#!/usr/bin/perl -w
use strict;
use warnings;

my @stack = ();

sub binop {
        my ($op,   $prec)   = @_;
        my ($rhop, $rhprec) = @{pop @stack};
        my ($lhop, $lhprec) = @{pop @stack};

        # Wrap operand(s) with lower precedence
        $lhop = '(' . $lhop . ')' if $prec > $lhprec;
        $rhop = '(' . $rhop . ')' if $prec > $rhprec;

        push @stack, [ $lhop . $op . $rhop, $prec ];
}

my %handlers = (
        'push'  => sub { push @stack, [shift, 999]; },
        'print' => sub { print "out: '" . ${@{pop @stack}}[0] . "'\n"; },
        'add'   => sub { binop '+', 1; },
        'mul'   => sub { binop '*', 2; }
);

while (<DATA>) {
        chomp;

        print "TOS='" . (@stack ? $stack[$#stack][0] : '-nil-') . "' "
        .       "stmt = '" . $_ . "'\n";

        my ($operator, $operand) = split;
        if ($handlers{$operator}) {
                $handlers{$operator}->($operand);
        }
        else {
                print "Unknown operator: '$operator' for line '$_'\n";
        }
}
__DATA__
push 3
push 5
add
push apples
add
push oranges
push 9
push bananas
push peels
mul
add
add
mul
print