7 Try::Tiny - minimal try/catch with proper preservation of $@
15 You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
16 conditions, avoiding quirks in Perl and common mistakes:
18 # handle errors with a catch handler
22 warn "caught error: $_"; # not $@
25 You can also use it like a standalone C<eval> to catch and ignore any error
26 conditions. Obviously, this is an extreme measure not to be undertaken
36 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
37 minimize common mistakes with eval blocks, and NOTHING else.
39 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
40 another call stack layer, and supports calling C<return> from the C<try> block to
41 return from the parent subroutine. These extra features come at a cost of a few
42 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
43 occasionally problematic, and the additional catch filtering uses L<Moose>
44 type constraints which may not be desirable either.
46 The main focus of this module is to provide simple and reliable error handling
47 for those having a hard time installing L<TryCatch>, but who still want to
48 write correct C<eval> blocks without 5 lines of boilerplate each time.
50 It's designed to work as correctly as possible in light of the various
51 pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
52 of error values (simple strings, references, objects, overloaded objects, etc).
54 If the C<try> block dies, it returns the value of the last statement executed in
55 the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
56 context or the empty list in list context. The following examples all
57 assign C<"bar"> to C<$x>:
59 my $x = try { die "foo" } catch { "bar" };
60 my $x = try { die "foo" } || "bar";
61 my $x = (try { die "foo" }) // "bar";
63 my $x = eval { die "foo" } || "bar";
65 You can add C<finally> blocks, yielding the following:
68 try { die 'foo' } finally { $x = 'bar' };
69 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
71 C<finally> blocks are always executed making them suitable for cleanup code
72 which cannot be handled using local. You can add as many C<finally> blocks to a
73 given C<try> block as you like.
75 Note that adding a C<finally> block without a preceding C<catch> block
76 suppresses any errors. This behaviour is consistent with using a standalone
77 C<eval>, but it is not consistent with C<try>/C<finally> patterns found in
78 other programming languages, such as Java, Python, Javascript or C#. If you
79 learnt the C<try>/C<finally> pattern from one of these languages, watch out for
84 All functions are exported by default using L<Exporter>.
86 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
87 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
93 Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
96 The mandatory subroutine is evaluated in the context of an C<eval> block.
98 If no error occurred the value from the first block is returned, preserving
101 If there was an error and the second subroutine was given it will be invoked
102 with the error in C<$_> (localized) and as that block's first and only
105 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
106 value it had before the C<try> block was executed.
108 Note that the error may be false, but if that happens the C<catch> block will
111 Once all execution is finished then the C<finally> block, if given, will execute.
115 Intended to be used in the second argument position of C<try>.
117 Returns a reference to the subroutine it was given but blessed as
118 C<Try::Tiny::Catch> which allows try to decode correctly what to do
119 with this code reference.
123 Inside the C<catch> block the caught error is stored in C<$_>, while previous
124 value of C<$@> is still available for use. This value may or may not be
125 meaningful depending on what happened before the C<try>, but it might be a good
126 idea to preserve it in an error stack.
128 For code that captures C<$@> when throwing new errors (i.e.
129 L<Class::Throwable>), you'll need to do:
150 Intended to be the second or third element of C<try>. C<finally> blocks are always
151 executed in the event of a successful C<try> or if C<catch> is run. This allows
152 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
155 When invoked, the C<finally> block is passed the error that was caught. If no
156 error was caught, it is passed nothing. (Note that the C<finally> block does not
157 localize C<$_> with the error, since unlike in a C<catch> block, there is no way
158 to know if C<$_ == undef> implies that there were no errors.) In other words,
159 the following code does just what you would expect:
164 # ...code run in case of error
167 print "The try block died with: @_\n";
169 print "The try block ran without error.\n";
173 B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will
174 not do anything about handling possible errors coming from code located in these
177 Furthermore B<exceptions in C<finally> blocks are not trappable and are unable
178 to influence the execution of your program>. This is due to limitation of
179 C<DESTROY>-based scope guards, which C<finally> is implemented on top of. This
180 may change in a future version of Try::Tiny.
182 In the same way C<catch()> blesses the code reference this subroutine does the same
183 except it bless them as C<Try::Tiny::Finally>.
189 There are a number of issues with C<eval>.
193 When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially
194 clobbering an error that is currently being caught.
196 This causes action at a distance, clearing previous errors your caller may have
199 C<$@> must be properly localized before invoking C<eval> in order to avoid this
202 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
203 also makes it impossible to capture the previous error before you die (for
204 instance when making exception objects with error stacks).
206 For this reason C<try> will actually set C<$@> to its previous value (the one
207 available before entering the C<try> block) in the beginning of the C<eval>
210 =head2 Localizing $@ silently masks errors
212 Inside an C<eval> block, C<die> behaves sort of like:
216 return_undef_from_eval();
219 This means that if you were polite and localized C<$@> you can't die in that
220 scope, or your error will be discarded (printing "Something's wrong" instead).
222 The workaround is very ugly:
233 =head2 $@ might not be a true value
241 because due to the previous caveats it may have been unset.
243 C<$@> could also be an overloaded error object that evaluates to false, but
244 that's asking for trouble anyway.
