3 # ABSTRACT: Minimal try/catch with proper preservation of $@
10 use Exporter 5.57 'import';
11 our @EXPORT = our @EXPORT_OK = qw(try catch finally);
14 $Carp::Internal{+__PACKAGE__}++;
17 my $su = $INC{'Sub/Util.pm'} && defined &Sub::Util::set_subname;
18 my $sn = $INC{'Sub/Name.pm'} && eval { Sub::Name->VERSION(0.08) };
20 $su = eval { require Sub::Util; } && defined &Sub::Util::set_subname;
22 $sn = eval { require Sub::Name; Sub::Name->VERSION(0.08) };
26 *_subname = $su ? \&Sub::Util::set_subname
27 : $sn ? \&Sub::Name::subname
29 *_HAS_SUBNAME = ($su || $sn) ? sub(){1} : sub(){0};
34 # Need to prototype as @ not $$ because of the way Perl evaluates the prototype.
35 # Keeping it at $$ means you only ever get 1 sub because we need to eval in a list
36 # context & not a scalar one
39 my ( $try, @code_refs ) = @_;
41 # we need to save this here, the eval block will be in scalar context due
43 my $wantarray = wantarray;
45 # work around perl bug by explicitly initializing these, due to the likelyhood
46 # this will be used in global destruction (perl rt#119311)
47 my ( $catch, @finally ) = ();
49 # find labeled blocks in the argument list.
50 # catch and finally tag the blocks by blessing a scalar reference to them.
51 foreach my $code_ref (@code_refs) {
53 if ( ref($code_ref) eq 'Try::Tiny::Catch' ) {
54 croak 'A try() may not be followed by multiple catch() blocks'
56 $catch = ${$code_ref};
57 } elsif ( ref($code_ref) eq 'Try::Tiny::Finally' ) {
58 push @finally, ${$code_ref};
61 'try() encountered an unexpected argument ('
62 . ( defined $code_ref ? $code_ref : 'undef' )
63 . ') - perhaps a missing semi-colon before or'
68 # FIXME consider using local $SIG{__DIE__} to accumulate all errors. It's
69 # not perfect, but we could provide a list of additional errors for
72 # name the blocks if we have Sub::Name installed
74 _subname("${caller}::try {...} " => $try)
77 # set up scope guards to invoke the finally blocks at the end.
78 # this should really be a function scope lexical variable instead of
79 # file scope + local but that causes issues with perls < 5.20 due to
81 local $_finally_guards{guards} = [
82 map { Try::Tiny::ScopeGuard->_new($_) }
86 # save the value of $@ so we can set $@ back to it in the beginning of the eval
87 # and restore $@ after the eval finishes
92 # failed will be true if the eval dies, because 1 will not be returned
94 my $failed = not eval {
97 # evaluate the try block in the correct context
100 } elsif ( defined $wantarray ) {
106 return 1; # properly set $failed to false
109 # preserve the current error and reset the original value of $@
113 # at this point $failed contains a true value if the eval died, even if some
114 # destructor overwrote $@ as the eval was unwinding.
116 # pass $error to the finally blocks
117 push @$_, $error for @{$_finally_guards{guards}};
119 # if we got an error, invoke the catch block.
121 # This works like given($error), but is backwards compatible and
122 # sets $_ in the dynamic scope for the body of C<$catch>
124 return $catch->($error);
127 # in case when() was used without an explicit return, the C<for>
128 # loop will be aborted and there's no useful return value
133 # no failure, $@ is back to what it was, everything is fine
134 return $wantarray ? @ret : $ret[0];
139 my ( $block, @rest ) = @_;
141 croak 'Useless bare catch()' unless wantarray;
144 _subname("${caller}::catch {...} " => $block)
147 bless(\$block, 'Try::Tiny::Catch'),
153 my ( $block, @rest ) = @_;
155 croak 'Useless bare finally()' unless wantarray;
158 _subname("${caller}::finally {...} " => $block)
161 bless(\$block, 'Try::Tiny::Finally'),
167 package # hide from PAUSE
168 Try::Tiny::ScopeGuard;
170 use constant UNSTABLE_DOLLARAT => ("$]" < '5.013002') ? 1 : 0;
178 my ($code, @args) = @{ $_[0] };
180 local $@ if UNSTABLE_DOLLARAT;
186 "Execution of finally() block $code resulted in an exception, which "
187 . '*CAN NOT BE PROPAGATED* due to fundamental limitations of Perl. '
188 . 'Your program will continue as if this event never took place. '
189 . "Original exception text follows:\n\n"
190 . (defined $@ ? $@ : '$@ left undefined...')
