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

my @stack = ();

sub binop {
        my $op = shift;
        my $prec = shift;

        my $lh = pop @stack;
        my ($lhop, $lhprec) = ($$lh[0], $$lh[1]);

        my $rh = pop @stack;
        my ($rhop, $rhprec) = ($$rh[0], $$rh[1]);

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

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

while (<DATA>) {
        chomp;

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

        my ($operator, $operand) = split;
        if ($operator eq 'push') {
                # A constant is atomic, so it has high precedence so
                # we don't wrap it
                push @stack, [$operand, 999];
        }
        elsif ($operator eq 'print') {
                my $op = pop @stack;
                my ($operand, $prec) = ($$op[0], $$op[1]);
                print "out: '" . $operand . "'\n";
        }
        elsif ($operator eq 'add') {
                binop '+', 1;
        }
        elsif ($operator eq 'mul') {
                binop '*', 2;
        }
}
__DATA__
push 3
push 5
add
push 7
push 9
add
mul
print

##</code><code>##

root@swill ~/PerlMonks
$ ./stack_to_infix2.pl
TOS='-nil-' stmt = 'push 3'
TOS='3' stmt = 'push 5'
TOS='5' stmt = 'add'
TOS='5+3' stmt = 'push 7'
TOS='7' stmt = 'push 9'
TOS='9' stmt = 'add'
TOS='9+7' stmt = 'mul'
TOS='(9+7)*(5+3)' stmt = 'print'
out: '(9+7)*(5+3)'

root@swill ~/PerlMonks
$

##</code><code>##

    my $op = pop @stack;
    my ($operand, $prec) = ($$op[0], $$op[1]);