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Date: Sat, 23 Nov 2024 22:22:40 +0000
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Subject: Re: [PHP-DEV] [RFC] Data Classes
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From: imsop.php@rwec.co.uk ("Rowan Tommins [IMSoP]")

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On 23/11/2024 20:27, Rob Landers wrote:
> Interesting! I actually found it to be intuitive.
>
> Think of it like this:
>
> function increment(array $array) {
>   $array[0]++;
> }
>
> $arr = [0];
> increment($arr);
> echo $arr[0]; // is 0
>
> We don't expect $arr to be any different outside of the function 
> because $arr is a value, not a reference.


My mental model, rightly or wrongly, is that passing to a parameter is a 
bit like an assignment to a local variable:

function increment(array) {
    $array = $__args[0];
    $array[0]++;
}

(This is explicitly how subroutine parameters work in Perl; I don't know 
if that's affected my mental model, or just means Larry Wall pictured it 
the same way.)

You can even assign a new value to it, like any other variable:

function whatever(array $array) {
    $array = 'not even an array any more';
}

But in PHP, $this isn't a parameter, and it's never possible to assign a 
new value to $this; so it feels completely alien to have a method where 
$this stops referring to the current object, and becomes a local variable.


>> I think it would be clearer to prevent direct modification of $this:
>>
>
> Not that I disagree (see the records RFC), but at that point, why not 
> make data classes implicitly readonly?


I'm only suggesting restricting mutation on $this, not on the object 
itself. $foo->x++ would still work, and have automatic copy-on-write; 
but $this->x++ would be an error on a data class, just as $this=$bar is 
an error on all existing objects.


>> That would still be compatible with Ilija's suggestion, which was to 
>> add special "mutating methods":
>>
> I actually find this appealing, but it is strange to me to allow this 
> syntax on classes. Is there precedent for that? Or is there a way we 
> can do it using "regular looking PHP"; or are structs the way to go?


The way I see it, it's just a third type of method, to add to the two we 
already have:

- instance methods: $this refers to the current instance
- static methods: $this is forbidden
- mutating methods: $this refers to the desired result of the mutation

In fact, it's a bit like __construct or __clone, where $this refers to 
the newly created/copied object, before anything else points to it.


-- 
Rowan Tommins
[IMSoP]

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    <div class="moz-cite-prefix">On 23/11/2024 20:27, Rob Landers wrote:<br>
    </div>
    <blockquote type="cite"
      cite="mid:3e4506d7-227e-4521-a0c8-68e71fdc42a1@app.fastmail.com">
      <meta http-equiv="content-type" content="text/html; charset=UTF-8">
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      <div>Interesting! I actually found it to be intuitive.<br>
      </div>
      <div><br>
      </div>
      <div>Think of it like this:<br>
      </div>
      <div><br>
      </div>
      <div>function increment(array $array) {<br>
      </div>
      <div>  $array[0]++;<br>
      </div>
      <div>}<br>
      </div>
      <div><br>
      </div>
      <div>$arr = [0];<br>
      </div>
      <div>increment($arr);<br>
      </div>
      <div>echo $arr[0]; // is 0<br>
      </div>
      <div><br>
      </div>
      <div>We don't expect $arr to be any different outside of the
        function because $arr is a value, not a reference.</div>
    </blockquote>
    <p><br>
    </p>
    <p>My mental model, rightly or wrongly, is that passing to a
      parameter is a bit like an assignment to a local variable:<br>
    </p>
    <p>function increment(array) { <br>
         $array = $__args[0];<br>
         $array[0]++;<br>
      }</p>
    <p>(This is explicitly how subroutine parameters work in Perl; I
      don't know if that's affected my mental model, or just means Larry
      Wall pictured it the same way.)</p>
    <p>You can even assign a new value to it, like any other variable:</p>
    <p>function whatever(array $array) {<br>
         $array = 'not even an array any more';<br>
      }<br>
    </p>
    <p>But in PHP, $this isn't a parameter, and it's never possible to
      assign a new value to $this; so it feels completely alien to have
      a method where $this stops referring to the current object, and
      becomes a local variable.<br>
    </p>
    <p><br>
    </p>
    <blockquote type="cite"
      cite="mid:3e4506d7-227e-4521-a0c8-68e71fdc42a1@app.fastmail.com">
      <blockquote type="cite" id="qt" style="">
        <p>I think it would be clearer to prevent direct modification of
          $this:<br>
        </p>
      </blockquote>
      <div><br>
      </div>
      <div>Not that I disagree (see the records RFC), but at that point,
        why not make data classes implicitly readonly?<br>
      </div>
    </blockquote>
    <p><br>
    </p>
    <p>I'm only suggesting restricting mutation on $this, not on the
      object itself. $foo-&gt;x++ would still work, and have automatic
      copy-on-write; but $this-&gt;x++ would be an error on a data
      class, just as $this=$bar is an error on all existing objects.</p>
    <p><br>
    </p>
    <blockquote type="cite"
      cite="mid:3e4506d7-227e-4521-a0c8-68e71fdc42a1@app.fastmail.com">
      <blockquote type="cite" id="qt" style="">
        <div>That would still be compatible with Ilija's suggestion,
          which was to add special "mutating methods":<br>
        </div>
        <br>
      </blockquote>
      <div>I actually find this appealing, but it is strange to me to
        allow this syntax on classes. Is there precedent for that? Or is
        there a way we can do it using "regular looking PHP"; or are
        structs the way to go?<br>
      </div>
    </blockquote>
    <p><br>
    </p>
    <p>The way I see it, it's just a third type of method, to add to the
      two we already have:</p>
    <p>- instance methods: $this refers to the current instance<br>
      - static methods: $this is forbidden<br>
      - mutating methods: $this refers to the desired result of the
      mutation<br>
    </p>
    <p>In fact, it's a bit like __construct or __clone, where $this
      refers to the newly created/copied object, before anything else
      points to it.<br>
    </p>
    <p><br>
    </p>
    <pre class="moz-signature" cols="72">-- 
Rowan Tommins
[IMSoP]</pre>
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