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Monad for CakePHP 2

This project refers to ircmaxell's monad-php library, and arranged that CakePHP can use Monad. The entry of his blog is also useful.


Basic usage is similar to monad-php. Values are "wrapped" in the monad via either the constructor: new Identity($value) or the unit() method on an existing instance: $monad->unit($value);

Functions can be called on the wrapped value using bind():

App::uses('Identity', 'Monaca.Lib');
$monad = new Identity(1);
$monad->bind(function($value) { var_dump($value); });
// Prints int(1)

All calls to bind return a new monad instance wrapping the return value of the function.

App::uses('Identity', 'Monaca.Lib');
$monad = new Identity(1);
$monad->bind(function($value) {
        return 2 * $value;
    })->bind(function($value) { 
// Prints int(2)

Additionally, "extracting" the raw value is supported as well (since this is PHP and not a pure functional language)...

App::uses('Identity', 'Monaca.Lib');
$monad = new Identity(1);
// Prints int(1)

Also, optional getter is useful. get($key), getOrElse($key, $default), getOrCall($key, callable $function), getOrThrow($key, Exception $ex), the first argument of these functions is to specify offset or key of array if the value might be array. If the value is non-array, use null for the first argument.

App::uses('Identity', 'Monaca.Lib');
$monad = new Identity(1);
// Prints int(1)

$monad = new Identity(array('foo'=>100));
// Prints int(100)

$monad = new Identity(null);
$default = 100;
var_dump($monad->getOrElse(null, $default));
// Prints int(100)

$monad = new Identity(null);
$function = function() { return 1; };
var_dump($monad->getOrCall(null, $function);
// Prints int(1)

$monad = new Identity(null);
$exception = new Exception('That is NULL!!!');
var_dump($monad->getOrThrow(null, $exception);
// Throws specified exception.

Maybe Monad

One of the first useful monads, is the Maybe monad. The value here is that it will only call the callback provided to bind() if the value it wraps is not null.

App::uses('Maybe', 'Monaca.Lib');
$monad = new Maybe(1);
$monad->bind(function($value) { var_dump($value); });
// prints int(1)

$monad = new Maybe(null);
$monad->bind(function($value) { var_dump($value); });
// prints nothing (callback never called)...

List Monad

This abstracts away the concept of a list of items (an array):

App::uses('ListMonad', 'Monaca.Lib');
$monad = new ListMonad(array(1, 2, 3, 4));
$doubled = $monad->bind(function($value) { return 2 * $value; });
// Prints array(2, 4, 6, 8)

Note that the passed in function gets called once per value, so it only ever deals with a single element, never the entire array...

It also works with any Traversable object (like iterators, etc). Just be aware that returning the new monad that's wrapped will alwyas become an array...

Hash Monad

This abstracts away the concept of a hash of items (an array):

App::uses('HashMonad', 'Monaca.Lib');
$monad = new HashMonad(array(
  'foo' => 1, 
  'bar' => 2,
  'buz' => 3
$doubled = $monad->bind(function($value) { return 2 * $value; });
// Prints array('foo' => 2, 'bar' => 4, 'buz' => 6)


These Monads can be composed together to do some really useful things:

$monad = new ListMonad(array(1, 2, 3, null, 4));
$newMonad = $monad->bind(function($value) { return new Maybe($value); });
$doubled = $newMonad->bind(function($value) { return 2 * $value; });
// Prints array(2, 4, 6, null, 8)

Or, what if you want to deal with multi-dimensional arrays?

$monad = new ListMonad(array(array(1, 2), array(3, 4), array(5, 6)));
$newMonad = $monad->bind(function($value) { return new ListMonad($value); });
$doubled = $newMonad->bind(function($value) { return 2 * $value; });
// Prints array(array(2, 4), array(6, 8), array(10, 12))

There also exist helper constants on each of the monads to get a callback to the unit method:

$newMonad = $monad->bind(Maybe::UNIT);
// Does the same thing as above 

Real World Example

Imagine that you want to traverse a multi-dimensional array to create a list of values of a particular sub-key. For example:

$posts = array(
    array("title" => "foo", "author" => array("name" => "Bob", "email" => "")),
    array("title" => "bar", "author" => array("name" => "Tom", "email" => "")),
    array("title" => "baz"),
    array("title" => "biz", "author" => array("name" => "Mark", "email" => "")),

What if we wanted to extract all author names from this data set. In traditional procedural programming, you'd likely have a number of loops and conditionals. With monads, it becomes quite simple.

First, we define a function to return a particular index of an array:

function index($key) {
    return function($array) use ($key) {
        return isset($array[$key]) ? $array[$key] : null;

Basically, this just creates a callback which will return a particular array key if it exists. With this, we have everything we need to get the list of authors.

$postMonad = new ListMonad($posts);
$names = $postMonad

Follow through and see what happens!

For CakePHP

Added MonadController which has useful functions.

  • retrieve session functions
  • retrieve configure functions
  • retrieve post/get parameters functions
  • the view can retrieve viewVars

Example code:

App::uses('MonadController', 'Monaca.Lib');

class SomeController extends MonadController {
  public function index() {
    // you can retrieve session params in optional ways
    $aSession = $this->getSession('a');
    $bSession = $this->getSessionOrElse('b', 'default value');
    $cSession = $this->getSessionOrCall('c', function() { LogError('No session c'); return 'default'; });
    $dSession = $this->getSessionOrThrow('d', new Exception('Session d should not empty'));
    // you can retrieve configure params in optional ways
    // Also you can use getConfigOrElse, getConfigOrCall, getConfigOrThrow 
    $aConfig = $this->getConfig('a');
    // you can retrieve post/get params in optional ways
    // Also you can use getPostOrElse, getPostOrCall, getPostOrThrow, getQueryOrElse, getQueryOrCall, getQueryOrThrow
    $aPost = $this->getPost('a');
    $aGet = $this->getQuery('a');
    // you can retrieve post or get params.
    // Also: getInputOrElse, getInputOrCall, getInputOrThrow
    $aInput = $this->getInput('a');
    $this->set('foo', 'bar');

// In view, you can retrieve viewVars in optional ways // also: getOrCall, getOrThrow

  <li><?php echo $this->getOrElse('foo', 'bar'); ?></li>

If your Controller cannot extend MonadController, you can implement these functions by using components and view.

App::uses('MonadComponent', 'Monaca.Controller/Component');
App::uses('MonadView', 'Monaca.View');

class FooController extends Controller {
  $components = array('Monaca.Monad');  // add Monaca.Monad
  public function __constructor($request = null, $response = null) {
    parent::__construct($request, $response);
    // Specify viewClass somewhere you want to use MonadView.
    $this->viewClass = 'Monaca.Monad';
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