Monads - PHP

collection.php

<?php

class CollectionMonad extends Monad {
	public function __construct(array $value) {
		$this->value = $value;
	}
	public function bind($callback) {
		$newValues = array();
		foreach ($this->value as $value) {
			$newValues[] = $this->callCallback($callback, $value);
		}
		return new CollectionMonad($newValues);
	}
}

error.php

<?php

class ErrorMonad extends Monad {
	protected $value;
        protected $error;
	public function __construct($value, $error) {
		$this->value = $value;
		$this->error = $error;
	}
	public function bind($callback) {
		$class = get_class($this);
                try {
			if (!$this->error) {
				return new $class($this->callCallback($callback, $this->value));
			}
			return new $class($this->value, $this->error);
		} catch (\Exception $e) {
			return new $class($this->value, $e->getMessage());
		}
	}
}

maybe.php

<?php

class MaybeMonad extends Monad {
	public function bind($callback) {
		if ($this->value) {
			return parent::bind($callback);
		}
		return new self(null);
	}
}

monad.php

<?php

class Monad {
	protected $value;
	public function __construct($value) {
		$this->value = $value;
	}
	public function bind($callback) {
		$class = get_class($this);
		return new $class($this->callCallback($callback, $this->value));
	}
	protected function callCallback($callback, $value) {
		if ($value instanceof Monad) {
			return $value->bind($callback);
		}
		return $callback($value);
	}
}