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 You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional
161 conditions, avoiding quirks in Perl and common mistakes:
163 # handle errors with a catch handler
167 warn "caught error: $_"; # not $@
170 You can also use it like a stanalone C<eval> to catch and ignore any error
171 conditions. Obviously, this is an extreme measure not to be undertaken
174 # just silence errors
181 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
182 minimize common mistakes with eval blocks, and NOTHING else.
184 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
185 another call stack layer, and supports calling C<return> from the try block to
186 return from the parent subroutine. These extra features come at a cost of a few
187 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
188 occasionally problematic, and the additional catch filtering uses L<Moose>
189 type constraints which may not be desirable either.
191 The main focus of this module is to provide simple and reliable error handling
192 for those having a hard time installing L<TryCatch>, but who still want to
193 write correct C<eval> blocks without 5 lines of boilerplate each time.
195 It's designed to work as correctly as possible in light of the various
196 pathological edge cases (see L<BACKGROUND>) and to be compatible with any style
197 of error values (simple strings, references, objects, overloaded objects, etc).
199 If the try block dies, it returns the value of the last statement executed in
200 the catch block, if there is one. Otherwise, it returns C<undef> in scalar
201 context or the empty list in list context. The following two examples both
202 assign C<"bar"> to C<$x>.
204 my $x = try { die "foo" } catch { "bar" };
206 my $x = eval { die "foo" } || "bar";
208 You can add finally blocks making the following true.
211 try { die 'foo' } finally { $x = 'bar' };
212 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
214 Finally blocks are always executed making them suitable for cleanup code
215 which cannot be handled using local. You can add as many finally blocks to a
216 given try block as you like.
220 All functions are exported by default using L<Exporter>.
222 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
223 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
229 Takes one mandatory try subroutine, an optional catch subroutine & finally
232 The mandatory subroutine is evaluated in the context of an C<eval> block.
234 If no error occurred the value from the first block is returned, preserving
237 If there was an error and the second subroutine was given it will be invoked
238 with the error in C<$_> (localized) and as that block's first and only
241 C<$@> does B<not> contain the error. Inside the C<catch> block it has the same
242 value it had before the C<try> block was executed.
244 Note that the error may be false, but if that happens the C<catch> block will
247 Once all execution is finished then the finally block if given will execute.
251 Intended to be used in the second argument position of C<try>.
253 Returns a reference to the subroutine it was given but blessed as
254 C<Try::Tiny::Catch> which allows try to decode correctly what to do
255 with this code reference.
259 Inside the catch block the caught error is stored in C<$_>, while previous
260 value of C<$@> is still available for use. This value may or may not be
261 meaningful depending on what happened before the C<try>, but it might be a good
262 idea to preserve it in an error stack.
264 For code that captures C<$@> when throwing new errors (i.e.
265 L<Class::Throwable>), you'll need to do:
286 Intended to be the second or third element of C<try>. Finally blocks are always
287 executed in the event of a successful C<try> or if C<catch> is run. This allows
288 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
291 When invoked, the finally block is passed the error that was caught. If no
292 error was caught, it is passed nothing. (Note that the finally block does not
293 localize C<$_> with the error, since unlike in a catch block, there is no way
294 to know if C<$_ == undef> implies that there were no errors.) In other words,
295 the following code does just what you would expect:
300 # ...code run in case of error
303 print "The try block died with: @_\n";
305 print "The try block ran without error.\n";
309 B<You must always do your own error handling in the finally block>. C<Try::Tiny> will
310 not do anything about handling possible errors coming from code located in these
313 In the same way C<catch()> blesses the code reference this subroutine does the same
314 except it bless them as C<Try::Tiny::Finally>.
320 There are a number of issues with C<eval>.
324 When you run an eval block and it succeeds, C<$@> will be cleared, potentially
325 clobbering an error that is currently being caught.
327 This causes action at a distance, clearing previous errors your caller may have
330 C<$@> must be properly localized before invoking C<eval> in order to avoid this
333 More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
334 also makes it impossible to capture the previous error before you die (for
335 instance when making exception objects with error stacks).
