PHP Map object

Map.php

<?php
/*
EXAMPLES:

$map = new Map(array('foo' => 'bar', 'baz' => 'quux'));
$map = $map->withDefaultValue('default value');
$map = $map->withDefault('strtoupper');

var_dump($map->apply(array('foo', 'baz', 'quux'))); // bar, quux, QUUX
var_dump(array_map($map, array('foo', 'baz', 'quux'))); // bar, quux, QUUX
var_dump($map['foo']); // bar
var_dump($map('quux')); // QUUX

*/
class Map implements ArrayAccess
{
    private $store = array();
    private $deftype = false;
    private $def = null;

    public function __construct(array $data = null)
    {
        if ($data !== null) {
            $this->store = $data;
        }
    }

    public function __invoke($offset)
    {
        return $this->offsetGet($offset);
    }

    public function offsetExists($offset)
    {
        return array_key_exists($offset, $this->store);
    }

    public function offsetGet($offset)
    {
        if (array_key_exists($offset, $this->store)) {
            return $this->store[$offset];
        }
        switch ($this->deftype) {
            case 'val': return $this->def;
            case 'fun': return call_user_func($this->def, $offset);
            default: throw new Exception('Key does not exist: '.$offset);
        }
    }

    public function offsetSet($offset, $value)
    {
        $this->store[$offset] = $value;
    }

    public function offsetUnset($offset)
    {
        unset($this->store[$offset]);
    }

    public function withDefaultValue($value)
    {
        $new = clone $this;
        $new->deftype = 'val';
        $new->def = $value;
        return $new;
    }

    public function withDefault(callable $value)
    {
        $new = clone $this;
        $new->deftype = 'fun';
        $new->def = $value;
        return $new;
    }

    public function apply(array $values)
    {
        return array_map($this, $values);
    }

}