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
53 subname("try {...}" => $try);
54 subname("catch {...} " => $catch) if $catch;
55 subname("finally {...} " => $_) foreach @finally;
57 # save the value of $@ so we can set $@ back to it in the beginning of the eval
58 # and restore $@ after the eval finishes
63 # failed will be true if the eval dies, because 1 will not be returned
65 my $failed = not eval {
68 # evaluate the try block in the correct context
71 } elsif ( defined $wantarray ) {
77 return 1; # properly set $fail to false
80 # preserve the current error and reset the original value of $@
84 # set up a scope guard to invoke the finally block at the end
86 map { Try::Tiny::ScopeGuard->_new($_, $failed ? $error : ()) }
89 # at this point $failed contains a true value if the eval died, even if some
90 # destructor overwrote $@ as the eval was unwinding.
92 # if we got an error, invoke the catch block.
94 # This works like given($error), but is backwards compatible and
95 # sets $_ in the dynamic scope for the body of C<$catch>
97 return $catch->($error);
100 # in case when() was used without an explicit return, the C<for>
101 # loop will be aborted and there's no useful return value
106 # no failure, $@ is back to what it was, everything is fine
107 return $wantarray ? @ret : $ret[0];
112 my ( $block, @rest ) = @_;
114 croak 'Useless bare catch()' unless wantarray;
117 bless(\$block, 'Try::Tiny::Catch'),
123 my ( $block, @rest ) = @_;
125 croak 'Useless bare finally()' unless wantarray;
128 bless(\$block, 'Try::Tiny::Finally'),
134 package # hide from PAUSE
135 Try::Tiny::ScopeGuard;
137 use constant UNSTABLE_DOLLARAT => ($] < '5.013002') ? 1 : 0;
145 my ($code, @args) = @{ $_[0] };
147 local $@ if UNSTABLE_DOLLARAT;
153 "Execution of finally() block $code resulted in an exception, which "
154 . '*CAN NOT BE PROPAGATED* due to fundamental limitations of Perl. '
155 . 'Your program will continue as if this event never took place. '
156 . "Original exception text follows:\n\n"
157 . (defined $@ ? $@ : '$@ left undefined...')
172 You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
173 conditions, avoiding quirks in Perl and common mistakes:
175 # handle errors with a catch handler
179 warn "caught error: $_"; # not $@
182 You can also use it like a standalone C<eval> to catch and ignore any error
183 conditions. Obviously, this is an extreme measure not to be undertaken
186 # just silence errors
193 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
194 minimize common mistakes with eval blocks, and NOTHING else.
196 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
197 another call stack layer, and supports calling C<return> from the C<try> block to
198 return from the parent subroutine. These extra features come at a cost of a few
199 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
200 occasionally problematic, and the additional catch filtering uses L<Moose>
201 type constraints which may not be desirable either.
203 The main focus of this module is to provide simple and reliable error handling
204 for those having a hard time installing L<TryCatch>, but who still want to
205 write correct C<eval> blocks without 5 lines of boilerplate each time.
207 It's designed to work as correctly as possible in light of the various
208 pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
209 of error values (simple strings, references, objects, overloaded objects, etc).
211 If the C<try> block dies, it returns the value of the last statement executed in
212 the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
213 context or the empty list in list context. The following examples all
214 assign C<"bar"> to C<$x>:
216 my $x = try { die "foo" } catch { "bar" };
217 my $x = try { die "foo" } || { "bar" };
218 my $x = (try { die "foo" }) // { "bar" };
220 my $x = eval { die "foo" } || "bar";
222 You can add C<finally> blocks, yielding the following:
225 try { die 'foo' } finally { $x = 'bar' };
226 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
228 C<finally> blocks are always executed making them suitable for cleanup code
229 which cannot be handled using local. You can add as many C<finally> blocks to a
230 given C<try> block as you like.
234 All functions are exported by default using L<Exporter>.
236 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
237 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
243 Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
246 The mandatory subroutine is evaluated in the context of an C<eval> block.
248 If no error occurred the value from the first block is returned, preserving
251 If there was an error and the second subroutine was given it will be invoked
252 with the error in C<$_> (localized) and as that block's first and only
255 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
256 value it had before the C<try> block was executed.
258 Note that the error may be false, but if that happens the C<catch> block will
261 Once all execution is finished then the C<finally> block, if given, will execute.
265 Intended to be used in the second argument position of C<try>.
267 Returns a reference to the subroutine it was given but blessed as
268 C<Try::Tiny::Catch> which allows try to decode correctly what to do
269 with this code reference.
273 Inside the C<catch> block the caught error is stored in C<$_>, while previous
274 value of C<$@> is still available for use. This value may or may not be
275 meaningful depending on what happened before the C<try>, but it might be a good
276 idea to preserve it in an error stack.
