Closure examples.

counter.js

// This works on Rhino.

var counter = (function () {
  var i = 1;
  return function () {
    return i++;
  };
})();

print(counter()); // 1
print(counter()); // 2
print(counter()); // 3
print(counter()); // 4
print(counter()); // 5

counter.php

<?php
$i = 1;
$counter = function () use (&$i) {
    return $i++;
};

echo $counter() . PHP_EOL; // 1
echo $counter() . PHP_EOL; // 2
echo $counter() . PHP_EOL; // 3
echo $counter() . PHP_EOL; // 4
echo $counter() . PHP_EOL; // 5

counter.pl

use strict;
use warnings;
use feature qw(say);

my $counter = sub {
    my $i = 1;
    return sub {
        return $i++;
    };
}->();

say $counter->(); # 1
say $counter->(); # 2
say $counter->(); # 3
say $counter->(); # 4
say $counter->(); # 5

counter.py

# Not closure but generator.

def counter():
  i = 0
  while (True):
    i += 1
    yield i

counter = counter()
print counter.next() # 1
print counter.next() # 2
print counter.next() # 3
print counter.next() # 4
print counter.next() # 5

counter.rb

counter = lambda do
  i = 0
  lambda { i += 1 }
end.call

puts counter.call # 1
puts counter.call # 2
puts counter.call # 3
puts counter.call # 4
puts counter.call # 5

counter_error.php

<?php
// This can't work because of parse error...

$counter = (function () {
    $i = 0;
    return function () use (&$i) {
        return $i++;
    };
})();

echo $counter() . PHP_EOL;
echo $counter() . PHP_EOL;
echo $counter() . PHP_EOL;
echo $counter() . PHP_EOL;
echo $counter() . PHP_EOL;

counter_invoke.php

<?php
class Counter
{
    protected $_i = 1;

    public function __invoke()
    {
        return $this->_i++;
    }
}

$counter = new Counter;
echo $counter() . PHP_EOL; // 1
echo $counter() . PHP_EOL; // 2
echo $counter() . PHP_EOL; // 3
echo $counter() . PHP_EOL; // 4
echo $counter() . PHP_EOL; // 5

counter_tostring.php

<?php
class Counter
{
    protected $_i = 1;

    public function __toString()
    {
        return (string) $this->_i++;
    }
}

$counter = new Counter;
echo $counter . PHP_EOL; // 1
echo $counter . PHP_EOL; // 2
echo $counter . PHP_EOL; // 3
echo $counter . PHP_EOL; // 4
echo $counter . PHP_EOL; // 5