7 use vars qw(@EXPORT @EXPORT_OK $VERSION);
11 $VERSION = eval $VERSION;
13 @EXPORT = @EXPORT_OK = qw(try catch);
16 my ( $try, $catch ) = @_;
18 # we need to save this here, the eval block will be in scalar context due
20 my $wantarray = wantarray;
22 my ( @ret, $error, $failed );
24 # FIXME consider using local $SIG{__DIE__} to accumilate all errors. It's
25 # not perfect, but we could provide a list of additional errors for
29 # localize $@ to prevent clobbering of previous value by a successful
33 # failed will be true if the eval dies, because 1 will not be returned
37 # evaluate the try block in the correct context
40 } elsif ( defined $wantarray ) {
46 return 1; # properly set $fail to false
49 # copy $@ to $error, when we leave this scope local $@ will revert $@
50 # back to its previous value
54 # at this point $failed contains a true value if the eval died even if some
55 # destructor overwrite $@ as the eval was unwinding.
57 # if we got an error, invoke the catch block.
59 # This works like given($error), but is backwards compatible and
60 # sets $_ in the dynamic scope for the body of C<$catch>
62 return $catch->($error);
68 # no failure, $@ is back to what it was, everything is fine
69 return $wantarray ? @ret : $ret[0];
86 Try::Tiny - minimal try/catch with proper localization of $@
90 # handle errors with a catch handler
94 warn "caught error: $_";
104 This module provides bare bones C<try>/C<catch> statements that are designed to
105 minimize common mistakes done with eval blocks (for instance assuming that
106 C<$@> is set to a true value on error, or clobbering previous values of C<$@>),
109 This is unlike L<TryCatch> which provides a nice syntax and avoids adding
110 another call stack layer, and supports calling C<return> from the try block to
111 return from the parent subroutine. These extra features come at a cost of a few
112 dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
113 occasionally problematic, and the additional catch filtering using L<Moose>
114 type constraints may not be desirable either.
116 The main focus of this module is to provide reliable but simple error handling
117 for those having a hard time installing L<TryCatch>, but who still want to
118 write correct C<eval> blocks without 5 lines of boilerplate each time.
120 It's designed to work as correctly as possible in light of the various
121 pathological edge cases (see L<BACKGROUND>) and to be compatible with any style
122 of error values (simple strings, references, objects, overloaded objects, etc).
126 All are exported by default using L<Exporter>.
128 In the future L<Sub::ExporteR> may be used to allow the keywords to be renamed,
129 but this technically does not satisfy Adam Kennedy's definition of "Tiny".
135 Takes one mandatory and one optional catch subroutine.
137 The mandatory subroutine is evaluated in the context of an C<eval> block.
139 If no error occured the value from the first block is returned.
141 If there was an error and the second subroutine was given it will be invoked
142 with the error in C<$_> (localized) and as that block's first and only
145 Note that the error may be false
149 Just retuns the subroutine it was given.
157 Intended to be used in the second argument position of C<try>.
163 There are a number of issues with C<eval>.
167 When you run an eval block and it succeeds, C<$@> will be cleared, potentially
168 cloberring an error that is currently being caught.
170 C<$@> must be properly localized before invoking C<eval> in order to avoid this issue.
172 =head2 Localizing $@ silently masks errors
174 Inside an eval block C<die> behaves sort of like:
178 return_undef_from_eval();
181 This means that if you were polite and localized C<$@> you can't die in that
182 scope while propagating your error.
184 The workaround is very ugly:
195 =head2 $@ might not be a true value
203 because due to the previous caveats it may have been unset. $@ could also an
204 overloaded error object that evaluates to false, but that's asking for trouble
207 The classic failure mode is:
209 sub Object::DESTROY {
214 my $obj = Object->new;
223 In this case since C<Object::DESTROY> is not localizing C<$@> but using eval it
224 will set C<$@> to C<"">.
226 The destructor is only fired after C<die> sets C<$@> to
227 C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has
230 The workaround for this is even uglier. Even though we can't save the value of
231 C<$@> from code that doesn't localize it but uses C<eval> in destructors, we
232 can at least be sure there was an error:
234 my $failed = not eval {
240 This is because an C<eval> that caught a C<die> will always behave like
241 C<return> with no arguments.
245 Using Perl 5.10 you can enable the C<given>/C<when> construct. The C<catch>
246 block is invoked in a topicalizer context (like a C<given> block).
248 Note that you can't return a useful value from C<catch> using the C<when>
251 This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to
252 concisely match errors:
257 when (qr/^Can't locate .*?\.pm in \@INC/) { } # ignore
267 Introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
268 will report this when using full stack traces. This is considered a feature.
272 The value of C<$_> in the C<catch> block is not guaranteed to be preserved,
273 there is no safe way to ensure this if C<eval> is used unhygenically in
274 destructors. It is guaranteed that C<catch> will be called, though.
284 Much more feature complete, more convenient semantics, but at the cost of
285 implementation complexity.
289 Exception object implementation with a C<try> statement. Does not localize
292 =item L<Exception::Class::TryCatch>
294 Provides a C<catch> statement, but properly calling C<eval> is your
297 The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the
298 issues with C<$@> but you still need to localize to prevent clobbering.
302 =head1 VERSION CONTROL
304 L<http://github.com/nothingmuch/try-tiny/>
308 Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt>
312 Copyright (c) 2009 Yuval Kogman. All rights reserved.
313 This program is free software; you can redistribute
314 it and/or modify it under the terms of the MIT license.