Re: [RFC] Data Classes

On 23/11/2024 16:05, Rob Landers wrote:

Your RFC doesn't discuss this - the changeName example shows behaviour *inside* the method, but not behaviour when *calling* it

An interesting observation, can you explain more as to what you mean?

Looking closer, there's a hint at what you expect to happen in your Rectangle example: $bigRectangle = $rectangle->resize(10, 20); assert($bigRectangle !== $rectangle); // true It seems that modifications to $this aren't visible outside the method, creating a purely local clone, which would be discarded if it wasn't returned (or saved somewhere). I can see the logic, but the result is a bit unintuitive: data class Example {    public function __construct(public int $x) {}    public function inc(): void {

     $this->x++;

  } } $foo = new Example(0); $foo->x++; $foo->inc(); echo $foo->x; // 1, not 2 I think it would be clearer to prevent direct modification of $this: data class Example {    public function __construct(public int $x) {}    public function inc(): void {

      $this->x++; // ERROR: Can not mutate $this in data class

   }    public function withInc(): static {

      $new = $this; // explicitly make a local copy of $this
      $new->x++; // copy-on-write separates $new from $this
      return $new;

   } } That would still be compatible with Ilija's suggestion, which was to add special "mutating methods": data class Example {

    public function __construct(public int $x) {}
    public mutating function inc(): void {
        $this->x++;
    }

} $foo = new Example(0); $foo->x++; $foo->inc!(); // copy-on-write triggered *before* the method is called echo $foo->x; // 2 -- Rowan Tommins [IMSoP]