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 push @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 $@
83 # back to its previous value
87 # set up a scope guard to invoke the finally block at the end
89 map { Try::Tiny::ScopeGuard->_new($_, $failed ? $error : ()) }
92 # at this point $failed contains a true value if the eval died, even if some
93 # destructor overwrote $@ as the eval was unwinding.
95 # if we got an error, invoke the catch block.
97 # This works like given($error), but is backwards compatible and
98 # sets $_ in the dynamic scope for the body of C<$catch>
100 return $catch->($error);
103 # in case when() was used without an explicit return, the C<for>
104 # loop will be aborted and there's no useful return value
109 # no failure, $@ is back to what it was, everything is fine
110 return $wantarray ? @ret : $ret[0];
115 my ( $block, @rest ) = @_;
118 bless(\$block, 'Try::Tiny::Catch'),
124 my ( $block, @rest ) = @_;
127 bless(\$block, 'Try::Tiny::Finally'),
133 package Try::Tiny::ScopeGuard;
141 my @guts = @{ shift() };
142 my $code = shift @guts;
155 Try::Tiny - minimal try/catch with proper localization of $@
159 # handle errors with a catch handler
163 warn "caught error: $_"; # not $@
166 # just silence errors
173 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
174 minimize common mistakes with eval blocks, and NOTHING else.
176 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
177 another call stack layer, and supports calling C<return> from the try block to
178 return from the parent subroutine. These extra features come at a cost of a few
179 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
180 occasionally problematic, and the additional catch filtering uses L<Moose>
181 type constraints which may not be desirable either.
183 The main focus of this module is to provide simple and reliable error handling
184 for those having a hard time installing L<TryCatch>, but who still want to
185 write correct C<eval> blocks without 5 lines of boilerplate each time.
187 It's designed to work as correctly as possible in light of the various
188 pathological edge cases (see L<BACKGROUND>) and to be compatible with any style
189 of error values (simple strings, references, objects, overloaded objects, etc).
191 If the try block dies, it returns the value of the last statement executed in
192 the catch block, if there is one. Otherwise, it returns C<undef> in scalar
193 context or the empty list in list context. The following two examples both
194 assign C<"bar"> to C<$x>.
196 my $x = try { die "foo" } catch { "bar" };
198 my $x = eval { die "foo" } || "bar";
200 You can add finally blocks making the following true.
203 try { die 'foo' } finally { $x = 'bar' };
204 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
206 Finally blocks are always executed making them suitable for cleanup code
207 which cannot be handled using local. You can add as many finaly blocks to a
208 given try block as you like.
212 All functions are exported by default using L<Exporter>.
214 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
215 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
221 Takes one mandatory try subroutine, an optional catch subroutine & finally
224 The mandatory subroutine is evaluated in the context of an C<eval> block.
226 If no error occurred the value from the first block is returned, preserving
229 If there was an error and the second subroutine was given it will be invoked
230 with the error in C<$_> (localized) and as that block's first and only
233 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
234 value it had before the C<try> block was executed.
236 Note that the error may be false, but if that happens the C<catch> block will
239 Once all execution is finished then the finally block if given will execute.
243 Intended to be used in the second argument position of C<try>.
245 Returns a reference to the subroutine it was given but blessed as
246 C<Try::Tiny::Catch> which allows try to decode correctly what to do
247 with this code reference.
251 Inside the catch block the caught error is stored in C<$_>, while previous
252 value of C<$@> is still available for use. This value may or may not be
253 meaningful depending on what happened before the C<try>, but it might be a good
254 idea to preserve it in an error stack.
256 For code that captures C<$@> when throwing new errors (i.e.
257 L<Class::Throwable>), you'll need to do:
278 Intended to be the second or third element of C<try>. Finally blocks are always
279 executed in the event of a successful C<try> or if C<catch> is run. This allows
280 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
283 When invoked, the finally block is passed the error that was caught. If no
284 error was caught, it is passed nothing. In other words, the following code
285 does just what you would expect:
290 # ...code run in case of error
293 print "The try block died with: @_\n";
295 print "The try block ran without error.\n";
299 B<You must always do your own error handling in the finally block>. C<Try::Tiny> will
300 not do anything about handling possible errors coming from code located in these
303 In the same way C<catch()> blesses the code reference this subroutine does the same
304 except it bless them as C<Try::Tiny::Finally>.
310 There are a number of issues with C<eval>.
314 When you run an eval block and it succeeds, C<$@> will be cleared, potentially
315 clobbering an error that is currently being caught.
317 This causes action at a distance, clearing previous errors your caller may have
320 C<$@> must be properly localized before invoking C<eval> in order to avoid this
323 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
324 also makes it impossible to capture the previous error before you die (for
325 instance when making exception objects with error stacks).
327 For this reason C<try> will actually set C<$@> to its previous value (before
328 the localization) in the beginning of the C<eval> block.
330 =head2 Localizing $@ silently masks errors
332 Inside an eval block C<die> behaves sort of like:
336 return_undef_from_eval();
339 This means that if you were polite and localized C<$@> 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 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 @$args }
445 C<return> returns from the C<try> block, not from the parent sub (note that
446 this is also how C<eval> works, but not how L<TryCatch> works):
449 try { return "foo" };
457 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
458 will not report this when using full stack traces, though, because
459 C<%Carp::Internal> is used. This lack of magic is considered a feature.
463 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
464 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
465 ensure this, since C<eval> may be used unhygenically in destructors. The only
466 guarantee is that the C<catch> will be called if an exception is thrown.
470 The return value of the C<catch> block is not ignored, so if testing the result
471 of the expression for truth on success, be sure to return a false value from
479 return; # avoid returning a true value;
486 C<$SIG{__DIE__}> is still in effect.
488 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
489 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
490 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
491 the scope of the error throwing code.
495 Lexical C<$_> may override the one set by C<catch>.
497 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
505 warn $_; # will print $foo, not the error
506 warn $_[0]; # instead, get the error like this
519 Much more feature complete, more convenient semantics, but at the cost of
520 implementation complexity.
524 Automatic error throwing for builtin functions and more. Also designed to
525 work well with C<given>/C<when>.
529 A lightweight role for rolling your own exception classes.
533 Exception object implementation with a C<try> statement. Does not localize
536 =item L<Exception::Class::TryCatch>
538 Provides a C<catch> statement, but properly calling C<eval> is your
541 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
542 issues with C<$@>, but you still need to localize to prevent clobbering.
546 =head1 LIGHTNING TALK
548 I gave a lightning talk about this module, you can see the slides (Firefox
551 L<http://nothingmuch.woobling.org/talks/takahashi.xul?data=yapc_asia_2009/try_tiny.txt>
555 L<http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
557 =head1 VERSION CONTROL
559 L<http://github.com/nothingmuch/try-tiny/>
563 Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt>
567 Copyright (c) 2009 Yuval Kogman. All rights reserved.
568 This program is free software; you can redistribute
569 it and/or modify it under the terms of the MIT license.