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 # hide from PAUSE
134 Try::Tiny::ScopeGuard;
142 my @guts = @{ shift() };
143 my $code = shift @guts;
156 Try::Tiny - minimal try/catch with proper localization of $@
160 # handle errors with a catch handler
164 warn "caught error: $_"; # not $@
167 # just silence errors
174 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
175 minimize common mistakes with eval blocks, and NOTHING else.
177 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
178 another call stack layer, and supports calling C<return> from the try block to
179 return from the parent subroutine. These extra features come at a cost of a few
180 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
181 occasionally problematic, and the additional catch filtering uses L<Moose>
182 type constraints which may not be desirable either.
184 The main focus of this module is to provide simple and reliable error handling
185 for those having a hard time installing L<TryCatch>, but who still want to
186 write correct C<eval> blocks without 5 lines of boilerplate each time.
188 It's designed to work as correctly as possible in light of the various
189 pathological edge cases (see L<BACKGROUND>) and to be compatible with any style
190 of error values (simple strings, references, objects, overloaded objects, etc).
192 If the try block dies, it returns the value of the last statement executed in
193 the catch block, if there is one. Otherwise, it returns C<undef> in scalar
194 context or the empty list in list context. The following two examples both
195 assign C<"bar"> to C<$x>.
197 my $x = try { die "foo" } catch { "bar" };
199 my $x = eval { die "foo" } || "bar";
201 You can add finally blocks making the following true.
204 try { die 'foo' } finally { $x = 'bar' };
205 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
207 Finally blocks are always executed making them suitable for cleanup code
208 which cannot be handled using local. You can add as many finally blocks to a
209 given try block as you like.
213 All functions are exported by default using L<Exporter>.
215 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
216 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
222 Takes one mandatory try subroutine, an optional catch subroutine & finally
225 The mandatory subroutine is evaluated in the context of an C<eval> block.
227 If no error occurred the value from the first block is returned, preserving
230 If there was an error and the second subroutine was given it will be invoked
231 with the error in C<$_> (localized) and as that block's first and only
234 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
235 value it had before the C<try> block was executed.
237 Note that the error may be false, but if that happens the C<catch> block will
240 Once all execution is finished then the finally block if given will execute.
244 Intended to be used in the second argument position of C<try>.
246 Returns a reference to the subroutine it was given but blessed as
247 C<Try::Tiny::Catch> which allows try to decode correctly what to do
248 with this code reference.
252 Inside the catch block the caught error is stored in C<$_>, while previous
253 value of C<$@> is still available for use. This value may or may not be
254 meaningful depending on what happened before the C<try>, but it might be a good
255 idea to preserve it in an error stack.
257 For code that captures C<$@> when throwing new errors (i.e.
258 L<Class::Throwable>), you'll need to do:
279 Intended to be the second or third element of C<try>. Finally blocks are always
280 executed in the event of a successful C<try> or if C<catch> is run. This allows
281 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
284 When invoked, the finally block is passed the error that was caught. If no
285 error was caught, it is passed nothing. In other words, the following code
286 does just what you would expect:
291 # ...code run in case of error
294 print "The try block died with: @_\n";
296 print "The try block ran without error.\n";
300 B<You must always do your own error handling in the finally block>. C<Try::Tiny> will
301 not do anything about handling possible errors coming from code located in these
304 In the same way C<catch()> blesses the code reference this subroutine does the same
305 except it bless them as C<Try::Tiny::Finally>.
311 There are a number of issues with C<eval>.
315 When you run an eval block and it succeeds, C<$@> will be cleared, potentially
316 clobbering an error that is currently being caught.
318 This causes action at a distance, clearing previous errors your caller may have
321 C<$@> must be properly localized before invoking C<eval> in order to avoid this
324 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
325 also makes it impossible to capture the previous error before you die (for
326 instance when making exception objects with error stacks).
328 For this reason C<try> will actually set C<$@> to its previous value (before
329 the localization) in the beginning of the C<eval> block.
331 =head2 Localizing $@ silently masks errors
333 Inside an eval block C<die> behaves sort of like:
337 return_undef_from_eval();
340 This means that if you were polite and localized C<$@> you can't die in that
341 scope, or your error will be discarded (printing "Something's wrong" instead).
343 The workaround is very ugly:
354 =head2 $@ might not be a true value
362 because due to the previous caveats it may have been unset.
364 C<$@> could also be an overloaded error object that evaluates to false, but
365 that's asking for trouble anyway.
367 The classic failure mode is:
369 sub Object::DESTROY {
374 my $obj = Object->new;
383 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
384 C<eval>, it will set C<$@> to C<"">.
386 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
387 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
388 been cleared by C<eval> in the destructor.
390 The workaround for this is even uglier than the previous ones. Even though we
391 can't save the value of C<$@> from code that doesn't localize, we can at least
392 be sure the eval was aborted due to an error:
394 my $failed = not eval {
400 This is because an C<eval> that caught a C<die> will always return a false
405 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
407 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
408 but note that you can't return a useful value from C<catch> using the C<when>
409 blocks without an explicit C<return>.
411 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
412 concisely match errors:
417 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
427 C<@_> is not available within the C<try> block, so you need to copy your
428 arglist. In case you want to work with argument values directly via C<@_>
429 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
432 my ( $self, @args ) = @_;
433 try { $self->bar(@args) }
441 try { $_ = $self->bar($_) for @$args }
446 C<return> returns from the C<try> block, not from the parent sub (note that
447 this is also how C<eval> works, but not how L<TryCatch> works):
450 try { return "foo" };
458 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
459 will not report this when using full stack traces, though, because
460 C<%Carp::Internal> is used. This lack of magic is considered a feature.
464 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
465 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
466 ensure this, since C<eval> may be used unhygenically in destructors. The only
467 guarantee is that the C<catch> will be called if an exception is thrown.
471 The return value of the C<catch> block is not ignored, so if testing the result
472 of the expression for truth on success, be sure to return a false value from
480 return; # avoid returning a true value;
487 C<$SIG{__DIE__}> is still in effect.
489 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
490 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
491 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
492 the scope of the error throwing code.
496 Lexical C<$_> may override the one set by C<catch>.
498 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
506 warn $_; # will print $foo, not the error
507 warn $_[0]; # instead, get the error like this
520 Much more feature complete, more convenient semantics, but at the cost of
521 implementation complexity.
525 Automatic error throwing for builtin functions and more. Also designed to
526 work well with C<given>/C<when>.
530 A lightweight role for rolling your own exception classes.
534 Exception object implementation with a C<try> statement. Does not localize
537 =item L<Exception::Class::TryCatch>
539 Provides a C<catch> statement, but properly calling C<eval> is your
542 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
543 issues with C<$@>, but you still need to localize to prevent clobbering.
547 =head1 LIGHTNING TALK
549 I gave a lightning talk about this module, you can see the slides (Firefox
552 L<http://nothingmuch.woobling.org/talks/takahashi.xul?data=yapc_asia_2009/try_tiny.txt>
556 L<http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
558 =head1 VERSION CONTROL
560 L<http://github.com/nothingmuch/try-tiny/>
564 Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt>
568 Copyright (c) 2009 Yuval Kogman. All rights reserved.
569 This program is free software; you can redistribute
570 it and/or modify it under the terms of the MIT license.