3 # ABSTRACT: minimal try/catch with proper preservation of $@
9 our @EXPORT = our @EXPORT_OK = qw(try catch finally);
12 $Carp::Internal{+__PACKAGE__}++;
14 BEGIN { eval "use Sub::Name; 1" or *{subname} = sub {1} }
16 # Need to prototype as @ not $$ because of the way Perl evaluates the prototype.
17 # Keeping it at $$ means you only ever get 1 sub because we need to eval in a list
18 # context & not a scalar one
21 my ( $try, @code_refs ) = @_;
23 # we need to save this here, the eval block will be in scalar context due
25 my $wantarray = wantarray;
27 my ( $catch, @finally );
29 # find labeled blocks in the argument list.
30 # catch and finally tag the blocks by blessing a scalar reference to them.
31 foreach my $code_ref (@code_refs) {
33 if ( ref($code_ref) eq 'Try::Tiny::Catch' ) {
34 croak 'A try() may not be followed by multiple catch() blocks'
36 $catch = ${$code_ref};
37 } elsif ( ref($code_ref) eq 'Try::Tiny::Finally' ) {
38 push @finally, ${$code_ref};
41 'try() encountered an unexpected argument ('
42 . ( defined $code_ref ? $code_ref : 'undef' )
43 . ') - perhaps a missing semi-colon before or'
48 # FIXME consider using local $SIG{__DIE__} to accumulate all errors. It's
49 # not perfect, but we could provide a list of additional errors for
52 # name the blocks if we have Sub::Name installed
54 subname("${caller}::try {...}" => $try);
55 subname("${caller}::catch {...} " => $catch) if $catch;
56 subname("${caller}::finally {...} " => $_) foreach @finally;
58 # save the value of $@ so we can set $@ back to it in the beginning of the eval
59 # and restore $@ after the eval finishes
64 # failed will be true if the eval dies, because 1 will not be returned
66 my $failed = not eval {
69 # evaluate the try block in the correct context
72 } elsif ( defined $wantarray ) {
78 return 1; # properly set $fail to false
81 # preserve the current error and reset the original value of $@
85 # set up a scope guard to invoke the finally block at the end
87 map { Try::Tiny::ScopeGuard->_new($_, $failed ? $error : ()) }
90 # at this point $failed contains a true value if the eval died, even if some
91 # destructor overwrote $@ as the eval was unwinding.
93 # if we got an error, invoke the catch block.
95 # This works like given($error), but is backwards compatible and
96 # sets $_ in the dynamic scope for the body of C<$catch>
98 return $catch->($error);
101 # in case when() was used without an explicit return, the C<for>
102 # loop will be aborted and there's no useful return value
107 # no failure, $@ is back to what it was, everything is fine
108 return $wantarray ? @ret : $ret[0];
113 my ( $block, @rest ) = @_;
115 croak 'Useless bare catch()' unless wantarray;
118 bless(\$block, 'Try::Tiny::Catch'),
124 my ( $block, @rest ) = @_;
126 croak 'Useless bare finally()' unless wantarray;
129 bless(\$block, 'Try::Tiny::Finally'),
135 package # hide from PAUSE
136 Try::Tiny::ScopeGuard;
138 use constant UNSTABLE_DOLLARAT => ($] < '5.013002') ? 1 : 0;
146 my ($code, @args) = @{ $_[0] };
148 local $@ if UNSTABLE_DOLLARAT;
154 "Execution of finally() block $code resulted in an exception, which "
155 . '*CAN NOT BE PROPAGATED* due to fundamental limitations of Perl. '
156 . 'Your program will continue as if this event never took place. '
157 . "Original exception text follows:\n\n"
158 . (defined $@ ? $@ : '$@ left undefined...')
