You can add as many finally blocks to a |
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208 | given try block as you like. |
7195fc08 |
209 | |
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210 | =head1 EXPORTS |
211 | |
1f7c5af6 |
212 | All functions are exported by default using L<Exporter>. |
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213 | |
7195fc08 |
214 | If you need to rename the C<try>, C<catch> or C<finally> keyword consider using |
6157bcb8 |
215 | L<Sub::Import> to get L<Sub::Exporter>'s flexibility. |
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216 | |
217 | =over 4 |
218 | |
7195fc08 |
219 | =item try (&;@) |
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220 | |
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221 | Takes one mandatory try subroutine, an optional catch subroutine & finally |
222 | subroutine. |
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223 | |
224 | The mandatory subroutine is evaluated in the context of an C<eval> block. |
225 | |
1f7c5af6 |
226 | If no error occurred the value from the first block is returned, preserving |
227 | list/scalar context. |
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228 | |
229 | If there was an error and the second subroutine was given it will be invoked |
230 | with the error in C<$_> (localized) and as that block's first and only |
231 | argument. |
232 | |
2dc64249 |
233 | C<$@> does B<not> contain the error. Inside the C<catch> block it has the same |
234 | value it had before the C<try> block was executed. |
235 | |
1f7c5af6 |
236 | Note that the error may be false, but if that happens the C<catch> block will |
1d64c1ad |
237 | still be invoked. |
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238 | |
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239 | Once all execution is finished then the finally block if given will execute. |
240 | |
241 | =item catch (&;$) |
1f7c5af6 |
242 | |
243 | Intended to be used in the second argument position of C<try>. |
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244 | |
7195fc08 |
245 | Returns a reference to the subroutine it was given but blessed as |
246 | C<Try::Tiny::Catch> which allows try to decode correctly what to do |
247 | with this code reference. |
3176feef |
248 | |
249 | catch { ... } |
250 | |
2dc64249 |
251 | Inside the catch block the caught error is stored in C<$_>, while previous |
252 | value of C<$@> is still available for use. This value may or may not be |
253 | meaningful depending on what happened before the C<try>, but it might be a good |
254 | idea to preserve it in an error stack. |
ac4f5f9f |
255 | |
0a0641f9 |
256 | For code that captures C<$@> when throwing new errors (i.e. |
257 | L<Class::Throwable>), you'll need to do: |
258 | |
259 | local $@ = $_; |
260 | |
7195fc08 |
261 | =item finally (&;$) |
262 | |
263 | try { ... } |
264 | catch { ... } |
265 | finally { ... }; |
266 | |
267 | Or |
268 | |
269 | try { ... } |
270 | finally { ... }; |
271 | |
272 | Or even |
273 | |
274 | try { ... } |
275 | finally { ... } |
276 | catch { ... }; |
277 | |
278 | Intended to be the second or third element of C<try>. Finally blocks are always |
279 | executed in the event of a successful C<try> or if C<catch> is run. This allows |
280 | you to locate cleanup code which cannot be done via C<local()> e.g. closing a file |
281 | handle. |
282 | |
d6e0f0df |
283 | When invoked, the finally block is passed the error that was caught. If no |
284 | error was caught, it is passed nothing. In other words, the following code |
285 | does just what you would expect: |
286 | |
287 | try { |
288 | die_sometimes(); |
289 | } catch { |
290 | # ...code run in case of error |
291 | } finally { |
292 | if (@_) { |
293 | print "The try block died with: @_\n"; |
294 | } else { |
295 | print "The try block ran without error.\n"; |
296 | } |
297 | }; |
298 | |
7195fc08 |
299 | B<You must always do your own error handling in the finally block>. C<Try::Tiny> will |
300 | not do anything about handling possible errors coming from code located in these |
301 | blocks. |
302 | |
303 | In the same way C<catch()> blesses the code reference this subroutine does the same |
304 | except it bless them as C<Try::Tiny::Finally>. |
305 | |
3176feef |
306 | =back |
307 | |
308 | =head1 BACKGROUND |
309 | |
310 | There are a number of issues with C<eval>. |
311 | |
