try to clarify a caveat

[p5sagit/Try-Tiny.git] / lib / Try / Tiny.pm

package Try::Tiny;

use strict;
#use warnings;

use vars qw(@EXPORT @EXPORT_OK $VERSION @ISA);

BEGIN {
	require Exporter;
	@ISA = qw(Exporter);
}

$VERSION = "0.02";

$VERSION = eval $VERSION;

@EXPORT = @EXPORT_OK = qw(try catch);

$Carp::Internal{+__PACKAGE__}++;

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<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 (&) {
	return $_[0];
}


__PACKAGE__

__END__

=pod

=head1 NAME

Try::Tiny - minimal try/catch with proper localization of $@

=head1 SYNOPSIS

	# handle errors with a catch handler
	try {
		die "foo";
	} catch {
		warn "caught error: $_";
	};

	# just silence errors
	try {
		die "foo";
	};

=head1 DESCRIPTION

This module provides bare bones C<try>/C<catch> 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 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 try block dies, it returns the value of the last statement executed in
the catch block, if there is one.  Otherwise, it returns C<undef> in scalar
context or the empty list in list context. The following two examples both
assign C<"bar"> to C<$x>.

	my $x = try { die "foo" } catch { "bar" };

	my $x = eval { die "foo" } || "bar";

=head1 EXPORTS

All functions are exported by default using L<Exporter>.

If you need to rename the C<try> or C<catch> keyword consider using
L<Sub::Import> to get L<Sub::Exporter>'s flexibility.

=over 4

=item try (&;$)

Takes one mandatory try subroutine and one optional catch 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.

Note that the error may be false, but if that happens the C<catch> block will
still be invoked.

=item catch (&)

Intended to be used in the second argument position of C<try>.

Just returns the subroutine it was given.

	catch { ... }

is the same as

	sub { ... }

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<try>, but it might be a good idea to preserve it in an error stack.

=back

=head1 BACKGROUND

There are a number of issues with C<eval>.

=head2 Clobbering $@

When you run an 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 begining 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 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 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, you need to name your args:

	sub foo {
		my ( $self, @args ) = @_;
		try { $self->bar(@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 bar {
		try { return "foo" };
		return "baz";
	}

	say bar(); # "baz"

=item *

C<try> introduces another caller stack frame. L<Sub::Uplevel> is not used. L<Carp>
will report this when using full stack traces. 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;

=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=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