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 $code_ref (@code_refs) {
32 if ( ref($code_ref) eq 'Try::Tiny::Catch' ) {
33 croak 'A try() may not be followed by multiple catch() blocks'
35 $catch = ${$code_ref};
36 } elsif ( ref($code_ref) eq 'Try::Tiny::Finally' ) {
37 push @finally, ${$code_ref};
40 'try() encountered an unexpected argument ('
41 . ( defined $code_ref ? $code_ref : 'undef' )
42 . ') - perhaps a missing semi-colon before or'
47 # FIXME consider using local $SIG{__DIE__} to accumulate all errors. It's
48 # not perfect, but we could provide a list of additional errors for
51 # save the value of $@ so we can set $@ back to it in the beginning of the eval
52 # and restore $@ after the eval finishes
57 # failed will be true if the eval dies, because 1 will not be returned
59 my $failed = not eval {
62 # evaluate the try block in the correct context
65 } elsif ( defined $wantarray ) {
71 return 1; # properly set $fail to false
74 # 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;
136 my @guts = @{ shift() };
137 my $code = shift @guts;
150 Try::Tiny - minimal try/catch with proper preservation of $@
154 You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
155 conditions, avoiding quirks in Perl and common mistakes:
157 # handle errors with a catch handler
161 warn "caught error: $_"; # not $@
164 You can also use it like a standalone C<eval> to catch and ignore any error
165 conditions. Obviously, this is an extreme measure not to be undertaken
168 # just silence errors
175 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
176 minimize common mistakes with eval blocks, and NOTHING else.
178 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
179 another call stack layer, and supports calling C<return> from the C<try> block to
180 return from the parent subroutine. These extra features come at a cost of a few
181 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
182 occasionally problematic, and the additional catch filtering uses L<Moose>
183 type constraints which may not be desirable either.
185 The main focus of this module is to provide simple and reliable error handling
186 for those having a hard time installing L<TryCatch>, but who still want to
187 write correct C<eval> blocks without 5 lines of boilerplate each time.
189 It's designed to work as correctly as possible in light of the various
190 pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
191 of error values (simple strings, references, objects, overloaded objects, etc).
193 If the C<try> block dies, it returns the value of the last statement executed in
194 the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
195 context or the empty list in list context. The following examples all
196 assign C<"bar"> to C<$x>:
198 my $x = try { die "foo" } catch { "bar" };
199 my $x = try { die "foo" } || { "bar" };
200 my $x = (try { die "foo" }) // { "bar" };
202 my $x = eval { die "foo" } || "bar";
204 You can add C<finally> blocks, yielding the following:
207 try { die 'foo' } finally { $x = 'bar' };
208 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
210 C<finally> blocks are always executed making them suitable for cleanup code
211 which cannot be handled using local. You can add as many C<finally> blocks to a
212 given C<try> block as you like.
216 All functions are exported by default using L<Exporter>.
218 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
219 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
225 Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
228 The mandatory subroutine is evaluated in the context of an C<eval> block.
230 If no error occurred the value from the first block is returned, preserving
233 If there was an error and the second subroutine was given it will be invoked
234 with the error in C<$_> (localized) and as that block's first and only
237 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
238 value it had before the C<try> block was executed.
240 Note that the error may be false, but if that happens the C<catch> block will
243 Once all execution is finished then the C<finally> block, if given, will execute.
247 Intended to be used in the second argument position of C<try>.
249 Returns a reference to the subroutine it was given but blessed as
250 C<Try::Tiny::Catch> which allows try to decode correctly what to do
251 with this code reference.
255 Inside the C<catch> block the caught error is stored in C<$_>, while previous
256 value of C<$@> is still available for use. This value may or may not be
257 meaningful depending on what happened before the C<try>, but it might be a good
258 idea to preserve it in an error stack.
260 For code that captures C<$@> when throwing new errors (i.e.
261 L<Class::Throwable>), you'll need to do:
282 Intended to be the second or third element of C<try>. C<finally> blocks are always
283 executed in the event of a successful C<try> or if C<catch> is run. This allows
284 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
287 When invoked, the C<finally> block is passed the error that was caught. If no
288 error was caught, it is passed nothing. (Note that the C<finally> block does not
289 localize C<$_> with the error, since unlike in a C<catch> block, there is no way
290 to know if C<$_ == undef> implies that there were no errors.) In other words,
291 the following code does just what you would expect:
296 # ...code run in case of error
299 print "The try block died with: @_\n";
301 print "The try block ran without error.\n";
305 B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will
306 not do anything about handling possible errors coming from code located in these
309 In the same way C<catch()> blesses the code reference this subroutine does the same
310 except it bless them as C<Try::Tiny::Finally>.