173 You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
174 conditions, avoiding quirks in Perl and common mistakes:
176 # handle errors with a catch handler
180 warn "caught error: $_"; # not $@
183 You can also use it like a standalone C<eval> to catch and ignore any error
184 conditions. Obviously, this is an extreme measure not to be undertaken
187 # just silence errors
194 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
195 minimize common mistakes with eval blocks, and NOTHING else.
197 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
198 another call stack layer, and supports calling C<return> from the C<try> block to
199 return from the parent subroutine. These extra features come at a cost of a few
200 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
201 occasionally problematic, and the additional catch filtering uses L<Moose>
202 type constraints which may not be desirable either.
204 The main focus of this module is to provide simple and reliable error handling
205 for those having a hard time installing L<TryCatch>, but who still want to
206 write correct C<eval> blocks without 5 lines of boilerplate each time.
208 It's designed to work as correctly as possible in light of the various
209 pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
210 of error values (simple strings, references, objects, overloaded objects, etc).
212 If the C<try> block dies, it returns the value of the last statement executed in
213 the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
214 context or the empty list in list context. The following examples all
215 assign C<"bar"> to C<$x>:
217 my $x = try { die "foo" } catch { "bar" };
218 my $x = try { die "foo" } || { "bar" };
219 my $x = (try { die "foo" }) // { "bar" };
221 my $x = eval { die "foo" } || "bar";
223 You can add C<finally> blocks, yielding the following:
226 try { die 'foo' } finally { $x = 'bar' };
227 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
229 C<finally> blocks are always executed making them suitable for cleanup code
230 which cannot be handled using local. You can add as many C<finally> blocks to a
231 given C<try> block as you like.
235 All functions are exported by default using L<Exporter>.
237 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
238 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
244 Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
247 The mandatory subroutine is evaluated in the context of an C<eval> block.
249 If no error occurred the value from the first block is returned, preserving
252 If there was an error and the second subroutine was given it will be invoked
253 with the error in C<$_> (localized) and as that block's first and only
256 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
257 value it had before the C<try> block was executed.
259 Note that the error may be false, but if that happens the C<catch> block will
262 Once all execution is finished then the C<finally> block, if given, will execute.
266 Intended to be used in the second argument position of C<try>.
268 Returns a reference to the subroutine it was given but blessed as
269 C<Try::Tiny::Catch> which allows try to decode correctly what to do
270 with this code reference.
274 Inside the C<catch> block the caught error is stored in C<$_>, while previous
275 value of C<$@> is still available for use. This value may or may not be
276 meaningful depending on what happened before the C<try>, but it might be a good
277 idea to preserve it in an error stack.
279 For code that captures C<$@> when throwing new errors (i.e.
280 L<Class::Throwable>), you'll need to do:
301 Intended to be the second or third element of C<try>. C<finally> blocks are always
302 executed in the event of a successful C<try> or if C<catch> is run. This allows
303 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
306 When invoked, the C<finally> block is passed the error that was caught. If no
307 error was caught, it is passed nothing. (Note that the C<finally> block does not
308 localize C<$_> with the error, since unlike in a C<catch> block, there is no way
309 to know if C<$_ == undef> implies that there were no errors.) In other words,
310 the following code does just what you would expect:
315 # ...code run in case of error
318 print "The try block died with: @_\n";
320 print "The try block ran without error.\n";
324 B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will
325 not do anything about handling possible errors coming from code located in these
328 Furthermore B<exceptions in C<finally> blocks are not trappable and are unable
329 to influence the execution of your program>. This is due to limitation of
330 C<DESTROY>-based scope guards, which C<finally> is implemented on top of. This
331 may change in a future version of Try::Tiny.
333 In the same way C<catch()> blesses the code reference this subroutine does the same
334 except it bless them as C<Try::Tiny::Finally>.
340 There are a number of issues with C<eval>.
344 When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially
345 clobbering an error that is currently being caught.
347 This causes action at a distance, clearing previous errors your caller may have
350 C<$@> must be properly localized before invoking C<eval> in order to avoid this
353 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
354 also makes it impossible to capture the previous error before you die (for
355 instance when making exception objects with error stacks).
