How to use autodie with non-builtins?

Question

The autodie documentation hints that it is possible to use it for other functions than those built-ins which it can handle by default, but there are no clear examples how to do that in it.

Specifically I would like to use it for the Imager module. A lot of the functions and methods of that can fail, and I would prefer if that wouldn't mean that my code will be littered with or die Imager|$image->errstr; phrases all over.

Of course, if there's another way than using autodie to achieve that, I would be interested in that too.

Assent

Answer 1

autodie only works with functions, not methods. This is because it's lexically scoped, and method lookup can't be lexically scoped. autodie::hints explains how to tell autodie about user-defined functions, but that won't do anything for methods.

I don't know of any way to get autodie-like behavior for methods, unless the module has that built in (e.g. DBI's RaiseError).

You could have a subroutine to do the check, but it wouldn't save all that much code, since you'd still have to pass it the correct object or class to call errstr on.

Canaletto

Answer 2

See autodie::hints

Sinistrorse

Thanks. If I understood that correctly, I just need to put the names of the functions into the argument list for use autodie if they signal failure by one of the default behaviours listed in the introduction. But it still doesn't seem to work. Imager is mostly OO. Is there anything special I have to do to make autodie work with methods? – Gluck

Answer 3

Here is an alternative technique that works with methods:

package SafeCall;
    use Carp ();
    sub AUTOLOAD {
        my ($method) = our $AUTOLOAD =~ /([^:']+)$/;   #'

        unshift @_, my $obj = ${shift @_};

        my $code = $obj->can($method)
            or Carp::croak "no method '$method' on $obj";

        &$code or Carp::croak $obj->can('errstr')
                              ? $obj->errstr
                              : "calling $method on $obj failed"
    }

And to use it:

package Image;
    sub new {bless {here => 'ok', also => 'can be looked up'}};
    sub fails    {$_[0]{not_here}}
    sub succeeds {$_[0]{here}}
    sub lookup   {$_[0]{$_[1]}}
    sub errstr   {'an error occurred'}

package main;
use 5.010; # for say()

my $safe = sub {bless \$_[0] => 'SafeCall'}; 
# storing the constructor in the scalar $safe allows $safe to be used
# as a method:  $obj->$safe->method

my $img = Image->new;

say $img->$safe->succeeds;       # prints 'ok'

say $safe->($img)->succeeds;     # or call this way (also prints 'ok')

say $img->$safe->lookup('also'); # prints 'can be looked up'

say $img->$safe->fails;      # dies with 'an error occurred at file.pl line ##'

Halford