312 | =head2 Clobbering $@ |
313 | |
314 | When you run an eval block and it succeeds, C<$@> will be cleared, potentially |
a717a876 |
315 | clobbering an error that is currently being caught. |
3176feef |
316 | |
1f7c5af6 |
317 | This causes action at a distance, clearing previous errors your caller may have |
318 | not yet handled. |
319 | |
320 | C<$@> must be properly localized before invoking C<eval> in order to avoid this |
321 | issue. |
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322 | |
8e5b4441 |
323 | More specifically, C<$@> is clobbered at the beginning of the C<eval>, which |
511c05ca |
324 | also makes it impossible to capture the previous error before you die (for |
325 | instance when making exception objects with error stacks). |
326 | |
327 | For this reason C<try> will actually set C<$@> to its previous value (before |
1d64c1ad |
328 | the localization) in the beginning of the C<eval> block. |
511c05ca |
329 | |
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330 | =head2 Localizing $@ silently masks errors |
331 | |
332 | Inside an eval block C<die> behaves sort of like: |
333 | |
334 | sub die { |
91254b51 |
335 | $@ = $_[0]; |
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336 | return_undef_from_eval(); |
337 | } |
338 | |
339 | This means that if you were polite and localized C<$@> you can't die in that |
1f7c5af6 |
340 | scope, or your error will be discarded (printing "Something's wrong" instead). |
3176feef |
341 | |
342 | The workaround is very ugly: |
343 | |
344 | my $error = do { |
345 | local $@; |
346 | eval { ... }; |
347 | $@; |
348 | }; |
349 | |
350 | ... |
351 | die $error; |
352 | |
353 | =head2 $@ might not be a true value |
354 | |
355 | This code is wrong: |
356 | |
357 | if ( $@ ) { |
358 | ... |
359 | } |
360 | |
1f7c5af6 |
361 | because due to the previous caveats it may have been unset. |
362 | |
1d64c1ad |
363 | C<$@> could also be an overloaded error object that evaluates to false, but |
364 | that's asking for trouble anyway. |
3176feef |
365 | |
366 | The classic failure mode is: |
367 | |
368 | sub Object::DESTROY { |
369 | eval { ... } |
370 | } |
371 | |
372 | eval { |
373 | my $obj = Object->new; |
374 | |
375 | die "foo"; |
376 | }; |
377 | |
378 | if ( $@ ) { |
379 | |
380 | } |
381 | |
1f7c5af6 |
382 | In this case since C<Object::DESTROY> is not localizing C<$@> but still uses |
1d64c1ad |
383 | C<eval>, it will set C<$@> to C<"">. |
3176feef |
384 | |
1f7c5af6 |
385 | The destructor is called when the stack is unwound, after C<die> sets C<$@> to |
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386 | C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has |
1f7c5af6 |
387 | been cleared by C<eval> in the destructor. |
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388 | |
1f7c5af6 |
389 | The workaround for this is even uglier than the previous ones. Even though we |
390 | can't save the value of C<$@> from code that doesn't localize, we can at least |
391 | be sure the eval was aborted due to an error: |
3176feef |
392 | |
393 | my $failed = not eval { |
394 | ... |
395 | |
396 | return 1; |
397 | }; |
398 | |
1f7c5af6 |
399 | This is because an C<eval> that caught a C<die> will always return a false |
400 | value. |
3176feef |
401 | |
f9b91e2c |
402 | =head1 SHINY SYNTAX |
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403 | |
1f7c5af6 |
404 | Using Perl 5.10 you can use L<perlsyn/"Switch statements">. |
3176feef |
405 | |
1f7c5af6 |
406 | The C<catch> block is invoked in a topicalizer context (like a C<given> block), |
407 | but note that you can't return a useful value from C<catch> using the C<when> |
27293e40 |
408 | blocks without an explicit C<return>. |
3176feef |
409 | |
410 | This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to |
411 | concisely match errors: |
412 | |
413 | try { |
414 | require Foo; |
415 | } catch { |
1f7c5af6 |
416 | when (/^Can't locate .*?\.pm in \@INC/) { } # ignore |
3176feef |
417 | default { die $_ } |
deb85b37 |
418 | }; |
3176feef |
419 | |
420 | =head1 CAVEATS |
421 | |
422 | =over 4 |
423 | |
424 | =item * |
425 | |
013dca8f |
426 | C<@_> is not available within the C<try> block, so you need to copy your |
427 | arglist. In case you want to work with argument values directly via C<@_> |
428 | aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference: |
318cb1eb |
429 | |
430 | sub foo { |
