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package Try::Tiny;
use strict;
#use warnings;
use vars qw(@EXPORT @EXPORT_OK $VERSION @ISA);
BEGIN {
require Exporter;
@ISA = qw(Exporter);
}
$VERSION = "0.12";
$VERSION = eval $VERSION;
@EXPORT = @EXPORT_OK = qw(try catch finally);
$Carp::Internal{+__PACKAGE__}++;
# 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];
}
}
sub catch (&;@) {
my ( $block, @rest ) = @_;
return (
bless(\$block, 'Try::Tiny::Catch'),
@rest,
);
}
sub finally (&;@) {
my ( $block, @rest ) = @_;
return (
bless(\$block, 'Try::Tiny::Finally'),
@rest,
);
}
{
package # hide from PAUSE
Try::Tiny::ScopeGuard;
sub _new {
shift;
bless [ @_ ];
}
sub DESTROY {
my @guts = @{ shift() };
my $code = shift @guts;
$code->(@guts);
}
}
__PACKAGE__
__END__
=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 $@
};
You can also use it like a standalone C<eval> to catch and ignore any error
conditions. Obviously, this is an extreme measure not to be undertaken
lightly:
# just silence errors
try {
die "foo";
};
=head1 DESCRIPTION
This module provides bare bones C<try>/C<catch>/C<finally> statements that are designed to
minimize common mistakes with eval blocks, and NOTHING else.
This is unlike L<TryCatch> which provides a nice syntax and avoids adding
another call stack layer, and supports calling C<return> from the C<try> block to
return from the parent subroutine. These extra features come at a cost of a few
dependencies, namely L<Devel::Declare> and L<Scope::Upper> which are
occasionally problematic, and the additional catch filtering uses L<Moose>
type constraints which may not be desirable either.
The main focus of this module is to provide simple and reliable error handling
for those having a hard time installing L<TryCatch>, but who still want to
write correct C<eval> blocks without 5 lines of boilerplate each time.
It's designed to work as correctly as possible in light of the various
pathological edge cases (see L</BACKGROUND>) and to be compatible with any style
of error values (simple strings, references, objects, overloaded objects, etc).
If the C<try> block dies, it returns the value of the last statement executed in
the C<catch> block, if there is one. Otherwise, it returns C<undef> in scalar
context or the empty list in list context. The following examples all
assign C<"bar"> to C<$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 { $x = 'bar' };
C<finally> blocks are always executed making them suitable for cleanup code
which cannot be handled using local. You can add as many C<finally> blocks to a
given C<try> block as you like.
=head1 EXPORTS
All functions are exported by default using L<Exporter>.
If you need to rename the C<try>, C<catch> or C<finally> keyword consider using
L<Sub::Import> to get L<Sub::Exporter>'s flexibility.
=over 4
=item try (&;@)
Takes one mandatory C<try> subroutine, an optional C<catch> subroutine and C<finally>
subroutine.
The mandatory subroutine is evaluated in the context of an C<eval> block.
If no error occurred the value from the first block is returned, preserving
list/scalar context.
If there was an error and the second subroutine was given it will be invoked
with the error in C<$_> (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.
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<$@> must be properly localized before invoking C<eval> in order to avoid this
issue.
More specifically, C<$@> is clobbered at the beginning of the C<eval>, which
also makes 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.
=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 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 SHINY SYNTAX
Using Perl 5.10 you can use L<perlsyn/"Switch statements">.
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 $_ }
};
=head1 CAVEATS
=over 4
=item *
C<@_> is not available within the C<try> block, so you need to copy your
arglist. In case you want to work with argument values directly via C<@_>
aliasing (i.e. allow C<$_[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 $success;
say "This text WILL NEVER appear!";
}
Note that if you have a C<catch> block, it must return C<undef> for this to work,
since if a C<catch> block exists, its return value is returned in place of C<undef>
when an exception is thrown.
=item *
C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
will not report this when using full stack traces, though, because
C<%Carp::Internal> is used. This lack of magic is considered a feature.
=item *
The value of C<$_> 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
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 $_[0]; # instead, get the error like this
}
}
}
=back
=head1 SEE ALSO
=over 4
=item L<TryCatch>
Much more feature complete, more convenient semantics, but at the cost of
implementation complexity.
=item L<autodie>
Automatic error throwing for builtin functions and more. Also designed to
work well with C<given>/C<when>.
=item L<Throwable>
A lightweight role for rolling your own exception classes.
=item L<Error>
Exception object implementation with a C<try> statement. Does not localize
C<$@>.
=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://nothingmuch.woobling.org/talks/takahashi.xul?data=yapc_asia_2009/try_tiny.txt>
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.
=cut
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