3 # ABSTRACT: minimal try/catch with proper preservation of $@
9 our @EXPORT = our @EXPORT_OK = qw(try catch finally);
12 $Carp::Internal{+__PACKAGE__}++;
14 # Need to prototype as @ not $$ because of the way Perl evaluates the prototype.
15 # Keeping it at $$ means you only ever get 1 sub because we need to eval in a list
16 # context & not a scalar one
19 my ( $try, @code_refs ) = @_;
21 # we need to save this here, the eval block will be in scalar context due
23 my $wantarray = wantarray;
25 my ( $catch, @finally );
27 # find labeled blocks in the argument list.
28 # catch and finally tag the blocks by blessing a scalar reference to them.
29 foreach my $code_ref (@code_refs) {
31 if ( ref($code_ref) eq 'Try::Tiny::Catch' ) {
32 croak 'A try() may not be followed by multiple catch() blocks'
34 $catch = ${$code_ref};
35 } elsif ( ref($code_ref) eq 'Try::Tiny::Finally' ) {
36 push @finally, ${$code_ref};
39 'try() encountered an unexpected argument ('
40 . ( defined $code_ref ? $code_ref : 'undef' )
41 . ') - perhaps a missing semi-colon before or'
46 # FIXME consider using local $SIG{__DIE__} to accumulate all errors. It's
47 # not perfect, but we could provide a list of additional errors for
50 # save the value of $@ so we can set $@ back to it in the beginning of the eval
51 # and restore $@ after the eval finishes
56 # failed will be true if the eval dies, because 1 will not be returned
58 my $failed = not eval {
61 # evaluate the try block in the correct context
64 } elsif ( defined $wantarray ) {
70 return 1; # properly set $fail to false
73 # preserve the current error and reset the original value of $@
77 # set up a scope guard to invoke the finally block at the end
79 map { Try::Tiny::ScopeGuard->_new($_, $failed ? $error : ()) }
82 # at this point $failed contains a true value if the eval died, even if some
83 # destructor overwrote $@ as the eval was unwinding.
85 # if we got an error, invoke the catch block.
87 # This works like given($error), but is backwards compatible and
88 # sets $_ in the dynamic scope for the body of C<$catch>
90 return $catch->($error);
93 # in case when() was used without an explicit return, the C<for>
94 # loop will be aborted and there's no useful return value
99 # no failure, $@ is back to what it was, everything is fine
100 return $wantarray ? @ret : $ret[0];
105 my ( $block, @rest ) = @_;
107 croak 'Useless bare catch()' unless defined wantarray;
110 bless(\$block, 'Try::Tiny::Catch'),
116 my ( $block, @rest ) = @_;
118 croak 'Useless bare finally()' unless defined wantarray;
121 bless(\$block, 'Try::Tiny::Finally'),
127 package # hide from PAUSE
128 Try::Tiny::ScopeGuard;
130 use constant UNSTABLE_DOLLARAT => ($] < '5.013002') ? 1 : 0;
138 my ($code, @args) = @{ $_[0] };
140 local $@ if UNSTABLE_DOLLARAT;
146 "Execution of finally() block $code resulted in an exception, which "
147 . '*CAN NOT BE PROPAGATED* due to fundamental limitations of Perl. '
148 . 'Your program will continue as if this event never took place. '
149 . "Original exception text follows:\n\n"
150 . (defined $@ ? $@ : '$@ left undefined...')
165 You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
166 conditions, avoiding quirks in Perl and common mistakes:
168 # handle errors with a catch handler
172 warn "caught error: $_"; # not $@
175 You can also use it like a standalone C<eval> to catch and ignore any error
176 conditions. Obviously, this is an extreme measure not to be undertaken
179 # just silence errors
186 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
187 minimize common mistakes with eval blocks, and NOTHING else.
189 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
190 another call stack layer, and supports calling C<return> from the C<try> block to
191 return from the parent subroutine. These extra features come at a cost of a few
192 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
193 occasionally problematic, and the additional catch filtering uses L<Moose>
194 type constraints which may not be desirable either.
196 The main focus of this module is to provide simple and reliable error handling
197 for those having a hard time installing L<TryCatch>, but who still want to
198 write correct C<eval> blocks without 5 lines of boilerplate each time.
200 It's designed to work as correctly as possible in light of the various
201 pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
202 of error values (simple strings, references, objects, overloaded objects, etc).
204 If the C<try> block dies, it returns the value of the last statement executed in
205 the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
206 context or the empty list in list context. The following examples all
207 assign C<"bar"> to C<$x>:
209 my $x = try { die "foo" } catch { "bar" };
210 my $x = try { die "foo" } || { "bar" };
211 my $x = (try { die "foo" }) // { "bar" };
213 my $x = eval { die "foo" } || "bar";
215 You can add C<finally> blocks, yielding the following:
218 try { die 'foo' } finally { $x = 'bar' };
219 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
221 C<finally> blocks are always executed making them suitable for cleanup code
222 which cannot be handled using local. You can add as many C<finally> blocks to a
223 given C<try> block as you like.
227 All functions are exported by default using L<Exporter>.
229 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
230 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
236 Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
239 The mandatory subroutine is evaluated in the context of an C<eval> block.
241 If no error occurred the value from the first block is returned, preserving
244 If there was an error and the second subroutine was given it will be invoked
245 with the error in C<$_> (localized) and as that block's first and only
248 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
249 value it had before the C<try> block was executed.
251 Note that the error may be false, but if that happens the C<catch> block will
254 Once all execution is finished then the C<finally> block, if given, will execute.
258 Intended to be used in the second argument position of C<try>.
260 Returns a reference to the subroutine it was given but blessed as
261 C<Try::Tiny::Catch> which allows try to decode correctly what to do
262 with this code reference.
