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package Try::Tiny; # git description: v0.29-2-g3b23a06 use 5.006; # ABSTRACT: Minimal try/catch with proper preservation of $@ our $VERSION = '0.30'; use strict; use warnings; use Exporter 5.57 'import'; our @EXPORT = our @EXPORT_OK = qw(try catch finally); use Carp; $Carp::Internal{+__PACKAGE__}++; BEGIN { my $su = $INC{'Sub/Util.pm'} && defined &Sub::Util::set_subname; my $sn = $INC{'Sub/Name.pm'} && eval { Sub::Name->VERSION(0.08) }; unless ($su || $sn) { $su = eval { require Sub::Util; } && defined &Sub::Util::set_subname; unless ($su) { $sn = eval { require Sub::Name; Sub::Name->VERSION(0.08) }; } } *_subname = $su ? \&Sub::Util::set_subname : $sn ? \&Sub::Name::subname : sub { $_[1] }; *_HAS_SUBNAME = ($su || $sn) ? sub(){1} : sub(){0}; } my %_finally_guards; # Need to prototype as @ not $$ because of the way Perl evaluates the prototype. # Keeping it at $$ means you only ever get 1 sub because we need to eval in a list # context & not a scalar one sub try (&;@) { my ( $try, @code_refs ) = @_; # we need to save this here, the eval block will be in scalar context due # to $failed my $wantarray = wantarray; # work around perl bug by explicitly initializing these, due to the likelyhood # this will be used in global destruction (perl rt#119311) my ( $catch, @finally ) = (); # find labeled blocks in the argument list. # catch and finally tag the blocks by blessing a scalar reference to them. foreach my $code_ref (@code_refs) { if ( ref($code_ref) eq 'Try::Tiny::Catch' ) { croak 'A try() may not be followed by multiple catch() blocks' if $catch; $catch = ${$code_ref}; } elsif ( ref($code_ref) eq 'Try::Tiny::Finally' ) { push @finally, ${$code_ref}; } else { croak( 'try() encountered an unexpected argument (' . ( defined $code_ref ? $code_ref : 'undef' ) . ') - perhaps a missing semi-colon before or' ); } } # FIXME consider using local $SIG{__DIE__} to accumulate all errors. It's # not perfect, but we could provide a list of additional errors for # $catch->(); # name the blocks if we have Sub::Name installed _subname(caller().'::try {...} ' => $try) if _HAS_SUBNAME; # set up scope guards to invoke the finally blocks at the end. # this should really be a function scope lexical variable instead of # file scope + local but that causes issues with perls < 5.20 due to # perl rt#119311 local $_finally_guards{guards} = [ map { Try::Tiny::ScopeGuard->_new($_) } @finally ]; # save the value of $@ so we can set $@ back to it in the beginning of the eval # and restore $@ after the eval finishes my $prev_error = $@; my ( @ret, $error ); # failed will be true if the eval dies, because 1 will not be returned # from the eval body my $failed = not eval { $@ = $prev_error; # evaluate the try block in the correct context if ( $wantarray ) { @ret = $try->(); } elsif ( defined $wantarray ) { $ret[0] = $try->(); } else { $try->(); }; return 1; # properly set $failed to false }; # preserve the current error and reset the original value of $@ $error = $@; $@ = $prev_error; # at this point $failed contains a true value if the eval died, even if some # destructor overwrote $@ as the eval was unwinding. if ( $failed ) { # pass $error to the finally blocks push @$_, $error for @{$_finally_guards{guards}}; # if we got an error, invoke the catch block. if ( $catch ) { # This works like given($error), but is backwards compatible and # sets $_ in the dynamic scope for the body of C<$catch> for ($error) { return $catch->($error); } # in case when() was used without an explicit return, the C<for> # loop will be aborted and there's no useful return value } return; } else { # no failure, $@ is back to what it was, everything is fine return $wantarray ? @ret : $ret[0]; } } sub catch (&;@) { my ( $block, @rest ) = @_; croak 'Useless bare catch()' unless wantarray; _subname(caller().'::catch {...} ' => $block) if _HAS_SUBNAME; return ( bless(\$block, 'Try::Tiny::Catch'), @rest, ); } sub finally (&;@) { my ( $block, @rest ) = @_; croak 'Useless bare finally()' unless wantarray; _subname(caller().'::finally {...} ' => $block) if _HAS_SUBNAME; return ( bless(\$block, 'Try::Tiny::Finally'), @rest, ); } { package # hide from PAUSE Try::Tiny::ScopeGuard; use constant UNSTABLE_DOLLARAT => ("$]" < '5.013002') ? 1 : 0; sub _new { shift; bless [ @_ ]; } sub DESTROY { my ($code, @args) = @{ $_[0] }; local $@ if UNSTABLE_DOLLARAT; eval { $code->(@args); 1; } or do { warn "Execution of finally() block $code resulted in an exception, which " . '*CAN NOT BE PROPAGATED* due to fundamental limitations of Perl. ' . 