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
203 L<before Perl version 5.14.0|perl5140delta/"Exception Handling">
204 C<$@> was clobbered at the beginning of the C<eval>, which
205 also made it impossible to capture the previous error before you die (for
206 instance when making exception objects with error stacks).
208 For this reason C<try> will actually set C<$@> to its previous value (the one
209 available before entering the C<try> block) in the beginning of the C<eval>
212 =head2 Localizing $@ silently masks errors
214 Inside an C<eval> block, C<die> behaves sort of like:
218 return_undef_from_eval();
221 This means that if you were polite and localized C<$@> you can't die in that
222 scope, or your error will be discarded (printing "Something's wrong" instead).
224 The workaround is very ugly:
235 =head2 $@ might not be a true value
243 because due to the previous caveats it may have been unset.
245 C<$@> could also be an overloaded error object that evaluates to false, but
246 that's asking for trouble anyway.
248 The classic failure mode (fixed in L<Perl 5.14.0|perl5140delta/"Exception Handling">) is:
250 sub Object::DESTROY {
255 my $obj = Object->new;
264 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
265 C<eval>, it will set C<$@> to C<"">.
267 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
268 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
269 been cleared by C<eval> in the destructor.
271 The workaround for this is even uglier than the previous ones. Even though we
272 can't save the value of C<$@> from code that doesn't localize, we can at least
273 be sure the C<eval> was aborted due to an error:
275 my $failed = not eval {
281 This is because an C<eval> that caught a C<die> will always return a false
284 =head1 ALTERNATE SYNTAX
286 Using Perl 5.10 you can use L<perlsyn/"Switch statements"> (but please don't,
287 because that syntax has since been deprecated because there was too much
288 unexpected magical behaviour).
290 =for stopwords topicalizer
292 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
293 but note that you can't return a useful value from C<catch> using the C<when>
294 blocks without an explicit C<return>.
296 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
297 concisely match errors:
302 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
312 C<@_> is not available within the C<try> block, so you need to copy your
313 argument list. In case you want to work with argument values directly via C<@_>
314 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
317 my ( $self, @args ) = @_;
318 try { $self->bar(@args) }
326 try { $_ = $self->bar($_) for @$args }
331 C<return> returns from the C<try> block, not from the parent sub (note that
332 this is also how C<eval> works, but not how L<TryCatch> works):
342 say "this text WILL be displayed, even though an exception is thrown";
345 Instead, you should capture the return value:
352 return unless $success;
354 say "This text WILL NEVER appear!";
357 sub parent_sub_with_catch {
363 # do something with $_
364 return undef; #see note
366 return unless $success;
368 say "This text WILL NEVER appear!";
371 Note that if you have a C<catch> block, it must return C<undef> for this to work,
372 since if a C<catch> block exists, its return value is returned in place of C<undef>
373 when an exception is thrown.
377 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
378 will not report this when using full stack traces, though, because
379 C<%Carp::Internal> is used. This lack of magic is considered a feature.
381 =for stopwords unhygienically
385 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
386 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
387 ensure this, since C<eval> may be used unhygienically in destructors. The only
388 guarantee is that the C<catch> will be called if an exception is thrown.
392 The return value of the C<catch> block is not ignored, so if testing the result
393 of the expression for truth on success, be sure to return a false value from
401 return; # avoid returning a true value;
408 C<$SIG{__DIE__}> is still in effect.
410 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
411 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
412 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
413 the scope of the error throwing code.
417 Lexical C<$_> may override the one set by C<catch>.
419 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
427 warn $_; # will print $foo, not the error
428 warn $_[0]; # instead, get the error like this
433 Note that this behavior was changed once again in
434 L<Perl5 version 18|https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
435 However, since the entirety of lexical C<$_> is now L<considered experimental
436 |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
437 is unclear whether the new version 18 behavior is final.
447 Much more feature complete, more convenient semantics, but at the cost of
448 implementation complexity.
452 Automatic error throwing for builtin functions and more. Also designed to
453 work well with C<given>/C<when>.
457 A lightweight role for rolling your own exception classes.
461 Exception object implementation with a C<try> statement. Does not localize
464 =item L<Exception::Class::TryCatch>
466 Provides a C<catch> statement, but properly calling C<eval> is your
469 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
470 issues with C<$@>, but you still need to localize to prevent clobbering.
474 =head1 LIGHTNING TALK
476 I gave a lightning talk about this module, you can see the slides (Firefox
479 L<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
483 L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
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 Karen Etheridge Peter Rabbitson Ricardo Signes Mark Fowler Graham Knop Lukas Mai Aristotle Pagaltzis Dagfinn Ilmari Mannsåker Paul Howarth Rudolf Leermakers anaxagoras awalker chromatic Alex cm-perl Andrew Yates David Lowe Glenn Hans Dieter Pearcey Jens Berthold Jonathan Yu Marc Mims Stosberg Pali
512 Karen Etheridge <ether@cpan.org>
516 Peter Rabbitson <ribasushi@cpan.org>
520 Ricardo Signes <rjbs@cpan.org>
524 Mark Fowler <mark@twoshortplanks.com>
528 Graham Knop <haarg@haarg.org>
532 Lukas Mai <l.mai@web.de>
536 Aristotle Pagaltzis <pagaltzis@gmx.de>
540 Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
544 Paul Howarth <paul@city-fan.org>
548 Rudolf Leermakers <rudolf@hatsuseno.org>
552 anaxagoras <walkeraj@gmail.com>
556 awalker <awalker@sourcefire.com>
560 chromatic <chromatic@wgz.org>
564 Alex <alex@koban.(none)>
568 cm-perl <cm-perl@users.noreply.github.com>
572 Andrew Yates <ayates@haddock.local>
576 David Lowe <davidl@lokku.com>
580 Glenn Fowler <cebjyre@cpan.org>
584 Hans Dieter Pearcey <hdp@weftsoar.net>
588 Jens Berthold <jens@jebecs.de>
592 Jonathan Yu <JAWNSY@cpan.org>
596 Marc Mims <marc@questright.com>
600 Mark Stosberg <mark@stosberg.com>
608 =head1 COPYRIGHT AND LICENCE
610 This software is Copyright (c) 2009 by יובל קוג'מן (Yuval Kogman).
612 This is free software, licensed under:
614 The MIT (X11) License