How do I handle errors in methods chains in Perl?

Question

What is the best way to deal with exceptions threw in a method chaining in Perl? I want to assign a value of 0 or undef if any of the methods chained throw an exception

Answer 1

You can write a scalar method that will wrap a method chain in error handling:

my $try = sub {
    @_ > 1 or return bless {ok => $_[0]} => 'Try';

    my ($self, $method) = splice @_, 0, 2;
    my $ret;
    eval {
        $ret = $self->$method(@_);
    1} or return bless {error => $@} => 'Try';
    bless {ok => $ret} => 'Try'
};

{package Try;
    use overload fallback => 1, '""' => sub {$_[0]{ok}};
    sub AUTOLOAD {
        my ($method) = our $AUTOLOAD =~ /([^:]+)$/;
        $_[0]{ok} ? $_[0]{ok}->$try($method, @_[1..$#_]) : $_[0]
    }
    sub DESTROY {}
    sub error {$_[0]{error}}
}

to use it:

{package Obj;
    sub new {bless [0]}
    sub set {$_[0][0] = $_[1]; $_[0]}
    sub add {$_[0][0] += ($_[1] || 1); $_[0]}
    sub show {print "Obj: $_[0][0]\n"}
    sub dies  {die "an error occured"}
}

my $obj = Obj->new;

say "ok 1" if $obj->$try(set => 5)->add->add->show; # prints "Obj 7"
                                                    # and "ok 1"

say "ok 2" if $obj->$try('dies')->add->add->show;   # prints nothing 

say $obj->$try('dies')->add->add->show->error;  # prints "an error occured..."

The first line of the $try method also allows the following syntax:

say "ok 3" if $obj->$try->set(5)->add->add->show;