PHP inheritance and different parameters in childs

Question

I'm trying to create something similar to generics in PHP. I want other programmers to be able to create their own supported data types, so I used design patterns called Fasade, Adapter and Factory.

I have a main class that is a factory that creates concrete objects that share a uniform interface.

Factory:

class ListFactory
{
    public function __construct(private readonly array $listClasses) {}

    public function create(string $type)
    {
        return new ($this->listClasses[$type]);
    }
}

The $listClasses variable is ideally injected via DI and contains all classes that implement the ListInterface in the format ['type' => 'className'] ...

So now I have an interface that defines only one method so far:

ListInterface:

interface ListInterface
{
    public function add($item): static;
}

and then so far I have two concrete classes that implement this interface:

IntList:

class IntList implements ListInterface
{
    private array $value = [];

    public function add($item): static
    {
        if (!is_int($item)) { /* throw exception */}
        $this->value[] = $item;
        return $this;
    }
}

String List:

class StringList implements ListInterface
{
    private array $value = [];

    public function add($item): static
    {
        if (!is_strnig($item)) { /* throw exception */}
        $this->value[] = $item;
        return $this;
    }
}

the question is how to make it so that I don't have to validate the data type with if, but I could write, for example, in the StringList class the add function like this:

public function add(string $item): static
{
    $this->value[] = $item;
    return $this;
}

If I try to do it now, I get an error like that add method in the StringList class must be compatible with the add method inside the ListInterface

Thanks

Answer 1

The signature must match as your error message stated. To make it easy for every add function of that interface you could use a trait and do the type checking in there.

interface ListInterface {
    public function add(mixed $item): static;
}

trait ListTrait {
    public function add(mixed $item): static
    {
        switch(gettype($item)) {
            case 'integer':
                if(!$this instanceof IntList) throw new Exception("Int expected");
                break;
            case 'string':
                if(!$this instanceof StringList) throw new Exception("String expected");
                break;
        }
        $this->value[] = $item;
        return $this;
    }
}

class IntList implements ListInterface
{
    use ListTrait;
    private array $value = [];
}

class StringList implements ListInterface
{
    use ListTrait;
    private array $value = [];
}

Example usage

$i = new IntList();
$i->add(123);
$i->add("123"); // Exception!

$s = new StringList();
$s->add("123");
$s->add(123); // Exception!