205 You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
206 conditions, avoiding quirks in Perl and common mistakes:
208 # handle errors with a catch handler
212 warn "caught error: $_"; # not $@
215 You can also use it like a standalone C<eval> to catch and ignore any error
216 conditions. Obviously, this is an extreme measure not to be undertaken
219 # just silence errors
226 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
227 minimize common mistakes with eval blocks, and NOTHING else.
229 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
230 another call stack layer, and supports calling C<return> from the C<try> block to
231 return from the parent subroutine. These extra features come at a cost of a few
232 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
233 occasionally problematic, and the additional catch filtering uses L<Moose>
234 type constraints which may not be desirable either.
236 The main focus of this module is to provide simple and reliable error handling
237 for those having a hard time installing L<TryCatch>, but who still want to
238 write correct C<eval> blocks without 5 lines of boilerplate each time.
240 It's designed to work as correctly as possible in light of the various
241 pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
242 of error values (simple strings, references, objects, overloaded objects, etc).
244 If the C<try> block dies, it returns the value of the last statement executed in
245 the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
246 context or the empty list in list context. The following examples all
247 assign C<"bar"> to C<$x>:
249 my $x = try { die "foo" } catch { "bar" };
250 my $x = try { die "foo" } || "bar";
251 my $x = (try { die "foo" }) // "bar";
253 my $x = eval { die "foo" } || "bar";
255 You can add C<finally> blocks, yielding the following:
258 try { die 'foo' } finally { $x = 'bar' };
259 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
261 C<finally> blocks are always executed making them suitable for cleanup code
262 which cannot be handled using local. You can add as many C<finally> blocks to a
263 given C<try> block as you like.
265 Note that adding a C<finally> block without a preceding C<catch> block
266 suppresses any errors. This behaviour is consistent with using a standalone
267 C<eval>, but it is not consistent with C<try>/C<finally> patterns found in
268 other programming languages, such as Java, Python, Javascript or C#. If you
269 learnt the C<try>/C<finally> pattern from one of these languages, watch out for
274 All functions are exported by default using L<Exporter>.
276 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
277 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
283 Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
286 The mandatory subroutine is evaluated in the context of an C<eval> block.
288 If no error occurred the value from the first block is returned, preserving
291 If there was an error and the second subroutine was given it will be invoked
292 with the error in C<$_> (localized) and as that block's first and only
295 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
296 value it had before the C<try> block was executed.
298 Note that the error may be false, but if that happens the C<catch> block will
301 Once all execution is finished then the C<finally> block, if given, will execute.
305 Intended to be used in the second argument position of C<try>.
307 Returns a reference to the subroutine it was given but blessed as
308 C<Try::Tiny::Catch> which allows try to decode correctly what to do
309 with this code reference.
313 Inside the C<catch> block the caught error is stored in C<$_>, while previous
314 value of C<$@> is still available for use. This value may or may not be
315 meaningful depending on what happened before the C<try>, but it might be a good
316 idea to preserve it in an error stack.
318 For code that captures C<$@> when throwing new errors (i.e.
319 L<Class::Throwable>), you'll need to do:
340 Intended to be the second or third element of C<try>. C<finally> blocks are always
341 executed in the event of a successful C<try> or if C<catch> is run. This allows
342 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
345 When invoked, the C<finally> block is passed the error that was caught. If no
346 error was caught, it is passed nothing. (Note that the C<finally> block does not
347 localize C<$_> with the error, since unlike in a C<catch> block, there is no way
348 to know if C<$_ == undef> implies that there were no errors.) In other words,
349 the following code does just what you would expect:
354 # ...code run in case of error
357 print "The try block died with: @_\n";
359 print "The try block ran without error.\n";
363 B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will
364 not do anything about handling possible errors coming from code located in these
367 Furthermore B<exceptions in C<finally> blocks are not trappable and are unable
368 to influence the execution of your program>. This is due to limitation of
369 C<DESTROY>-based scope guards, which C<finally> is implemented on top of. This
370 may change in a future version of Try::Tiny.
