MonadPHP

This is a basic Monad library for PHP.

Usage

Values are "wrapped" in the monad via either the constructor: new MonadPHP\Identity($value) or the unit() method on an existing instance: $monad->unit($value);

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

use MonadPHP\Identity;
$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.

 use MonadPHP\Identity;
$monad = new Identity(1);
$monad->bind(function($value) {
        return 2 * $value;
    })->bind(function($value) { 
        var_dump($value); 
    });
// Prints int(2)

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

use MonadPHP\Identity;
$monad = new Identity(1);
var_dump($monad->extract());
// Prints int(1)

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.

use MonadPHP\Maybe;
$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)...

The included Chain monad does the same thing, but providing a short-cut implementation for objects:

use MonadPHP\Chain;
$monad = new Chain($someChainableObject);
$obj = $monad->call1()->call2()->nonExistantMethod()->call4()->extract();
var_dump($obj);
// null

This can prevent errors when used with chaining...

List Monad

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

use MonadPHP\ListMonad;
$monad = new ListMonad(array(1, 2, 3, 4));
$doubled = $monad->bind(function($value) { return 2 * $value; });
var_dump($doubled->extract());
// 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...

Composition

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

use MonadPHP\ListMonad;
use MonadPHP\Maybe;

$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 * $value; });
var_dump($doubled->extract());
// Prints array(2, 4, 6, null, 8)

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

use MonadPHP\ListMonad;
$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 * $value; });
var_dump($doubled->extract());
// 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" => "bob@example.com")),
    array("title" => "bar", "author" => array("name" => "Tom", "email" => "tom@example.com")),
    array("title" => "baz"),
    array("title" => "biz", "author" => array("name" => "Mark", "email" => "mark@example.com")),
);

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.

$postMonad = new MonadPHP\ListMonad($posts);
$names = $postMonad
    ->bind(MonadPHP\Maybe::unit)
    ->bind(index("author"))
    ->bind(index("name"))
    ->extract();