278 For code that captures C<$@> when throwing new errors (i.e.
279 L<Class::Throwable>), you'll need to do:
300 Intended to be the second or third element of C<try>. C<finally> blocks are always
301 executed in the event of a successful C<try> or if C<catch> is run. This allows
302 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
305 When invoked, the C<finally> block is passed the error that was caught. If no
306 error was caught, it is passed nothing. (Note that the C<finally> block does not
307 localize C<$_> with the error, since unlike in a C<catch> block, there is no way
308 to know if C<$_ == undef> implies that there were no errors.) In other words,
309 the following code does just what you would expect:
314 # ...code run in case of error
317 print "The try block died with: @_\n";
319 print "The try block ran without error.\n";
323 B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will
324 not do anything about handling possible errors coming from code located in these
327 Furthermore B<exceptions in C<finally> blocks are not trappable and are unable
328 to influence the execution of your program>. This is due to limitation of
329 C<DESTROY>-based scope guards, which C<finally> is implemented on top of. This
330 may change in a future version of Try::Tiny.
332 In the same way C<catch()> blesses the code reference this subroutine does the same
333 except it bless them as C<Try::Tiny::Finally>.
339 There are a number of issues with C<eval>.
343 When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially
344 clobbering an error that is currently being caught.
346 This causes action at a distance, clearing previous errors your caller may have
349 C<$@> must be properly localized before invoking C<eval> in order to avoid this
352 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
353 also makes it impossible to capture the previous error before you die (for
354 instance when making exception objects with error stacks).
356 For this reason C<try> will actually set C<$@> to its previous value (the one
357 available before entering the C<try> block) in the beginning of the C<eval>
360 =head2 Localizing $@ silently masks errors
362 Inside an C<eval> block, C<die> behaves sort of like:
366 return_undef_from_eval();
369 This means that if you were polite and localized C<$@> you can't die in that
370 scope, or your error will be discarded (printing "Something's wrong" instead).
372 The workaround is very ugly:
383 =head2 $@ might not be a true value
391 because due to the previous caveats it may have been unset.
393 C<$@> could also be an overloaded error object that evaluates to false, but
394 that's asking for trouble anyway.
396 The classic failure mode is:
398 sub Object::DESTROY {
403 my $obj = Object->new;
412 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
413 C<eval>, it will set C<$@> to C<"">.
415 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
416 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
417 been cleared by C<eval> in the destructor.
419 The workaround for this is even uglier than the previous ones. Even though we
420 can't save the value of C<$@> from code that doesn't localize, we can at least
421 be sure the C<eval> was aborted due to an error:
423 my $failed = not eval {
429 This is because an C<eval> that caught a C<die> will always return a false
434 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
436 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
437 but note that you can't return a useful value from C<catch> using the C<when>
438 blocks without an explicit C<return>.
440 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
441 concisely match errors:
446 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
456 C<@_> is not available within the C<try> block, so you need to copy your
457 arglist. In case you want to work with argument values directly via C<@_>
458 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
461 my ( $self, @args ) = @_;
462 try { $self->bar(@args) }
470 try { $_ = $self->bar($_) for @$args }
475 C<return> returns from the C<try> block, not from the parent sub (note that
476 this is also how C<eval> works, but not how L<TryCatch> works):
486 say "this text WILL be displayed, even though an exception is thrown";
489 Instead, you should capture the return value:
496 return unless $success;
498 say "This text WILL NEVER appear!";
501 Note that if you have a C<catch> block, it must return C<undef> for this to work,
502 since if a C<catch> block exists, its return value is returned in place of C<undef>
503 when an exception is thrown.
507 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
508 will not report this when using full stack traces, though, because
509 C<%Carp::Internal> is used. This lack of magic is considered a feature.
513 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
514 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
515 ensure this, since C<eval> may be used unhygenically in destructors. The only
516 guarantee is that the C<catch> will be called if an exception is thrown.
520 The return value of the C<catch> block is not ignored, so if testing the result
521 of the expression for truth on success, be sure to return a false value from
529 return; # avoid returning a true value;
536 C<$SIG{__DIE__}> is still in effect.
538 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
539 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
540 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
541 the scope of the error throwing code.
545 Lexical C<$_> may override the one set by C<catch>.
547 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
555 warn $_; # will print $foo, not the error
556 warn $_[0]; # instead, get the error like this
561 Note that this behavior was changed once again in L<Perl5 version 18
562 |https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
563 However, since the entirety of lexical C<$_> is now L<considired experimental
564 |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
565 is unclear whether the new version 18 behavior is final.
575 Much more feature complete, more convenient semantics, but at the cost of
576 implementation complexity.
580 Automatic error throwing for builtin functions and more. Also designed to
581 work well with C<given>/C<when>.
585 A lightweight role for rolling your own exception classes.
589 Exception object implementation with a C<try> statement. Does not localize
592 =item L<Exception::Class::TryCatch>
594 Provides a C<catch> statement, but properly calling C<eval> is your
597 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
598 issues with C<$@>, but you still need to localize to prevent clobbering.
602 =head1 LIGHTNING TALK
604 I gave a lightning talk about this module, you can see the slides (Firefox
607 L<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
611 L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
613 =head1 VERSION CONTROL
615 L<http://github.com/doy/try-tiny/>