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 vars qw(@EXPORT @EXPORT_OK $VERSION @ISA);
+use Exporter 5.57 'import';
+our @EXPORT = our @EXPORT_OK = qw(try catch finally);
-BEGIN {
- require Exporter;
- @ISA = qw(Exporter);
-}
-
-$VERSION = "0.12";
+use Carp;
+$Carp::Internal{+__PACKAGE__}++;
-$VERSION = eval $VERSION;
+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) };
+ }
+ }
-@EXPORT = @EXPORT_OK = qw(try catch finally);
+ *_subname = $su ? \&Sub::Util::set_subname
+ : $sn ? \&Sub::Name::subname
+ : sub { $_[1] };
+ *_HAS_SUBNAME = ($su || $sn) ? sub(){1} : sub(){0};
+}
-$Carp::Internal{+__PACKAGE__}++;
+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;
-
- 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) {
- next unless $code_ref;
-
- my $ref = ref($code_ref);
-
- if ( $ref eq 'Try::Tiny::Catch' ) {
- $catch = ${$code_ref};
- } elsif ( $ref eq 'Try::Tiny::Finally' ) {
- push @finally, ${$code_ref};
- } else {
- use Carp;
- confess("Unknown code ref type given '${ref}'. Check your usage & try again");
- }
- }
-
- # 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 = $@;
- }
-
- # set up a scope guard to invoke the finally block at the end
- my @guards =
- map { Try::Tiny::ScopeGuard->_new($_, $failed ? $error : ()) }
- @finally;
-
- # 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<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];
- }
+ 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 ) = @_;
+ my ( $block, @rest ) = @_;
- return (
- bless(\$block, 'Try::Tiny::Catch'),
- @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 ) = @_;
+ my ( $block, @rest ) = @_;
+
+ croak 'Useless bare finally()' unless wantarray;
- return (
- bless(\$block, 'Try::Tiny::Finally'),
- @rest,
- );
+ _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 @guts = @{ shift() };
- my $code = shift @guts;
- $code->(@guts);
+ my ($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"
+ ;
+ }
}
}
=pod
-=head1 NAME
-
-Try::Tiny - minimal try/catch with proper localization of $@
-
=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 $@
- };
+ # handle errors with a catch handler
+ try {
+ die "foo";
+ } catch {
+ warn "caught error: $_"; # not $@
+ };
You can also use it like a standalone C<eval> to catch and ignore any error
conditions. Obviously, this is an extreme measure not to be undertaken
lightly:
- # just silence errors
- try {
- die "foo";
- };
+ # just silence errors
+ try {
+ die "foo";
+ };
=head1 DESCRIPTION
context or the empty list in list context. The following examples all
assign C<"bar"> to C<$x>:
- my $x = try { die "foo" } catch { "bar" };
- my $x = try { die "foo" } || { "bar" };
- my $x = (try { die "foo" }) // { "bar" };
+ my $x = 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 = 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' };
+ my $x;
+ try { die 'foo' } finally { $x = 'bar' };
+ try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };
C<finally> blocks are always executed making them suitable for cleanup code
which cannot be handled using local. You can add as many C<finally> blocks to a
given C<try> block as you like.
+Note that adding a C<finally> block without a preceding C<catch> block
+suppresses any errors. This behaviour is consistent with using a standalone
+C<eval>, but it is not consistent with C<try>/C<finally> patterns found in
+other programming languages, such as Java, Python, Javascript or C#. If you
+learnt the C<try>/C<finally> pattern from one of these languages, watch out for
+this.
+
=head1 EXPORTS
All functions are exported by default using L<Exporter>.
Once all execution is finished then the C<finally> block, if given, will execute.
-=item catch (&;$)
+=item catch (&;@)
Intended to be used in the second argument position of C<try>.
C<Try::Tiny::Catch> which allows try to decode correctly what to do
with this code reference.
- catch { ... }
+ 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
For code that captures C<$@> when throwing new errors (i.e.
L<Class::Throwable>), you'll need to do:
- local $@ = $_;
+ local $@ = $_;
-=item finally (&;$)
+=item finally (&;@)
try { ... }
catch { ... }
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>.
C<$@> must be properly localized before invoking C<eval> in order to avoid this
issue.
-More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
-also makes it impossible to capture the previous error before you die (for
+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 (before
-the localization) in the beginning of the C<eval> block.
+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();
- }
+ 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<Perl 5.14.0|perl5140delta/"Exception Handling">) 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 ( $@ ) {
- }
+ }
In this case since C<Object::DESTROY> is not localizing C<$@> but still uses
C<eval>, it will set C<$@> to C<"">.
can't save the value of C<$@> 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 {
- ...
+ my $failed = not eval {
+ ...
- return 1;
- };
+ return 1;
+ };
This is because an C<eval> that caught a C<die> will always return a false
value.
-=head1 SHINY SYNTAX
+=head1 ALTERNATE SYNTAX
-Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
+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>
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 $_ }
- };
+ try {
+ require Foo;
+ } catch {
+ when (/^Can't locate .*?\.pm in \@INC/) { } # ignore
+ default { die $_ }
+ };
=head1 CAVEATS
=item *
C<@_> is not available within the C<try> block, so you need to copy your
-arglist. In case you want to work with argument values directly via C<@_>
+argument list. In case you want to work with argument values directly via C<@_>
aliasing (i.e. allow C<$_[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 }
- }
+ sub bar_in_place {
+ my $self = shift;
+ my $args = \@_;
+ try { $_ = $self->bar($_) for @$args }
+ }
=item *
this is also how C<eval> works, but not how L<TryCatch> works):
sub parent_sub {
- try {
- die;
- }
- catch {
- return;
- };
+ try {
+ die;
+ }
+ catch {
+ return;
+ };
- say "this text WILL be displayed, even though an exception is thrown";
+ 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;
+ my $success = try {
+ die;
+ 1;
+ };
+ return unless $success;
- say "This text WILL NEVER appear!";
+ 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 $success;
+
+ say "This text WILL NEVER appear!";
}
Note that if you have a C<catch> block, it must return C<undef> for this to work,
will not report this when using full stack traces, though, because
C<%Carp::Internal> is used. This lack of magic is considered a feature.
+=for stopwords unhygienically
+
=item *
The value of C<$_> 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 unhygenically in destructors. The only
+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 *
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 {
- ...
+ my $obj = try {
+ MightFail->new;
+ } catch {
+ ...
- return; # avoid returning a true value;
- };
+ return; # avoid returning a true value;
+ };
- return unless $obj;
+ return unless $obj;
=item *
For example Perl 5.10's C<given> form uses a lexical C<$_>, creating some
confusing behavior:
- given ($foo) {
- when (...) {
- try {
- ...
- } catch {
- warn $_; # will print $foo, not the error
- warn $_[0]; # instead, get the error like this
- }
- }
- }
+ given ($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
I gave a lightning talk about this module, you can see the slides (Firefox
only):
-L<http://nothingmuch.woobling.org/talks/takahashi.xul?data=yapc_asia_2009/try_tiny.txt>
+L<http://web.archive.org/web/20100628040134/http://nothingmuch.woobling.org/talks/takahashi.xul>
Or read the source:
-L<http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
-
-=head1 VERSION CONTROL
-
-L<http://github.com/nothingmuch/try-tiny/>
-
-=head1 AUTHOR
-
-Yuval Kogman E<lt>nothingmuch@woobling.orgE<gt>
-
-=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<http://web.archive.org/web/20100305133605/http://nothingmuch.woobling.org/talks/yapc_asia_2009/try_tiny.yml>
=cut
-
projects / p5sagit/Try-Tiny.git / blobdiff