#!/usr/bin/perl

use strict;
use warnings;

package LispTree;
use Moose;
use overload '""' => \&to_s;

has 'op'    => ( is => 'ro' );
has 'l'     => ( is => 'rw' );
has 'r'     => ( is => 'rw' );

sub to_s {
    my $self = shift;
    my $o  = '(' . $self->op . ' ';
       $o .= ref $self->l ? $self->{l}->to_s : $self->l;
       $o .= ' ';
       $o .= ref $self->r ? $self->{r}->to_s : $self->r;
       $o .= ')';
    return $o;
}

sub to_rpn {
    my $self = shift;
    my $o  = ref $self->l ? $self->{l}->to_rpn : $self->l;
       $o .= ' ';
       $o .= ref $self->r ? $self->{r}->to_rpn : $self->r;
       $o .= ' ' . $self->op;
    return $o;
}

package main;

my $root = LispTree->new( op => '+' );
$root->l(LispTree->new( op => '+', l => 6, r => 6 ));

my $leaf = LispTree->new( op => '*', l => 15, r => 2 ); 
$root->r($leaf);

print $root->to_rpn . "\n";