7 $VERSION = eval $VERSION if $VERSION =~ /_/;
10 our @EXPORT = our @EXPORT_OK = qw(try catch finally);
13 $Carp::Internal{+__PACKAGE__}++;
15 # Need to prototype as @ not $$ because of the way Perl evaluates the prototype.
16 # Keeping it at $$ means you only ever get 1 sub because we need to eval in a list
17 # context & not a scalar one
20 my ( $try, @code_refs ) = @_;
22 # we need to save this here, the eval block will be in scalar context due
24 my $wantarray = wantarray;
26 my ( $catch, @finally );
28 # find labeled blocks in the argument list.
29 # catch and finally tag the blocks by blessing a scalar reference to them.
30 foreach my $code_ref (@code_refs) {
31 next unless $code_ref;
33 my $ref = ref($code_ref);
35 if ( $ref eq 'Try::Tiny::Catch' ) {
36 croak 'A try() may not be followed by multiple catch() blocks'
38 $catch = ${$code_ref};
39 } elsif ( $ref eq 'Try::Tiny::Finally' ) {
40 push @finally, ${$code_ref};
42 confess("Unknown code ref type given '${ref}'. Check your usage & try again");
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 # reset the original value of $@
76 # set up a scope guard to invoke the finally block at the end
78 map { Try::Tiny::ScopeGuard->_new($_, $failed ? $error : ()) }
81 # at this point $failed contains a true value if the eval died, even if some
82 # destructor overwrote $@ as the eval was unwinding.
84 # if we got an error, invoke the catch block.
86 # This works like given($error), but is backwards compatible and
87 # sets $_ in the dynamic scope for the body of C<$catch>
89 return $catch->($error);
92 # in case when() was used without an explicit return, the C<for>
93 # loop will be aborted and there's no useful return value
98 # no failure, $@ is back to what it was, everything is fine
99 return $wantarray ? @ret : $ret[0];
104 my ( $block, @rest ) = @_;
106 croak 'Useless bare catch()' unless defined wantarray;
109 bless(\$block, 'Try::Tiny::Catch'),
115 my ( $block, @rest ) = @_;
117 croak 'Useless bare finally()' unless defined wantarray;
120 bless(\$block, 'Try::Tiny::Finally'),
126 package # hide from PAUSE
127 Try::Tiny::ScopeGuard;
135 my @guts = @{ shift() };
136 my $code = shift @guts;
149 Try::Tiny - minimal try/catch with proper preservation of $@
153 You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
154 conditions, avoiding quirks in Perl and common mistakes:
156 # handle errors with a catch handler
160 warn "caught error: $_"; # not $@
163 You can also use it like a standalone C<eval> to catch and ignore any error
164 conditions. Obviously, this is an extreme measure not to be undertaken
167 # just silence errors
174 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
175 minimize common mistakes with eval blocks, and NOTHING else.
177 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
178 another call stack layer, and supports calling C<return> from the C<try> block to
179 return from the parent subroutine. These extra features come at a cost of a few
180 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
181 occasionally problematic, and the additional catch filtering uses L<Moose>
182 type constraints which may not be desirable either.
184 The main focus of this module is to provide simple and reliable error handling
185 for those having a hard time installing L<TryCatch>, but who still want to
186 write correct C<eval> blocks without 5 lines of boilerplate each time.
188 It's designed to work as correctly as possible in light of the various
189 pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
190 of error values (simple strings, references, objects, overloaded objects, etc).
192 If the C<try> block dies, it returns the value of the last statement executed in
193 the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
194 context or the empty list in list context. The following examples all
195 assign C<"bar"> to C<$x>:
197 my $x = try { die "foo" } catch { "bar" };
198 my $x = try { die "foo" } || { "bar" };
199 my $x = (try { die "foo" }) // { "bar" };
201 my $x = eval { die "foo" } || "bar";
203 You can add C<finally> blocks, yielding the following:
206 try { die 'foo' } finally { $x = 'bar' };
207 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
209 C<finally> blocks are always executed making them suitable for cleanup code
210 which cannot be handled using local. You can add as many C<finally> blocks to a
211 given C<try> block as you like.
215 All functions are exported by default using L<Exporter>.
217 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
218 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
224 Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
227 The mandatory subroutine is evaluated in the context of an C<eval> block.
229 If no error occurred the value from the first block is returned, preserving
232 If there was an error and the second subroutine was given it will be invoked
233 with the error in C<$_> (localized) and as that block's first and only
236 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
237 value it had before the C<try> block was executed.
239 Note that the error may be false, but if that happens the C<catch> block will
242 Once all execution is finished then the C<finally> block, if given, will execute.
246 Intended to be used in the second argument position of C<try>.
248 Returns a reference to the subroutine it was given but blessed as
249 C<Try::Tiny::Catch> which allows try to decode correctly what to do
250 with this code reference.
254 Inside the C<catch> block the caught error is stored in C<$_>, while previous
255 value of C<$@> is still available for use. This value may or may not be
256 meaningful depending on what happened before the C<try>, but it might be a good
257 idea to preserve it in an error stack.
