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 $catch = ${$code_ref};
37 } elsif ( $ref eq 'Try::Tiny::Finally' ) {
38 push @finally, ${$code_ref};
40 confess("Unknown code ref type given '${ref}'. Check your usage & try again");
44 # FIXME consider using local $SIG{__DIE__} to accumulate all errors. It's
45 # not perfect, but we could provide a list of additional errors for
48 # save the value of $@ so we can set $@ back to it in the beginning of the eval
49 # and restore $@ after the eval finishes
54 # failed will be true if the eval dies, because 1 will not be returned
56 my $failed = not eval {
59 # evaluate the try block in the correct context
62 } elsif ( defined $wantarray ) {
68 return 1; # properly set $fail to false
71 # reset the original value of $@
74 # set up a scope guard to invoke the finally block at the end
76 map { Try::Tiny::ScopeGuard->_new($_, $failed ? $error : ()) }
79 # at this point $failed contains a true value if the eval died, even if some
80 # destructor overwrote $@ as the eval was unwinding.
82 # if we got an error, invoke the catch block.
84 # This works like given($error), but is backwards compatible and
85 # sets $_ in the dynamic scope for the body of C<$catch>
87 return $catch->($error);
90 # in case when() was used without an explicit return, the C<for>
91 # loop will be aborted and there's no useful return value
96 # no failure, $@ is back to what it was, everything is fine
97 return $wantarray ? @ret : $ret[0];
102 my ( $block, @rest ) = @_;
105 bless(\$block, 'Try::Tiny::Catch'),
111 my ( $block, @rest ) = @_;
114 bless(\$block, 'Try::Tiny::Finally'),
120 package # hide from PAUSE
121 Try::Tiny::ScopeGuard;
129 my @guts = @{ shift() };
130 my $code = shift @guts;
143 Try::Tiny - minimal try/catch with proper preservation of $@
147 You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
148 conditions, avoiding quirks in Perl and common mistakes:
150 # handle errors with a catch handler
154 warn "caught error: $_"; # not $@
157 You can also use it like a standalone C<eval> to catch and ignore any error
158 conditions. Obviously, this is an extreme measure not to be undertaken
161 # just silence errors
168 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
169 minimize common mistakes with eval blocks, and NOTHING else.
171 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
172 another call stack layer, and supports calling C<return> from the C<try> block to
173 return from the parent subroutine. These extra features come at a cost of a few
174 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
175 occasionally problematic, and the additional catch filtering uses L<Moose>
176 type constraints which may not be desirable either.
178 The main focus of this module is to provide simple and reliable error handling
179 for those having a hard time installing L<TryCatch>, but who still want to
180 write correct C<eval> blocks without 5 lines of boilerplate each time.
182 It's designed to work as correctly as possible in light of the various
183 pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
184 of error values (simple strings, references, objects, overloaded objects, etc).
186 If the C<try> block dies, it returns the value of the last statement executed in
187 the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
188 context or the empty list in list context. The following examples all
189 assign C<"bar"> to C<$x>:
191 my $x = try { die "foo" } catch { "bar" };
192 my $x = try { die "foo" } || { "bar" };
193 my $x = (try { die "foo" }) // { "bar" };
195 my $x = eval { die "foo" } || "bar";
197 You can add C<finally> blocks, yielding the following:
200 try { die 'foo' } finally { $x = 'bar' };
201 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
203 C<finally> blocks are always executed making them suitable for cleanup code
204 which cannot be handled using local. You can add as many C<finally> blocks to a
205 given C<try> block as you like.
209 All functions are exported by default using L<Exporter>.
211 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
212 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
218 Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
221 The mandatory subroutine is evaluated in the context of an C<eval> block.
223 If no error occurred the value from the first block is returned, preserving
226 If there was an error and the second subroutine was given it will be invoked
227 with the error in C<$_> (localized) and as that block's first and only
230 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
231 value it had before the C<try> block was executed.
233 Note that the error may be false, but if that happens the C<catch> block will
236 Once all execution is finished then the C<finally> block, if given, will execute.
240 Intended to be used in the second argument position of C<try>.
242 Returns a reference to the subroutine it was given but blessed as
243 C<Try::Tiny::Catch> which allows try to decode correctly what to do
244 with this code reference.
248 Inside the C<catch> block the caught error is stored in C<$_>, while previous
249 value of C<$@> is still available for use. This value may or may not be
250 meaningful depending on what happened before the C<try>, but it might be a good
251 idea to preserve it in an error stack.
