#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

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

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

my $operations = {
    '*' => sub { $_[0] * $_[1] },
    '+' => sub { $_[0] + $_[1] },
    '/' => sub { $_[0] / $_[1] },
    '-' => sub { $_[0] - $_[1] },
};

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

sub calc {
    my $self = shift;
    my $l = blessed $self->l ? $self->{l}->calc : $self->l;
    my $r = blessed $self->r ? $self->{r}->calc : $self->r;
    return $operations->{$self->op}->( $l, $r );
}

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 . "\n" x 2 . $root->to_rpn . "\n" . $root->calc . "\n";