The Maybe Monad [1] in PHP inspired by Scala's implementation of this concept which they call Option [2]. [1] http://en.wikipedia.org/wiki/Monad_%28functional_programming%29#The_Maybe_monad [2] http://www.scala-lang.org/api/current/scala/Option.html

Option.php

<?php

interface Option/*[T]*/ {
  /**
   * @return T
   */
  public function get();

  /**
   * @param => T  $f
   * @return T
   */
  public function getOrElseLazy($f);

  /**
   * @param mixed  $alt
   * @return Option[mixed]
   */
  public function orElse($alt);

  /**
   * @param T => U  $f
   * @return Option[U]
   */
  public function map($f);

  /**
   * @return T
   */
  public function orNull();

  /**
   * @return bool
   */
  public function isDefined();

  /**
   * @return bool
   */
  public function isEmpty();
}

class None implements Option {
  public function get() {
    invariant_violation('Trying to get value from None');
  }

  public function getOrElse($alt) {
    return $alt;
  }

  public function getOrElseLazy($f) {
    return $f();
  }

  public function orElse($alt) {
    return option($alt);
  }

  public function map($f) {
    return none();
  }

  public function orNull() {
    return null;
  }

  public function isDefined() {
    return false;
  }

  public function isEmpty() {
    return true;
  }

  public function __toString() {
    return null;
  }
}

class Some {
  private $v;

  public function __construct($v) {
    $this->v = $v;
  }

  public function get() {
    return $this->v;
  }

  public function getOrElse($alt) {
    return $this->v;
  }

  public function orElse($alt) {
    return $this;
  }

  public function getOrElseLazy($f) {
    return $this->v;
  }

  public function orNull() {
    return $this->v;
  }

  public function map($f) {
    return new Some($f($this->v));
  }

  public function isDefined() {
    return true;
  }

  public function isEmpty() {
    return false;
  }

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

function none() {
  static $instance = null;
  if (!$instance) {
    $instance = new None();
  }
  return $instance;
}

function some($v) {
  return new Some($v);
}

function option($v) {
  if ($v === null) {
    return none();
  }
  return new Some($v);
}

Awesome. Thank you!

thank you