6 use vars qw(@EXPORT @EXPORT_OK $VERSION @ISA);
15 $VERSION = eval $VERSION;
17 @EXPORT = @EXPORT_OK = qw(try catch finally);
19 $Carp::Internal{+__PACKAGE__}++;
21 # Need to prototype as @ not $$ because of the way Perl evaluates the prototype.
22 # Keeping it at $$ means you only ever get 1 sub because we need to eval in a list
23 # context & not a scalar one
26 my ( $try, @code_refs ) = @_;
28 # we need to save this here, the eval block will be in scalar context due
30 my $wantarray = wantarray;
32 my ( $catch, $finally );
34 # find labeled blocks in the argument list.
35 # catch and finally tag the blocks by blessing a scalar reference to them.
36 foreach my $code_ref (@code_refs) {
37 next unless $code_ref;
39 my $ref = ref($code_ref);
41 if ( $ref eq 'Try::Tiny::Catch' ) {
42 $catch = ${$code_ref};
43 } elsif ( $ref eq 'Try::Tiny::Finally' ) {
44 $finally = ${$code_ref};
47 confess("Unknown code ref type given '${ref}'. Check your usage & try again");
51 # save the value of $@ so we can set $@ back to it in the beginning of the eval
54 my ( @ret, $error, $failed );
56 # FIXME consider using local $SIG{__DIE__} to accumulate all errors. It's
57 # not perfect, but we could provide a list of additional errors for
61 # localize $@ to prevent clobbering of previous value by a successful
65 # failed will be true if the eval dies, because 1 will not be returned
70 # evaluate the try block in the correct context
73 } elsif ( defined $wantarray ) {
79 return 1; # properly set $fail to false
82 # copy $@ to $error; when we leave this scope, local $@ will revert $@
83 # back to its previous value
87 # set up a scope guard to invoke the finally block at the end
88 my $guard = $finally && bless \$finally, "Try::Tiny::ScopeGuard";
90 # at this point $failed contains a true value if the eval died, even if some
91 # destructor overwrote $@ as the eval was unwinding.
93 # if we got an error, invoke the catch block.
95 # This works like given($error), but is backwards compatible and
96 # sets $_ in the dynamic scope for the body of C<$catch>
98 return $catch->($error);
101 # in case when() was used without an explicit return, the C<for>
102 # loop will be aborted and there's no useful return value
107 # no failure, $@ is back to what it was, everything is fine
108 return $wantarray ? @ret : $ret[0];
113 my ( $block, @rest ) = @_;
116 bless(\$block, 'Try::Tiny::Catch'),
122 my ( $block, @rest ) = @_;
125 bless(\$block, 'Try::Tiny::Finally'),
130 sub Try::Tiny::ScopeGuard::DESTROY {
143 Try::Tiny - minimal try/catch with proper localization of $@
147 # handle errors with a catch handler
151 warn "caught error: $_";
154 # just silence errors
161 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
162 minimize common mistakes with eval blocks, and NOTHING else.
164 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
165 another call stack layer, and supports calling C<return> from the try block to
166 return from the parent subroutine. These extra features come at a cost of a few
167 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
168 occasionally problematic, and the additional catch filtering uses L<Moose>
169 type constraints which may not be desirable either.
171 The main focus of this module is to provide simple and reliable error handling
172 for those having a hard time installing L<TryCatch>, but who still want to
173 write correct C<eval> blocks without 5 lines of boilerplate each time.
175 It's designed to work as correctly as possible in light of the various
176 pathological edge cases (see L<BACKGROUND>) and to be compatible with any style
177 of error values (simple strings, references, objects, overloaded objects, etc).
179 If the try block dies, it returns the value of the last statement executed in
180 the catch block, if there is one. Otherwise, it returns C<undef> in scalar
181 context or the empty list in list context. The following two examples both
182 assign C<"bar"> to C<$x>.
184 my $x = try { die "foo" } catch { "bar" };
186 my $x = eval { die "foo" } || "bar";
188 You can add finally blocks making the following true.
191 try { die 'foo' } finally { $x = 'bar' };
192 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
194 Finally blocks are always executed making them suitable for cleanup code
195 which cannot be handled using local.
199 All functions are exported by default using L<Exporter>.
201 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
202 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
208 Takes one mandatory try subroutine, an optional catch subroutine & finally
211 The mandatory subroutine is evaluated in the context of an C<eval> block.
213 If no error occurred the value from the first block is returned, preserving
216 If there was an error and the second subroutine was given it will be invoked
217 with the error in C<$_> (localized) and as that block's first and only
220 Note that the error may be false, but if that happens the C<catch> block will
223 Once all execution is finished then the finally block if given will execute.
227 Intended to be used in the second argument position of C<try>.
229 Returns a reference to the subroutine it was given but blessed as
230 C<Try::Tiny::Catch> which allows try to decode correctly what to do
231 with this code reference.
