X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=lib%2FTry%2FTiny.pm;h=501b006aebf5c84d1ece671d09c6acca48eba491;hb=b1ab4ef09b6eef778693ceac185ad4d10a595542;hp=dd330c918f53e8d38363fc059ef6c1a2890ef53f;hpb=57c50f41df82a5993fbbbbb8551dcaecdcbce28e;p=p5sagit%2FTry-Tiny.git diff --git a/lib/Try/Tiny.pm b/lib/Try/Tiny.pm index dd330c9..501b006 100644 --- a/lib/Try/Tiny.pm +++ b/lib/Try/Tiny.pm @@ -1,92 +1,195 @@ package Try::Tiny; +use 5.006; +# ABSTRACT: Minimal try/catch with proper preservation of $@ + +our $VERSION = '0.29'; use strict; -#use warnings; +use warnings; + +use Exporter 5.57 'import'; +our @EXPORT = our @EXPORT_OK = qw(try catch finally); -use vars qw(@EXPORT @EXPORT_OK $VERSION @ISA); +use Carp; +$Carp::Internal{+__PACKAGE__}++; BEGIN { - require Exporter; - @ISA = qw(Exporter); + 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 || $sn) ? sub(){1} : sub(){0}; } -$VERSION = "0.02"; - -$VERSION = eval $VERSION; +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]; + } +} -@EXPORT = @EXPORT_OK = qw(try catch); +sub catch (&;@) { + my ( $block, @rest ) = @_; -$Carp::Internal{+__PACKAGE__}++; + croak 'Useless bare catch()' unless wantarray; -sub try (&;$) { - my ( $try, $catch ) = @_; - - # we need to save this here, the eval block will be in scalar context due - # to $failed - my $wantarray = wantarray; - - # save the value of $@ so we can set $@ back to it in the beginning of the eval - my $prev_error = $@; - - my ( @ret, $error, $failed ); - - # 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->(); - - { - # localize $@ to prevent clobbering of previous value by a successful - # eval. - local $@; - - # failed will be true if the eval dies, because 1 will not be returned - # from the eval body - $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 $fail to false - }; - - # copy $@ to $error; when we leave this scope, local $@ will revert $@ - # back to its previous value - $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 ) { - # 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]; - } + _subname(caller().'::catch {...} ' => $block) + if _HAS_SUBNAME; + return ( + bless(\$block, 'Try::Tiny::Catch'), + @rest, + ); } -sub catch (&) { - return $_[0]; +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__ @@ -94,31 +197,34 @@ __END__ =pod -=head1 NAME +=head1 SYNOPSIS -Try::Tiny - minimal try/catch with proper localization of $@ +You can use Try::Tiny's C and C to expect and handle exceptional +conditions, avoiding quirks in Perl and common mistakes: -=head1 SYNOPSIS + # handle errors with a catch handler + try { + die "foo"; + } catch { + warn "caught error: $_"; # not $@ + }; - # handle errors with a catch handler - try { - die "foo"; - } catch { - warn "caught error: $_"; - }; +You can also use it like a standalone C to catch and ignore any error +conditions. Obviously, this is an extreme measure not to be undertaken +lightly: - # just silence errors - try { - die "foo"; - }; + # just silence errors + try { + die "foo"; + }; =head1 DESCRIPTION -This module provides bare bones C/C statements that are designed to +This module provides bare bones C/C/C statements that are designed to minimize common mistakes with eval blocks, and NOTHING else. This is unlike L which provides a nice syntax and avoids adding -another call stack layer, and supports calling C from the try block to +another call stack layer, and supports calling C from the C block to return from the parent subroutine. These extra features come at a cost of a few dependencies, namely L and L which are occasionally problematic, and the additional catch filtering uses L @@ -129,30 +235,50 @@ for those having a hard time installing L, but who still want to write correct C blocks without 5 lines of boilerplate each time. It's designed to work as correctly as possible in light of the various -pathological edge cases (see L) and to be compatible with any style +pathological edge cases (see L) and to be compatible with any style of error values (simple strings, references, objects, overloaded objects, etc). -If the try block dies, it returns the value of the last statement executed in -the catch block, if there is one. Otherwise, it returns C in scalar -context or the empty list in list context. The following two examples both -assign C<"bar"> to C<$x>. +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"; - my $x = try { die "foo" } catch { "bar" }; +You can add C blocks, yielding the following: - my $x = eval { die "foo" } || "bar"; + my $x; + try { die 'foo' } finally { $x = 'bar' }; + try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' }; + +C blocks are always executed making them suitable for cleanup code +which cannot be handled using local. You can add as many C blocks to a +given C block as you like. + +Note that adding a C block without a preceding C block +suppresses any errors. This behaviour is consistent with using a standalone +C, but it is not consistent with C/C patterns found in +other programming languages, such as Java, Python, Javascript or C#. If you +learnt the C/C pattern from one of these languages, watch out for +this. =head1 EXPORTS All functions are exported by default using L. -If you need to rename the C or C keyword consider using +If you need to rename the C, C or C keyword consider using L to get L's flexibility. =over 4 -=item try (&;$) +=item try (&;@) -Takes one mandatory try subroutine and one optional catch subroutine. +Takes one mandatory C subroutine, an optional C subroutine and C +subroutine. The mandatory subroutine is evaluated in the context of an C block. @@ -163,24 +289,85 @@ 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. -=item catch (&) +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. -Just returns the subroutine it was