'Your program will continue as if this event never took place. ' . "Original exception text follows:\n\n" . (defined $@ ? $@ : '$@ left undefined...') . "\n" ; } } } __PACKAGE__ __END__ =pod =encoding UTF-8 =head1 NAME Try::Tiny - Minimal try/catch with proper preservation of $@ =head1 VERSION version 0.30 =head1 SYNOPSIS You can use Try::Tiny's C<try> and C<catch> to expect and handle exceptional conditions, avoiding quirks in Perl and common mistakes: # handle errors with a catch handler try { die "foo"; } catch { warn "caught error: $_"; # not $@ }; You can also use it like a standalone C<eval> to catch and ignore any error conditions. Obviously, this is an extreme measure not to be undertaken lightly: # just silence errors try { die "foo"; }; =head1 DESCRIPTION This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to minimize common mistakes with eval blocks, and NOTHING else. This is unlike L<TryCatch> which provides a nice syntax and avoids adding another call stack layer, and supports calling C<return> from the C<try> block to return from the parent subroutine. These extra features come at a cost of a few dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are occasionally problematic, and the additional catch filtering uses L<Moose> type constraints which may not be desirable either. The main focus of this module is to provide simple and reliable error handling for those having a hard time installing L<TryCatch>, but who still want to write correct C<eval> blocks without 5 lines of boilerplate each time. It's designed to work as correctly as possible in light of the various pathological edge cases (see L</BACKGROUND>) and to be compatible with any style of error values (simple strings, references, objects, overloaded objects, etc). If the C<try> block dies, it returns the value of the last statement executed in the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar context or the empty list in list context. The following examples all assign C<"bar"> to C<$x>: my $x = try { die "foo" } catch { "bar" }; my $x = try { die "foo" } || "bar"; my $x = (try { die "foo" }) // "bar"; my $x = eval { die "foo" } || "bar"; You can add C<finally> blocks, yielding the following: my $x; try { die 'foo' } finally { $x = 'bar' }; try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' }; C<finally> blocks are always executed making them suitable for cleanup code which cannot be handled using local. You can add as many C<finally> blocks to a given C<try> block as you like. Note that adding a C<finally> block without a preceding C<catch> block suppresses any errors. This behaviour is consistent with using a standalone C<eval>, but it is not consistent with C<try>/C<finally> patterns found in other programming languages, such as Java, Python, Javascript or C#. If you learnt the C<try>/C<finally> pattern from one of these languages, watch out for this. =head1 EXPORTS All functions are exported by default using L<Exporter>. If you need to rename the C<try>, C<catch> or C<finally> keyword consider using L<Sub::Import> to get L<Sub::Exporter>'s flexibility. =over 4 =item try (&;@) Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally> subroutine. The mandatory subroutine is evaluated in the context of an C<eval> block. If no error occurred the value from the first block is returned, preserving list/scalar context. If there was an error and the second subroutine was given it will be invoked with the error in C<$_> (localized) and as that block's first and only argument. C<$@> does B<not> contain the error. Inside the C<catch> block it has the same value it had before the C<try> block was executed. Note that the error may be false, but if that happens the C<catch> block will still be invoked. Once all execution is finished then the C<finally> block, if given, will execute. =item catch (&;@) Intended to be used in the second argument position of C<try>. Returns a reference to the subroutine it was given but blessed as C<Try::Tiny::Catch> which allows try to decode correctly what to do with this code reference. catch { ... } Inside the C<catch> block the caught error is stored in C<$_>, while previous value of C<$@> is still available for use. This value may or may not be meaningful depending on what happened before the C<try>, but it might be a good idea to preserve it in an error stack. For code that captures C<$@> when throwing new errors (i.e. L<Class::Throwable>), you'll need to do: local $@ = $_; =item finally (&;@) try { ... } catch { ... } finally { ... }; Or try { ... } finally { ... }; Or even try { ... } finally { ... } catch { ... }; Intended to be the second or third element of C<try>. C<finally> blocks are always executed in the event of a successful C<try> or if C<catch> is run. This allows you to locate cleanup code which cannot be done via C<local()> e.g. closing a file handle. When invoked, the C<finally> block is passed the error that was caught. If no error was caught, it is passed nothing. (Note that the C<finally> block does not localize C<$_> with the error, since unlike in a C<catch> block, there is no way to know if C<$_ == undef> implies that there were no errors.) In other words, the following code does just what you would expect: try { die_sometimes(); } catch { # ...code run in case of error } finally { if (@_) { print "The try block died with: @_\n"; } else { print "The try block ran without error.