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

use Math::BigInt;
use Tk;

sub factorial {
  my ($n) = @_;
  return 1 if $n == 0;
  no warnings 'recursion';
  return factorial($n-1) * $n
}

my $mw = MainWindow->new(-title => 'Factorial Calculator');

my $result;
my $value;
my $v = $mw->Entry(
    -width        => 6,
    -relief       => 'sunken',
    -textvariable => \$value,
);

my $calculate = sub {
    # Stringify to prevent the conversion to int or float!
    $result = "" . factorial(Math::BigInt->new($value));
};
$v->bind('<Return>', $calculate);

my $desc = $mw->Label(-text => 'Factorial is:',);
my $r    = $mw->Label(-textvariable => \$result,);
my $calc = $mw->Button(-text => 'Calculate', -command => $calculate);
my $q    = $mw->Button(-text => 'QUIT', -command => sub { $mw->DESTROY },);

Tk::grid($v, $desc, $r,   -padx => 10, -pady => 10);
Tk::grid($calc, '-', '-', -padx => 10, -pady => 5);
Tk::grid($q, '-', '-',    -padx => 10, -pady => 5);

Tk::MainLoop();