PHP immutable set

set.class.php

<?php

function set() {
  return new Set(func_get_args());
}

class Set implements Countable, Iterator {
  private $data = array();

  function __construct($data) {
    $this->data = $this->unique_array($data);
  }

  /** Countable implementation **/
  function count  () { return count($this->data);                 }
  /** End Countable implementation **/

  /** Iterator implementation **/
  function current() { return current($this->data);               }
  function key    () { return $this->hash(current($this->data));  }
  function next   () { next($this->data); return $this;           }
  function rewind () { reset($this->data); return $this;          }
  function valid  () { return TRUE;                               }
  /** End Iterator implementation **/

  /** "Mutation" operations return a new Set **/
  function map($fn) {
    return new Set(array_map($fn, $this->data));
  }

  function filter($fn) {
    return new Set(array_filter($this->data, $fn));
  }

  function append($value) {
    return new Set(array_merge($this->data, func_get_args()));
  }

  function remove($value) {
    $args = func_get_args();
    return $this->filter(function($x) use ($args) {
      return !in_array($x, $args);
    });
  }

  function concat(Set $that) {
    return new Set(array_merge($this->data), $that->as_array());
  }
  /** End "Mutation" operations **/

  /** Boolean existence test **/
  function contains($value) {
    return isset($this->data[$this->hash($value)]);
  }

  /** Native array **/
  function as_array() {
    return array_values($this->data);
  }

  /** String for prettyprint **/
  function __toString() {
    return sprintf('Set(%s)', implode(', ',
      array_map(array($this, 'inspect'), $this->data)
    ));
  }

  /** Key is hash of serialized value **/
  private function hash($data) {
    return sha1(serialize($data));
  }

  /** If we allow data to be passed in by reference (e.g nested arrays) **/
  /** Then there is a risk it will get modified from outside.           **/
  private function copy($data) {
    return unserialize(serialize($data));
  }

  /** Convert an arbitrary array into one with unique values keyed on hash **/
  private function unique_array($arr) {
    return array_combine(
      array_map(array($this, 'hash'), $arr),
      array_map(array($this, 'copy'), $arr)
    );
  }

  /** Output variable type and value for prettyprint **/
  private function inspect($var) {
    ob_start();
    var_dump($var);
    return trim(ob_get_clean());
  }

}