266 Inside the C<catch> block the caught error is stored in C<$_>, while previous
267 value of C<$@> is still available for use. This value may or may not be
268 meaningful depending on what happened before the C<try>, but it might be a good
269 idea to preserve it in an error stack.
271 For code that captures C<$@> when throwing new errors (i.e.
272 L<Class::Throwable>), you'll need to do:
293 Intended to be the second or third element of C<try>. C<finally> blocks are always
294 executed in the event of a successful C<try> or if C<catch> is run. This allows
295 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
298 When invoked, the C<finally> block is passed the error that was caught. If no
299 error was caught, it is passed nothing. (Note that the C<finally> block does not
300 localize C<$_> with the error, since unlike in a C<catch> block, there is no way
301 to know if C<$_ == undef> implies that there were no errors.) In other words,
302 the following code does just what you would expect:
307 # ...code run in case of error
310 print "The try block died with: @_\n";
312 print "The try block ran without error.\n";
316 B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will
317 not do anything about handling possible errors coming from code located in these
320 Furthermore B<exceptions in C<finally> blocks are not trappable and are unable
321 to influence the execution of your program>. This is due to limitation of
322 C<DESTROY>-based scope guards, which C<finally> is implemented on top of. This
323 may change in a future version of Try::Tiny.
325 In the same way C<catch()> blesses the code reference this subroutine does the same
326 except it bless them as C<Try::Tiny::Finally>.
332 There are a number of issues with C<eval>.
336 When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially
337 clobbering an error that is currently being caught.
339 This causes action at a distance, clearing previous errors your caller may have
342 C<$@> must be properly localized before invoking C<eval> in order to avoid this
345 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
346 also makes it impossible to capture the previous error before you die (for
347 instance when making exception objects with error stacks).
349 For this reason C<try> will actually set C<$@> to its previous value (the one
350 available before entering the C<try> block) in the beginning of the C<eval>
353 =head2 Localizing $@ silently masks errors
355 Inside an C<eval> block, C<die> behaves sort of like:
359 return_undef_from_eval();
362 This means that if you were polite and localized C<$@> you can't die in that
363 scope, or your error will be discarded (printing "Something's wrong" instead).
365 The workaround is very ugly:
376 =head2 $@ might not be a true value
384 because due to the previous caveats it may have been unset.
386 C<$@> could also be an overloaded error object that evaluates to false, but
387 that's asking for trouble anyway.
389 The classic failure mode is:
391 sub Object::DESTROY {
396 my $obj = Object->new;
405 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
406 C<eval>, it will set C<$@> to C<"">.
408 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
409 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
410 been cleared by C<eval> in the destructor.
412 The workaround for this is even uglier than the previous ones. Even though we
413 can't save the value of C<$@> from code that doesn't localize, we can at least
414 be sure the C<eval> was aborted due to an error:
416 my $failed = not eval {
422 This is because an C<eval> that caught a C<die> will always return a false
427 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
429 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
430 but note that you can't return a useful value from C<catch> using the C<when>
431 blocks without an explicit C<return>.
433 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
434 concisely match errors:
439 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
449 C<@_> is not available within the C<try> block, so you need to copy your
450 arglist. In case you want to work with argument values directly via C<@_>
451 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
454 my ( $self, @args ) = @_;
455 try { $self->bar(@args) }
463 try { $_ = $self->bar($_) for @$args }
468 C<return> returns from the C<try> block, not from the parent sub (note that
469 this is also how C<eval> works, but not how L<TryCatch> works):
479 say "this text WILL be displayed, even though an exception is thrown";
482 Instead, you should capture the return value:
489 return unless $success;
491 say "This text WILL NEVER appear!";
494 Note that if you have a C<catch> block, it must return C<undef> for this to work,
495 since if a C<catch> block exists, its return value is returned in place of C<undef>
496 when an exception is thrown.
500 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
501 will not report this when using full stack traces, though, because
502 C<%Carp::Internal> is used. This lack of magic is considered a feature.
506 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
507 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
508 ensure this, since C<eval> may be used unhygenically in destructors. The only
509 guarantee is that the C<catch> will be called if an exception is thrown.
513 The return value of the C<catch> block is not ignored, so if testing the result
514 of the expression for truth on success, be sure to return a false value from
522 return; # avoid returning a true value;
529 C<$SIG{__DIE__}> is still in effect.
531 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
532 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
533 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
534 the scope of the error throwing code.
538 Lexical C<$_> may override the one set by C<catch>.
540 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
548 warn $_; # will print $foo, not the error
549 warn $_[0]; # instead, get the error like this
554 Note that this behavior was changed once again in L<Perl5 version 18
555 |https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
556 However, since the entirety of lexical C<$_> is now L<considired experimental
557 |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
558 is unclear whether the new version 18 behavior is final.
568 Much more feature complete, more convenient semantics, but at the cost of
569 implementation complexity.
573 Automatic error throwing for builtin functions and more. Also designed to
574 work well with C<given>/C<when>.
578 A lightweight role for rolling your own exception classes.
582 Exception object implementation with a C<try> statement. Does not localize
585 =item L<Exception::Class::TryCatch>
587 Provides a C<catch> statement, but properly calling C<eval> is your
590 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
591 issues with C<$@>, but you still need to localize to prevent clobbering.
595 =head1 LIGHTNING TALK
597 I gave a lightning talk about this module, you can see the slides (Firefox
600 L<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
604 L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
606 =head1 VERSION CONTROL
608 L<http://github.com/doy/try-tiny/>