246 The classic failure mode is:
248 sub Object::DESTROY {
253 my $obj = Object->new;
262 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
263 C<eval>, it will set C<$@> to C<"">.
265 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
266 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
267 been cleared by C<eval> in the destructor.
269 The workaround for this is even uglier than the previous ones. Even though we
270 can't save the value of C<$@> from code that doesn't localize, we can at least
271 be sure the C<eval> was aborted due to an error:
273 my $failed = not eval {
279 This is because an C<eval> that caught a C<die> will always return a false
284 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
286 =for stopwords topicalizer
288 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
289 but note that you can't return a useful value from C<catch> using the C<when>
290 blocks without an explicit C<return>.
292 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
293 concisely match errors:
298 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
308 C<@_> is not available within the C<try> block, so you need to copy your
309 argument list. In case you want to work with argument values directly via C<@_>
310 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
313 my ( $self, @args ) = @_;
314 try { $self->bar(@args) }
322 try { $_ = $self->bar($_) for @$args }
327 C<return> returns from the C<try> block, not from the parent sub (note that
328 this is also how C<eval> works, but not how L<TryCatch> works):
338 say "this text WILL be displayed, even though an exception is thrown";
341 Instead, you should capture the return value:
348 return unless $success;
350 say "This text WILL NEVER appear!";
353 sub parent_sub_with_catch {
359 # do something with $_
360 return undef; #see note
362 return unless $success;
364 say "This text WILL NEVER appear!";
367 Note that if you have a C<catch> block, it must return C<undef> for this to work,
368 since if a C<catch> block exists, its return value is returned in place of C<undef>
369 when an exception is thrown.
373 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
374 will not report this when using full stack traces, though, because
375 C<%Carp::Internal> is used. This lack of magic is considered a feature.
377 =for stopwords unhygienically
381 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
382 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
383 ensure this, since C<eval> may be used unhygienically in destructors. The only
384 guarantee is that the C<catch> will be called if an exception is thrown.
388 The return value of the C<catch> block is not ignored, so if testing the result
389 of the expression for truth on success, be sure to return a false value from
397 return; # avoid returning a true value;
404 C<$SIG{__DIE__}> is still in effect.
406 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
407 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
408 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
409 the scope of the error throwing code.
413 Lexical C<$_> may override the one set by C<catch>.
415 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
423 warn $_; # will print $foo, not the error
424 warn $_[0]; # instead, get the error like this
429 Note that this behavior was changed once again in L<Perl5 version 18
430 |https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
431 However, since the entirety of lexical C<$_> is now L<considered experimental
432 |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
433 is unclear whether the new version 18 behavior is final.
443 Much more feature complete, more convenient semantics, but at the cost of
444 implementation complexity.
448 Automatic error throwing for builtin functions and more. Also designed to
449 work well with C<given>/C<when>.
453 A lightweight role for rolling your own exception classes.
457 Exception object implementation with a C<try> statement. Does not localize
460 =item L<Exception::Class::TryCatch>
462 Provides a C<catch> statement, but properly calling C<eval> is your
465 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
466 issues with C<$@>, but you still need to localize to prevent clobbering.
470 =head1 LIGHTNING TALK
472 I gave a lightning talk about this module, you can see the slides (Firefox
475 L<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
479 L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
481 =head1 VERSION CONTROL
483 L<http://github.com/doy/try-tiny/>
487 Bugs may be submitted through L<the RT bug tracker|https://rt.cpan.org/Public/Dist/Display.html?Name=Try-Tiny>
488 (or L<bug-Try-Tiny@rt.cpan.org|mailto:bug-Try-Tiny@rt.cpan.org>).
496 יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
500 Jesse Luehrs <doy@tozt.net>
506 =for stopwords Peter Rabbitson Karen Etheridge Ricardo Signes Mark Fowler Graham Knop Dagfinn Ilmari Mannsåker Paul Howarth Rudolf Leermakers anaxagoras awalker chromatic Alex cm-perl Andrew Yates David Lowe Glenn Hans Dieter Pearcey Jonathan Yu Marc Mims Stosberg
512 Peter Rabbitson <ribasushi@cpan.org>
516 Karen Etheridge <ether@cpan.org>
520 Ricardo Signes <rjbs@cpan.org>
524 Mark Fowler <mark@twoshortplanks.com>
528 Graham Knop <haarg@haarg.org>
532 Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
536 Paul Howarth <paul@city-fan.org>
540 Rudolf Leermakers <rudolf@hatsuseno.org>
544 anaxagoras <walkeraj@gmail.com>
548 awalker <awalker@sourcefire.com>
552 chromatic <chromatic@wgz.org>
556 Alex <alex@koban.(none)>
560 cm-perl <cm-perl@users.noreply.github.com>
564 Andrew Yates <ayates@haddock.local>
568 David Lowe <davidl@lokku.com>
572 Glenn Fowler <cebjyre@cpan.org>
576 Hans Dieter Pearcey <hdp@weftsoar.net>
580 Jonathan Yu <JAWNSY@cpan.org>
584 Marc Mims <marc@questright.com>
588 Mark Stosberg <mark@stosberg.com>
592 =head1 COPYRIGHT AND LICENCE
594 This software is Copyright (c) 2009 by יובל קוג'מן (Yuval Kogman).
596 This is free software, licensed under:
598 The MIT (X11) License