Monad.php

<?php

class Only{
    protected $_container = null;
    
    public function __construct($value){
        $this->_container = $value;
    }
    
    public function get(){
        return $this->_container;
    }
    
    public static function pack($value){
        return new static($value);
    }
    
    public function map($func){
        return $func($this->_container);
    }
}

//First Law: Left Identity
//Result of function applied to value
//should be the same as wrapped value being applied
//to value
$addTwo = function($value){return $value + 2;};

var_dump($addTwo(2) == Only::pack(2)->map($addTwo));

//Second Law: Right Identity
//Passing in a function that does not operate on
//supplied value, but only calls pack() must return
//same value as what is bound in monad ie 2
$only = Only::pack(2);
$only_applied = $only->map(function($value){
   return Only::pack($value); 
});
var_dump($only->get() === $only_applied->get());


//Third Law: Nesting map() calls must produce the same
//result as when they are called subsequentially
$addTwo = function($value){return Only::pack($value + 2);};
$subtractTwo = function($value){return Only::pack($value - 2);};

$only = Only::pack(2)->map($addTwo)->map($subtractTwo);
$onlyDirr = Only::pack(2)->map(function($value) use ($addTwo, $subtractTwo){
    return $addTwo($value)->map(function($value)use($subtractTwo){
        return $subtractTwo($value);
    });
});
var_dump($only->get() === $onlyDirr->get());