Documentation fixes

[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 = "0.11";

$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