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 # at this point $failed contains a true value if the eval died, even if some
88 # destructor overwrote $@ as the eval was unwinding.
90 # if we got an error, invoke the catch block.
92 # This works like given($error), but is backwards compatible and
93 # sets $_ in the dynamic scope for the body of C<$catch>
95 my $catch_return = $catch->($error);
97 # Finally blocks run after all other blocks so it is executed here
98 $finally->() if ( $finally );
100 #And return whatever catch returned
101 return $catch_return;
104 # in case when() was used without an explicit return, the C<for>
105 # loop will be aborted and there's no useful return value
110 # Execute finally block once we decided we worked
111 $finally->() if ( $finally );
113 # no failure, $@ is back to what it was, everything is fine
114 return $wantarray ? @ret : $ret[0];
119 my ( $block, @rest ) = @_;
122 bless(\$block, 'Try::Tiny::Catch'),
128 my ( $block, @rest ) = @_;
131 bless(\$block, 'Try::Tiny::Finally'),
144 Try::Tiny - minimal try/catch with proper localization of $@
148 # handle errors with a catch handler
152 warn "caught error: $_";
155 # just silence errors
162 This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
163 minimize common mistakes with eval blocks, and NOTHING else.
165 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
166 another call stack layer, and supports calling C<return> from the try block to
167 return from the parent subroutine. These extra features come at a cost of a few
168 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
169 occasionally problematic, and the additional catch filtering uses L<Moose>
170 type constraints which may not be desirable either.
172 The main focus of this module is to provide simple and reliable error handling
173 for those having a hard time installing L<TryCatch>, but who still want to
174 write correct C<eval> blocks without 5 lines of boilerplate each time.
176 It's designed to work as correctly as possible in light of the various
177 pathological edge cases (see L<BACKGROUND>) and to be compatible with any style
178 of error values (simple strings, references, objects, overloaded objects, etc).
180 If the try block dies, it returns the value of the last statement executed in
181 the catch block, if there is one. Otherwise, it returns C<undef> in scalar
182 context or the empty list in list context. The following two examples both
183 assign C<"bar"> to C<$x>.
185 my $x = try { die "foo" } catch { "bar" };
187 my $x = eval { die "foo" } || "bar";
189 You can add finally blocks making the following true.
192 try { die 'foo' } finally { $x = 'bar' };
193 try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
195 Finally blocks are always executed making them suitable for cleanup code
196 which cannot be handled using local.
200 All functions are exported by default using L<Exporter>.
202 If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
203 L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
209 Takes one mandatory try subroutine, an optional catch subroutine & finally
212 The mandatory subroutine is evaluated in the context of an C<eval> block.
214 If no error occurred the value from the first block is returned, preserving
217 If there was an error and the second subroutine was given it will be invoked
218 with the error in C<$_> (localized) and as that block's first and only
221 Note that the error may be false, but if that happens the C<catch> block will
224 Once all execution is finished then the finally block if given will execute.
228 Intended to be used in the second argument position of C<try>.
230 Returns a reference to the subroutine it was given but blessed as
231 C<Try::Tiny::Catch> which allows try to decode correctly what to do
232 with this code reference.
236 Inside the catch block the previous value of C<$@> is still available for use.
237 This value may or may not be meaningful depending on what happened before the
238 C<try>, but it might be a good idea to preserve it in an error stack.
257 Intended to be the second or third element of C<try>. Finally blocks are always
258 executed in the event of a successful C<try> or if C<catch> is run. This allows
259 you to locate cleanup code which cannot be done via C<local()> e.g. closing a file
262 B<You must always do your own error handling in the finally block>. C<Try::Tiny> will
263 not do anything about handling possible errors coming from code located in these
266 In the same way C<catch()> blesses the code reference this subroutine does the same
267 except it bless them as C<Try::Tiny::Finally>.
273 There are a number of issues with C<eval>.
277 When you run an eval block and it succeeds, C<$@> will be cleared, potentially
278 clobbering an error that is currently being caught.
280 This causes action at a distance, clearing previous errors your caller may have
283 C<$@> must be properly localized before invoking C<eval> in order to avoid this
286 More specifically, C<$@> is clobbered at the begining of the C<eval>, which
287 also makes it impossible to capture the previous error before you die (for
288 instance when making exception objects with error stacks).
290 For this reason C<try> will actually set C<$@> to its previous value (before
291 the localization) in the beginning of the C<eval> block.
293 =head2 Localizing $@ silently masks errors
295 Inside an eval block C<die> behaves sort of like:
299 return_undef_from_eval();
302 This means that if you were polite and localized C<$@> you can't die in that
303 scope, or your error will be discarded (printing "Something's wrong" instead).
305 The workaround is very ugly:
316 =head2 $@ might not be a true value
324 because due to the previous caveats it may have been unset.
326 C<$@> could also be an overloaded error object that evaluates to false, but
327 that's asking for trouble anyway.
329 The classic failure mode is:
331 sub Object::DESTROY {
336 my $obj = Object->new;
345 In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
346 C<eval>, it will set C<$@> to C<"">.
348 The destructor is called when the stack is unwound, after C<die> sets C<$@> to
349 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
350 been cleared by C<eval> in the destructor.
352 The workaround for this is even uglier than the previous ones. Even though we
353 can't save the value of C<$@> from code that doesn't localize, we can at least
354 be sure the eval was aborted due to an error:
356 my $failed = not eval {
362 This is because an C<eval> that caught a C<die> will always return a false
367 Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
369 The C<catch> block is invoked in a topicalizer context (like a C<given> block),
370 but note that you can't return a useful value from C<catch> using the C<when>
371 blocks without an explicit C<return>.
373 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
374 concisely match errors:
379 when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
389 C<@_> is not available, you need to name your args:
392 my ( $self, @args ) = @_;
393 try { $self->bar(@args) }
398 C<return> returns from the C<try> block, not from the parent sub (note that
399 this is also how C<eval> works, but not how L<TryCatch> works):
402 try { return "foo" };
410 C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
411 will report this when using full stack traces. This lack of magic is considered
416 The value of C<$_> in the C<catch> block is not guaranteed to be the value of
417 the exception thrown (C<$@>) in the C<try> block. There is no safe way to
418 ensure this, since C<eval> may be used unhygenically in destructors. The only
419 guarantee is that the C<catch> will be called if an exception is thrown.
423 The return value of the C<catch> block is not ignored, so if testing the result
424 of the expression for truth on success, be sure to return a false value from
432 return; # avoid returning a true value;
445 Much more feature complete, more convenient semantics, but at the cost of
446 implementation complexity.
450 Automatic error throwing for builtin functions and more. Also designed to
451 work well with C<given>/C<when>.
455 A lightweight role for rolling your own exception classes.
459 Exception object implementation with a C<try> statement. Does not localize
462 =item L<Exception::Class::TryCatch>
464 Provides a C<catch> statement, but properly calling C<eval> is your
467 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
468 issues with C<$@>, but you still need to localize to prevent clobbering.
472 =head1 LIGHTNING TALK
474 I gave a lightning talk about this module, you can see the slides (Firefox
477 L<http://nothingmuch.woobling.org/talks/takahashi.xul?data=try_tiny.txt>
481 L<http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
483 =head1 VERSION CONTROL
485 L<http://github.com/nothingmuch/try-tiny/>
489 Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt>
493 Copyright (c) 2009 Yuval Kogman. All rights reserved.
494 This program is free software; you can redistribute
495 it and/or modify it under the terms of the MIT license.