372 In the same way C<catch()> blesses the code reference this subroutine does the same
373 except it bless them as C<Try::Tiny::Finally>.
379 There are a number of issues with C<eval>.
383 When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially
384 clobbering an error that is currently being caught.
386 This causes action at a distance, clearing previous errors your caller may have
389 C<$@> must be properly localized before invoking C<eval> in order to avoid this
392 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
393 also makes it impossible to capture the previous error before you die (for
394 instance when making exception objects with error stacks).
396 For this reason C<try> will actually set C<$@> to its previous value (the one
397 available before entering the C<try> block) in the beginning of the C<eval>
400 =head2 Localizing $@ silently masks errors
402 Inside an C<eval> block, C<die> behaves sort of like:
406 return_undef_from_eval();
409 This means that if you were polite and localized C<$@> you can't die in that
410 scope, or your error will be discarded (printing "Something's wrong" instead).
412 The workaround is very ugly:
423 =head2 $@ might not be a true value
431 because due to the previous caveats it may have been unset.
433 C<$@> could also be an overloaded error object that evaluates to false, but
434 that's asking for trouble anyway.
436 The classic failure mode is:
438 sub Object::DESTROY {
443 my $obj = Object->new;
452 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
453 C<eval>, it will set C<$@> to C<"">.
455 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
456 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
457 been cleared by C<eval> in the destructor.
459 The workaround for this is even uglier than the previous ones. Even though we
460 can't save the value of C<$@> from code that doesn't localize, we can at least
461 be sure the C<eval> was aborted due to an error:
463 my $failed = not eval {
469 This is because an C<eval> that caught a C<die> will always return a false
474 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
476 =for stopwords topicalizer
478 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
479 but note that you can't return a useful value from C<catch> using the C<when>
480 blocks without an explicit C<return>.
482 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
483 concisely match errors:
488 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
498 C<@_> is not available within the C<try> block, so you need to copy your
499 argument list. In case you want to work with argument values directly via C<@_>
500 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
503 my ( $self, @args ) = @_;
504 try { $self->bar(@args) }
512 try { $_ = $self->bar($_) for @$args }
517 C<return> returns from the C<try> block, not from the parent sub (note that
518 this is also how C<eval> works, but not how L<TryCatch> works):
528 say "this text WILL be displayed, even though an exception is thrown";
531 Instead, you should capture the return value:
538 return unless $success;
540 say "This text WILL NEVER appear!";
543 sub parent_sub_with_catch {
549 # do something with $_
550 return undef; #see note
552 return unless $success;
554 say "This text WILL NEVER appear!";
557 Note that if you have a C<catch> block, it must return C<undef> for this to work,
558 since if a C<catch> block exists, its return value is returned in place of C<undef>
559 when an exception is thrown.
563 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
564 will not report this when using full stack traces, though, because
565 C<%Carp::Internal> is used. This lack of magic is considered a feature.
567 =for stopwords unhygienically
571 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
572 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
573 ensure this, since C<eval> may be used unhygienically in destructors. The only
574 guarantee is that the C<catch> will be called if an exception is thrown.
578 The return value of the C<catch> block is not ignored, so if testing the result
579 of the expression for truth on success, be sure to return a false value from
587 return; # avoid returning a true value;
594 C<$SIG{__DIE__}> is still in effect.
596 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
597 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
598 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
599 the scope of the error throwing code.
603 Lexical C<$_> may override the one set by C<catch>.
605 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
613 warn $_; # will print $foo, not the error
614 warn $_[0]; # instead, get the error like this
619 Note that this behavior was changed once again in L<Perl5 version 18
620 |https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
621 However, since the entirety of lexical C<$_> is now L<considered experimental
622 |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
623 is unclear whether the new version 18 behavior is final.
633 Much more feature complete, more convenient semantics, but at the cost of
634 implementation complexity.
638 Automatic error throwing for builtin functions and more. Also designed to
639 work well with C<given>/C<when>.
643 A lightweight role for rolling your own exception classes.
647 Exception object implementation with a C<try> statement. Does not localize
650 =item L<Exception::Class::TryCatch>
652 Provides a C<catch> statement, but properly calling C<eval> is your
655 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
656 issues with C<$@>, but you still need to localize to prevent clobbering.
660 =head1 LIGHTNING TALK
662 I gave a lightning talk about this module, you can see the slides (Firefox
665 L<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
669 L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>