431 | my ( $self, @args ) = @_; |
432 | try { $self->bar(@args) } |
433 | } |
013dca8f |
434 | |
435 | or |
436 | |
437 | sub bar_in_place { |
438 | my $self = shift; |
439 | my $args = \@_; |
440 | try { $_ = $self->bar($_) for @$args } |
441 | } |
318cb1eb |
442 | |
443 | =item * |
444 | |
445 | C<return> returns from the C<try> block, not from the parent sub (note that |
446 | this is also how C<eval> works, but not how L<TryCatch> works): |
447 | |
448 | sub bar { |
449 | try { return "foo" }; |
450 | return "baz"; |
451 | } |
452 | |
453 | say bar(); # "baz" |
454 | |
455 | =item * |
456 | |
1f7c5af6 |
457 | C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp> |
c12e626f |
458 | will not report this when using full stack traces, though, because |
459 | C<%Carp::Internal> is used. This lack of magic is considered a feature. |
3176feef |
460 | |
461 | =item * |
462 | |
57c50f41 |
463 | The value of C<$_> in the C<catch> block is not guaranteed to be the value of |
464 | the exception thrown (C<$@>) in the C<try> block. There is no safe way to |
465 | ensure this, since C<eval> may be used unhygenically in destructors. The only |
466 | guarantee is that the C<catch> will be called if an exception is thrown. |
3176feef |
467 | |
a5cd5f73 |
468 | =item * |
469 | |
470 | The return value of the C<catch> block is not ignored, so if testing the result |
471 | of the expression for truth on success, be sure to return a false value from |
472 | the C<catch> block: |
473 | |
474 | my $obj = try { |
475 | MightFail->new; |
476 | } catch { |
477 | ... |
478 | |
479 | return; # avoid returning a true value; |
480 | }; |
481 | |
482 | return unless $obj; |
483 | |
eaca95b7 |
484 | =item * |
485 | |
486 | C<$SIG{__DIE__}> is still in effect. |
487 | |
488 | Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of |
489 | C<eval> blocks, since it isn't people have grown to rely on it. Therefore in |
490 | the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for |
491 | the scope of the error throwing code. |
492 | |
cbfb5327 |
493 | =item * |
494 | |
495 | Lexical C<$_> may override the one set by C<catch>. |
496 | |
497 | For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some |
498 | confusing behavior: |
499 | |
500 | given ($foo) { |
501 | when (...) { |
502 | try { |
503 | ... |
504 | } catch { |
505 | warn $_; # will print $foo, not the error |
506 | warn $_[0]; # instead, get the error like this |
507 | } |
508 | } |
509 | } |
510 | |
3176feef |
511 | =back |
512 | |
513 | =head1 SEE ALSO |
514 | |
515 | =over 4 |
516 | |
517 | =item L<TryCatch> |
518 | |
519 | Much more feature complete, more convenient semantics, but at the cost of |
520 | implementation complexity. |
521 | |
9bc603cb |
522 | =item L<autodie> |
523 | |
524 | Automatic error throwing for builtin functions and more. Also designed to |
525 | work well with C<given>/C<when>. |
526 | |
f8227e43 |
527 | =item L<Throwable> |
528 | |
529 | A lightweight role for rolling your own exception classes. |
530 | |
3176feef |
531 | =item L<Error> |
532 | |
533 | Exception object implementation with a C<try> statement. Does not localize |
534 | C<$@>. |
535 | |
536 | =item L<Exception::Class::TryCatch> |
537 | |
538 | Provides a C<catch> statement, but properly calling C<eval> is your |
539 | responsibility. |
540 | |
541 | The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the |
1d64c1ad |
542 | issues with C<$@>, but you still need to localize to prevent clobbering. |
3176feef |
543 | |
544 | =back |
545 | |
faecd5a0 |
546 | =head1 LIGHTNING TALK |
547 | |
548 | I gave a lightning talk about this module, you can see the slides (Firefox |
549 | only): |
550 | |
e9140680 |
551 | L<http://nothingmuch.woobling.org/talks/takahashi.xul?data=yapc_asia_2009/try_tiny.txt> |
faecd5a0 |
552 | |
553 | Or read the source: |
554 | |
555 | L<http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml> |
556 | |
3176feef |
557 | =head1 VERSION CONTROL |
558 | |
559 | L<http://github.com/nothingmuch/try-tiny/> |
560 | |
561 | =head1 AUTHOR |
562 | |
563 | Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt> |
564 | |
565 | =head1 COPYRIGHT |
566 | |
c4e1eb12 |
567 | Copyright (c) 2009 Yuval Kogman. All rights reserved. |
3176feef |
568 | This program is free software; you can redistribute |
c4e1eb12 |
569 | it and/or modify it under the terms of the MIT license. |
3176feef |
570 | |
571 | =cut |
572 | |