253 For code that captures C<$@> when throwing new errors (i.e.
254 L<Class::Throwable>), you'll need to do:
275 Intended to be the second or third element of C<try>. C<finally> blocks are always
276 executed in the event of a successful C<try> or if C<catch> is run. This allows
277 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
280 When invoked, the C<finally> block is passed the error that was caught. If no
281 error was caught, it is passed nothing. (Note that the C<finally> block does not
282 localize C<$_> with the error, since unlike in a C<catch> block, there is no way
283 to know if C<$_ == undef> implies that there were no errors.) In other words,
284 the following code does just what you would expect:
289 # ...code run in case of error
292 print "The try block died with: @_\n";
294 print "The try block ran without error.\n";
298 B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will
299 not do anything about handling possible errors coming from code located in these
302 In the same way C<catch()> blesses the code reference this subroutine does the same
303 except it bless them as C<Try::Tiny::Finally>.
309 There are a number of issues with C<eval>.
313 When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially
314 clobbering an error that is currently being caught.
316 This causes action at a distance, clearing previous errors your caller may have
319 C<$@> must be properly localized before invoking C<eval> in order to avoid this
322 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
323 also makes it impossible to capture the previous error before you die (for
324 instance when making exception objects with error stacks).
326 For this reason C<try> will actually set C<$@> to its previous value (the one
327 available before entering the C<try> block) in the beginning of the C<eval>
330 =head2 Localizing $@ silently masks errors
332 Inside an C<eval> block, C<die> behaves sort of like:
336 return_undef_from_eval();
339 This means that if you were polite and localized C<$@> you can't die in that
340 scope, or your error will be discarded (printing "Something's wrong" instead).
342 The workaround is very ugly:
353 =head2 $@ might not be a true value
361 because due to the previous caveats it may have been unset.
363 C<$@> could also be an overloaded error object that evaluates to false, but
364 that's asking for trouble anyway.
366 The classic failure mode is:
368 sub Object::DESTROY {
373 my $obj = Object->new;
382 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
383 C<eval>, it will set C<$@> to C<"">.
385 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
386 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
387 been cleared by C<eval> in the destructor.
389 The workaround for this is even uglier than the previous ones. Even though we
390 can't save the value of C<$@> from code that doesn't localize, we can at least
391 be sure the C<eval> was aborted due to an error:
393 my $failed = not eval {
399 This is because an C<eval> that caught a C<die> will always return a false
404 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
406 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
407 but note that you can't return a useful value from C<catch> using the C<when>
408 blocks without an explicit C<return>.
410 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
411 concisely match errors:
416 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
426 C<@_> is not available within the C<try> block, so you need to copy your
427 arglist. In case you want to work with argument values directly via C<@_>
428 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
431 my ( $self, @args ) = @_;
432 try { $self->bar(@args) }
440 try { $_ = $self->bar($_) for @$args }
445 C<return> returns from the C<try> block, not from the parent sub (note that
446 this is also how C<eval> works, but not how L<TryCatch> works):
456 say "this text WILL be displayed, even though an exception is thrown";
459 Instead, you should capture the return value:
466 return unless $success;
468 say "This text WILL NEVER appear!";
471 Note that if you have a C<catch> block, it must return C<undef> for this to work,
472 since if a C<catch> block exists, its return value is returned in place of C<undef>
473 when an exception is thrown.
477 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
478 will not report this when using full stack traces, though, because
479 C<%Carp::Internal> is used. This lack of magic is considered a feature.
483 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
484 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
485 ensure this, since C<eval> may be used unhygenically in destructors. The only
486 guarantee is that the C<catch> will be called if an exception is thrown.
490 The return value of the C<catch> block is not ignored, so if testing the result
491 of the expression for truth on success, be sure to return a false value from
499 return; # avoid returning a true value;
506 C<$SIG{__DIE__}> is still in effect.
508 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
509 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
510 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
511 the scope of the error throwing code.
515 Lexical C<$_> may override the one set by C<catch>.
517 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
525 warn $_; # will print $foo, not the error
526 warn $_[0]; # instead, get the error like this
531 Note that this behavior was changed once again in L<Perl5 version 18
532 |https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
533 However, since the entirety of lexical C<$_> is now L<considired experimental
534 |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
535 is unclear whether the new version 18 behavior is final.
545 Much more feature complete, more convenient semantics, but at the cost of
546 implementation complexity.
550 Automatic error throwing for builtin functions and more. Also designed to
551 work well with C<given>/C<when>.
555 A lightweight role for rolling your own exception classes.
559 Exception object implementation with a C<try> statement. Does not localize
562 =item L<Exception::Class::TryCatch>
564 Provides a C<catch> statement, but properly calling C<eval> is your
567 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
568 issues with C<$@>, but you still need to localize to prevent clobbering.
572 =head1 LIGHTNING TALK
574 I gave a lightning talk about this module, you can see the slides (Firefox
577 L<web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
581 L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
583 =head1 VERSION CONTROL
585 L<http://github.com/nothingmuch/try-tiny/>
589 Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt>
593 Copyright (c) 2009 Yuval Kogman. All rights reserved.
594 This program is free software; you can redistribute
595 it and/or modify it under the terms of the MIT license.