But your Haskell-ripoff is a at least a step forward - thanks.

With enough hacking you can make anything look declarative!

use v5.14;
use strict;
use warnings;

# Ugly hack code
#
package MooseX::AttributeShouldIgnoreFalse {
    BEGIN { $INC{'MooseX/AttributeShouldIgnoreFalse.pm'} = __FILE__ };
    use Sub::Exporter -setup => {
        exports => ['attribute_should_ignore_false'],
        groups  => {
            default => ['attribute_should_ignore_false'],
        },
    };
    sub attribute_should_ignore_false {
        my @attrs = @_;
        my $meta  = (scalar caller)->meta;
        $meta->add_around_method_modifier(BUILDARGS => sub {
            my $orig = shift;
            my $self = shift;
            my $parm = $self->$orig(@_);
            $parm->{$_} || delete $parm->{$_} for @attrs;
            $parm;
        });
    }
}

# Lovely declarative code
#
package Foo {
    use Moose;
    use MooseX::AttributeShouldIgnoreFalse;
    has foo => (is => 'ro', default => 'whatever');
    attribute_should_ignore_false 'foo';
}

my $value = 'hello';
print Foo->new(foo => $value)->dump;

$value = '';
print Foo->new(foo => $value)->dump;
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package Cow { use Moo; has name => (is => 'lazy', default => sub { 'Mooington' }) } say Cow->new->name

Were getting here :-)

Ideally I would want to declare everything at the attriute-level, e.g.:

has foo => (is => "ro", default => { value => "whatever", when => "fal
+se" });
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Can you do that as well?

I am not so much interested in the feature itself, but it is interesting to see some examples of Moose-metaprogramming at work...

Thanks!