\n"; } }; B<You must always do your own error handling in the C<finally> block>. C<Try::Tiny> will not do anything about handling possible errors coming from code located in these blocks. Furthermore B<exceptions in C<finally> blocks are not trappable and are unable to influence the execution of your program>. This is due to limitation of C<DESTROY>-based scope guards, which C<finally> is implemented on top of. This may change in a future version of Try::Tiny. In the same way C<catch()> blesses the code reference this subroutine does the same except it bless them as C<Try::Tiny::Finally>. =back =head1 BACKGROUND There are a number of issues with C<eval>. =head2 Clobbering $@ When you run an C<eval> block and it succeeds, C<$@> will be cleared, potentially clobbering an error that is currently being caught. This causes action at a distance, clearing previous errors your caller may have not yet handled. C<$@> must be properly localized before invoking C<eval> in order to avoid this issue. More specifically, L<before Perl version 5.14.0|perl5140delta/"Exception Handling"> C<$@> was clobbered at the beginning of the C<eval>, which also made it impossible to capture the previous error before you die (for instance when making exception objects with error stacks). For this reason C<try> will actually set C<$@> to its previous value (the one available before entering the C<try> block) in the beginning of the C<eval> block. =head2 Localizing $@ silently masks errors Inside an C<eval> block, C<die> behaves sort of like: sub die { $@ = $_[0]; return_undef_from_eval(); } This means that if you were polite and localized C<$@> you can't die in that scope, or your error will be discarded (printing "Something's wrong" instead). The workaround is very ugly: my $error = do { local $@; eval { ... }; $@; }; ... die $error; =head2 $@ might not be a true value This code is wrong: if ( $@ ) { ... } because due to the previous caveats it may have been unset. C<$@> could also be an overloaded error object that evaluates to false, but that's asking for trouble anyway. The classic failure mode (fixed in L<Perl 5.14.0|perl5140delta/"Exception Handling">) is: sub Object::DESTROY { eval { ... } } eval { my $obj = Object->new; die "foo"; }; if ( $@ ) { } In this case since C<Object::DESTROY> is not localizing C<$@> but still uses C<eval>, it will set C<$@> to C<"">. The destructor is called when the stack is unwound, after C<die> sets C<$@> to C<"foo at Foo.pm line 42\n">, so by the time C<if ( $@ )> is evaluated it has been cleared by C<eval> in the destructor. The workaround for this is even uglier than the previous ones. Even though we can't save the value of C<$@> from code that doesn't localize, we can at least be sure the C<eval> was aborted due to an error: my $failed = not eval { ... return 1; }; This is because an C<eval> that caught a C<die> will always return a false value. =head1 ALTERNATE SYNTAX Using Perl 5.10 you can use L<perlsyn/"Switch statements"> (but please don't, because that syntax has since been deprecated because there was too much unexpected magical behaviour). =for stopwords topicalizer The C<catch> block is invoked in a topicalizer context (like a C<given> block), but note that you can't return a useful value from C<catch> using the C<when> blocks without an explicit C<return>. This is somewhat similar to Perl 6's C<CATCH> blocks. You can use it to concisely match errors: try { require Foo; } catch { when (/^Can't locate .*?\.pm in \@INC/) { } # ignore default { die $_ } }; =head1 CAVEATS =over 4 =item * C<@_> is not available within the C<try> block, so you need to copy your argument list. In case you want to work with argument values directly via C<@_> aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference: sub foo { my ( $self, @args ) = @_; try { $self->bar(@args) } } or sub bar_in_place { my $self = shift; my $args = \@_; try { $_ = $self->bar($_) for @$args } } =item * C<return> returns from the C<try> block, not from the parent sub (note that this is also how C<eval> works, but not how L<TryCatch> works): sub parent_sub { try { die; } catch { return; }; say "this text WILL be displayed, even though an exception is thrown"; } Instead, you should capture the return value: sub parent_sub { my $success = try { die; 1; }; return unless $success; say "This text WILL NEVER appear!"; } # OR sub parent_sub_with_catch { my $success = try { die; 1; } catch { # do something with $_ return undef; #see note }; return unless $success; say "This