use strict;
use v5.10;

use MooseX::Declare;
use Moose::Util::TypeConstraints;

subtype 'PositiveInt'
    => as 'Int'
    => where { $_ > 0 }
    => message { "Number '$_' is not positive" };
    

class BankAccount {
    use MooseX::StrictConstructor;
    
    has balance => ( isa => 'Int', is => 'rw', default => 0 );
    
    method deposit ( PositiveInt :$amount ) {
        $self->balance( $self->balance + $amount );
    }
}


my $a = BankAccount->new( balance => 100 );

$a->deposit( amount => 50 );

say $a->balance;