Functors.php

<?php

interface Functor{
    public function __invoke($function);
}

abstract class AFunctor implements Functor{
    protected $_storage = null;
    public function __construct($data = null){
        $this->set($data);
    }
    
    public function get(){
        return $this->_storage;
    }
    
    public function set($data){
        $this->_storage = $data;
    }
}

class JustANumber extends AFunctor{
    public function __invoke($function){
        return new JustANumber($function($this->_storage));
    }
}

class JustAnArray extends AFunctor{
    public function __invoke($function){
        return new JustAnArray(array_map(function($value) use ($function){
            return $function($value);
        }, $this->_storage));
    }
}

function fMap($function, $functor){
    if(!$functor instanceof Functor){
        throw new Exception('Not a functor');
    }
    return $functor($function);
}

$square = function($num){return $num*$num;};

var_dump(fmap($square, new JustANumber(2)));
var_dump(fmap($square, new JustAnArray(array(1,2,3,4))));