text WILL NEVER appear!"; } Note that if you have a C<catch> block, it must return C<undef> for this to work, since if a C<catch> block exists, its return value is returned in place of C<undef> when an exception is thrown. =item * C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp> will not report this when using full stack traces, though, because C<%Carp::Internal> is used. This lack of magic is considered a feature. =for stopwords unhygienically =item * The value of C<$_> in the C<catch> block is not guaranteed to be the value of the exception thrown (C<$@>) in the C<try> block. There is no safe way to ensure this, since C<eval> may be used unhygienically in destructors. The only guarantee is that the C<catch> will be called if an exception is thrown. =item * The return value of the C<catch> block is not ignored, so if testing the result of the expression for truth on success, be sure to return a false value from the C<catch> block: my $obj = try { MightFail->new; } catch { ... return; # avoid returning a true value; }; return unless $obj; =item * C<$SIG{__DIE__}> is still in effect. Though it can be argued that C<$SIG{__DIE__}> should be disabled inside of C<eval> blocks, since it isn't people have grown to rely on it. Therefore in the interests of compatibility, C<try> does not disable C<$SIG{__DIE__}> for the scope of the error throwing code. =item * Lexical C<$_> may override the one set by C<catch>. For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some confusing behavior: given ($foo) { when (...) { try { ... } catch { warn $_; # will print $foo, not the error warn $_[0]; # instead, get the error like this } } } Note that this behavior was changed once again in L<Perl5 version 18|https://metacpan.org/module/perldelta#given-now-aliases-the-global-_>. However, since the entirety of lexical C<$_> is now L<considered experimental |https://metacpan.org/module/perldelta#Lexical-_-is-now-experimental>, it is unclear whether the new version 18 behavior is final. =back =head1 SEE ALSO =over 4 =item L<TryCatch> Much more feature complete, more convenient semantics, but at the cost of implementation complexity. =item L<autodie> Automatic error throwing for builtin functions and more. Also designed to work well with C<given>/C<when>. =item L<Throwable> A lightweight role for rolling your own exception classes. =item L<Error> Exception object implementation with a C<try> statement. Does not localize C<$@>. =item L<Exception::Class::TryCatch> Provides a C<catch> statement, but properly calling C<eval> is your responsibility. The C<try> keyword pushes C<$@> onto an error stack, avoiding some of the issues with C<$@>, but you still need to localize to prevent clobbering. =back =head1 LIGHTNING TALK I gave a lightning talk about this module, you can see the slides (Firefox only): L<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul> Or read the source: L<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml> =head1 SUPPORT Bugs may be submitted through L<the RT bug tracker|https://rt.cpan.org/Public/Dist/Display.html?Name=Try-Tiny> (or L<bug-Try-Tiny@rt.cpan.org|mailto:bug-Try-Tiny@rt.cpan.org>). =head1 AUTHORS =over 4 =item * יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org> =item * Jesse Luehrs <doy@tozt.net> =back =head1 CONTRIBUTORS =for stopwords Karen Etheridge Peter Rabbitson Ricardo Signes Mark Fowler Graham Knop Lukas Mai Aristotle Pagaltzis Dagfinn Ilmari Mannsåker Paul Howarth Rudolf Leermakers anaxagoras awalker chromatic Alex cm-perl Andrew Yates David Lowe Glenn Hans Dieter Pearcey Jens Berthold Jonathan Yu Marc Mims Stosberg Pali =over 4 =item * Karen Etheridge <ether@cpan.org> =item * Peter Rabbitson <ribasushi@cpan.org> =item * Ricardo Signes <rjbs@cpan.org> =item * Mark Fowler <mark@twoshortplanks.com> =item * Graham Knop <haarg@haarg.org> =item * Lukas Mai <l.mai@web.de> =item * Aristotle Pagaltzis <pagaltzis@gmx.de> =item * Dagfinn Ilmari Mannsåker <ilmari@ilmari.org> =item * Paul Howarth <paul@city-fan.org> =item * Rudolf Leermakers <rudolf@hatsuseno.org> =item * anaxagoras <walkeraj@gmail.com> =item * awalker <awalker@sourcefire.com> =item * chromatic <chromatic@wgz.org> =item * Alex <alex@koban.(none)> =item * cm-perl <cm-perl@users.noreply.github.com> =item * Andrew Yates <ayates@haddock.local> =item * David Lowe <davidl@lokku.com> =item * Glenn Fowler <cebjyre@cpan.org> =item * Hans Dieter Pearcey <hdp@weftsoar.net> =item * Jens Berthold <jens@jebecs.de> =item * Jonathan Yu <JAWNSY@cpan.org> =item * Marc Mims <marc@questright.com> =item * Mark Stosberg <mark@stosberg.com> =item * Pali <pali@cpan.org> =back =head1 COPYRIGHT AND LICENCE This software is Copyright (c) 2009 by יובל קוג'מן (Yuval Kogman). This is free software, licensed under: The MIT (X11) License =cut

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