better example for returning from a try block (forwardever)

[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 = "0.10";

$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 stanalone 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 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";

You can add finally blocks making the following true.

	my $x;
	try { die 'foo' } finally { $x = 'bar' };
	try { die 'foo' } catch { warn "Got a die: $_" } finally { $x = 'bar' };

Finally blocks are always executed making them suitable for cleanup code
which cannot be handled using local.  You can add as many finally blocks to a
given 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 try subroutine, an optional catch subroutine & 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 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 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>. 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 finally block is passed the error that was caught.  If no
error was caught, it is passed nothing.  (Note that the finally block does not
localize C<$_> with the error, since unlike in a catch block, there is no way
to know if C<$_ == 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 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 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 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 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 catch block, it must return undef for this to work,
since if a catch block exists, its return value is returned in place of 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