235 Inside the catch block the previous value of C<$@> is still available for use.
236 This value may or may not be meaningful depending on what happened before the
237 C<try>, but it might be a good idea to preserve it in an error stack.
239 For code that captures C<$@> when throwing new errors (i.e.
240 L<Class::Throwable>), you'll need to do:
261 Intended to be the second or third element of C<try>. Finally blocks are always
262 executed in the event of a successful C<try> or if C<catch> is run. This allows
263 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
266 B<You must always do your own error handling in the finally block>. C<Try::Tiny> will
267 not do anything about handling possible errors coming from code located in these
270 In the same way C<catch()> blesses the code reference this subroutine does the same
271 except it bless them as C<Try::Tiny::Finally>.
277 There are a number of issues with C<eval>.
281 When you run an eval block and it succeeds, C<$@> will be cleared, potentially
282 clobbering an error that is currently being caught.
284 This causes action at a distance, clearing previous errors your caller may have
287 C<$@> must be properly localized before invoking C<eval> in order to avoid this
290 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
291 also makes it impossible to capture the previous error before you die (for
292 instance when making exception objects with error stacks).
294 For this reason C<try> will actually set C<$@> to its previous value (before
295 the localization) in the beginning of the C<eval> block.
297 =head2 Localizing $@ silently masks errors
299 Inside an eval block C<die> behaves sort of like:
303 return_undef_from_eval();
306 This means that if you were polite and localized C<$@> you can't die in that
307 scope, or your error will be discarded (printing "Something's wrong" instead).
309 The workaround is very ugly:
320 =head2 $@ might not be a true value
328 because due to the previous caveats it may have been unset.
330 C<$@> could also be an overloaded error object that evaluates to false, but
331 that's asking for trouble anyway.
333 The classic failure mode is:
335 sub Object::DESTROY {
340 my $obj = Object->new;
349 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
350 C<eval>, it will set C<$@> to C<"">.
352 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
353 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
354 been cleared by C<eval> in the destructor.
356 The workaround for this is even uglier than the previous ones. Even though we
357 can't save the value of C<$@> from code that doesn't localize, we can at least
358 be sure the eval was aborted due to an error:
360 my $failed = not eval {
366 This is because an C<eval> that caught a C<die> will always return a false
371 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
373 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
374 but note that you can't return a useful value from C<catch> using the C<when>
375 blocks without an explicit C<return>.
377 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
378 concisely match errors:
383 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
393 C<@_> is not available, you need to name your args:
396 my ( $self, @args ) = @_;
397 try { $self->bar(@args) }
402 C<return> returns from the C<try> block, not from the parent sub (note that
403 this is also how C<eval> works, but not how L<TryCatch> works):
406 try { return "foo" };
414 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
415 will not report this when using full stack traces, though, because
416 C<%Carp::Internal> is used. This lack of magic is considered a feature.
420 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
421 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
422 ensure this, since C<eval> may be used unhygenically in destructors. The only
423 guarantee is that the C<catch> will be called if an exception is thrown.
427 The return value of the C<catch> block is not ignored, so if testing the result
428 of the expression for truth on success, be sure to return a false value from
436 return; # avoid returning a true value;
443 C<$SIG{__DIE__}> is still in effect.
445 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
446 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
447 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
448 the scope of the error throwing code.
452 Lexical C<$_> may override the one set by C<catch>.
454 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
462 warn $_; # will print $foo, not the error
463 warn $_[0]; # instead, get the error like this
476 Much more feature complete, more convenient semantics, but at the cost of
477 implementation complexity.
481 Automatic error throwing for builtin functions and more. Also designed to
482 work well with C<given>/C<when>.
486 A lightweight role for rolling your own exception classes.
490 Exception object implementation with a C<try> statement. Does not localize
493 =item L<Exception::Class::TryCatch>
495 Provides a C<catch> statement, but properly calling C<eval> is your
498 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
499 issues with C<$@>, but you still need to localize to prevent clobbering.
503 =head1 LIGHTNING TALK
505 I gave a lightning talk about this module, you can see the slides (Firefox
508 L<http://nothingmuch.woobling.org/talks/takahashi.xul?data=yapc_asia_2009/try_tiny.txt>
512 L<http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
514 =head1 VERSION CONTROL
516 L<http://github.com/nothingmuch/try-tiny/>
520 Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt>
524 Copyright (c) 2009 Yuval Kogman. All rights reserved.
525 This program is free software; you can redistribute
526 it and/or modify it under the terms of the MIT license.