The type is specified using isa, and you need lazy => 1 to make sure the default isn't calculated too soon. 

use Moose;

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

has 'y' => (
    is      => 'rw',
    isa     => 'Num',
    lazy    => 1,
    default => sub { sqrt($_[0])->x) },
);

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In reply to Re^2: Moose and BUILD by ikegami
in thread Moose and BUILD by tj_thompson

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