316 There are a number of issues with C<eval>.
320 When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially
321 clobbering an error that is currently being caught.
323 This causes action at a distance, clearing previous errors your caller may have
326 C<$@> must be properly localized before invoking C<eval> in order to avoid this
329 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
330 also makes it impossible to capture the previous error before you die (for
331 instance when making exception objects with error stacks).
333 For this reason C<try> will actually set C<$@> to its previous value (the one
334 available before entering the C<try> block) in the beginning of the C<eval>
337 =head2 Localizing $@ silently masks errors
339 Inside an C<eval> block, C<die> behaves sort of like:
343 return_undef_from_eval();
346 This means that if you were polite and localized C<$@> you can't die in that
347 scope, or your error will be discarded (printing "Something's wrong" instead).
349 The workaround is very ugly:
360 =head2 $@ might not be a true value
368 because due to the previous caveats it may have been unset.
370 C<$@> could also be an overloaded error object that evaluates to false, but
371 that's asking for trouble anyway.
373 The classic failure mode is:
375 sub Object::DESTROY {
380 my $obj = Object->new;
389 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
390 C<eval>, it will set C<$@> to C<"">.
392 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
393 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
394 been cleared by C<eval> in the destructor.
396 The workaround for this is even uglier than the previous ones. Even though we
397 can't save the value of C<$@> from code that doesn't localize, we can at least
398 be sure the C<eval> was aborted due to an error:
400 my $failed = not eval {
406 This is because an C<eval> that caught a C<die> will always return a false
411 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
413 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
414 but note that you can't return a useful value from C<catch> using the C<when>
415 blocks without an explicit C<return>.
417 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
418 concisely match errors:
423 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
433 C<@_> is not available within the C<try> block, so you need to copy your
434 arglist. In case you want to work with argument values directly via C<@_>
435 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
438 my ( $self, @args ) = @_;
439 try { $self->bar(@args) }
447 try { $_ = $self->bar($_) for @$args }
452 C<return> returns from the C<try> block, not from the parent sub (note that
453 this is also how C<eval> works, but not how L<TryCatch> works):
463 say "this text WILL be displayed, even though an exception is thrown";
466 Instead, you should capture the return value:
473 return unless $success;
475 say "This text WILL NEVER appear!";
478 Note that if you have a C<catch> block, it must return C<undef> for this to work,
479 since if a C<catch> block exists, its return value is returned in place of C<undef>
480 when an exception is thrown.
484 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
485 will not report this when using full stack traces, though, because
486 C<%Carp::Internal> is used. This lack of magic is considered a feature.
490 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
491 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
492 ensure this, since C<eval> may be used unhygenically in destructors. The only
493 guarantee is that the C<catch> will be called if an exception is thrown.
497 The return value of the C<catch> block is not ignored, so if testing the result
498 of the expression for truth on success, be sure to return a false value from
506 return; # avoid returning a true value;
513 C<$SIG{__DIE__}> is still in effect.
515 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
516 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
517 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
518 the scope of the error throwing code.
522 Lexical C<$_> may override the one set by C<catch>.
524 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
532 warn $_; # will print $foo, not the error
533 warn $_[0]; # instead, get the error like this
538 Note that this behavior was changed once again in L<Perl5 version 18
539 |https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>.
540 However, since the entirety of lexical C<$_> is now L<considired experimental
541 |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it
542 is unclear whether the new version 18 behavior is final.
552 Much more feature complete, more convenient semantics, but at the cost of
553 implementation complexity.
557 Automatic error throwing for builtin functions and more. Also designed to
558 work well with C<given>/C<when>.
562 A lightweight role for rolling your own exception classes.
566 Exception object implementation with a C<try> statement. Does not localize
569 =item L<Exception::Class::TryCatch>
571 Provides a C<catch> statement, but properly calling C<eval> is your
574 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
575 issues with C<$@>, but you still need to localize to prevent clobbering.
579 =head1 LIGHTNING TALK
581 I gave a lightning talk about this module, you can see the slides (Firefox
584 L<web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
588 L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
590 =head1 VERSION CONTROL
592 L<http://github.com/nothingmuch/try-tiny/>
596 Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt>
600 Copyright (c) 2009 Yuval Kogman. All rights reserved.
601 This program is free software; you can redistribute
602 it and/or modify it under the terms of the MIT license.