337 For this reason C<try> will actually set C<$@> to its previous value (before
338 the localization) in the beginning of the C<eval> block.
340 =head2 Localizing $@ silently masks errors
342 Inside an eval block C<die> behaves sort of like:
346 return_undef_from_eval();
349 This means that if you were polite and localized C<$@> you can't die in that
350 scope, or your error will be discarded (printing "Something's wrong" instead).
352 The workaround is very ugly:
363 =head2 $@ might not be a true value
371 because due to the previous caveats it may have been unset.
373 C<$@> could also be an overloaded error object that evaluates to false, but
374 that's asking for trouble anyway.
376 The classic failure mode is:
378 sub Object::DESTROY {
383 my $obj = Object->new;
392 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
393 C<eval>, it will set C<$@> to C<"">.
395 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
396 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
397 been cleared by C<eval> in the destructor.
399 The workaround for this is even uglier than the previous ones. Even though we
400 can't save the value of C<$@> from code that doesn't localize, we can at least
401 be sure the eval was aborted due to an error:
403 my $failed = not eval {
409 This is because an C<eval> that caught a C<die> will always return a false
414 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
416 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
417 but note that you can't return a useful value from C<catch> using the C<when>
418 blocks without an explicit C<return>.
420 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
421 concisely match errors:
426 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
436 C<@_> is not available within the C<try> block, so you need to copy your
437 arglist. In case you want to work with argument values directly via C<@_>
438 aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference:
441 my ( $self, @args ) = @_;
442 try { $self->bar(@args) }
450 try { $_ = $self->bar($_) for @$args }
455 C<return> returns from the C<try> block, not from the parent sub (note that
456 this is also how C<eval> works, but not how L<TryCatch> works):
466 say "this text WILL be displayed, even though an exception is thrown";
469 Instead, you should capture the return value:
476 return unless $success;
478 say "This text WILL NEVER appear!";
481 Note that if you have a catch block, it must return undef for this to work,
482 since if a catch block exists, its return value is returned in place of undef
483 when an exception is thrown.
487 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
488 will not report this when using full stack traces, though, because
489 C<%Carp::Internal> is used. This lack of magic is considered a feature.
493 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
494 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
495 ensure this, since C<eval> may be used unhygenically in destructors. The only
496 guarantee is that the C<catch> will be called if an exception is thrown.
500 The return value of the C<catch> block is not ignored, so if testing the result
501 of the expression for truth on success, be sure to return a false value from
509 return; # avoid returning a true value;
516 C<$SIG{__DIE__}> is still in effect.
518 Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of
519 C<eval> blocks, since it isn't people have grown to rely on it. Therefore in
520 the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for
521 the scope of the error throwing code.
525 Lexical C<$_> may override the one set by C<catch>.
527 For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
535 warn $_; # will print $foo, not the error
536 warn $_[0]; # instead, get the error like this
549 Much more feature complete, more convenient semantics, but at the cost of
550 implementation complexity.
554 Automatic error throwing for builtin functions and more. Also designed to
555 work well with C<given>/C<when>.
559 A lightweight role for rolling your own exception classes.
563 Exception object implementation with a C<try> statement. Does not localize
566 =item L<Exception::Class::TryCatch>
568 Provides a C<catch> statement, but properly calling C<eval> is your
571 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
572 issues with C<$@>, but you still need to localize to prevent clobbering.
576 =head1 LIGHTNING TALK
578 I gave a lightning talk about this module, you can see the slides (Firefox
581 L<http://nothingmuch.woobling.org/talks/takahashi.xul?data=yapc_asia_2009/try_tiny.txt>
585 L<http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
587 =head1 VERSION CONTROL
589 L<http://github.com/nothingmuch/try-tiny/>
593 Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt>
597 Copyright (c) 2009 Yuval Kogman. All rights reserved.
598 This program is free software; you can redistribute
599 it and/or modify it under the terms of the MIT license.