Playing around with functors in PHP

functor.php

<?php

interface Functor
{
	// fmap :: Functor f => (a -> b) -> f a -> f b
	public function fmap(callable $fn): Functor;
}

class Fn implements Functor
{
	private $fn;
	
	public function __construct(callable $fn)
	{
		$this->fn = $fn;
	}
	
	/**
	 * Mapping over functions is just function composition!
	 */
	public function fmap(callable $fn): Functor
	{		
		return new Fn(function ($arg) use ($fn) {
			return call_user_func($this->fn, $fn($arg));
		});
	}
	
	public function __invoke($arg)
	{
		return call_user_func($this->fn, $arg);
	}
}

// EXAMPLE

class User 
{
	public $age;
	public $salary;
	
	public function __construct($age, $salary)
	{
		$this->age = $age;
		$this->salary = $salary;
	}
}

$users = [
	new User(23, 430000),
	new User(31, 280000),
	new User(54, 580000),
	new User(63, 750000),
	new User(18, 230000),
];

// filterOlderThan25 :: [User] -> [User]
$filterOlderThan25 = new Fn(function (array $users) { 
	return array_filter($users, function (User $user) { return $user->age >= 25; });
});

// extractSalaries :: [User] -> [Int]
$extractSalaries = new Fn(function (array $users) { 
	return array_map(function (User $user) { return $user->salary; }, $users);
});

// calculateAverage :: [Int] -> Double
$calculateAverage = new Fn(function (array $data) {
	return array_sum($data) / count($data);
});

// program :: [User] -> Double
$program = $calculateAverage
	->fmap($extractSalaries)
	->fmap($filterOlderThan25);
	
echo $program($users);
// 536666.66666667