How to make Data::Printer show stringified values in internals? - perl

Question

I have some complex object structures, and I use Data::Printer to inspect them. One case where it's not helpful enough is this: when an object (container) has a field that's another object (child), the child shows up in DDP's output only as class name. I wish to also see the stringified value of the child.
Let's have an example:
{
package Child;
use Moo;
use overload '""' => "stringify";
has 'value', is => 'ro';
sub stringify {
my $self = shift;
return "<Child:" . $self->value . ">";
}
}
{
package Container;
use Moo;
has 'child', is => 'ro';
}
my $child_x = Child->new(value => 'x');
print "stringified child x: $child_x\n";
my $child_y = Child->new(value => 'y');
print "stringified child y: $child_y\n";
my $container_x = Container->new(child => $child_x);
my $container_y = Container->new(child => $child_y);
use DDP;
print "ddp x: " . p($container_x) . "\n";
print "ddp y: " . p($container_y) . "\n";
Output:
stringified child x: <Child:x>
stringified child y: <Child:y>
ddp x: Container {
Parents Moo::Object
public methods (2) : child, new
private methods (0)
internals: {
child Child # <- note this
}
}
ddp y: Container {
Parents Moo::Object
public methods (2) : child, new
private methods (0)
internals: {
child Child # <- and this
}
}
As you see, the children are indistinguishable in the output. I'd like to see the stringification in that place, either in addition to or instead of the class name.

Answer 1

Per the Data::Printer docs,
Data::Printer offers you the ability to use filters to override any
kind of data display. The filters are placed on a hash, where keys are
the types - or class names - and values are anonymous subs that
receive two arguments: the item itself as first parameter, and the
properties hashref (in case your filter wants to read from it). This
lets you quickly override the way Data::Printer handles and displays
data types and, in particular, objects.

Answer 2

As Dave pointed out, we can define the filter when importing Data::Printer:
use DDP filters => {
'Child' => sub { "$_[0]" }
};
And even better way would be to use the _data_printer feature (because it's a pain to type out the filters definition every time I import DDP):
{
package Child;
...
sub _data_printer {
my $self = shift;
return "$self";
}
}
Both ways display the stringified value in internals:
ddp x: Container {
Parents Moo::Object
public methods (2) : child, new
private methods (0)
internals: {
child <Child:x>
}
}