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 # 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 and C 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 $@ }; You can also use it like a standalone C to catch and ignore any error conditions. Obviously, this is an extreme measure not to be undertaken lightly: # just silence errors try { die "foo"; }; =head1 DESCRIPTION This module provides bare bones C/C/C statements that are designed to minimize common mistakes with eval blocks, and NOTHING else. This is unlike L which provides a nice syntax and avoids adding another call stack layer, and supports calling C from the C block to return from the parent subroutine. These extra features come at a cost of a few dependencies, namely L and L which are occasionally problematic, and the additional catch filtering uses L type constraints which may not be desirable either. The main focus of this module is to provide simple and reliable error handling for those having a hard time installing L, but who still want to write correct C blocks without 5 lines of boilerplate each time. It's designed to work as correctly as possible in light of the various pathological edge cases (see L) and to be compatible with any style of error values (simple strings, references, objects, overloaded objects, etc). If the C block dies, it returns the value of the last statement executed in the C block, if there is one. Otherwise, it returns C in scalar context or the empty list in list context. The following examples all assign C<"bar"> to C<$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 blocks, yielding the following: my $x; try { die 'foo' } finally { $x = 'bar' }; try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' }; C blocks are always executed making them suitable for cleanup code which cannot be handled using local. You can add as many C blocks to a given C block as you like. Note that adding a C block without a preceding C block suppresses any errors. This behaviour is consistent with using a standalone C, but it is not consistent with C/C patterns found in other programming languages, such as Java, Python, Javascript or C#. If you learnt the C/C pattern from one of these languages, watch out for this. =head1 EXPORTS All functions are exported by default using L. If you need to rename the C, C or C keyword consider using L to get L's flexibility. =over 4 =item try (&;@) Takes one mandatory C subroutine, an optional C subroutine and C subroutine. The mandatory subroutine is evaluated in the context of an C block. If no error occurred the value from the first block is returned, preserving list/scalar context. If there was an error and the second subroutine was given it will be invoked with the error in C<$_> (localized) and as that block's first and only argument. C<$@> does B contain the error. Inside the C block it has the same value it had before the C block was executed. Note that the error may be false, but if that happens the C block will still be invoked. Once all execution is finished then the C block, if given, will execute. =item catch (&;@) Intended to be used in the second argument position of C. Returns a reference to the subroutine it was given but blessed as C which allows try to decode correctly what to do with this code reference. catch { ... } Inside the C block the caught error is stored in C<$_>, while previous value of C<$@> is still available for use. This value may or may not be meaningful depending on what happened before the C, but it might be a good idea to preserve it in an error stack. For code that captures C<$@> when throwing new errors (i.e. L), you'll need to do: local $@ = $_; =item finally (&;@) try { ... } catch { ... } finally { ... }; Or try { ... } finally { ... }; Or even try { ... } finally { ... } catch { ... }; Intended to be the second or third element of C. C blocks are always executed in the event of a successful C or if C is run. This allows you to locate cleanup code which cannot be done via C e.g. closing a file handle. When invoked, the C block is passed the error that was caught. If no error was caught, it is passed nothing. (Note that the C block does not localize C<$_> with the error, since unlike in a C 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 block>. C will not do anything about handling possible errors coming from code located in these blocks. Furthermore B blocks are not trappable and are unable to influence the execution of your program>. This is due to limitation of C-based scope guards, which C is implemented on top of. This may change in a future version of Try::Tiny. In the same way C blesses the code reference this subroutine does the same except it bless them as C. =back =head1 BACKGROUND There are a number of issues with C. =head2 Clobbering $@ When you run an C block and it succeeds, C<$@> 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<$@> must be properly localized before invoking C in order to avoid this issue. More specifically, L C<$@> was clobbered at the beginning of the C, 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 will actually set C<$@> to its previous value (the one available before entering the C block) in the beginning of the C block. =head2 Localizing $@ silently masks errors Inside an C block, C 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) is: sub Object::DESTROY { eval { ... } } eval { my $obj = Object->new; die "foo"; }; if ( $@ ) { } In this case since C is not localizing C<$@> but still uses C, it will set C<$@> to C<"">. The destructor is called when the stack is unwound, after C sets C<$@> to C<"foo at Foo.pm line 42\n">, so by the time C is evaluated it has been cleared by C 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 was aborted due to an error: my $failed = not eval { ... return 1; }; This is because an C that caught a C will always return a false value. =head1 ALTERNATE SYNTAX Using Perl 5.10 you can use L (but please don't, because that syntax has since been deprecated because there was too much unexpected magical behaviour). =for stopwords topicalizer The C block is invoked in a topicalizer context (like a C block), but note that you can't return a useful value from C using the C blocks without an explicit C. This is somewhat similar to Perl 6's C blocks. You can use it to concisely match errors: try { require Foo; } catch { when (/^Can't locate .*?\.pm in \@INC/) { } # ignore default { die $_ } }; =head1 CAVEATS =over 4 =item * C<@_> is not available within the C block, so you need to copy your argument list. In case you want to work with argument values directly via C<@_> aliasing (i.e. allow C<$_[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 returns from the C block, not from the parent sub (note that this is also how C works, but not how L 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 $success; say "This text WILL NEVER appear!"; } Note that if you have a C block, it must return C for this to work, since if a C block exists, its return value is returned in place of C when an exception is thrown. =item * C introduces another caller stack frame. L is not used. L will not report this when using full stack traces, though, because C<%Carp::Internal> is used. This lack of magic is considered a feature. =for stopwords unhygienically =item * The value of C<$_> in the C block is not guaranteed to be the value of the exception thrown (C<$@>) in the C block. There is no safe way to ensure this, since C may be used unhygienically in destructors. The only guarantee is that the C will be called if an exception is thrown. =item * The return value of the C 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 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<$SIG{__DIE__}> should be disabled inside of C blocks, since it isn't people have grown to rely on it. Therefore in the interests of compatibility, C does not disable C<$SIG{__DIE__}> for the scope of the error throwing code. =item * Lexical C<$_> may override the one set by C. For example Perl 5.10's C form uses a lexical C<$_>, creating some confusing behavior: given ($foo) { when (...) { try { ... } catch { warn $_; # will print $foo, not the error warn $_[0]; # instead, get the error like this } } } Note that this behavior was changed once again in L. However, since the entirety of lexical C<$_> is now L, it is unclear whether the new version 18 behavior is final. =back =head1 SEE ALSO =over 4 =item L Much more feature complete, more convenient semantics, but at the cost of implementation complexity. =item L Automatic error throwing for builtin functions and more. Also designed to work well with C/C. =item L A lightweight role for rolling your own exception classes. =item L Exception object implementation with a C statement. Does not localize C<$@>. =item L Provides a C statement, but properly calling C is your responsibility. The C keyword pushes C<$@> 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 Or read the source: L =head1 SUPPORT Bugs may be submitted through L (or L). =head1 AUTHORS =over 4 =item * יובל קוג'מן (Yuval Kogman) =item * Jesse Luehrs =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 =item * Peter Rabbitson =item * Ricardo Signes =item * Mark Fowler =item * Graham Knop =item * Lukas Mai =item * Aristotle Pagaltzis =item * Dagfinn Ilmari Mannsåker =item * Paul Howarth =item * Rudolf Leermakers =item * anaxagoras =item * awalker =item * chromatic =item * Alex =item * cm-perl =item * Andrew Yates =item * David Lowe =item * Glenn Fowler =item * Hans Dieter Pearcey =item * Jens Berthold =item * Jonathan Yu =item * Marc Mims =item * Mark Stosberg =item * Pali =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