259 For code that captures C<$@> when throwing new errors (i.e.
260 L<Class::Throwable>), you'll need to do:
281 Intended to be the second or third element of C<try>. C<finally> blocks are always
282 executed in the event of a successful C<try> or if C<catch> is run. This allows
283 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
286 When invoked, the C<finally> block is passed the error that was caught. If no
287 error was caught, it is passed nothing. (Note that the C<finally> block does not
288 localize C<$_> with the error, since unlike in a C<catch> block, there is no way
289 to know if C<$_ == undef> implies that there were no errors.) In other words,
290 the following code does just what you would expect:
295 # ...code run in case of error
298 print "The try block died with: @_\n";
300 print "The try block ran without error.\n";
304 B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will
305 not do anything about handling possible errors coming from code located in these
308 In the same way C<catch()> blesses the code reference this subroutine does the same
309 except it bless them as C<Try::Tiny::Finally>.
315 There are a number of issues with C<eval>.
319 When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially
320 clobbering an error that is currently being caught.
322 This causes action at a distance, clearing previous errors your caller may have
325 C<$@> must be properly localized before invoking C<eval> in order to avoid this
328 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
329 also makes it impossible to capture the previous error before you die (for
330 instance when making exception objects with error stacks).
332 For this reason C<try> will actually set C<$@> to its previous value (the one
333 available before entering the C<try> block) in the beginning of the C<eval>
336 =head2 Localizing $@ silently masks errors
338 Inside an C<eval> block, C<die> behaves sort of like:
342 return_undef_from_eval();
345 This means that if you were polite and localized C<$@> you can't die in that
346 scope, or your error will be discarded (printing "Something's wrong" instead).
348 The workaround is very ugly:
359 =head2 $@ might not be a true value
367 because due to the previous caveats it may have been unset.
369 C<$@> could also be an overloaded error object that evaluates to false, but
370 that's asking for trouble anyway.
372 The classic failure mode is:
374 sub Object::DESTROY {
379 my $obj = Object->new;
388 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
389 C<eval>, it will set C<$@> to C<"">.
391 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
392 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
393 been cleared by C<eval> in the destructor.
395 The workaround for this is even uglier than the previous ones. Even though we
396 can't save the value of C<$@> from code that doesn't localize, we can at least
397 be sure the C<eval> was aborted due to an error:
399 my $failed = not eval {
405 This is because an C<eval> that caught a C<die> will always return a false
410 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
412 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
413 but note that you can't return a useful value from C<catch> using the C<when>
414 blocks without an explicit C<return>.
416 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
417 concisely match errors:
422 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
432 C<@_> is not available within the C<try> block, so you need to copy your
433 arglist. In case you want to work with argument values directly via C<@_>
434 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
437 my ( $self, @args ) = @_;
438 try { $self->bar(@args) }
446 try { $_ = $self->bar($_) for @$args }
451 C<return> returns from the C<try> block, not from the parent sub (note that
452 this is also how C<eval> works, but not how L<TryCatch> works):
462 say "this text WILL be displayed, even though an exception is thrown";
465 Instead, you should capture the return value:
472 return unless $success;
474 say "This text WILL NEVER appear!";
477 Note that if you have a C<catch> block, it must return C<undef> for this to work,
478 since if a C<catch> block exists, its return value is returned in place of C<undef>
479 when an exception is thrown.
483 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
484 will not report this when using full stack traces, though, because
485 C<%Carp::Internal> is used. This lack of magic is considered a feature.
489 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
490 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
491 ensure this, since C<eval> may be used unhygenically in destructors. The only
492 guarantee is that the C<catch> will be called if an exception is thrown.
496 The return value of the C<catch> block is not ignored, so if testing the result
497 of the expression for truth on success, be sure to return a false value from
505 return; # avoid returning a true value;
512 C<$SIG{__DIE__}> is still in effect.
514 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
515 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
516 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
517 the scope of the error throwing code.
521 Lexical C<$_> may override the one set by C<catch>.
523 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
531 warn $_; # will print $foo, not the error
532 warn $_[0]; # instead, get the error like this
537 Note that this behavior was changed once again in L<Perl5 version 18
538 |https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
539 However, since the entirety of lexical C<$_> is now L<considired experimental
540 |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
541 is unclear whether the new version 18 behavior is final.
551 Much more feature complete, more convenient semantics, but at the cost of
552 implementation complexity.
556 Automatic error throwing for builtin functions and more. Also designed to
557 work well with C<given>/C<when>.
561 A lightweight role for rolling your own exception classes.
565 Exception object implementation with a C<try> statement. Does not localize
568 =item L<Exception::Class::TryCatch>
570 Provides a C<catch> statement, but properly calling C<eval> is your
573 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
574 issues with C<$@>, but you still need to localize to prevent clobbering.
578 =head1 LIGHTNING TALK
580 I gave a lightning talk about this module, you can see the slides (Firefox
583 L<web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
587 L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
589 =head1 VERSION CONTROL
591 L<http://github.com/nothingmuch/try-tiny/>
595 Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt>
599 Copyright (c) 2009 Yuval Kogman. All rights reserved.
600 This program is free software; you can redistribute
601 it and/or modify it under the terms of the MIT license.