357 For this reason C<try> will actually set C<$@> to its previous value (the one
358 available before entering the C<try> block) in the beginning of the C<eval>
361 =head2 Localizing $@ silently masks errors
363 Inside an C<eval> block, C<die> behaves sort of like:
367 return_undef_from_eval();
370 This means that if you were polite and localized C<$@> you can't die in that
371 scope, or your error will be discarded (printing "Something's wrong" instead).
373 The workaround is very ugly:
384 =head2 $@ might not be a true value
392 because due to the previous caveats it may have been unset.
394 C<$@> could also be an overloaded error object that evaluates to false, but
395 that's asking for trouble anyway.
397 The classic failure mode is:
399 sub Object::DESTROY {
404 my $obj = Object->new;
413 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
414 C<eval>, it will set C<$@> to C<"">.
416 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
417 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
418 been cleared by C<eval> in the destructor.
420 The workaround for this is even uglier than the previous ones. Even though we
421 can't save the value of C<$@> from code that doesn't localize, we can at least
422 be sure the C<eval> was aborted due to an error:
424 my $failed = not eval {
430 This is because an C<eval> that caught a C<die> will always return a false
435 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
437 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
438 but note that you can't return a useful value from C<catch> using the C<when>
439 blocks without an explicit C<return>.
441 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
442 concisely match errors:
447 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
457 C<@_> is not available within the C<try> block, so you need to copy your
458 arglist. In case you want to work with argument values directly via C<@_>
459 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
462 my ( $self, @args ) = @_;
463 try { $self->bar(@args) }
471 try { $_ = $self->bar($_) for @$args }
476 C<return> returns from the C<try> block, not from the parent sub (note that
477 this is also how C<eval> works, but not how L<TryCatch> works):
487 say "this text WILL be displayed, even though an exception is thrown";
490 Instead, you should capture the return value:
497 return unless $success;
499 say "This text WILL NEVER appear!";
502 Note that if you have a C<catch> block, it must return C<undef> for this to work,
503 since if a C<catch> block exists, its return value is returned in place of C<undef>
504 when an exception is thrown.
508 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
509 will not report this when using full stack traces, though, because
510 C<%Carp::Internal> is used. This lack of magic is considered a feature.
514 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
515 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
516 ensure this, since C<eval> may be used unhygenically in destructors. The only
517 guarantee is that the C<catch> will be called if an exception is thrown.
521 The return value of the C<catch> block is not ignored, so if testing the result
522 of the expression for truth on success, be sure to return a false value from
530 return; # avoid returning a true value;
537 C<$SIG{__DIE__}> is still in effect.
539 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
540 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
541 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
542 the scope of the error throwing code.
546 Lexical C<$_> may override the one set by C<catch>.
548 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
556 warn $_; # will print $foo, not the error
557 warn $_[0]; # instead, get the error like this
562 Note that this behavior was changed once again in L<Perl5 version 18
563 |https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
564 However, since the entirety of lexical C<$_> is now L<considired experimental
565 |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
566 is unclear whether the new version 18 behavior is final.
576 Much more feature complete, more convenient semantics, but at the cost of
577 implementation complexity.
581 Automatic error throwing for builtin functions and more. Also designed to
582 work well with C<given>/C<when>.
586 A lightweight role for rolling your own exception classes.
590 Exception object implementation with a C<try> statement. Does not localize
593 =item L<Exception::Class::TryCatch>
595 Provides a C<catch> statement, but properly calling C<eval> is your
598 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
599 issues with C<$@>, but you still need to localize to prevent clobbering.
603 =head1 LIGHTNING TALK
605 I gave a lightning talk about this module, you can see the slides (Firefox
608 L<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
612 L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
614 =head1 VERSION CONTROL
616 L<http://github.com/doy/try-tiny/>