Is it possible to make a unique value in PHP? - php

Question

I'm making a array class and want a value to be able to be returned by a higher order function. The idea is that its a instance constant or method returned value such that I can skip the value in a map.
In other languages making an array or some compound value, like ['skip'] will make it pointer equal such that I can then use the operator for pointer equal and it will not be equal to other arrays with the exact same content, but my problem is that ['skip'] === ['skip'] is true so even with === the two values are the same.
Here is an example of usage of my code where I accedentally have the same value as I used to skip:
namespace Test;
use Common\Domain\Collection;
$arr = new Collection();
$arr[] = 1;
$arr[] = 2;
$arr[] = 3;
$arr[] = 4;
echo count($arr); // prints 4
$arr2 = $arr->map(function ($v) {
return $v % 2 == 0 ? Collection::SKIP : ["skip"];
});
echo count($arr2); // prints 0, but should be 2
Is there a way to get a unique value or work around this somehow?
Here is code that implements Collection:
namespace Common\Domain;;
class Collection implements \Iterator, \Countable, \ArrayAccess
{
const SKIP = ["skip"];
private $arr = [];
public function map(callable $fn, bool $keepKeys = false) :Collection
{
$arr = new static();
$nOrder = 0;
foreach($this->arr as $key => $value) {
$result = call_user_func($fn, $value, $key, $nOrder, $this);
if($result !== self::SKIP) {
if($keepKeys) {
$arr[$key] = $result;
} else {
$arr[] = $result;
}
}
}
return $arr;
}
// implementation of interfaces \Iterator, \Countable, \ArrayAccess
public function current()
{
return current($this->arr);
}
public function next()
{
next($this->arr);
}
public function key()
{
return key($this->arr);
}
public function valid()
{
return isset($this->arr[$this->key()]);
}
public function rewind()
{
reset($this->arr);
}
public function count()
{
return count($this->arr);
}
public function offsetExists($offset)
{
return array_key_exists($offset, $this->arr);
}
public function offsetGet($offset)
{
return $this->arr[$offset];
}
public function offsetSet($offset, $value)
{
$this->arr[$offset] = $value;
}
public function offsetUnset($offset)
{
unset($this->arr[$offset]);
}
}

Answer 1

I guess you are looking for Java-type enumerations, which doesn't exist in PHP. My best guess on your problem would be to use an object instead of a constant, that you would instantiate statically for a convenient use. Then, in the loop of your map function, you check the value with an instanceof instead of the basic equality operator, against the class you defined.
So, here :
class UniqueValue
{
public static function get()
{
return new self();
}
}
Then :
$arr2 = $arr->map(function ($v) {
return $v % 2 == 0 ? UniqueValue::get() : ["skip"];
});
And inside your collection :
public function map(callable $fn, bool $keepKeys = false) :Collection
{
$arr = new static();
$nOrder = 0;
foreach($this->arr as $key => $value) {
$result = call_user_func($fn, $value, $key, $nOrder, $this);
if($result ! instanceof UniqueValue) {
if($keepKeys) {
$arr[$key] = $result;
} else {
$arr[] = $result;
}
}
}
return $arr;
}
This is the quickest approach I can think of. If your array contains data from "outside" I don't think it's possible in any way that it matches against a class check from your own code.

Answer 2

I would solve this by implementing another method for this. The method delete would map a function over the collection and remove any elements where the function returns false.
e.g.
class Collection
{
// ...
public function delete($func)
{
$result = new static();
foreach($this->arr as $item)
{
if($func($item) !== false) $result[] = $item;
}
}
}
// example
$arr = new Collection();
$arr[] = 1;
$arr[] = 2;
$arr[] = 3;
$arr[] = 4;
echo count($arr); // prints 4
$arr2 = $arr->delete(function ($v) {
return $v % 2 ? true : false;
});
var_dump($arr2); // prints [2, 4]