package Foo;

use Moose;

has 'y' => (
    is => 'Num',
    is => 'rw'
);

has 'x' => (
    is => 'Num',
    is => 'rw'
);

sub init_y {
    my ($self, $value) = @_;
    $self->y(sqrt($self->x));
}

sub BUILD {
    my ($self) = @_;

    print STDERR "Foo::BUILD running\n";
    $self->init_y;
}

1;