given. +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 $@ = $_; + +=item finally (&;@) + + try { ... } + catch { ... } + finally { ... }; + +Or + + try { ... } + finally { ... }; + +Or even - catch { ... } + try { ... } + finally { ... } + catch { ... }; -is the same as +Intended to be the second or third element of C. C blocks are always +executed in the event of a successful C or if C is run. This allows +you to locate cleanup code which cannot be done via C e.g. closing a file +handle. - sub { ... } +When invoked, the C block is passed the error that was caught. If no +error was caught, it is passed nothing. (Note that the C block does not +localize C<$_> with the error, since unlike in a C block, there is no way +to know if C<$_ == undef> implies that there were no errors.) In other words, +the following code does just what you would expect: -Inside the catch block the previous value of C<$@> 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. + 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 @@ -190,7 +377,7 @@ There are a number of issues with C. =head2 Clobbering $@ -When you run an eval block and it succeeds, C<$@> will be cleared, potentially +When you run an C block and it succeeds, C<$@> will be cleared, potentially clobbering an error that is currently being caught. This causes action at a distance, clearing previous errors your caller may have @@ -199,64 +386,67 @@ not yet handled. C<$@> must be properly localized before invoking C in order to avoid this issue. -More specifically, C<$@> is clobbered at the begining of the C, which -also makes it impossible to capture the previous error before you die (for +More specifically, +L +C<$@> 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<$@> to its previous value (before -the localization) in the beginning of the C block. +For this reason C will actually set C<$@> 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 eval block C behaves sort of like: +Inside an C block, C behaves sort of like: - sub die { - $@ = $_[0]; - return_undef_from_eval(); - } + 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 { ... }; - $@; - }; + my $error = do { + local $@; + eval { ... }; + $@; + }; - ... - die $error; + ... + die $error; =head2 $@ might not be a true value This code is wrong: - if ( $@ ) { - ... - } + 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 is: +The classic failure mode (fixed in L) is: - sub Object::DESTROY { - eval { ... } - } + sub Object::DESTROY { + eval { ... } + } - eval { - my $obj = Object->new; + eval { + my $obj = Object->new; - die "foo"; - }; + die "foo"; + }; - if ( $@ ) { + if ( $@ ) { - } + } In this case since C is not localizing C<$@> but still uses C, it will set C<$@> to C<"">. @@ -267,20 +457,24 @@ been cleared by C in the destructor. The workaround for this is even uglier than the previous ones. Even though we can't save the value of C<$@> from code that doesn't localize, we can at least -be sure the eval was aborted due to an error: +be sure the C was aborted due to an error: - my $failed = not eval { - ... + my $failed = not eval { + ... - return 1; - }; + return 1; + }; This is because an C that caught a C will always return a false value. -=head1 SHINY SYNTAX +=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). -Using Perl 5.10 you can use L. +=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 @@ -289,12 +483,12 @@ 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 $_ } - }; + try { + require Foo; + } catch { + when (/^Can't locate .*?\.pm in \@INC/) { } # ignore + default { die $_ } + }; =head1 CAVEATS @@ -302,36 +496,82 @@ concisely match errors: =item * -C<@_> is not available, you need to name your args: +C<@_> is not available within the C block, so you need to copy your +argument list. In case you want to work with argument values directly via C<@_> +aliasing (i.e. allow C<$_[1] = "foo">), you need to pass C<@_> by reference: - sub foo { - my ( $self, @args ) = @_; - try { $self->bar(@args) } - } + 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 bar { - try { return "foo" }; - return "baz"; - } - - say bar(); # "baz" + 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 $success; + + say "This text WILL NEVER appear!"; + } + +Note that if you have a C block, it must return C for this to work, +since if a C block exists, its return value is returned in place of C +when an exception is thrown. =item * C introduces another caller stack frame. L is not used. L -will report this when using full stack traces. This lack of magic is considered -a feature. +will not report this when using full stack traces, though, because +C<%Carp::Internal> is used. This lack of magic is considered a feature. + +=for stopwords unhygienically =item * The value of C<$_> 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 unhygenically in destructors. The only +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 * @@ -340,15 +580,48 @@ 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 { - ... + my $obj = try { + MightFail->new; + } catch { + ... + + return; # avoid returning a true value; + }; + + return unless $obj; - return; # avoid returning a true value; - }; +=item * + +C<$SIG{__DIE__}> is still in effect. - return unless $obj; +Though it can be argued that C<$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 @@ -390,25 +663,10 @@ issues with C<$@>, but you still need to localize to prevent clobbering. I gave a lightning talk about this module, you can see the slides (Firefox only): -L +L Or read the source: -L - -=head1 VERSION CONTROL - -L - -=head1 AUTHOR - -Yuval Kogman Enothingmuch@woobling.orgE - -=head1 COPYRIGHT - - Copyright (c) 2009 Yuval Kogman. All rights reserved. - This program is free software; you can redistribute - it and/or modify it under the terms of the MIT license. +L =cut -