#!/usr/bin/perl -w
use strict;
use re 'eval';

use Do_notation;

my $question="Div Div Con 18 Con 2 Div Con 6 Con 3";

my @ans=ev($question)->(0);
print "ans=@ans\n";

sub ev
{
	my $term = $_[0];
	my $con  = qr{Con (\d+)};
	my $div; $div = qr{Div ($con|(??{$div})) ($con|(??{$div}))};
	
	if($term =~ /^$con/)
	{
		return Return($1);
	}
	elsif($term=~/^$div/)
	{
		my $numer=$1; my $denom=$3;
		return DO {
					my $n <- ev($numer);
					my $d <- ev($denom);
					my $g <- tick();
					Return( $n / $d );
			      }
	}else{die "Syntax Error: $term";}
}

sub Return
{
    my $val = $_[0];
    sub {
            my $state = $_[0];
            ($val, $state)
        }
}

sub Bind
{
    my $func1 = $_[0];
    sub {
            my $func2 = $_[0];
            sub {
                    my $initial_state=$_[0];
                    (my $a, my $next_state) = $func1->($initial_state);
                    (my $b, my $final_state)=($func2->($a))->($next_state);
                    ($b, $final_state)
                }
        }
}

sub tick 
{
	sub{
			(undef,$_[0]+1)
		}
}