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Date: Tue, 2 Apr 2024 02:17:32 +0200
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Subject: [PHP-DEV] [RFC][Concept] Data classes (a.k.a. structs)
To: PHP internals <internals@lists.php.net>
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From: tovilo.ilija@gmail.com (Ilija Tovilo)

Hi everyone!

I'd like to introduce an idea I've played around with for a couple of
weeks: Data classes, sometimes called structs in other languages (e.g.
Swift and C#).

In a nutshell, data classes are classes with value semantics.
Instances of data classes are implicitly copied when assigned to a
variable, or when passed to a function. When the new instance is
modified, the original instance remains untouched. This might sound
familiar: It's exactly how arrays work in PHP.

```php
$a = [1, 2, 3];
$b = $a;
$b[] = 4;
var_dump($a); // [1, 2, 3]
var_dump($b); // [1, 2, 3, 4]
```

You may think that copying the array on each assignment is expensive,
and you would be right. PHP uses a trick called copy-on-write, or CoW
for short. `$a` and `$b` actually share the same array until `$b[] =
4;` modifies it. It's only at this point that the array is copied and
replaced in `$b`, so that the modification doesn't affect `$a`. As
long as a variable is the sole owner of a value, or none of the
variables modify the value, no copy is needed. Data classes use the
same mechanism.

But why value semantics in the first place? There are two major flaws
with by-reference semantics for data structures:

1. It's very easy to forget cloning data that is referenced somewhere
else before modifying it. This will lead to "spooky actions at a
distance". Having recently used JavaScript (where all data structures
have by-reference semantics) for an educational IR optimizer,
accidental mutations of shared arrays/maps/sets were my primary source
of bugs.
2. Defensive cloning (to avoid issue 1) will lead to useless work when
the value is not referenced anywhere else.

PHP offers readonly properties and classes to address issue 1.
However, they further promote issue 2 by making it impossible to
modify values without cloning them first, even if we know they are not
referenced anywhere else. Some APIs further exacerbate the issue by
requiring multiple copies for multiple modifications (e.g.
`$response->withStatus(200)->withHeader('X-foo', 'foo');`).

As you may have noticed, arrays already solve both of these issues
through CoW. Data classes allow implementing arbitrary data structures
with the same value semantics in core, extensions or userland. For
example, a `Vector` data class may look something like the following:

```php
data class Vector {
    private $values;

    public function __construct(...$values) {
        $this->values = $values;
    }

    public mutating function append($value) {
        $this->values[] = $value;
    }
}

$a = new Vector(1, 2, 3);
$b = $a;
$b->append!(4);
var_dump($a); // Vector(1, 2, 3)
var_dump($b); // Vector(1, 2, 3, 4)
```

An internal Vector implementation might offer a faster and stricter
alternative to arrays (e.g. Vector from php-ds).

Some other things to note about data classes:

* Data classes are ordinary classes, and as such may implement
interfaces, methods and more. I have not decided whether they should
support inheritance.
* Mutating method calls on data classes use a slightly different
syntax: `$vector->append!(42)`. All methods mutating `$this` must be
marked as `mutating`. The reason for this is twofold: 1. It signals to
the caller that the value is modified. 2. It allows `$vector` to be
cloned before knowing whether the method `append` is modifying, which
hugely reduces implementation complexity in the engine.
* Data classes customize identity (`===`) comparison, in the same way
arrays do. Two data objects are identical if all their properties are
identical (including order for dynamic properties).
* Sharing data classes by-reference is possible using references, as
you would for arrays.
* We may decide to auto-implement `__toString` for data classes,
amongst other things. I am still undecided whether this is useful for
PHP.
* Data classes protect from interior mutability. More concretely,
mutating nested data objects stored in a `readonly` property is not
legal, whereas it would be if they were ordinary objects.
* In the future, it should be possible to allow using data classes in
`SplObjectStorage`. However, because hashing is complex, this will be
postponed to a separate RFC.

One known gotcha is that we cannot trivially enforce placement of
`modfying` on methods without a performance hit. It is the
responsibility of the user to correctly mark such methods.

Here's a fully functional PoC, excluding JIT:
https://github.com/php/php-src/pull/13800

Let me know what you think. I will start working on an RFC draft once
work on property hooks concludes.

Ilija